The contrasting epidemiology of granulomatosis with polyangiitis (Wegener's) and microscopic polyangiitis.

Objectives: Granulomatosis with polyangiitis (Wegener's) (GPA) and microscopic polyangiitis (MPA) are uncommon and have unknown aetiology. The aim of the study was to investigate the epidemiology of GPA and MPA in a stable, well-defined population looking for differences in the pattern of occurrence, which might suggest a different aetiology.

Methods: Since 1988, we have maintained a prospective register of all patients with systemic vasculitis attending the Norfolk and Norwich University Hospital. Patients presenting with new-onset GPA and MPA as defined by the European Medicines Agency algorithm and registered with general practitioners in the former Norwich Health Authority area between 1988 and 2010 were identified. The population in 2008 was estimated to be 459 000 (221 000 males).

Results: One hundred and eleven GPA and 58 MPA incident cases were identified during 1988-2010. The overall annual incidence of GPA and MPA was 11.3/million and 5.9/million, respectively. There was evidence of a cyclical pattern of occurrence with a periodicity of 7.6 years for GPA with a peak incidence of 28.3/million in 2005 and the lowest in 2002 (2.2/million). Other lesser peaks occurred in 1990 and 1996. While the peak incidence of MPA was in 2008 (15.2/million), there was no convincing evidence of periodicity. The incidence of cANCA/PR3- or pANCA/MPO-positive vasculitis showed a similar pattern to GPA and MPA, respectively.

Conclusion: This study lends support to the notion that the aetiology of GPA and MPA may be distinct conditions with different aetiologies. The cyclical incidence of GPA is possibly an indication for the influence of infection.