Dermatopathology: Pattern Recognition and Clinicopathologic Correlation
The pattern approach
Dermatopathology questions reward a systematic pattern approach. First decide the reaction pattern: spongiotic, psoriasiform, lichenoid, interface, vesiculobullous, granulomatous, or panniculitic. Then assess the level of the process (epidermis, dermis, subcutis) and the inflammatory cell composition. This narrows the differential before you commit to a diagnosis.
Key discriminators
Spongiotic dermatitis shows intercellular edema; eosinophils suggest allergic contact or drug reaction. Psoriasiform hyperplasia with parakeratosis and neutrophils in the stratum corneum suggests psoriasis. Lichenoid dermatitis shows a band-like lymphocytic infiltrate with basal vacuolar change; lichen planus has wedge-shaped hypergranulosis and saw-tooth rete ridges. Interface dermatitis with dyskeratosis raises lupus or erythema multiforme. For bullous diseases, the split level is decisive: intraepidermal (pemphigus) vs subepidermal (pemphigoid, dermatitis herpetiformis).
Special studies and traps
Direct immunofluorescence is essential for autoimmune bullous disease and lupus. Immunohistochemistry (S100, Melan-A, SOX10, cytokeratins) distinguishes melanoma from carcinoma. The common trap is overreliance on one feature; diagnosis requires correlation with clinical appearance. Another trap is misreading a benign mimic (e.g., pseudolymphoma vs lymphoma) without clonality studies.
Worked example: A biopsy shows subepidermal blister with linear IgG and C3 at the basement membrane and numerous eosinophils. This is bullous pemphigoid. The trap is pemphigus vulgaris, which would show intraepidermal acantholysis with intercellular IgG.
Rule of thumb: Name the reaction pattern first, then the cell type, then correlate clinically.