Erythema Migrans and Interface Changes: More Than a Fortuitous Association.

The histology of erythema (chronicum) migrans (ECM) is classically described as a nonspecific perivascular infiltrate with a variable number of plasma cells and eosinophils. However, deviations from this pattern were described, such as focal interface changes or spongiosis, potentially posing a clinicopathological challenge. In this study, cases submitted with a serologically confirmed, clinically unequivocal, or highly suspicious diagnosis of ECM/Lyme disease between January 01, 2016, and September 01, 2018, were retrieved from the electronic database system and reviewed to delineate the histopathologic features of ECM. The series consisted of 14 cases. A superficial perivascular lymphocytic infiltrate was noted in all biopsies, accompanied by a deep and/or interstitial inflammatory infiltrate in 9 cases (64%). The inflammation ranged from relatively sparse to dense and prominent. At least focal interface changes were noted in 12 biopsies (86%). Eosinophils and plasma cells were noted in 7 (50%) and 10 (71%) cases, respectively. From a histologic standpoint, ECM is a protean entity and may manifest with a variable density of perivascular and/or interstitial lymphocytic infiltrate admixed with eosinophils and/or plasma cells and accompanied by focal interface dermatitis. Within the appropriate clinical context, ECM should be considered in the differential diagnosis of focal interface and/or sparse perivascular dermatitis.