Erythema Nodosum, Early Stage-A Subcutaneous Variant of Leukocytoclastic Vasculitis? Clinicopathological Correlation in a Series of 13 Patients.

Erythema nodosum (EN) is considered to represent a septal panniculitis. In a period from January 2000 until June 2018, we clinically and histopathologically investigated 124 patients with EN, 13 (10.5%) of them in an early stage demonstrating features of a leukocytoclastic vasculitis (LCV) around postcapillary venules of the subcutaneous fat. Three of these patients presented with EN on the lower legs and Sweet syndrome on the head/neck, arms, or trunk. 19.3% and 70.2% of patients demonstrated "classic" features of subacute and chronic forms of EN, respectively. Histopathologically, in cases of early EN apart from septally accentuated vascular damage and neutrophils with nuclear dust, eosinophils were evident in 5 specimens as well, in one case even with flame figures as seen in Wells syndrome. The inflammation spilled over to the dermis and lobular panniculus in 12 and 10 specimens, respectively. From the same time period and for comparison, we investigated 497 cases of "classic" LCV. Depending on the degree of vascular damage and the presence of neutrophils and nuclear dust, 65.8% presented with acute, 18.9% with subacute, and 15.3% with late-stage disease. In the latter, only a few neutrophils but rather lymphocytes and macrophages were present. Four hundred forty patients revealed an involvement of the deep dermis; of those, in 342, a septal inflammation was present as well, whereas in 94, the process was purely dermal. The subcutis was missing for evaluation in 61 cases. These results indicate a closer relationship between EN and LCV than previously considered.