Granulomatous Skin Diseases

Granulomatous skin diseases comprise a diverse group of cutaneous disorders that share the common histopathological feature of granulomatous inflammation. Granulomas are organized collections of activated macrophages that have transformed into specialized cells called epithelioid cells, often accompanied by multinucleated giant cells (formed by fusion of epithelioid cells), surrounded by T-lymphocytes, and may contain varying degrees of necrosis or palisading patterns. This inflammatory pattern develops in response to persistent antigenic stimulation that the body cannot clear with conventional acute inflammation, including infectious organisms, foreign materials, and as a manifestation of autoimmune or idiopathic inflammatory disorders. Recognition and proper classification of granulomatous skin diseases is essential because treatment depends entirely on identifying the underlying cause — antimicrobial therapy for infections versus immunomodulatory treatment for non-infectious causes.

Pathogenesis and granuloma formation: 1) Granulomas develop when antigen persists despite acute inflammatory response — typical antigens include poorly degradable mycobacterial cell wall components, fungal cell walls, foreign materials, autoantigens, or unidentified triggers; 2) Initial response involves macrophage activation by interferon-gamma and other Th1 cytokines from antigen-presenting cell processing; 3) Macrophages transform into epithelioid cells with abundant cytoplasm and indistinct cell borders, similar to epithelial cells; 4) Multinucleated giant cells form by fusion of multiple epithelioid cells — Langhans giant cells (nuclei arranged peripherally in horseshoe pattern, classic in tuberculosis and sarcoidosis), foreign body giant cells (nuclei scattered throughout cytoplasm), Touton giant cells (nuclei in central wreath surrounded by foamy cytoplasm, in xanthogranulomas), Wegener-type giant cells; 5) T-lymphocytes (CD4+ Th1 type) form a cuff around the granuloma, providing cytokine support including interferon-gamma, IL-2, TNF-alpha, IL-12; 6) Macrophages may also fuse with epithelioid cells; 7) Granulomas may evolve to: caseating necrosis (central liquefactive necrosis with cheese-like appearance, classic for tuberculosis), palisading pattern (histiocytes arranged radially around central degenerated collagen or necrobiosis, characteristic of granuloma annulare and necrobiosis lipoidica), suppurative (with central neutrophilic infiltrate, characteristic of deep fungal and atypical mycobacterial infections), foreign body type (with engulfed material visible, sometimes polarizable under polarized light); 8) Cytokine network includes TNF-alpha (essential for granuloma maintenance — explains worsening of granulomatous diseases with TNF blockers, particularly tuberculosis reactivation), IFN-gamma, IL-12, IL-18, IL-1, IL-6.

Histopathological classification: 1) Sarcoidal granuloma — well-formed compact granulomas of epithelioid cells with sparse lymphocytic cuff (naked granulomas), no caseation, asteroid bodies and Schaumann bodies may be present in giant cells, characteristic of sarcoidosis but also drug-induced, foreign body, beryllium granuloma, infectious in some atypical mycobacterial infections; 2) Tuberculoid granuloma — epithelioid granulomas with prominent peripheral lymphocyte cuff, central caseation necrosis (classic for tuberculosis), Langhans giant cells, may have acid-fast bacilli on Ziehl-Neelsen stain (tuberculosis, mycobacteria — abundant in lepromatous leprosy), special stains essential for diagnosis; 3) Necrobiotic granuloma — palisading histiocytes arranged radially around central degenerated collagen with mucin deposition (granuloma annulare) or central degenerated collagen with neutrophil debris (necrobiosis lipoidica) or central fibrinoid necrosis (rheumatoid nodule); 4) Suppurative granuloma — granulomatous inflammation with central neutrophilic infiltrate, characteristic of deep fungal infections (sporotrichosis with cigar-shaped budding yeasts, blastomycosis with broad-based budding yeasts, coccidioidomycosis with spherules), atypical mycobacteria (M. marinum, M. abscessus), botryomycosis (granular sulfur granules of Staphylococcus); 5) Foreign body granuloma — multinucleated giant cells (foreign body type) engulfing visible foreign material, often birefringent under polarized light (silica, talc, sutures, dermal fillers, tattoo pigments).

Major granulomatous skin diseases: 1) Sarcoidosis — multisystem disorder of unknown etiology characterized by non-caseating sarcoidal granulomas in multiple organs; cutaneous involvement in 25-30 percent of patients with various morphologic patterns: a) Papular sarcoidosis — small skin-colored or red papules; b) Plaque sarcoidosis — annular or annular-erythematous plaques; c) Nodular sarcoidosis — subcutaneous nodules; d) Lupus pernio — characteristic violaceous plaques on nose, ears, cheeks (associated with lung fibrosis, ENT involvement, chronic disease); e) Scar sarcoidosis — sarcoidosis developing in old scars (vaccinia scars, surgical scars, tattoos); f) Subcutaneous sarcoidosis (Darier-Roussy); systemic features include lung involvement (95 percent — bilateral hilar lymphadenopathy, pulmonary infiltrates, fibrosis), eye involvement (uveitis, conjunctival nodules), liver and spleen involvement, lymphadenopathy, hypercalcemia (vitamin D activation by macrophages), elevated angiotensin-converting enzyme (ACE) level; treatment with topical and intralesional corticosteroids, hydroxychloroquine, methotrexate, anti-TNF biologics; 2) Granuloma annulare — common idiopathic granulomatous condition characterized by ring-shaped (annular) plaques with raised edges and skin-colored to erythematous color; localized form most common (90 percent of cases — typically dorsa of hands, feet, ankles), generalized form (10 percent — more widespread, may be associated with diabetes mellitus, dyslipidemia), perforating GA (transepidermal elimination of necrobiotic material), subcutaneous GA (deeper nodules in children); usually self-limited (months to 2 years), treatment includes topical corticosteroids, intralesional corticosteroids, topical calcineurin inhibitors, cryotherapy, phototherapy, hydroxychloroquine, methotrexate; 3) Necrobiosis lipoidica — characterized by yellow-brown atrophic plaques with telangiectasia and brown borders, typically on shins (95 percent on lower extremities); strongly associated with diabetes mellitus (60-70 percent of patients have diabetes, more in type 1 — formerly called necrobiosis lipoidica diabeticorum); ulceration in 30 percent; treatment includes topical and intralesional corticosteroids, topical tacrolimus, pentoxifylline, hydroxychloroquine, anti-TNF therapy, JAK inhibitors emerging; 4) Tuberculosis cutis — multiple forms based on host immunity and inoculation route: a) Lupus vulgaris — most common form, slowly progressive plaque on face, neck, with apple-jelly nodules on diascopy; b) Scrofuloderma — caseating cervical lymph nodes with overlying skin breakdown and fistulas; c) Tuberculosis verrucosa cutis — warty plaque from exogenous inoculation; d) Miliary tuberculosis cutis; treatment with anti-tuberculous therapy 6-9 months; 5) Leprosy — chronic mycobacterial infection of skin and peripheral nerves caused by Mycobacterium leprae; multiple forms based on cell-mediated immunity: a) Tuberculoid leprosy — strong cell-mediated immunity with localized skin lesions and nerve involvement, few bacilli; b) Lepromatous leprosy — weak cell-mediated immunity with diffuse infiltration, leonine facies, abundant bacilli; c) Borderline forms intermediate; characteristic skin and peripheral nerve thickening; treatment with multidrug therapy (rifampin, dapsone, clofazimine) by WHO regimens; 6) Foreign body granulomas — caused by introduced material (silica from broken glass, talc, dermal fillers like silicone or hyaluronic acid, suture material, tattoo pigments, beryllium); 7) Cutaneous leishmaniasis — caused by Leishmania species transmitted by sandfly bites; chronic ulcerated nodule typically on exposed skin; granulomatous inflammation with characteristic Leishman bodies in macrophages; treatment with topical paromomycin, intralesional pentavalent antimony, oral miltefosine, systemic liposomal amphotericin B for diffuse forms; 8) Other granulomatous conditions — granulomatous rosacea, deep fungal infections, rheumatoid nodules, palisaded neutrophilic and granulomatous dermatitis (PNGD), interstitial granulomatous dermatitis (IGD).