Introduction:

Allergic contact dermatitis (ACD) has a wide spectrum of clinical presentations, which mimic diverse dermatological conditions. When patch tests do not identify that the relevant allergens or treatment is not effective, a skin biopsy is warranted. However, there are few descriptive series on the histopathology of ACD. The purpose of this study was to characterize microscopic changes in ACD and to identify features that may help in the differential diagnosis.

Methods:

We retrospectively included 20 skin biopsies of ACD cases. Slides were reviewed, and microscopic changes analyzed.

Results:

We reported a clinicopathologic concordance of 80%. The main histological differential diagnosis was drug eruption (DE). Common epidermal findings included acanthosis (95%), parakeratosis (85%), and spongiosis (80%). Necrotic keratinocytes were observed in only three cases. The most common dermal change was the presence of a superficial perivascular inflammatory infiltrate. Lymphocytes were present in all cases and eosinophils in 80% of the biopsies, although in much smaller number. Neutrophils and atypical lymphocytes were absent.

Conclusion:

In ACD, isolated pathological findings are nonspecific and clinicopathological correlation is essential. Eosinophilic spongiosis is the typical pattern, but findings depend on the stage of evolution. Several histological features (including parakeratosis and epidermal hyperplasia with signs of spongiosis, superficial dermal infiltrate, absence of both apoptotic keratinocytes, and vacuolar degeneration), may aid in the differential diagnosis with DE.

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