Dermatopathology: Pattern Analysis and Diagnostic Pitfalls
Pattern recognition
Dermatopathology is pattern-based. At scanning magnification, classify the biopsy: spongiotic, psoriasiform, lichenoid/interface, vesiculobullous, granulomatous, vasculopathic, or panniculitic. Only then examine cytology - whether the infiltrate is lymphocytic, neutrophilic, eosinophilic, or mixed; whether it is perivascular, periadnexal, or diffuse; and whether there is epidermal hyperplasia, atrophy, or necrosis.
High-yield patterns: spongiotic dermatitis shows intercellular edema and vesicle formation (eczema, contact dermatitis, pityriasis rosea); psoriasiform hyperplasia with thinned suprapapillary plates and Munro microabscesses suggests psoriasis; lichenoid interface dermatitis with basal vacuolization and necrotic keratinocytes suggests lichen planus, lupus, or a lichenoid drug reaction. Bullous disorders require direct immunofluorescence: linear IgG/C3 along the basement membrane in bullous pemphigoid, intercellular IgG in pemphigus, granular IgA in dermatitis herpetiformis.
Traps: a "busy dermis" may be a dermatofibroma, so look for collagen trapping at the periphery; lichenoid drug eruptions can closely mimic lichen planus, so ask about new medications; deep perivascular infiltrates should prompt consideration of lupus tumidus or perniosis. Special stains - PAS for fungi, Fite for mycobacteria, elastic van Gieson for vasculitis - should be ordered when the differential demands them.
Rule of thumb: diagnose the pattern first, then the disease. Worked example: a biopsy shows interface dermatitis with necrotic keratinocytes and a superficial perivascular lymphocytic infiltrate. The differential includes lichen planus, lichenoid drug eruption, and lupus erythematosus; clinical correlation, medication history, and direct immunofluorescence distinguish them.