Erythema nodosum in northern Finland between 1996 and 2019: A register-based study.

Erythema nodosum (EN) is seen at any age with varying and often unidentified etiology. We studied the etiology and characteristics of EN in Northern Finland. Medical records of all patients with a diagnosis code for EN between 1996 and 2019 from Oulu University Hospital were retrieved and analyzed. There were in total 142 EN cases with a female predominance (n = 112, 72.9%). The mean age of the patients was 35.9 years. There were five cases diagnosed with EN in those younger than 2 years of age. Almost one third had EN nodules in multiple anatomical locations. In addition to skin findings, systemic symptoms were common (81.0%), and seen more often in men (p < 0.05). In children and adolescents, the most common etiological factors were gastroenteritis caused by 'Yersinia, Salmonella or Campylobacter', followed by inflammatory bowel diseases and hormonal contraception. Bacterial infections were the most common etiological factor among adults. In 28.2% of the cases there was no identified causative factor. In this study, EN was seen surprisingly often in small children. Etiological factors varied markedly among different age groups and symptoms differed between the sexes in adults. These aspects should be taken into account when diagnosing EN patients.