Objectives: The aim was to describe the clinical characteristics and epidemiology of hypocomplementaemic urticarial vasculitis (HUV; anti-C1q vasculitis) in two geographically defined areas of Sweden.

Methods: In the health-care districts surrounding Skåne University Hospital (mean population 950 560) and Linköping University Hospital (mean population 428 503), all incident cases of HUV residing within the study areas at the onset of disease were identified during the years 2000-15. The diagnosis of HUV was confirmed by review of medical records. Only patients meeting the proposed diagnostic HUV criteria and/or the 2012 Chapel Hill consensus definitions in combination with an ever-positive anti-C1q antibody test were included.

Results: Sixteen patients (14 females) were identified during the study period. The median (interquartile range) age at diagnosis was 51 (40.7-56.7) years. Median (interquartile range) time of follow-up from diagnosis to 31 December 2015, or death, was 94 (46.5-136.2) months. The most frequent manifestations at diagnosis were urticaria (100%), arthritis (88%), followed by biopsy-proven glomerulonephritis (19%), episcleritis/scleritis (19%) and recurrent abdominal pain (13%). The annual incidence rate per million inhabitants was estimated as 0.7 (95% CI: 0.4, 1.1). Sixty-three per cent suffered from pulmonary disease at the last follow-up. Two patients died during the follow-up period. One patient underwent lung transplantation, and two patients proceeded to end-stage renal disease. The point prevalence on 31 December 2015 was 9.5/million (95% CI: 4.5, 14.5).

Conclusion: Hypocomplementaemic urticarial vasculitis constitutes a rare, but not always benign condition. Renal and lung manifestations were severe in some cases, highlighting the need for careful screening and monitoring of this potentially serious condition.