Dermatopathology: Pattern Diagnosis and Reports That Change Care
Dermatopathology in the OED tests pattern recognition and safe reporting: can you classify the reaction pattern, request the right special stain, and communicate margins and prognostic features clearly?
Core rules
- Read at low power first. Define the pattern: lichenoid interface, spongiotic, psoriasiform, vesiculobullous, granulomatous, vasculopathic, panniculitic or neoplastic. Then assess epidermal change, dermal infiltrate composition and depth, and adnexal involvement.
- Lichenoid dermatitis shows a band-like lymphocytic infiltrate obscuring the dermoepidermal junction with basal vacuolar change and apoptotic keratinocytes; the differential includes lichen planus, drug eruption, lupus and lichenoid keratosis. Spongiotic dermatitis shows intercellular oedema, eosinophils and parakeratosis; clinical correlation separates eczema from drug rash.
- Direct immunofluorescence: pemphigus shows intercellular IgG; pemphigoid shows linear IgG/C3 along the basement membrane; dermatitis herpetiformis shows granular IgA in dermal papillae; lupus shows granular IgG/C3 at the junction.
- Melanoma reporting: Breslow thickness, ulceration, mitotic rate for thin lesions, margins, lymphovascular invasion, perineural invasion and regression. Use Breslow-based staging, not Clark level alone.
- Special stains: PAS for fungi, Fite or acid-fast for mycobacteria, CD3/CD20 for lymphoid, Melan-A/HMB-45/S100/SOX10 for melanocytic, cytokeratin for epithelial.
- Clinicopathological correlation is mandatory; a silent biopsy does not exclude disease.
Worked example
A 52-year-old woman has a violaceous annular rash on the wrist. Biopsy shows a dense band-like lymphocytic infiltrate with basal vacuolar change and many apoptotic keratinocytes. That is a lichenoid interface pattern; the differential includes lichen planus and lichenoid drug eruption. Review drugs; if no culprit, treat as lichen planus. Do not call it lupus without interface mucin, periadnexal infiltrate or positive immunofluorescence.
Traps
Do not diagnose melanoma without assessing the deep margin and Breslow. Do not mistake a pseudolymphoma for lymphoma; use clonality and clinical context. Do not ignore a negative stain when infection is likely. Always state whether margins are involved.