Patients and methods

The study was hospital based in two regions of Europe—Norwich Health Authority (NHA) Norfolk, UK (latitude 52°N), and Hospital Xeral-Calde, Lugo, Spain (43°N). The NHA provided administrative services to a group of general practices covering a population of 413 500. Patients were included if they were registered with one of these practices. Hospital services were provided by a single institution—Norfolk and Norwich Hospital. In Lugo the Hospital Xeral-Calde is the only referral centre for a population of almost 250 000 people and serves a defined population. The studies were both performed between 1 January 1988 and 31 December 1998. Patients with vasculitis diagnosed before or after the study period were excluded.

Results

A total of 127 patients presented with new onset PSV (86 Norwich, 41 Lugo) in the two populations over an 11 year period. The overall annual incidence of PSV in adults was 18.9/million in Norwich and 18.3/million in Lugo (table 1). There was a greater incidence of WG in Norwich (10.6/million) than in Lugo (4.9/million), whereas MPA was more common in Lugo (table 1). The incidence of PAN and CSS was higher in Norwich. There was no significant change in incidence between 1988–92 and 1993–98 in either Norwich (15.5/million (95% CI 10.6 to 21.9) and 21.0/million (95% CI 15.6 to 27.4)) or Lugo (18.6/million (95% CI 11.2 to 29.1) and 18.0/million (95% CI 11.3 to 27.2). A rolling triennial average did not disclose any cyclical changes in incidence. In Norwich, there was an age-specific increase in incidence peaking in the 65–74 year group; the age distribution was much flatter in Lugo with an almost equal incidence between ages 45 and 74 (fig 1). In Norwich the peak incidence was 52.9/million (35.6 to 75.5), and in Lugo 34.1/million (16.3 to 62.8). The incidence was greater in men than women for all groups except for Wegener's granulomatosis in Lugo, where there was female preponderance (table1).

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Figure 1

Age-specific incidence of primary vasculitis in Norwich and Lugo, Spain.

Discussion

We have previously reported the epidemiology of vasculitis in Norwich and Lugo5 ,6 using different methods. In this study we harmonised our methods and therefore could compare directly the incidence of PSV in our two populations. In both centres a single referral centre provides secondary care to its local population and this population can be easily identified. Referrals to outside hospitals are rare because both populations are relatively geographically isolated. Patients with PSV are sufficiently ill to be referred for secondary care and are therefore not managed in the community, by either general practitioners or private physicians. The data provide minimum estimates of the incidence of PSV in our communities. No significant change (although there was an upward trend) in incidence occurred during our study, suggesting that there were no major changes in the completeness of our data collection or referral patterns.

The overall incidence of PSV was similar in Norwich and Lugo, but there were trends towards differences in incidence for each condition. There was a greater incidence of WG in Norwich and MPA in Lugo. This supports the idea (suggested anecdotally by doctors interested in vasculitis) that WG is more common in the north, whereas MPA is more common in the south of Europe. CSS and PAN were both more common in Norwich. These differences reflect, we believe, genuine variations in incidence between populations rather than differences in classification or patient selection. By applying the same classification criteria/definitions to a defined population it is possible to identify differences in incidence between populations. For the reasons mentioned above we believe that our figures represent a population based incidence, but as with any epidemiology study we cannot be absolutely confident that we identified all patients.

The latitudinal trends in incidence for WG and MPA are supported by other studies. Koldingsnes from Tromso in North Norway reported a higher annual incidence of WG (15.0/million) than we did from the UK.7 In the south MPA is particularly common in Kuwait, with an annual incidence of 24/million.8 Methodological differences make it difficult to compare these populations directly with ours.

The aetiopathogenesis of PSV is unknown, but genetic and environmental factors are likely to be important. MPA and WG can sometimes be difficult to distinguish clinically and it has been suggested that they have a common pathogenesis. These data, however, point towards a difference in the pathogenesis of WG compared with MPA.

The age and sex distribution of the populations of Norwich and Lugo are broadly similar. Both populations are mainly Caucasoid, but there is a significant component of Celtic ancestry in Lugo. The HLA make up of the Lugo population differs from that found in other Mediterranean areas and has a similar proportion of HLA-DRB1 alleles to that in the UK population.9 No clear HLA association has been shown with any type of PSV,10 suggesting that environmental factors may be more important than genetic factors. There was a trend towards an increase in incidence in Norwich but not Lugo, suggesting that any environmental factors were probably unchanged during the period of the study. We were unable to find any evidence for cyclical changes in incidence as recently suggested in a retrospective hospital based study from Sweden.11 In the 1980s it was suggested that there was an increase in incidence of WG. This does not seem to have continued and was probably due to the introduction of routine antineutrophil cytoplasmic antibody testing and greater awareness of doctors.12

Further studies conducted in other parts of the world may yield interesting clues to the pathogenesis of these so far enigmatic diseases, and permit dissection of the interplay between genes and environment.