The spectrum of histopathologic findings in cutaneous eruptions associated with influenza A (H1N1) infection.

Influenza A (H1N1), like many other viral infections, has been associated with cutaneous eruptions. Differential diagnoses in a viral exanthem generally include spongiotic dermatitis, urticaria and drug reaction. The aim of this series was to retrospectively review three cases (five biopsies) involving patients with a clinical history of H1N1 and an accompanying rash, and to evaluate whether unique histopathologic and immunohistochemical features exist among these patients' cutaneous eruptions. Findings among all cases included a sparse superficial perivascular infiltrate, and interestingly, scattered interstitial and prominent intravascular neutrophils. Two cases demonstrated mild spongiosis and mild interface change. Immunohistochemistry in all cases revealed a CD4-predominant lymphocytic infiltrate of the dermis with a sparse intraepidermal population of admixed CD4 and CD8 positive lymphocytes. Many changes found in the cutaneous eruption associated with H1N1 are similar to those of other viral eruptions, including a mild perivascular lymphocytic infiltrate, mild spongiosis and mild interface change; however, sparse dermal and intravascular neutrophils and intraepidermal lymphocytes appear to be the features unique to these cases of H1N1-associated cutaneous eruptions. Such a distinction may prove diagnostically important in the clinical setting and useful in the surveillance of this historically pandemic virus.