Dermatopathology: Pattern Recognition and Diagnostic Pitfalls
Read at low power first
Dermatopathology is pattern recognition before cytology. At low power, ask whether the process is epidermal, dermal, or subcutaneous; whether inflammation is interface, spongiotic, psoriasiform, lichenoid, vacuolar, granulomatous, or nodular; and whether there is alopecia, panniculitis, or a proliferation. Then examine cytology: lymphocytes, eosinophils, neutrophils, plasma cells, atypical melanocytes, or keratinocyte atypia.
Core concepts
Know the lichenoid interface pattern and its differential, including lichen planus, lichenoid drug eruption, lupus erythematosus, and hepatitis C-associated disease. Recognize spongiotic dermatitis with eosinophils as a clue to allergic contact or drug reaction. In alopecia, distinguish scarring from non-scarring by loss of follicular openings and follicular dropout. Direct immunofluorescence is essential for autoimmune blistering disease and lupus. Special stains include PAS for fungi, Fite for mycobacteria, Alcian blue for mucin, and Fontana-Masson for melanin. Immunohistochemistry such as SOX10, Melan-A, cytokeratin, CD3, and CD20 helps resolve poorly differentiated tumors.
Traps
Tangential sectioning can mimic atypia; crush artifact and edge artifact can obscure architecture. A single atypical cell does not make melanoma. Margin assessment is difficult in fragmented specimens. Beware of diagnosing malignancy in a healing or inflamed lesion without clinicopathologic correlation.
Worked example
A pruritic, violaceous, flat-topped papule shows bandlike lymphocytic infiltrate, basal vacuolar change, and Civatte bodies. That is lichenoid interface dermatitis. Before signing out lichen planus, review medications, check for photodistribution, and consider lupus serology if there is periadnexal inflammation or mucin.