Langerhans Cell Histiocytosis

Synonym (classic terms for congenital self-limited variant): Hashimoto–Pritzker disease, congenital self-healing reticulohistiocytosis

• Crusted papules, vesiculopustules, erosions, or nodules; occasionally petechiae or purpura.
• May be limited to the skin; less commonly associated with extracutaneous involvement.
• Cutaneous infiltrates of S100+/CD1a+ Langerhans cells.
• Purely cutaneous form often resolves spontaneously within weeks to months.
• Both cutaneous and extracutaneous RELAPSES may occur; patients require LONGITUDINAL EVALUATION.

• Rare; can be evident at birth or present during the neonatal period with crusted papules, pustules, papulovesicles, nodules, erosions, and ulcerations (see Ch. 91).
• Hemangioma-like papules, petechiae, purpura, atrophy, milia, and mucosal erosions may also be seen.
• Lesions are often widespread but can be SOLITARY, and may continue to develop and evolve over the first days to weeks of life.
• LCH lesions can develop ANYWHERE on the skin surface, INCLUDING the palms and soles.
• Differential diagnosis: neonatal varicella or HSV infection, congenital candidiasis, congenital syphilis, intrauterine GVHD, and the early-onset form of AI.

• Proliferation of LANGERHANS CELLS — LARGE, OVOID cells with "COFFEE BEAN" NUCLEI — in the superficial dermis and occasionally in the epidermis (EPIDERMOTROPISM).
• Positive staining with S100, CD1a, and CD207 (LANGERIN).
• Ultrastructurally: characteristic BIRBECK GRANULES.

• Prompt evaluation for systemic involvement, including:
• Complete physical examination
• Chemistry panel including liver function tests
• Complete blood cell count with differential
• Urine osmolality
• Abdominal ultrasound
• Skeletal survey
• Chest X-ray
• Referral to ONCOLOGY with consideration of bone marrow aspiration or biopsy is recommended.
• Distinction between purely cutaneous and multisystem disease is NOT possible based solely on skin findings.
• The histologic and immunohistochemical features of "congenital self-healing reticulohistiocytosis" and disseminated LCH are INDISTINGUISHABLE — now thought to represent different ends of the LCH CLINICAL SPECTRUM.

• Disease limited to the skin: lesions usually resolve spontaneously over weeks to months; prognosis is VERY GOOD.
• RELAPSES of cutaneous or extracutaneous disease (e.g. diabetes insipidus due to PITUITARY involvement) may occur months or years later → importance of longitudinal evaluation.
• Prognosis of MULTISYSTEM congenital LCH is LESS FAVORABLE; systemic therapy is generally required.
• In the majority of patients, lesional Langerhans cells harbor somatic activating mutations in BRAF V600E or genes encoding additional kinases in the RAS–MAP–kinase pathway (see Ch. 113).
• BRAF V600E is MORE FREQUENT in YOUNGER children and has been associated with MULTISYSTEM disease and a GREATER LIKELIHOOD OF RELAPSE.
• Traditional therapy for multisystem LCH: VINBLASTINE + PREDNISONE; there is an emerging role for targeted treatment with BRAF and MEK inhibitors.

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• Granulomatous reactions = chronic inflammation patterns, may take long time to develop and respond to treatment; occur with inflammatory disorders (autoimmune, autoinflammatory), medication reactions, malignancies, infections
• Necrobiosis lipoidica (NL) = granulomatous disease discussed in Ch. 21 (strong diabetes association); reactive granulomatous dermatitides (PNGD, IGD, IGDR) discussed in Ch. 8 with connective tissue diseases

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• Relatively common idiopathic dermis/subcutaneous tissue disorder; all races/ages, women 3x more often than men
• Most cases spontaneously resolve leaving normal skin, but elastic tissue loss may leave atrophic lesions resembling middermal elastolysis or anetoderma
• ≥20-year follow-up: lesions usually heal, patients remain healthy without unusual disease development; GA may recur in significant subset (particularly generalized lesions); may exhibit isomorphic (Koebner) response, may affect healed herpes zoster areas; post-biopsy lesions sometimes spontaneously resolve
• Can be reactive to underlying disorders/medications, but in most patients is benign self-limited skin-only condition
• Many morphologies; vast majority localized disease, subset generalized, children may have subcutaneous lesions; other morphologies rare; some patients photodistributed GA (overlap with annular elastolytic giant cell granuloma); patients usually exhibit primarily one clinical type (except subcutaneous form, where typical localized GA may co-occur)

• Affects children and young-to-middle-aged adults; usually 1-few lesions at a time; usually lateral/dorsal fingers/hands or dorsal feet, occasionally elbows/ankles (rarely eyelid or even a Becker nevus)
• Erythematous-to-violaceous thinly-bordered plaques/papules, spread peripherally with central involution → roughly annular lesions; overlying skin usually completely normal; may coalesce into scalloped patterns/firm plaques
• Never ulcerate, virtually always resolve without residua (some atrophy, possibly treatment-related); develop slowly, often involute spontaneously; >50% clear within 2 years but may recur (some progress to generalized disease)

• "Disseminated" and "generalized" often used interchangeably; originally disseminated = >10 lesions, generalized = multiple trunk+extremity lesions; some now use disseminated for predominantly papular, generalized for multiple annular plaques (distinctions based on small series, unclear significance)
• Mostly women in 4th-6th decades (highest rates in 50s), also common pattern in younger patients; diffuse symmetric papular/annular eruption, may number in hundreds
• Favors nape of neck, upper trunk, proximal extremities, rarely >5cm diameter; palms/soles/eyelids may be affected; face/genital area usually spared
• Occasional sun-exposure trigger (see actinic granuloma below); some asymptomatic, others pruritic; spontaneous clearing at variable times, relapses possible; average duration 3-4 years (range 4 months to >10 years)

• Significantly more common in women, usually age 30-70; flat/slightly palpable erythematous or red-brown lesions, especially upper medial thighs, intertriginous areas, bathing-trunk distribution
• Individual lesions average ≥several cm (may be much larger); careful palpation may reveal small papules; stretching skin may reveal papules/small annular lesions

• Most common in children, boys 2x more than girls; onset age 1 to adolescence (one congenital case reported); lower legs (especially dorsal foot) favored, also distal upper extremity or scalp; multiple lesions usual
• Often history of preceding trauma; skin-colored deep dermal/subcutaneous nodules up to several cm; superficial papular/annular lesions present in ~1/4 of subcutaneous GA patients
• Generally asymptomatic, resolve over a few years; major clinical problem = misdiagnosis as "rheumatoid nodule" triggering unnecessary rheumatologic workup
• Unusual variant localized to penis/scrotum (atypical GA location); adult women without RA may develop similar periarticular lesions

• Usually dorsal hands, papules with central keratotic core (transepidermal elimination of degenerated central material, may clinically resemble pustule)
• Atypical GA forms may warrant evaluation for underlying hematologic dyscrasia, HIV, or other rare disease associations

• Doesn't resemble other GA forms — often clinically missed; can be chronic but often acute; sudden onset painful hand/foot lesions ± scattered other-site lesions
• Lateral, dorsal, marginal hands (less so feet) affected; tender to palpation; palmar lesions dusky, may vaguely resemble erythema multiforme
• Associated arthralgias, diarrhea, feverish feeling ("cytokine storm" features); ESR may be elevated (even >50mm/h); resolves over months, sometimes after systemic corticosteroid or hydroxychloroquine therapy

• Higher rates of thyroid dysfunction, diabetes, hyperlipidemia than controls; case-control study suggests higher atopy rates (allergic rhinitis, asthma, eczema); higher autoimmune disease rates (RA, lupus)
• Significant patient quality-of-life impact; higher anxiety rates than general population

• Rare co-occurrence, reported in multiple case reports (mostly patients >65); neoplasm diagnosis usually predates GA diagnosis (can precede it); some "atypical" lesions painful, uncommon morphologic variants, or treatment-resistant
• Larger higher-quality epidemiology studies: no association with solid malignancy, but small increased hematologic disorder rate persists

• Occurs at all HIV disease stages (possibly cutaneous immune dysregulation); typically papular; generalized GA more common (60%) than localized (40%); photodistributed and perforating lesions may also occur

• Reported after bee sting, waxing-induced pseudofolliculitis, medical spa mesotherapy injections, BCG immunization
• Two infectious disease groups with GA-like lesions: borreliosis and tuberculosis (TB)
• Lyme disease (US) and Borrelia infections (Europe) rarely show interstitial granulomatous inflammation; clinically (Europe) lesions resemble morphea more than GA; appropriate antibiotic treatment may not resolve skin lesions despite infection evidence
• Tuberculid can closely resemble disseminated GA (histology may show caseous necrosis in granuloma centers); TB treatment resolves skin lesions — evaluate for TB/anti-TB treatment in appropriate patients
• Beyond VZV-site GA, viral triggers rare but reported following SARS-CoV-2 infection
• Medications can trigger interstitial granulomatous reactions including GA and mimickers (see Ch. 8); biologic agents (especially TNF inhibitors, also IL-17 inhibitors and others) rarely induce GA; newer anticancer agents (BRAF inhibitors, especially checkpoint inhibitors) reported to cause granulomatous lesions including GA
• Smoking history ever associated with increased GA odds

• Anterior and chronic intermediate uveitis described in some GA patients — unilateral or bilateral, mild/topical-therapy-responsive or aggressive with visual impairment
• Uveitis frequency in GA patients too low to recommend universal ophthalmologist screening; question GA patients about visual symptoms (ophthalmologic evaluation if present)
• Granulomatous skin lesions + uveitis may indicate sarcoidosis — careful histology review + further screening (e.g., chest imaging) may be warranted

• Two histopathologic patterns often coexist: classic pattern = palisading granuloma (histiocytes/epithelioid cells surrounding central altered-collagen zone), often with mucin deposition, fibrin, and nuclear dust in degenerated foci
• Lesions most often upper/middle reticular dermis but may involve deep dermis/subcutis; peripheral leukocytoclastic vasculitis rarely found; IgM and C3 in skin lesion vessels found in ~half of patients
• Second pattern = interstitial inflammation (may be isolated or adjacent to palisaded lesions, more common than classic palisading granuloma); patchy dermal histiocyte/mononuclear-cell infiltrate ± occasional neutrophils between collagen bundles; interstitial mucin often present
• Interstitial pattern alone sufficient for GA diagnosis, but further sectioning may reveal typical palisaded granulomas; elastophagocytosis rarely seen, resolution with residual middermal elastolysis reported

• Biopsy occasionally reported to cause lesion involution; no treatment required in many mild cases (asymptomatic, spontaneous resolution possible); numerous modalities reported effective (no single uniformly efficacious "treatment of choice")
• Localized cases: intralesional triamcinolone = reasonable initial treatment (effective, many relapse within 3-7 months); superpotent topical corticosteroids or topical calcineurin inhibitors occasionally effective (especially macular lesions); limited reports of topical JAK inhibitors and imiquimod helpful; excimer laser, fractional photothermolysis, light cryosurgical destruction, PDT efficacious for localized disease
• Generalized GA: major therapeutic challenge; may still benefit from local treatment (intralesional Kenalog for cosmetically concerning/exposed sites); systemic treatment needs 3-6 months to demonstrate efficacy/failure
• Two most-reported modalities: phototherapy and antimalarials; PUVA used in initial reports, NB-UVB also beneficial in recent reports; caution with phototherapy in photodistributed GA/possible AEGCG features; particularly effective for patch-type GA (occurs in photoprotected areas)
• Antimalarials used for decades — chloroquine initially, hydroxychloroquine good response rates more recently (chloroquine successfully used in hydroxychloroquine failures)
• Systemic corticosteroids very effective but high doses needed + usual immediate relapse on tapering make approach untenable; dyslipidemia/metabolic syndrome may relatively contraindicate
• Many systemic agents reported effective but few tested in large/blinded/controlled trials; GA may clear then recur when therapy stopped
• Dapsone: often used for phototherapy/antimalarial failures/intolerance, helpful in ~50% but many develop treatment-limiting adverse effects
• Severe disease: TNF inhibitors — etanercept, infliximab, adalimumab all reported effective (most experience with adalimumab); oral JAK inhibitors recently effective in small open-label study; dupilumab helpful in 2 single case reports
• Alternate treatments: doxycycline; rifampin+ofloxacin+minocycline combination monthly; sulfasalazine; pentoxifylline 400mg TID; high-dose nicotinamide, potassium iodide, oral calcitriol
• Methotrexate for refractory cases; fumaric acid esters over 1-18 months effective (combination with PUVA gives highest phototherapy response level); oral retinoids (especially isotretinoin 0.5mg/kg or slightly more); oral vitamin E showed improvement in 21-patient study
• Cyclosporine, IFN-γ, hydroxyurea effective in small series but toxicity limits use to significant GA cases

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• AEGCG and actinic granuloma unified by histopathologic appearance; "annular elastolytic granuloma (AEG)" proposed as alternative unifying term
• Unclear whether this represents a GA variant (most frequent on sun-damaged skin) or distinct disease; possibly some "facial annular sarcoidosis" and non-diabetes-associated facial NL cases belong in this category

• Pattern 1 (Miescher granuloma): single asymptomatic atrophic-appearing yellow thin forehead plaque; fine wrinkling + elasticity loss within ring; clinically resembles facial NL more than GA
• Pattern 2: multiple extensor upper-extremity (sometimes trunk) lesions, more frequent women, favors sun-exposed areas; active erythematous-to-yellow/orange border with central clearing/whiter centers; papular variant described
• Vast majority adults (children/infant affected rarely); most otherwise well, rare temporal arteritis associations reported; AEGCG described with acute myelogenous leukemia (resolved with remission, recurred with relapse) and pleomorphic CTCL
• As with GA, AEGCG lesions may heal with elastic tissue loss and skin laxity/anetoderma features; condition is chronic

• May represent same disorder as AEGCG; papules/plaques on sun-exposed skin, frequently numerous, may coalesce to cover much exposed skin
• Onset after significant sun exposure history + distribution should suggest diagnosis; few lesions may occur on sun-protected sites or spill over from affected areas
• Rarely open comedones, scarring, milia formation clinically; may associate with transepidermal elimination of damaged connective tissue or elastic tissue loss around follicular ostia (Favre-Racouchot-like appearance)
• Repigmentation of gray hairs and solar damage improvement within lesion centers described; affects older adults (usually >50), can be intensely pruritic
• NOT associated with diabetes mellitus, but numerous reports of occurrence with temporal arteritis (speculated vasculitis also caused by actinic connective-tissue injury around temporal artery); both GA lesions and temporal arteritis reported following VZV infection (virus not believed causative for vasculitis); conjunctival involvement reported

• Characteristic histology: largely interstitial dermal macrophage infiltrate, well-formed palisaded granulomas uncommon; numerous (often large) multinucleated giant cells; mucin scant/lacking
• Macrophages characteristically contain fragments of actinically damaged elastic tissue (elastophagocytosis); typical histology + classic clinical features supports specific diagnosis, but neither clinical nor histologic grounds alone are diagnostic
• Some AEGCG/actinic-granuloma-clinical-feature cases show typical GA or even sarcoidosis histology — suggests clinical/histologic spectrum; other granulomatous disorders can also show elastophagocytosis

• Difficult; aggressive sun protection for sun-exposed-skin lesions; topical/intralesional corticosteroids and topical calcineurin inhibitors for individual lesions
• Many respond to systemic corticosteroids but relapse immediately on tapering/discontinuation; oral antimalarials and dapsone sometimes effective; insulin improved diabetic control AND actinic granuloma in one patient
• Other anecdotal treatments: oral retinoids, tetracycline-class antibiotics, fumaric acid, PUVA, pentoxifylline, tranilast, cyclosporine, methotrexate; TNF inhibitors beneficial for recalcitrant cases

• Asymptomatic annular facial plaques, men > women; <10 cases reported; may be photosensitive; resolves spontaneously weeks-months without scarring
• Histology: early lesions — neutrophilic multifocal infiltrate around sebaceous glands with some sebocyte necrosis; later lesions — primarily lymphohistiocytic inflammation
• Clinical differential: GA/AEGCG, tinea faciei, pemphigus foliaceus, gyrate erythema, lupus erythematosus

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• Most frequent in central Africa (common there), rare elsewhere; adults >40, more common women; upper trunk/arms and sun-exposed areas most frequent
• Begins as small papules evolving within 1 year into round/oval plaques up to 15cm; active edge elevated up to 4mm, center slightly depressed/hypopigmented
• Pruritus possible, coalescing lesions form unusual polycyclic shapes; chronic course; most importantly separated from tuberculoid leprosy
• Histology resembles GA but with prominent multinucleated giant cells containing phagocytosed connective tissue; decreased elastic tissue in affected areas
• Shares many features with AEGCG/actinic granuloma/sun-exposed-skin GA — may be identical to these disorders, possibly not a distinct entity

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• Rare multisystem disease with prominent skin findings, unknown cause, strong paraproteinemia association; some consider it part of "adult orbital xanthogranulomatous disease (AOXGD)" spectrum (also includes adult-onset xanthogranuloma [AOX] and adult-onset asthma with periocular xanthogranuloma [AAPOX])
• Gradually progressive, equal sex distribution, average onset age ~50 (range 25->80); periorbital area most common site (>60% of patients); multicentric involvement typical; may be localized or initially present in scars

• Violaceous plaques/nodules with prominent yellow (xanthomatous) component; periorbital early lesions may be mistaken for xanthelasma (NXG more deep/firm/indurated, may extend into orbit and ulcerate)
• Trunk/proximal extremities: violaceous-to-orange-red plaques with active red border, atrophic center, superficial telangiectasias; skin lesions ulcerate in 50% of cases → atrophic scarring
• Acral nodules may occur (some solely subcutaneous); extensive vulvar-region-limited lesions described
• Extracutaneous involvement uncommon, most often eyes — burning/itching/pain around/in eyes; diplopia and inflammation in various eye compartments (conjunctivitis, keratitis, scleritis, uveitis, iritis, ectropion, proptosis); ulceration/scarring/eye distortion may cause visual occlusion, blindness possible
• Lymphadenopathy, hepatosplenomegaly, mucosal/esophageal/myocardial/pulmonary lesions may occur; granulomas may be metabolically active producing 1α-hydroxylase → increased 1,25-dihydroxyvitamin D → hypercalcemia

• Monoclonal IgG (usually κ) paraproteinemia in 80% of cases (other paraproteins reported); thrombocytopenia, neutrophilia, neutropenia, eosinophilia possible
• Bone marrow: leukopenia, plasmacytosis (25-50%), or frank myeloma (10-20%); myelodysplastic syndrome may be present/develop (CLL, Hodgkin or non-Hodgkin lymphoma); many diagnosed with MGUS
• Important to confer with hematology re: NXG-myeloma significance/relationship; NXG predates myeloma/MDS diagnosis by average 2.5-5 years (reported up to 20 years preceding eventual myeloma diagnosis)

• Extensive degenerated collagen zones surrounded by palisaded macrophages of various forms: foamy, Touton cells, epithelioid, giant cells (sometimes >50 nuclei); peripheral foamy cytoplasm common
• Atypical multinucleated giant cells with polarized nuclei (clustered at one cell end) seen in ≥80% of cases; process extends into fat, obliterating fat lobules
• Cholesterol clefts and extracellular lipid deposits may be prominent (not universal, not required for diagnosis); perivascular/interstitial lymphocyte+plasma cell infiltrate, lymphoid follicles present (polytypic in skin)
• Histologic differential: NL and other histiocytoses; NXG has more atypical/Touton giant cells, lymphoid nodules, cholesterol clefts; associated MGUS (as in plane xanthomas with paraproteinemia) appears to enhance intracellular cholesterol accumulation in macrophages/histiocytes

• Directed at paraprotein/underlying malignancy — treating malignancy may resolve NXG lesions; numerous reports of IV immunoglobulin response (may work by partially treating associated paraproteinemia); systemic corticosteroids also beneficial
• Other treatments: intralesional/topical corticosteroids, laser therapy, topical mechlorethamine, topical crisaborole, topical tofacitinib 2% cream, hydroxychloroquine, dapsone, IFN-α, systemic chemotherapy (chlorambucil, cyclophosphamide, melphalan, fludarabine, cladribine), plasmapheresis, local radiation therapy (eye lesions)
• Rituximab, extracorporeal photopheresis, thalidomide, lenalidomide have induced remissions; infliximab, azathioprine, methotrexate also used; simple excision an option but lesions may recur

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• Chronic multisystem inflammatory disease, granuloma formation in most affected tissues; worldwide, all ages/ethnicities/socioeconomic classes (prevalence/patterns vary)
• Europe: most prevalent Scandinavia (Sweden 64/100,000); UK 20/100,000; France/Germany ~10/100,000; lower Spain/Japan 1.4/100,000
• US: southeastern states and certain urban centers (NYC, Detroit, Washington DC) highest prevalence; marked racial variation — 10.9/100,000 White vs. 35.5/100,000 Black persons
• Women slightly more affected than men, highest incidence Black women ages 30-39; lifetime risk 0.85% White vs. 2.4% Black US residents
• Onset most frequent ages 20-40, second peak ages 65-69; late-onset sarcoidosis patients 5x more frequently women, have uveitis, specific skin lesions in 1/3 of cases

• IL-2 and IFN-γ-secreting CD4+ Th cells important (also Th1/Th17 cytokines, particularly TNF-α); underlying cause remains mystery despite genetic associations
• HLA-DRB1 variants → increased risk; HLA-DQB10201 and HLA-DRB10301 strongly associated with acute disease + good prognosis; HLA-B8/DR3 may associate with Löfgren syndrome
• TNF promoter mutations associated with EN in White sarcoidosis patients; lymphotoxin-α (LTA) intron 1 variant associated with EN in female White patients; IL-23 receptor polymorphisms associated with sarcoidal uveitis
• ANXA11, BTNL2, CCR5 gene polymorphisms identified in specific populations; family studies identified additional risk genes (JAK2, BACH2, NCF1, others)

• Present in 25-30% of sarcoidosis patients; classified specific (granulomas on biopsy) or nonspecific (reactive inflammation like EN, no classic granulomas on biopsy)
• Timing: ~20% skin lesions before systemic disease; ~50% simultaneous; ~30% skin lesions after systemic disease (sometimes by up to 10 years — often coincides with systemic corticosteroid tapering for pulmonary sarcoidosis)
• Varied cutaneous manifestations: common morphologies (papules, nodules, plaques, subcutaneous nodules, tattoo/scar-associated lesions, hypopigmented lesions); less common (ichthyosiform, erythrodermic, ulcerative)
• Face involvement common (esp. around nose/eyes/mouth); lesions can develop around prior scars, tattoos, remote foreign body deposits (e.g., knees from childhood bike-riding injuries)
• Lupus pernio: plaque/nodular sarcoidosis of nose/central face, can scale, deeply infiltrate, damage underlying structures, be disfiguring
• Special-site lesions (genital, mucosal, alopecic, nail) uncommonly occur; sarcoidal lesions usually multiple, firm, elastic on palpation, extend entire dermal thickness; overlying epidermis may be slightly thinned, discolored, telangiectatic, or scaly
• Color: pink/red/violaceous in some, frequently faint dull red/purple/brown/yellow tints (varies by stage and underlying skin tone); usually asymptomatic (subset itch)
• Racial difference in cutaneous lesion frequency: White patients — EN as common as specific manifestations (both ~10% of White sarcoidosis patients); Black patients — EN much less common, specific manifestations in ≥50%
• Skin lesions generally don't correlate with systemic involvement extent/nature or prognosis — EXCEPTIONS: EN (good prognosis), subcutaneous sarcoidosis, and lupus pernio

• Most common nonspecific cutaneous sarcoidosis finding; rare in sarcoidosis beginning after age 65
• Löfgren syndrome: fever, polyarthralgia, uveitis, bilateral hilar adenopathy, fatigue, EN — frequent in White Scandinavians, rare in Black Americans
• Typical red warm tender anterior shin subcutaneous nodules, most frequent young women; face/upper back/extensor upper extremities less frequently involved; strikingly elevated ESR (often >50mm/h)
• Good prognosis — sarcoidosis involutes within 2 years of onset in 80% of patients; absence of EN = risk factor for persistent disease activity; Sweet syndrome may rarely be seen as nonspecific sarcoidosis association

• Most common cutaneous sarcoidosis morphology, usually <1cm; localized or generalized (small papules predominate = "miliary sarcoid")
• Especially numerous face/eyelids/neck/shoulders; plaques from papule expansion/coalescence; lesions involute to faint macules over time; rarely prominent hyperkeratosis giving verrucous appearance
• "Papular sarcoidosis of the knee" distinctive (disease may be limited to this site) — granulomas often contain foreign bodies, potentially representing remote childhood injury
• Papular lesions along alar rim in Black patients may be first lupus pernio evidence — portends poor prognosis

• Papular lesions coalesce or arrange in annular patterns, usually red-brown hue, indurated on palpation; central clearing with hypopigmentation/atrophy/scarring possible
• Favors head (including scalp) and neck, usually associated with chronic sarcoidosis; alopecia may result in lesion center
• Annular plaques can preferentially develop in sun-exposed areas — distinguish from annular elastolytic granulomas

• Flat-surfaced, slightly elevated, greatest frequency cheeks/limbs/trunk symmetrically; superficial nodules may be superimposed, plaque coalescence → serpiginous lesions
• Scalp involvement may cause permanent alopecia; alopecia in an annular plaque with raised border should raise sarcoidosis suspicion

• Brown-to-violaceous smooth shiny plaques, head/neck (esp. nose, cheeks, lips, forehead, ears); can be very disfiguring
• Nasal mucosa/underlying bone involvement can cause nasal perforation/bridge collapse (resembling granulomatosis with polyangiitis); upper aerodigestive tract involvement common — ENT evaluation recommended
• 3/4 of lupus pernio patients have chronic fibrotic respiratory tract involvement; 43% have bone granulomas (punched-out cysts, most often fingers); 37% have chronic ocular lesions; sinus sarcoid associated with lupus pernio in 50% of cases
• Typically women in 4th-5th decade; skin lesions rarely spontaneously involute; may resemble rhinophyma
• Important to diagnose correctly — sarcoidal lesion ulceration may occur with laser treatment (even pulsed dye laser); chronic recalcitrant course, may need morphology-specific treatment

• Few-to-numerous 0.5-3cm deep-seated trunk/extremity nodules, only rarely face; overlying epidermis normal (30%), erythematous (50%), or slightly violaceous (10%); usually asymptomatic
• ~90% have multiple lesions, upper extremity most frequently affected (virtually 100%); upper-extremity lesions tend to form indurated linear bands from elbow to hand (cubital forearm side) — extensive involvement may simulate chronic cellulitis; biopsy usually required for diagnosis
• 90% of subcutaneous sarcoid patients also have systemic involvement (usually bilateral hilar adenopathy) — though hilar adenopathy is the most common lung finding generally, so it's unclear whether subcutaneous morphology itself confers specific prognosis beyond that noted for lupus pernio/EN/Löfgren

• Infiltration/elevation of tattoos and old flat scars = two scar sarcoid variants; previously flat scars become raised, erythematous/violaceous (may be confused with hypertrophic scars)
• Tattoo infiltration may be first sarcoidosis manifestation, confused with granulomatous hypersensitivity to tattoo pigment or mycobacterial infection; papulonodular reactions particularly common within black tattoos (but sarcoidosis can develop in any color/tattoo)
• One study: 8% of tattoo reactions may represent sarcoidosis; 20% of sarcoidosis patients develop some tattoo adverse reaction (majority likely sarcoid-related)
• Cosmetic tattooing (e.g., dermatology office) may cause sarcoidal granulomas in pulmonary sarcoidosis patients; other cosmetic procedures (particularly hyaluronic acid injections) can also become complicated by sarcoidal lesion development in sarcoidosis patients
• HCV patients on IFN therapy at high risk for sarcoidosis, can develop disfiguring sarcoidal reactions after cosmetic filler injections; similar granulomatous reactions may occur post-ear-piercing earlobe (localized granulomatous allergic dermatitis to metals — titanium, nickel, cobalt, zinc, gold, palladium all possible allergens); exclude sarcoidosis in such localized reactions (initial screening chest radiograph)
• 22-77% of cutaneous sarcoidosis biopsies contain polarizable foreign material — scar sarcoidosis very common; polarizable material presence doesn't confirm "foreign body granuloma" diagnosis but should prompt systemic sarcoidosis evaluation; if foreign material found, carefully exclude infection if no other sarcoidosis features present
• Foreign material seems to be nidus favoring sarcoidal granuloma development; scar sarcoid sometimes occurs with acute disease + EN (especially small knee papules)

• May be earliest sign, usually diagnosed in darkly pigmented races; few mm to >1cm, favor extremities; appear macular but dermal/subcutaneous component often palpable; exclude leprosy

• Very rare (~0.5% of sarcoidosis patients); primarily Black patients, also well-recognized in Japanese patients; presenting finding in 1/3 of cases (except Japan, usually a late finding in known sarcoidosis)
• Ulcerations de novo or in sarcoidal plaques; favors lower extremities, most patients have lesions in more than one region; trauma may be inciting event
• Clinical appearance may not be specific but skin biopsies diagnostic; lupus pernio may also be present; many have multisystem sarcoidosis (infrequently no other evidence found)
• Biopsies may show necrosis in granuloma centers; NL and ulcerative sarcoidosis share overlapping clinical features and sometimes histology — differentiation may require extracutaneous disease evaluation (present in sarcoidosis, absent in NL)
• Methotrexate (therapeutic for sarcoidosis) may also cause ulceration in sarcoidosis patients

• Extremely rare; diffuse infiltrative erythroderma, begins as erythematous scaling patches merging over large body areas; biopsy confirmatory, but "apple jelly" papules on diascopy throughout erythroderma suggest clinical diagnosis
• Exclude diffuse reactive granulomatous dermatitis in myelodysplastic disorder setting

• Resembles ichthyosis vulgaris/acquired ichthyosis, fine scaling usually distal extremities; virtually always non-White (especially Black) patients
• Almost all have/will develop systemic disease; in 75%, skin lesions follow or occur simultaneously with systemic sarcoidosis diagnosis; no palpable component but biopsy reveals dermal noncaseating granulomas

• Multiple morphologies: plaques extending into scalp → scarring hair loss; macular lesions (1-several cm) closely resembling alopecia areata (may be permanent or reversible)
• Diffuse alopecia, seborrheic-dermatitis-resembling scaly plaques, cicatricial lesions resembling discoid LE or pseudopelade also occur; biopsy of all forms reveals dermal granulomas, sometimes follicular structure loss; syringotropic involvement may cause hypohidrosis

• Extremely rare, substantial fibrosis simulating morphea; <10 cases described; most often localized, resembles linear morphea; biopsy shows noncaseating granulomas

• Can affect any nail compartment — onycholysis, subungual hyperkeratosis (nail bed), brittle nails, pitting, ridging, trachyonychia, distal matrix involvement, even pterygium (matrix destruction); nails may be hyperpigmented
• Sarcoidal dactylitis, phalangeal bone disease, intrathoracic sarcoidosis often accompany; "drumstick" dactylitis associated with lupus pernio

• Mouth: pinhead-sized papules, may group/fuse into flat plaque; hard palate, tongue, buccal mucosa, posterior pharynx may be involved (may simulate Fordyce spots)
• Lupus pernio cases: nasal mucosa frequently involved; rarely with ulcerative sarcoidosis, oral mucosa involved; gingival infiltration → "strawberry gums" simulating granulomatosis with polyangiitis

• Can involve virtually every organ, protean presentations; many cases asymptomatic (found incidentally on chest radiograph); fever may be only symptom or accompanied by weight loss/fatigue/malaise
• Intrathoracic lesions (parenchymal lung + hilar adenopathy) = most common manifestation, 90% of cases; all cutaneous sarcoidosis patients (even asymptomatic respiratory) should get annual chest radiograph + PFTs
• Pulmonary radiograph staging: Stage 0 normal; I bilateral hilar/paratracheal adenopathy; II adenopathy + pulmonary infiltrates; III pulmonary infiltrates only; IV pulmonary fibrosis
• Panda sign: gallium uptake in nasopharynx/lacrimal/parotid glands; lambda sign: uptake in paratracheal lymph nodes — presumptive sarcoidosis evidence
• Lymphadenopathy (mediastinal/hilar, generalized, or cervical/axillary) may be initial sign or occur during course; slight sarcoidosis-lymphoma association — new lymph node changes in diagnosed sarcoidosis warrant thorough evaluation
• Polyarthralgia with acute sarcoidosis or chronic disease component; chronic arthritis possible; osseous involvement often present in chronic disease — characteristic radiographic hand/foot phalangeal changes (round punched-out lytic cystic lesions, frequent with lupus pernio) = epithelioid granulomas
• Ocular involvement in 25-30% — all sarcoidosis patients (even asymptomatic) should have routine ophthalmologic exams; treat any eye findings (even asymptomatic ocular sarcoidosis can cause blindness)
• Anterior uveitis = most common ocular manifestation; lacrimal gland unilateral/bilateral painless nodular swelling; nodular painless iris lesions; retina/choroid/sclera/optic nerve lesions possible; optic neuritis with vision/color vision loss = emergency
• Ophthalmic disease highly correlated with systemic involvement; conjunctival biopsy positive in ~50% (easy diagnostic sampling site)
• Heerfordt syndrome (uveoparotid fever): parotid + lacrimal gland enlargement + uveitis + fever, usually 2-6 months untreated duration; facial nerve palsy and CNS disease frequently seen with it; Mikulicz syndrome = bilateral sarcoidosis of parotid/submandibular/sublingual/lacrimal glands
• Hepatic involvement: clinically apparent in ~20%, but blind liver biopsy shows granulomas in 60%, autopsy studies suggest 50-70% have hepatic granulomas; hepatomegaly with elevated alkaline phosphatase, biliary cirrhosis with hypercholesterolemia, portal hypertension with esophageal varices; liver biopsy with hepatic granulomas = excellent diagnostic confirmation
• Renal disease: direct granuloma involvement or secondary to hypercalcemia (macrophages have large 25-hydroxyvitamin D-1α-hydroxylase amounts converting to active 1,25-dihydroxyvitamin D); patients may appear "vitamin D deficient" if both forms not checked; nephrolithiasis may result
• Cardiac involvement: ~10-15% of cases (higher at autopsy); most common symptom = sudden death; Black patients significantly more likely to have cardiac involvement; baseline cardiac evaluation (history + ECG) recommended for all; ECG abnormalities/palpitations → cardiology referral for Holter monitor or cardiac MRI/PET; fatal arrhythmias and heart failure can develop
• Neurosarcoidosis: 5-10% of patients; numerous presentations — focal cranial nerve involvement (most often facial nerve palsy), aseptic meningitis, seizures, psychiatric changes, stroke, space-occupying lesions; tends chronic/relapsing with higher mortality; Uhthoff phenomenon = vision loss after heat exposure; MRI ± gadolinium useful for CNS lesion detection/therapy monitoring
• Other associations: elevated lymphoma risk; thyroid disorders common (~25%, nonspecific symptoms may be mistaken for sarcoidosis complaints); sarcoidosis-psoriasis association; possibly increased skin cancer risk (NMSC and melanoma, varies by skin type, not replicated in large population studies); eruptive large atypical dermatofibromas reported in one sarcoidosis patient
• ACE levels: little diagnostic utility — elevated with all granulomatous diseases (including infectious); suggestive but not diagnostic; normal ACE doesn't rule out sarcoidosis, elevated doesn't necessarily indicate multisystem involvement; ACE >3x upper limit of normal may suggest sarcoidosis
• F18-FDG PET more accurate for extent-of-involvement identification, being studied for treatment-response monitoring; ultrasound used to quantify granuloma burden (potential future monitoring tool)

• Rare, very age-dependent clinical features; older children (10-15) typically have lung/lymph node/eye involvement; calcium abnormalities in 30% of pediatric sarcoidosis; older children develop specific skin lesions at same rate as adults (~30%); one presentation resembles granulomatous periorificial dermatitis
• Blau syndrome: NOD2 gene mutations, early-onset sarcoidosis (<age 5), more common White patients; triad of skin/joint/eye involvement (often confused with juvenile RA)
• Skin lesions = typically small papules, first clinical feature in >half of patients (median onset age 1); often generalized, may be flat-topped (lichenoid appearance), red-brown to tan, clusters or linear arrays; face may show confluent lesions; lung involvement rare (helps distinguish from sarcoidosis)

• Identical histology across all affected tissues; characteristic finding = "naked tubercle" (collections of large pale-staining epithelioid histiocytes)
• Small necrosis foci possible in granuloma centers; multinucleate giant cells (sometimes with inclusions — asteroid bodies, Schaumann bodies) may be present
• Classically few lymphocytes around granulomas (may be numerous); granulomas nodular, diffuse, or tubular along neurovascular structures; perifollicular/periadnexal involvement seen
• Broad histologic differential — cannot definitively diagnose sarcoidosis histologically; metal allergic granulomas histologically identical; other foreign body granulomas (esp. silica), granulomatous rosacea, granulomatous secondary syphilis, tuberculoid leprosy, atypical mycobacterial infections, leishmaniasis may closely simulate

• Established by demonstrating consistent involvement in ≥2 different organ systems, usually histologically or via characteristic radiologic findings (gallium scans, PET, MRI)
• Cutaneous sarcoidal granulomas with no prior sarcoidosis history → first test = chest radiograph (abnormal → further pulmonary evaluation); ophthalmologic evaluation also performed (asymptomatic ocular involvement common — every patient should see ophthalmologist)
• Blind minor salivary gland biopsy shows sarcoidal granulomas in ~50% of systemic sarcoidosis patients; organ-specific guidelines otherwise exist; PET can guide biopsy site selection
• Skin = most accessible organ with varied presentation — dermatologists should have low biopsy threshold to spare more invasive procedures

• Reported with various spontaneous/iatrogenic immunologic aberrations; ataxia-telangiectasia, SCID, CVID (B-cell and T-cell defect primary immunodeficiencies) predispose to cutaneous sarcoidal granulomas
• These show low CD4/CD8 ratio histologically (contrast with classic sarcoidosis, CD4+ T-cell rich); rubella virus recently identified as potential antigenic trigger in CVID/other immunodeficiency-associated cutaneous granulomas
• Sarcoidosis-lymphoma syndrome (especially Hodgkin disease association); B-cell lymphoma, chronic myeloid/lymphoid leukemia, MALT lymphoma all described with sarcoidosis
• Sarcoidosis patients ~40-60% more likely to develop malignancy (solid tumors incl. skin cancers [3-fold risk], renal cancer, rarely endocrine tumors); lymphoma/solid tumor patients' enlarged nodes may show sarcoidal granulomas without tumor — important to sample nodes to avoid unnecessary therapy
• Sézary syndrome with extensive cutaneous granulomas described; granulomas may also arise from treatments (IFN therapy, mogamulizumab-associated granulomatous dermatitis)
• Medication-induced systemic sarcoidosis (typically pulmonary+cutaneous constellation): TNF inhibitors (especially etanercept, also adalimumab, infliximab) can trigger sarcoidosis (ironic — also often therapeutic, analogous to TNF inhibitors and psoriasis); IL-17 inhibitors and other biologics rarely reported; emerging dupilumab-triggered granulomatous reaction reports
• IFN therapy (usually for HCV) — cutaneous lesions (60%), pulmonary findings (75%), or both/other sarcoidosis features in 5% of HCV-IFN-treated patients; ribavirin addition may increase risk; >80% resolve after treatment discontinuation
• HIV treatment with HAART → sarcoidosis or tattoo granulomas appearance; sarcoidosis now well-recognized IRIS feature; rarely reported following COVID-19 (connection strength uncertain)
• HSCT (allogeneic/autologous) associated with pulmonary sarcoidosis appearance — hilar adenopathy post-HSCT-for-malignancy may be misinterpreted as recurrent/metastatic disease (inappropriate treatment risk)
• Other sarcoidosis-causing medications: alemtuzumab (anti-CD52, for CTCL), rituximab, tocilizumab, omalizumab, vemurafenib (BRAF inhibitor, alone or with ERK inhibitors), checkpoint inhibitors (ipilimumab for melanoma); PD-1 inhibitors cause range of granulomatous reactions from skin-only to multiorgan sarcoidosis-resembling disease
• Sarcoidosis patients at increased risk for other immune-mediated/chronic inflammatory diseases: SLE, autoimmune chronic hepatitis, multiple sclerosis, celiac disease, thyroid disease, IBD

• Granulomatous secondary syphilis may closely simulate clinically/histologically; Blau syndrome (AD granulomatous disease) similar to childhood sarcoidosis but distinguished by lack of pulmonary involvement; granulomatous CTCL usually distinguished histologically and by sarcoidosis's pulmonary involvement presence

• Numerous therapies reported beneficial (usually anecdotal); virtually no data on optimal therapy per cutaneous manifestation type; cutaneous disease may spontaneously remit
• Most skin lesions asymptomatic — major treatment indication = cosmetic; begin by evaluating for systemic disease (treating it may clear skin lesions); otherwise stepwise approach based on extent/severity/progression rapidity
• Systemic corticosteroids almost always beneficial but doses needed for cutaneous control (usually >10mg/day) too high for ideal long-term use
• Limited skin disease: intralesional triamcinolone 2.5-10mg/mL (or higher) very effective; thinner lesions — superpotent topical corticosteroids or topical tacrolimus; topical retinoids used for anti-inflammatory and especially cosmetic properties; recent reports show topical JAK inhibitor efficacy
• Local procedures: pulsed dye laser (repeated), PDT, phototherapy, CO2 laser remodeling effective in appropriate cases — but caution (skin trauma can induce new lesions, existing lesions may worsen/ulcerate post-laser); phototherapy/PDT benefit often transient with common relapse after stopping
• Severe lupus pernio: nasal skin excision + flap reconstruction can dramatically improve (usually only once inflammation adequately controlled)
• Antimalarials (chloroquine 250mg/day, or hydroxychloroquine 200-400mg/day generally limited to <5mg/kg/d) — ~3/4 of patients respond partially/completely; associated joint disease/hypercalcemia may also improve; can reduce required steroid dose; combinable with antibiotic treatment
• Tetracycline antibiotics (minocycline more effective/better data but doxycycline may be better tolerated without pigmentary change risk; both 100mg BID) for skin-lesion patients not needing systemic treatment — ~2/3-3/4 respond to minocycline, 1/4 have complete resolution; drug pigmentation specifically within granulomatous inflammation sites may occur
• CLEAR regimen (more aggressive antibiotics): levofloxacin 500mg/day + ethambutol 25mg/kg/day (up to 1200mg) + azithromycin 250mg/day + rifampin 10mg/kg/day (up to 300mg/day) — single small study; maximum response after several months
• Pentoxifylline mild benefit for pulmonary sarcoidosis, anecdotal mild skin disease help; apremilast beneficial in small 17-patient skin disease study
• Methotrexate 15-25mg/week efficacious for antiinflammatory-option failures, may help challenging morphologies (severe lupus pernio, ulcerative sarcoidosis); methotrexate-induced hepatitis in 15% of treated sarcoidosis patients (hepatic sarcoidosis possible confounder) — hepatology consultation may be required
• TNF = important granuloma-formation cytokine; TNF inhibitors (particularly infliximab, adalimumab) effective for refractory cutaneous/systemic sarcoidosis (higher dosing than psoriasis)
• Etanercept studied for pulmonary sarcoid, trial stopped early (biologic with most TNF-induced-sarcoidosis reports) — most experts avoid it, favor other agents; adalimumab and infliximab both highly effective for skin sarcoidosis
• Golimumab: nonstatistically significant improvement trend in one large double-blind placebo-controlled trial; infliximab particularly beneficial for severe lupus pernio (only 10-30% respond to other agents vs. 80-90%+ respond to infliximab); combination therapy for severe refractory cutaneous disease
• JAK inhibitors: topical formulations helpful in limited case reports; systemic JAK inhibitors successfully treat multiorgan sarcoidosis — Damsky et al: tofacitinib 5mg BID improved cutaneous/multiorgan sarcoidosis while steroid-sparing; additional studies ongoing
• Systemic corticosteroid indications: acute systemic involvement (fever, weight loss), active eye disease, myocardial sarcoidal involvement, active pulmonary disease with functional disability, hypersplenism, hypercalcemia, symptomatic CNS involvement
• Acthar gel may benefit recalcitrant advanced systemic sarcoidosis or refractory lupus pernio (including TNF-inhibitor failures)
• Alternate options for treatment failures: thalidomide 50-200mg/day (improvement after months, though small 40-patient RCT failed to show benefit; used less with biologics available; venous thrombosis risk especially >100mg/day; teratogen; option when methotrexate contraindicated); lenalidomide also reported beneficial
• Azathioprine/mycophenolate mofetil often used for extracutaneous disease, less effective than methotrexate for skin disease but reasonable for select patients; leflunomide similar to methotrexate, useful for methotrexate GI intolerance (~75% response rate)
• Retinoids (principally isotretinoin 0.5-1.0mg/kg) beneficial for some, response only after ≥6 weeks; cyclophosphamide for refractory disease; rituximab for recalcitrant pulmonary disease; fumaric acid esters improve cutaneous sarcoidosis (consider when other agents fail); recent sirolimus study showed early promising results for cutaneous sarcoidosis

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• Disorders with infiltrates containing Langerhans cells (X-type histiocytoses) or non-Langerhans cell histiocytes (non-X histiocytoses)
• Zelger/Burgdorf classification (refined by Weitzman/Jaffe) — 3 large families:

1. Langerhans cell (X-type) histiocytosis (LCH)
2. Non-LCH histiocytoses of juvenile xanthogranuloma (JXG) family (dermal dendritic cell phenotype: factor XIIIa+, fascin+, MS-1+, CD68+)
3. Multicentric reticulohistiocytosis (MRH) and sinus histiocytosis with massive lymphadenopathy (SHML; Rosai-Dorfman disease [RDD]) — thought NOT in JXG non-X family

• Diagnosis historically established by typical clinical features + compatible histology + typical evolution
• >100 different histiocyte/macrophage-dendritic-cell disorders described (each rare) — revised classification with 5 categories:
• L group (Langerhans): LCH, indeterminate cell histiocytosis (ICH), Erdheim-Chester disease (ECD)
• C group (cutaneous/mucosal non-LCH): XG family (JXG, adult xanthogranuloma [AXG], solitary reticulohistiocytoma [SRH], benign cephalic histiocytosis [BCH], generalized eruptive histiocytosis [GEH], progressive nodular histiocytosis [PNH]); non-XG family (cutaneous RDD, NXG); significant-systemic-component group (xanthoma disseminatum [XD], MRH)
• M group (malignant histiocytoses): primary/secondary malignant histiocytosis (discussed Ch. 27)
• R group (RDD): familial RDD, nodal RDD, extranodal RDD, neoplastic/autoimmune-associated RDD
• H group: hemophagocytic lymphohistiocytosis

• MAP kinase pathway mutations found in most histiocytosis lesions; >90% of LCH and ECD cases, >70% of JXGs harbor MAP kinase pathway mutations
• BRAF mutations described commonly in LCH and ECD; mutations in CSF1R, ALK, RET, NTRK, RAS, RAF, MAP2K and other kinase genes reported — illuminates etiopathogenesis, provides rationale for targeted therapeutics
• BRAF V600E mutation patients may respond to vemurafenib; RET or ALK mutation patients respond favorably to selpercatinib or crizotinib respectively; mutation-directed therapy should involve experienced oncology consultation

• Overwhelming "cytokine storm," complication of multiple disorders (infections, malignancy, rheumatologic disorders); "macrophage activation syndrome (MAS)" term used when HLH occurs with certain rheumatologic disorders (particularly juvenile idiopathic arthritis)
• Underlying disease + high fever, organomegaly, cytopenias, markedly elevated ferritin, elevated triglycerides/LDH, hemophagocytosis on bone marrow aspirate
• Interleukin inhibitors being explored as therapeutic options given intense cytokine inflammation

• Zelger/Burgdorf "xanthogranuloma family" classification relies on monocyte/macrophage lesion morphology; Weitzman/Jaffe refined with IHC features; no single morphologic cell type absolutely characterizes these disorders (histiocyte appearance can change within a disorder)
• 3 clinical groups: primarily/only skin (JXG); skin + major systemic component (ECD); primarily systemic with occasional skin lesions (RDD, SHML)
• Conceptual spectrum: BCH → JXG → ECD → GEH → XD → PNH; spectrum beginning = localized benign disorders; progressing = more generalized but still benign; end = visceral involvement, less likely to involute
• Parallels histiocyte histologic appearance progression: scalloped → vacuolated → xanthomatized → spindled (though many morphologies can be seen in any disease)

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• Most common non-LCH; 70% diagnosed within first year of life, ~1/3 congenital; mean onset age 22 months, median 5 months (early-onset proclivity); ~80% solitary
• Male:female ratio 1.4:1; adult-onset lesions tend late 20s-early 30s with equal sex distribution; 10x more common in White than Black persons (occurs in all races)

• Well-demarcated, firm, rubbery, round-oval dermal papules/nodules, often <1cm; early lesions pink-to-red with yellow tinge, become tan-brown over time
• Dermoscopy: orange-yellow background, subtle erythematous border with branched/linear vessels from edge to center, "clouds" of paler yellow areas (xanthomatized histiocytes); "setting sun" pattern (yellow center + peripheral linear branching vessels)
• Most asymptomatic; head/neck most common, then upper trunk/upper extremities
• 3 size types: small papular (2-5mm), large nodular (5-20mm), giant XG (>20mm); small type more numerous; nodules may coalesce into giant plaques; one patient often has both types (proposed ocular-involvement risk with micronodular type and internal-involvement risk with macronodular type has been refuted)
• Skin lesions spontaneously regress within 3-6 years in children (adults: usually persistent); hyperpigmentation, atrophy, or anetoderma may remain after resolution

• Hyperkeratotic nodules; macronodular tumors 2-10cm; clustered (agminated) forms; linear lesions; flat plaquelike lesions; pedunculated/cylindrical exophytic lesions
• Atypical sites: genitalia, lips, oral cavity, palms, soles, earlobes, fingers
• Most common location after dermis = subcutaneous tissue (again head/neck most common); ~15% of JXGs present this way, usually solitary mobile mass up to 3cm; subcutaneous JXG typically appears before age 1 (often before 3 months)
• Oral JXG may develop infancy/childhood, most frequently solitary tongue/lip/palate lesion

• Uncommon — visceral involvement with either multiple cutaneous lesions or solitary extracutaneous lesion; visceral disease = only 4% of childhood JXGs, 5-10% of all JXG cases
• Ocular involvement: ~0.3-0.4% of children with multiple JXGs; only 41% of ocular-JXG children have skin lesions; skin lesions appear after eye lesions in 45% of cases; 92% of eye lesions occur first 2 years of life
• Most common ocular location = iris (tumor, unilateral glaucoma, unilateral uveitis with spontaneous hyphema, or heterochromia iridis); eyelid or posterior eye also possible; ocular screening recommended for children with multiple cutaneous lesions before age 2
• Mass lesions of nasal/orbital/paranasal sinus region can erode orbit, extend to skull; other extracutaneous sites (frequency order): lung (respiratory distress, nodular chest radiograph opacities), liver (hepatomegaly, rarely fatal giant cell hepatitis), testis (mass), rarely CNS/kidney/spleen/retroperitoneum
• Other extracutaneous evaluations not indicated unless symptoms/findings suggest presence; extracutaneous lesions also spontaneously regress; if surgery required, extracutaneous lesions tend not to recur even with incomplete excision
• Disseminated/systemic JXG: rare life-threatening visceral burden; in 25% no skin lesions found (or skin appears after systemic disease identified); progressive CNS/liver/bone marrow involvement mandates aggressive therapy (usually LCH protocols); bone marrow involvement can produce HLH with profound cytopenias; locally aggressive tumors may be radiated

• Reported with neurofibromatosis (NF-1) and juvenile myelomonocytic leukemia (JMML); NF-1+JXG patients reported 20-32x more likely to develop JMML (recently questioned)
• 739-patient NF-1 review (20 years): 14 had malignancy, JXGs found in 4/14 vs. 6/29 controls — suggests JXG doesn't confer increased malignancy risk in NF-1
• JMML occurs infancy/early childhood — café au lait macules often only NF-1 finding at that time; all 3 conditions sometimes co-occur (males 3:1 predominance, usually maternal NF-1 history)
• Children with JXG should be examined for NF-1 stigmata; if found (especially boy with maternal NF-1 history), pediatrician should watch for possible JMML
• Nevus anemicus may occur with JXG+NF-1 — nevus anemicus in young infant with JXG should alert to possible NF-1
• Rarely JXG associated with mastocytosis or childhood ALL (leukemia and JXG can share clonality; JXG lesions may occur after leukemia treatment or concurrently); Wiskott-Aldrich syndrome reported with multiple JXGs
• Multiple XGs rare in adults, unusual to occur eruptively; ≥6 cases associated with hematologic malignancy (CLL, essential thrombocytosis, large B-cell lymphoma, adult T-cell lymphoma/leukemia, monoclonal gammopathy); adult systemic JXG involvement also rare, usually needs histologic confirmation

• Nonencapsulated but circumscribed proliferations in upper/middle reticular dermis, may extend to subcutis or abut epidermis with no grenz zone; no epidermotropism
• Histology varies with lesion age: very early — mononuclear cells with abundant amphophilic cytoplasm (poorly lipidized/vacuolated); later — more vacuolated, multinucleated forms appear
• Mature lesions: foam cells, multinucleated foam cells (Touton giant cells — characteristic but not specific), foreign body giant cells; inflammatory infiltrate = lymphocytes, eosinophils, neutrophils (lacks plasma cells); fibrosis in older lesions
• Soft tissue/visceral JXGs: more monomorphous cytology, may have very few Touton giant cells, prominent spindle cell appearance possible
• IHC especially valuable for extracutaneous JXG confirmation: factor XIIIa+, vimentin+, fascin+, MS-1+, CD68+; NOT CD1a+, S-100+, or other Langerhans-cell-specific markers
• MAP kinase pathway gene mutations reported (as with other histiocytoses): BRAF V600E, NTRK1 fusions, MAP2K1 and CSF1R mutations; one study showed ALK translocations more common than BRAF V600E in systemic JXG

• Most cases: observation; most resolved by age 6 (often normal or slightly hyperpigmented skin remaining); adult spontaneous involution slower — local surgical removal considerable
• Intravitreal bevacizumab injection may treat iris JXG; widespread systemic JXG — mutation-sequencing-based targeted treatment (e.g., vemurafenib for BRAF V600E-mutated systemic JXG; crizotinib for ALK-mutated histiocytosis)
• Patterns of involvement by JXG and LCH are similar (childhood onset, primary cutaneous involvement; visceral disease usually liver/bone/lungs); without histologic confirmation, isolated bone JXG likely misdiagnosed as isolated LCH (much more common)
• Clinical similarity may occur because both diseases arise from antigen-presenting dendritic cells (JXG = dermal dendrocyte proliferation, LCH = Langerhans cell proliferation)
• Clinical features favoring JXG: lack of crusting/scale, lesion distribution/size uniformity; histology definitive in difficult cases — JXGs negative for CD1a (Langerhans marker), usually negative for S-100 (unlike LCH, though few S-100+ cells possible)
• JXG may appear in patient who also has LCH; one PI3KCD-mutation JXG case reported; JXG lacks LCH's BRAF mutations
• Differential: benign cephalic histiocytosis (may be difficult to distinguish clinically/histologically — flatter lesions, mainly head/neck); papular xanthoma (histologically distinguishable); mastocytosis (urticates on scratching — Darier sign — distinguishable histologically); solitary childhood JXG must be distinguished from Spitz nevus (usually requires biopsy)

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• Rare, equal boys/girls, all races; usual onset 2 months-3 years (rarely to 5 years), 50% begin ages 5-12 months
• Begins on head virtually always (cheeks, eyelids, forehead, ears), may later appear neck/upper trunk (less often more caudad); always multiple but often few (5-20, can be >100)
• Slightly raised reddish-yellow papules 2-4mm; may coalesce into reticulate appearance; asymptomatic; mucosa/viscera not involved
• Spontaneously involutes over 2-8 years, leaving hyperpigmented macules; some cases evolve to JXG; one patient developed generalized eruptive histiocytoma years after BCH involution — supports spectrum concept (all derive from same dermal dendritic cell type)
• Histology: diffuse dermal monomorphous macrophage infiltration, CD68+/factor XIIIa+, negative S-100/CD1a; one case report showed improvement with topical 1% rapamycin

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• Very rare, usually presents young adulthood; diagnostic criteria:

1. Widespread, erythematous, essentially symmetric papules (especially trunk/proximal extremities, sparing flexors, rarely mucous membranes; NO visceral involvement)
2. Progressive new lesion development (often crops) over several years, eventual spontaneous involution to hyperpigmented macules
3. Benign histology of monomorphous vacuolated macrophages

• Lesions appear in crops, may be grouped/clustered; skin-colored, brown, or violaceous; rare in childhood (may be difficult to distinguish from widespread BCH there, if indeed separate)
• May suddenly appear weeks after bacterial/viral illness (adults and children); adults — may associate with underlying malignancy (usually leukemia/lymphoma)
• Distinguished from XD by lack of visceral disease, benign course, and macrophage scalloped appearance in XD
• Histology: dermal infiltrate of monomorphous vacuolated macrophages/mononuclear histiocytes; GEH cells vimentin+, CD68+, usually factor XIIIa+; negative S-100/CD1a
• Unpredictable natural history — complete resolution in some, persistence in others; some progress to widespread XGs, XD, or PNH (supports spectrum concept)
• Childhood: may need no treatment; adulthood: PUVA or isotretinoin considered; described as both reactive condition and clonally related to underlying hematoproliferative disorder
• Reported with chronic myelomonocytic leukemia, FIP1L1-PDGFRA-positive chronic eosinophilic leukemia, and NTRK1 gene rearrangement

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• Very rare, potentially progressive non-LCH, preferentially males (2:1) in childhood/young adulthood
• Insidious onset of small yellowish-red-to-brown discrete disseminated papules/nodules, characteristically eyelids and flexural areas (axillary/inguinal folds, antecubital/popliteal fossae)
• Over years, increase in number → coalescent xanthomatous plaques/nodules

• 30-50% have mucous membrane involvement, most often oropharynx (dysphagia), larynx (dysphonia, airway obstruction), conjunctiva/cornea (blindness)
• Diabetes insipidus (usually transient) in 40% (30% at presentation); CNS involvement (epilepsy, hydrocephalus, ataxia) can occur; synovitis, inflammatory arthritis, osteolytic bone lesions described
• Disseminated xanthosiderohistiocytosis variant: keloidal lesions + histologic iron phagocytosis; may have underlying paraproteinemia (should screen)

• ~1/3 spontaneous complete remission, 1/3 partial remission, 1/3 persistent/progressive disease; complete-remission patients lack systemic disease evidence besides diabetes insipidus
• Spontaneous remission often leaves atrophy/anetoderma from local elastic tissue loss

• Serum lipids abnormal in 20% (may cause confusion with hyperlipidemic xanthomatosis); early lesions show surprisingly nonfoamy, scalloped macrophages
• Later lesions: xanthoma cells, Touton giant cells, often mild inflammatory infiltrate (lymphocytes, plasma cells, neutrophils); macrophages CD68+/factor XIIIa+
• Disseminated xanthosiderohistiocytosis variant: keloidal-consistency lesions, annular borders, cephalad distribution, extensive iron/lipid deposition in macrophages/connective tissue

• Progressive XD → considerable morbidity, can be fatal → aggressive therapy indicated; lipid-lowering therapy improved lesions in a few patients; cladribine (2-chlorodeoxyadenosine) shown benefit in some
• Corticosteroids, statins (topical/systemic), fenofibrate, doxycycline, cyclosporine, cyclophosphamide used with varying efficacy (limited reports); IL-1 receptor antagonist used for one multisystem/recalcitrant patient (based on ECD response reports)
• One case with BRAF V600E mutation suggests targeted therapy option for select patients; NB-UVB phototherapy used; F18-FDG PET/CT useful for defining disease extent/treatment response of visceral lesions; surgical removal and laser therapy (including nonablative 1450nm diode laser) described for individual lesions

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• Very rare, equal sex distribution, usually onset ages 40-60; two lesion types: superficial papules and deeper larger subcutaneous nodules
• Superficial: small xanthomatous papules up to 5mm, diffusely distributed but SPARE flexors (unlike XD, which favors flexors)
• Deep lesions: up to 5cm, associated with pain/ulceration/disfigurement; smaller lesions may evolve to larger over time; face lesions may coalesce → leonine facies + ectropion
• New lesions progressively appear; individual spontaneous resolution possible but general involution of ALL lesions does not occur; mucosal lesions unusual but can involve conjunctiva/larynx
• Iron deficiency anemia can occur (nodule histiocytes accumulate iron, reducing body iron stores)
• Histology: superficial lesions show foamy macrophages; deeper lesions show densely cellular spindle-shaped histiocyte proliferation with multinucleated giant cells (deep spindled-histiocyte lesions = diagnostic PNH feature)
• Treatment: local excision for symptomatic lesions; cryosurgery improved some lesions; KRAS-mutation patient successfully treated with MEK inhibitor cobimetinib; baricitinib beneficial in one case

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• Rare, poorly defined non-LCH; any age but usually early childhood or post-adolescence; usually solitary, men 4:1 over women
• Small yellow papule 1-10mm; if multiple, generalized (not grouped), doesn't favor flexors; normal lipid profile
• Erosive arthritis (resembling MRH) reported in one child and one adult
• Histology: dermal xanthomatized foamy macrophage aggregates with Touton giant cells; inflammatory cells scant/absent; CD68+ but factor XIIIa NEGATIVE (distinguishes from JXG)
• Differential: normolipemic plane xanthomas, normolipemic papuloeruptive xanthomatosis; infants — spontaneous involution natural history; one adult treated successfully with doxycycline
• "Plaquelike papular xanthoma": 2 reports of solitary red-yellow plaques with superficial foamy macrophage/Touton cell infiltrate and clear grenz zone

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• Rare non-LCH, primarily visceral disorder with cutaneous lesions in 20-30% (half at presentation); onset any age childhood to 9th decade
• Characteristic feature: bilateral symmetric sclerosis of long bone metaphyseal/diaphyseal regions (pathognomonic radiologic finding)
• Diabetes insipidus may occur (pituitary involvement) and retroperitoneal fibrosis affecting kidneys; despite normal gross appearance, many internal organs affected
• Progressive course — infiltration of many visceral organs followed by fibrosis, often fatal (usually pulmonary fibrosis/cardiac failure); 1-year survival 96%, 5-year survival 68%
• Skin lesions: red-brown or xanthomatous 2-15mm papules/nodules; favor eyelids (as xanthomas), axilla, groin, neck, trunk (inframammary), face (similar to XD lesions)
• 71-patient review, 15 with skin involvement: 8 xanthelasma-like, 2 xanthomas, 5 subcutaneous nodules, 1 GA-like lesions

• As with LCH, significant % have BRAF V600E mutation; most have activating MAP kinase pathway mutations; some patients have simultaneous LCH+ECD ("mixed histiocytosis," apparently mutation-driven); less often NRAS mutations
• Mutation identification unlocks targeted therapy options; historically initial treatment = IFN-α; unresponsive patients — anakinra or infliximab, also sirolimus and tocilizumab
• BRAF V600E mutation → BRAF inhibitor (vemurafenib) effective; MEK inhibitors for other MAP kinase pathway activating mutations (beneficial responses even without known mutation); ALK or RET fusion → targeted inhibitors; cladribine and imatinib options if no mutation detected

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• Rare AD or X-linked hereditary disorder, primarily women (rarely males); few-to-numerous skin-colored-to-red-brown pinhead-to-pea-sized papules, face/arms/forearms/hands/legs
• Affected family members may show differing clinical patterns; onset 2nd decade, slow progression, no spontaneous involution tendency
• One report: boy with dermal nodules resolving then developing persistent progressive-mucinous-histiocytosis plaques; visceral/mucosal lesions not reported
• Histology: middermal proliferation of spindle-shaped and epithelioid monocytes; superficial telangiectatic vessels, increased mast cells; abundant alcian-blue-demonstrated mucin (acid mucopolysaccharides)
• Distinguished from other non-LCH by familial pattern, lack of lipidized/multinucleated cells, mucin presence; IHC: CD68+ consistently, negative CD1a/S-100/CD34; one report required electron microscopy for diagnosis
• One case: gain-of-function PDGFRB mutation in affected family (suggests imatinib treatment option); thalidomide beneficial in one case; CO2 ablative laser helped another

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• Rare, histiocytes within dilated lymphatic vessels usually in superficial papillary/reticular dermis; usually women; erythematous-to-violaceous macules/patches, often livedoid, generally limbs/torso
• Can occur with systemic diseases — typically rheumatoid arthritis, also Crohn's disease (groin/perianal lesions), and with neoplasms (breast, colorectal, melanoma, rarely others)
• Can relate to metallic joint implants, reported with silicone breast implant; typically described with RA but can occur with osteoarthritis (tends to occur near affected joints)

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• Usually appears 1st-2nd decade of life as febrile illness with massive cervical (and often other) lymphadenopathy, polyclonal hyperglobulinemia, leukocytosis, anemia, elevated ESR
• Males and people of African descent especially susceptible; 5 subgroups: classical (nodal), familial, extranodal, neoplasia-associated, immune-disease-associated
• Extranodal involvement in 43% of cases (skin = most common site); ~10% of SHML patients have skin lesions, 3% have skin-only detectable disease ("cutaneous sinus histiocytosis"/"cutaneous RDD")
• Skin lesions: isolated or disseminated yellow-brown papules/pustules/nodules or macular erythema; large annular GA-resembling lesions may occur; most cutaneous RDD patients older (age 40-60)

• Superficial/deep perivascular lymphocyte+plasma cell infiltrate; nodular/diffuse dermal infiltration by large foamy histiocytes
• Emperipolesis (intact lymphocytes/plasma cells within histiocyte cytoplasm) = important diagnostic feature; foamy histiocytes may be seen in dermal lymphatic channels
• Cutaneous histology sometimes very nonspecific except emperipolesis — diagnosis may only become clear evaluating lymph node/other organ involvement
• IHC/EM useful: infiltrating cells CD4+, factor XIIIa+, S-100+, but NO Birbeck granules; OCT2 recently reported as consistent RDD marker (not other histiocytic entities); increased IgG4 plasma cells and some fibrosis common — RDD can coexist with IgG4-related disease

• Cause unknown; numerous reports identify HHV-6 in involved lymph nodes; molecular studies show gain-of-function MAP kinase pathway mutations (NRAS, KRAS, MAP2K1, rarely BRAF) in ~1/3 of cases — suggests subset represents clonal process (as with other histiocytic disorders)
• Germline mutations reported in some familial cases (e.g., SLC29A3); usually clears spontaneously — no treatment required generally
• Numerous agents used with variable success, indicated only if condition risks death or significant complication (usually vital organ compression): radiation, systemic corticosteroids, thalidomide
• Single/multiagent chemotherapy mixed response, but methotrexate alone successful; skin lesions — cryotherapy, topical corticosteroids, acitretin, intralesional corticosteroids may be tried

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• Two distinct forms: reticulohistiocytoma and MRH; "diffuse cutaneous reticulohistiocytosis" believed by some to be distinct entity, by others MRH without arthritis
• Two forms have identical histology but distinct clinical manifestations; MAP kinase pathway mutations reported (BRAF V600E in reticulohistiocytoma case, also RAF1+TSC2 case; MAP2K1 and TET2 mutations in MRH case)

• Usually solitary firm dermal lesion <1cm, favors trunk/extremities; solitary and multiple lesions without systemic involvement described (contrast MRH), mainly adult men, rarely children
• Cyclin D1 staining described in one report (not specific to this histiocytosis type)

Overview: multisystem disease, usual onset ~age 50 (range 6-86); 2x more common women; all races; primary manifestations = skin lesions + potentially destructive arthritis

• Joint disease occurs first in ~40%, skin lesions precede joint symptoms in 20%, simultaneous in 40%

Clinical Features: few to few hundred firm skin-colored-to-red-brown papules/nodules, mostly 2-10mm (some several cm); most frequent fingers/hands with paronychial tendency; ~half show nailfold arrangement ("coral bead" appearance, may associate with nail dystrophy)

• Upper body (arms, scalp, face, ears, neck) also common; ~90% have face+hand lesions; nodular/papular pinna involvement and symmetric joint-area distribution characteristic
• Arm/elbow/knee nodules may resemble rheumatoid nodules; diffuse erythematous lesions possible (simulating erythroderma); may present initially with macular/minimally-infiltrated sun-exposed-site erythema (simulating photodermatitis or dermatomyositis)
• Small papules often present (useful for confirming MRH diagnosis); lesions may ulcerate; xanthelasma in 30%; atypical patchy face/upper-limb hypopigmentation noted; ~10% complain of pruritus (not localized to lesions, may precede appearance)

Mucous Membrane/Systemic: mucous membrane involvement in 1/3 (most common lips/tongue; also gingiva, palate, buccal mucosa, nasopharynx, larynx, sclera); esophageal lesions can cause dysphagia; 1/3 have hypercholesterolemia+xanthelasma; rheumatoid factor usually negative

Osteoarticular Changes (most important aspect): no association between skin eruption extent/severity and joint disease course

• Inflammatory symmetric polyarticular arthritis affecting many joints (hands, knees, shoulders, wrists, hips, ankles, elbows, feet, spine); can be rapidly destructive/mutilating with phalanx/digit absorption and telescopic shortening — "doigt en lorgnette" (opera-glass fingers)
• Earlier reports: ≥50% developed arthritis mutilans (now reduced to ~11%); infiltrating cells identical in skin and joints microscopically/immunophenotypically
• Variable course — complete involution in many cases after ~8 years, but joint destruction permanent (severe disability cause); may resemble RA and psoriatic arthritis; weight loss and fever in 1/3

Associations: ~15% have associated autoimmune disorders (thyroiditis, Sjögren syndrome, ulcerative colitis, vitiligo reported)

• 25-33% of reported cases have associated malignancy — every MRH patient should get careful history/exam + complete age-appropriate cancer screening repeated regularly (similar to dermatomyositis protocol)
• No specific tumor type association; reported cancers: breast, GI tract, GU tract, melanoma, leukemia, lymphoma; skin lesions usually appear before malignancy diagnosis (synchronous behavior only occasionally reported)
• One case: TB identified, TB treatment resolved the MRH
• Other organ/tissue involvement possible: bone, muscle, lymph nodes, liver, myocardium, pericardium, lungs, pleura, stomach; myocardial involvement may be fatal; usual interstitial pneumonia or typical MRH cells may be seen in pulmonary lesions

Histology: skin lesions centered middermis, tend to occupy much/all of dermis; mononuclear and multinucleate monocyte/macrophage infiltrating cells

• Giant cells most characteristic — abundant smooth/slightly granular eosinophilic/amphophilic "ground-glass" cytoplasm, darker centrally than peripherally; PAS+ after diastase digestion
• Overlying epidermis may be thinned but usually separated from dermal process by narrow grenz zone; polymorphous infiltrate (lymphocytes, neutrophils, eosinophils, plasma cells) characteristic
• IHC: monocyte/macrophage cells CD68+, vimentin+, CD163+; MRH cells in skin AND joints stain TRAP+ and cathepsin K+ (osteoclast markers) — explains MRH response to bisphosphonates (cause osteoclast apoptosis, taken up by reticuloendothelial system)

Treatment: given aggressive arthritis, consider early/adequate treatment — but associated malignancy common and can worsen with immunosuppression (same for underlying asymptomatic TB); screen for both conditions initially, treat adequately if found

• Spontaneous remissions common, making treatment efficacy hard to determine; major goal = prevent joint destruction/disability
• Sequencing/targeted therapeutics revolutionizing other histiocytic disease management, promise for MRH with targetable mutations
• TNF inhibitors and bisphosphonates frequently used (often combined with other agents); two systemic approaches: (1) systemic corticosteroids + methotrexate (or leflunomide) + TNF inhibitor (infliximab more effective than etanercept, probably initial TNF-inhibitor choice); (2) immunosuppressives + bisphosphonate (logical given osteoclastic infiltrating-cell phenotype, appears joint-sparing)
• Refractory cases: bisphosphonate + TNF inhibitor + methotrexate + systemic corticosteroids combination considered; tocilizumab used (granulomatous lesions involve TNF-α, IL-1, IL-6 inflammation)
• Azathioprine, thalidomide, cyclophosphamide, other immunosuppressives reported in isolated cases; skin-only patients — no treatment required; PUVA, antimalarials, topical nitrogen mustard, low-dose methotrexate, bisphosphonate, or TNF inhibitor considered for severe symptoms
• UV light reported to induce disease lesions — photodistributed-lesion or dermatomyositis-like-presentation patients should avoid light treatment

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• Rare histiocytosis, cells stain variably for Langerhans markers (S-100, CD1a — one S-100-negative case reported) but NEGATIVE for langerin (CD207), no Birbeck granules on EM
• Cells may be CD68+ (suggesting monocyte/macrophage lineage); exact origin unclear
• One 3-case series showed ETV3-NCOA2 translocations (suggests distinct entity); subsequent cases used NGS to identify this fusion for diagnosis; ETV3 encodes transcriptional repressor in macrophage growth arrest, NCOA2 encodes white/brown adipose tissue balance transcription factor
• BRAF V600E mutations found in some cases more recently (as with other histiocytoses); affects children and adults
• Solitary and multiple lesions, color yellow to red-brown; papules/plaques/nodules 3mm-10cm; nonspecific clinical features resembling many non-LCH papular lesions; conjunctival involvement reported
• Solitary malignant tumors with similar IHC described, clinically resembling atypical fibroxanthoma
• Benign course in vast majority, no therapy required; UVB, PUVA, total-skin electron beam therapy clear skin lesions; pravastatin, thalidomide (alone or with isotretinoin), methotrexate effective
• Many treated with LCH-similar chemotherapeutic agents but equivocal response; targeted therapy option if mutations detected; delgocitinib 0.5% ointment used in one case
• Solitary malignant-histology lesions managed with surgical excision (adequate margins); adjunctive therapy/sentinel node sampling utility unknown
• Postscabetic nodules and rarely post-pityriasis-rosea lesions may contain cells immunohistologically identical to indeterminate cells

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• Familial inherited syndrome or acquired secondary/systemic infiltrative process; characteristic/diagnostic cell = histiocytic cell with cytoplasmic granules staining blue-green (Giemsa) or blue (May-Gruenwald)
• Infiltrates marrow, spleen, liver, lymph nodes, lungs, and (in some patients) skin; skin lesions: papules/nodules, facial waxy plaques, eyelid swelling, patchy-gray face/upper-trunk pigmentation
• Similar histologic findings in: myelogenous leukemia, light-chain deposition disease, adult Niemann-Pick disease (type B), sphingomyelinase deficiency, apolipoprotein E gene mutations, and following prolonged IV fat supplementation or liposomal amphotericin B use
• Unifying feature: abnormal lipid metabolism by infiltrating histiocytes; seen in the infiltrate of one CTCL patient

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• L group ("X-type histiocytoses" historically) caused by Langerhans cells; rare, characterized by Langerhans cell proliferation, many organs can be affected
• Prognosis based on involvement location — reticuloendothelial system involvement (bone marrow, liver, spleen) = higher-risk/worst prognosis; skin/bone/pituitary-only = best outcomes
• Onset age important — childhood and adult LCH considered separately; adults more likely mucocutaneous lesions, 2x more likely to reactivate (63% vs. 37%), more likely to die (24% vs. 11%)
• Disease pattern may evolve/relapse to another pattern (especially in younger children) — up to 50% of children <1 year with skin-limited LCH progress to multisystem disease; repeated evaluation/close follow-up required
• Spectrum: solitary benign autoinvoluting lesions at one end, multicentric multiorgan visceral+skin disease at other; classified single-system LCH (single organ; only 12% of children with single-system LCH have skin lesions) or multisystem LCH (cutaneous involvement more common)

• Langerhans cells appear to be myeloid dendritic cells, not necessarily epidermal Langerhans cells; most LCH cases show clonality and telomere shortening
• Activating MAP kinase pathway mutations (including BRAF V600E) in >80% of patients — suggests clonal neoplastic condition; adult LCH patients may have myeloid and solid cancers supporting neoplastic phenotype
• Associated leukemia cells may share surface markers and be clonally related; but IL-17A elevated and IL-17A receptor status determines disease extent (preliminary evidence) — suggests LCH is a "hybrid" condition with both neoplastic and immunologic dysregulation features, helping explain variable outcomes (spontaneous involution to progressive/fatal)

• Dense dermal Langerhans cell infiltrate — superficial/immediately subepidermal (small papules/scaly patches clinically), folliculocentric, or deep/diffuse (papular/nodular lesions)
• Langerhans cells: abundant amphophilic cytoplasm, eccentric round or kidney-bean-shaped nucleus; frequent exocytosis into overlying epidermis (extensive → macroscopic vesicles, secondary erosion)
• Dermal infiltrate accompanied by neutrophils, eosinophils, lymphocytes, plasma cells; dermal edema/hemorrhage characteristic
• Larger/older lesions: infiltrating histiocytic cells become foamy, fibrosis possible; may lack specific Langerhans marker immunoreactivity and resemble JXG; nuclear atypia/mitotic indices don't predict prognosis, not reproducible
• IHC diagnostic: S-100+, CD1a+, Langerin (CD207)+ (protein expressed in Birbeck granule); EM rarely required given this marker panel

• International terminology/treatment protocol standardization improved management, enabled rapid investigational efficacy determination (e.g., vemurafenib with BRAF V600E identification)

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• Autoinvoluting self-limited LCH form (one end of LCH spectrum); usually present at birth or very soon after (one 8-year-old case reported)
• Two forms: solitary and multinodular variant; solitary/generalized lesions anywhere on skin, 0.2-2.5cm (may grow postnatally, exceptionally up to 8cm)
• Papules/nodules at presentation ± erosion/ulceration; red, brown, pink, or dusky; rarely hemorrhagic bullae (must rule out herpes simplex/bacterial infection via culture — these also cause hemorrhagic papules/blisters)
• Lesions >1cm characteristically ulcerate as they resolve; asymptomatic, spontaneously involute over first 6 months, leaving normal skin or atrophic scarring from ulcerated nodules
• Internal involvement rarely reported (mucosa, even lungs — which also spontaneously involuted) — makes distinguishing CSHR from autoinvoluting LCH difficult
• Histology: skin lesions composed of Langerhans cells, no histologic features identify this variant specifically; because systemic LCH may present identically clinically/histologically, systemic evaluation recommended (physical exam, CBC, LFTs, bone radiology); BRAF and other MAPK pathway genes may be mutated
• Regular follow-up required — recurrences (especially within first year) occur in ~10% of cases (as with other LCH forms)

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• Boys slightly more affected than girls; incidence ~2.6/million children (7/million <age 1, 2/million ages 1-4, ~1/million ages 5-14)
• Children conceived via IVF before 2002 appear to have increased LCH risk; neonatal disease in 6% of cases (sometimes unrecognized if asymptomatic internal organ involvement without function effect)
• Overall: bone lesions in ~2/3, skin disease in ~1/3; one study: 68% skin-limited initially, 32% multisystem initially, progression from skin-only to systemic in 28% of cases; scaly papules = most common morphology; only 10% have neither skin nor bone involvement
• <1 year: skin involved in 3/4, ear/bone in ~1/3; worse prognosis (2/3 have multisystem disease, half with liver/lung/bone marrow involvement)
• Ages 1-4: bone disease most common, but ≥2/3 have multisystem disease; ages 5-14: bone disease almost always seen, multisystem disease in <20%

• Peak presentation ages 20-35; multisystem disease in 1/3-2/3 of adult LCH; bones most common organ involved (12% have disease limited to 1-several bones); skin/mucosal involvement second most common
• One single-center 25-adult study: 20 skin-limited, 5 progressed to systemic over median 6.8 years; diabetes insipidus in 20% (other endocrine abnormalities from hypothalamic-pituitary involvement) — persist after LCH treatment, require constant monitoring

• ~10% of children have skin-only single-organ disease; 50% of multisystem-LCH children have skin involvement; almost 90% of multisystem-LCH children <1 year have skin lesions
• Skin disease pattern doesn't predict systemic disease presence/extent
• Most common childhood pattern: tiny red/red-brown/yellow papules with characteristic hemorrhagic top + petechial component, widespread but favoring intertriginous areas, behind ears, scalp
• Superficial resemblance to seborrheic dermatitis (esp. infants) but focal hemorrhage differentiates; papules often folliculocentric; may erode/weep; this pattern in children frequently associated with multisystem disease
• Rare variant (more common infants): vesicles that rupture easily → widespread erosions — may be confused with other bullous diseases (congenital candidiasis, herpesvirus infections, bullous impetigo, bullous mastocytosis, primary immunobullous diseases, EB) — culture to differentiate (especially neonates, where infections can be lethal); vesicles result from large intraepidermal Langerhans cell collections (Tzanck smear may demonstrate)
• Less common: slightly larger papules up to 1cm, yellow-red, resembling xanthomas/XGs (may be numerous/widespread); rare lichen-planopilaris-resembling variant; prurigo-nodularis-like lesion case reported
• Congenital hemorrhagic lesions resembling "blueberry muffin" babies reported (biopsy shows typical LCH)
• Nail changes uncommon but occur: nail dystrophy, nail bed purpura, nail plate loss, paronychia (fingernails and toenails); most nail-involvement patients have multisystem disease
• Genital-restricted LCH rare, but vulvar/inguinal/perianal disease may be initial manifestation — tends painful/ulcerative, may simulate hidradenitis suppurativa or cutaneous Crohn's disease (axillary/scalp involvement may also be present)
• Adults: papular or diffuse skin lesions (sometimes both forms at different sites); acneiform chest/back lesions identical clinically to acne vulgaris; xanthomatous lesions possible
• Repeatedly reported adult pattern: red erosive intertriginous eruption closely resembling deficiency dermatitis or cutaneous Crohn's disease; usually hemorrhagic/petechial component (as in pediatric lesions); favors groin/inframammary areas especially elderly women
• Nodular scabies lesions can closely simulate LCH (including Langerhans/indeterminate cells on EM dermal infiltrate, S-100/CD1a staining); larger papules resemble JXG and xanthomas

• May be involved in children — painful mucosal ulcerations, often buccal mucosa; alveolar bone lesions = most common oral lesions, osteolytic → significant periodontitis; gingival ulceration possible
• Teeth detach from underlying bone, appear "floating" on x-ray; seek palpable masses/gingival lesions, complete dental evaluation in all patients; cervical adenopathy common; bilateral parotid swelling may occur

• Bone = most commonly involved organ in pediatric LCH; lesions asymptomatic or painful; skull most often involved, then long bones, then flat bones; tend to occur in older children/young adults
• Treated with curettage, intralesional corticosteroids, or radiation; endocrine dysfunction (usually diabetes insipidus) more common with skull bone disease and extensive disease — diabetes insipidus is a common long-term sequela of LCH recovery

• Bone marrow involvement → cytopenias (may present as skin purpura); liver involvement direct (Langerhans cell infiltration) or indirect (enlarged porta hepatis nodes → obstructive disease); either pattern can cause biliary cirrhosis
• Pulmonary disease (diffuse micronodular infiltrates + cysts) less frequent in children than adults; pulmonary LCH average onset 3rd decade — diffuse micronodular pattern on chest radiograph may progress to cyst formation (honeycomb lung), large bullae, pneumothorax
• >90% of adults with pulmonary LCH are tobacco/marijuana smokers; pneumothorax in 25% of cases; high-resolution CT useful for diagnosis; lung transplantation may be required; unclear if isolated pulmonary LCH is reactive process or LCH variant

• Outcome/therapy determined by involvement extent and (more importantly) affected organ function
• Mortality: <1 year multisystem disease ~50% (worst); ages 1-4 ~30% or lower; ages ≥5 only 6%; ear and lung involvement = poor prognostic finding in multisystem disease
• Early initial response to multidrug chemotherapy in childhood multisystem LCH = important survival predictor (92% survival in responders vs. 11% in nonresponders after 6 weeks of treatment)
• Baseline/repeated evaluation important — one organ system may resolve while disease progresses in another; skin lesions may spontaneously resolve then recur years later — regular follow-up required
• Classification: single-system vs. multisystem LCH; multisystem further stratified by high-risk organ involvement (bone marrow, liver, spleen, CNS) vs. without; multisystem patients should be referred to oncology
• BRAF V600E mutation → BRAF inhibitor vemurafenib shows excellent efficacy; generally vinblastine + corticosteroid used as initial treatment (reactivation remains a problem)
• Localized skin disease managed by dermatologist: topical steroids, topical nitrogen mustard/carmustine, PUVA, NB-UVB (or excimer laser), thalidomide, methotrexate, azathioprine depending on extent; 5% imiquimod may be effective for limited lesions; oral retinoids (acitretin, isotretinoin) adjunctive if no contraindications

• Associated lymphomas, solid tumors, myelodysplasias occur with LCH — ALL and MDS may precede LCH appearance; AML and ALL may follow it
• Some cutaneous/systemic lymphoma cases show Langerhans cell aggregates in affected tissue — unclear if LCH-lymphoma coexistence or reactive Langerhans cell proliferation within lymphoma-affected tissue; cases with same MAP kinase pathway mutations in both neoplasms reported

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