Patients hospitalized because of small vessel vasculitides with renal involvement in the period 1975-95: organ involvement, anti-neutrophil cytoplasmic antibodies patterns, seasonal attack rates and fluctuation of annual frequencies.

Objectives: To study organ involvement, anti-neutrophil cytoplasmic antibodies (ANCA) patterns, trends in yearly frequencies and seasonal variations of symptom onset in patients hospitalized because of small vessel vasculitides during a 21 year period (1975-95).

Design: A retrospective investigation was conducted of 138 patients hospitalized with a diagnosis of small vessel vasculitides, as defined by the Chapel Hill Consensus Conference, within the County of Orebro, a mixed urban and rural area of central Sweden.

Setting: Orebro Medical Center Hospital, Orebro, Sweden and two district hospitals within the County of Orebro, Sweden.

Results: During the studied period there were 19 patients with a diagnosis of Wegener's granulomatosis (WG), 70 patients with microscopic polyangiitis (MPA), 36 patients with renal limited vasculitis (RLV), two with Churge-Strauss vasculitis (C-S), seven with Henoch-Schönleins purpura (HSP) and four with essential cryoglobulinemic vasculitis (ECV). Renal involvement was present in 123 patients (89.1%). A positive c- and/or pANCA was found in nearly 90% of the 111 patients where sera were available. Calculations of frequency data, restricted to the primary catchment area for patients with ANCA associated vasculitis and renal involvement (WG, MPA, RLV) during a 21-year period (1975-95) gave a mean annual frequency of 1.6 per 100,000 adults (95% CI: 1.2-3.1); for this group of patients with the inclusion of those with C-S, HSP and ECV during the last 10 year period (1986-95) gave a mean annual frequency of 2.5 per 100,000 adults (95% CI: 1.7-3.4), for male adults 3 per 100,000 (95% CI: 1.6-4.4), and female adults 1.9 (95% CI: 0.9-2.8). A frequency peak of 6.3 per 100,000 was seen for men aged 55-64. A periodic fluctuation of the frequencies with peaks every 3-4 years was noted for patients with ANCA related vasculitis (WG, MPA, RLV) during the 21-year period 1975-95. Onset of symptoms was predominantly noticed during the winter months (December-February) for patients with a positive cANCA.

Conclusion: The observed frequencies in our study of patients with small vessel vasculitides were higher than those previously documented. We also showed a periodic fluctuation of the annual frequencies and a seasonal variation of symptom onset.