Microscopic features of experimentally induced allergic contact dermatitis (ACD) have been reported; however, the histologic characteristics of environmentally induced ACD have not been definitively elucidated.

This pilot study was conducted to describe the histologic features most useful in differentiating ACD from other forms of eczematous dermatitis.

A retrospective chart review of patients who underwent evaluation for contact dermatitis was performed. The results of skin biopsy were reviewed in a blinded manner, and two subgroups were compared: gold standard ACD cases and control cases (eczematous dermatitis with negative results of patch testing).

Of 317 cases reviewed, 39 fulfilled the inclusion criteria. Many histologic similarities were noted: both subgroups had substantial acanthosis and lymphocytic infiltration. In addition, present to a moderate degree in both groups were hyperkeratosis, spongiosis, eosinophils, and dermal dendritic fibrohistiocytic (DFH) cells. However, compared with the control cases, eosinophilic spongiosis and multinucleate dermal DFH cells were found to a considerably greater degree in the standard ACD cases.

Eosinophilic spongiosis and multinucleate dermal DFH cells, in the presence of acanthosis, lymphocytic infiltrate, dermal eosinophils, and hyperkeratosis, are particularly suggestive of ACD. While such findings alone are not diagnostic of ACD, the presence of these combined histologic features supports the pursuit of patch testing in cases of chronic eczematous dermatitis.