Primary renal vasculitis in Norfolk - increasing incidence or increasing recognition?

Background. The incidence of renal vasculitis has previously been estimated using histological definitions or only a single clinical diagnosis, e.g. Wegener's Granulomatosis (WG). Our hospital is the single referral centre for the former Norwich Health Authority (NHA) which encompasses a stable, homogenous, well-defined and studied population. We estimated the overall incidence of primary renal vasculitis and the incidence within individual clinical disease classifications. Methods. All cases of primary renal vasculitis diagnosed within the NHA over 66 months (1992-1997) were identified by review of renal biopsies, the Norfolk Vasculitis Register, hospital discharge summaries and plasmapheresis records. Patients were classified using the 1990 American College of Rheumatology criteria for Polyarteritis Nodosa (PAN), Churg Strauss Syndrome (CSS) and Henoch-Schonlein Purpura; the Chapel Hill Consensus Conference Definitions for Microscopic Polyangiitis (mPA) and the Lanham criteria for CSS. incidence figures were calculated using the NHA adult population of 413747 (1994). Ninety-five per cent confidence intervals (C.I.) were calculated using the poisson distribution. Results. The overall annual incidence for primary renal vasculitis was 18/million (C.I. 12.9-24.4). The annual incidence of renal involvement of individual diseases was as follows: WG 7.9/million (95% C.I. 4.7-12.5); mPA 7.5/million (95% C.I. 4.4-12.0); PAN 7.0/million (95% C.I. 4.0-11.4); HSP 3.1/million (95% C.I. 1.2-6.3); CSS 1.3/million (95% C.I. 0.3-3.9). Conclusions. The annual incidence for primary renal vasculitis overall and the individual subtypes in Norfolk is much higher than previous European estimates. This may reflect an increasing incidence in primary renal vasculitis with time or underestimation in previous studies. However the incidence of renal vasculitis in our population is markedly lower than reported in Kuwait. There may therefore be true variation in incidence between populations which could have implications for the aetiology of primary vasculitis.