We found that wide variation exists in the incidence of SAH. The overall incidence of SAH was approximately 9 per 100
000 person‐years but varied significantly by region, with doubled rates in Japan and Finland and far lower rates in South and Central America. The incidence was higher in women and increased with age. The gender distribution varied with age. At young ages, incidence was higher in men, while after the age of 55 years, the incidence was higher in women. The incidence of SAH has probably decreased slightly over the past 45 years.
Several factors may contribute to the higher incidence in Finland and Japan, but the extent of their contributions remains speculative. In Japan and Finland, a higher risk of rupture of intracranial aneurysms is described.65
Genetic factors may also play an important role in both Japan and Finland.
The relatively older age in Japan may be another explanation. Global statistics report the Japanese as being the oldest population in the world, with a median age of 43 years in 2005.66
However, this older age cannot entirely explain the high incidence, because age specific incidences were also higher in Japan than in the reference population. Another explanation may be better case finding, but case finding in the studies from Japan was not more exhaustive than in other regions. Five of the seven studies did not describe regular contacts with general practitioners, and none mentioned contacting rehabilitation facilities or nursing homes as a case finding method. However, the majority of studies from Japan examined instances of sudden death more extensively than studies from other regions. Most studies from Japan used in addition to autopsy, neuroimaging of patients who had died suddenly or during transportation to the hospital. Probably more patients dying early after SAH were detected by scrutinising all of these events, which increased the incidence of SAH compared with studies in which such instances of sudden death were not examined in this way. However, sudden death accounts for only 12% of all SAH patients67
; more extensive examination of patients dying early may contribute to, but cannot entirely explain, the higher incidence in Japan. The proportion of patients in whom the diagnosis of SAH was confirmed by CT scanning was almost 100% in Japan. However, a large proportion of patients investigated by means of CT does not lead to a higher incidence. In our previous review, we found a higher
percentage of CT use to be associated with a lower
incidence of SAH and in recent studies we found no relation between the proportion of patients investigated by means of CT and the reported incidence. The greater use of neuroimaging in Japan is therefore unlikely to be an explanation for the high incidence rates reported in Japan.
Age adjusted incidences were also higher in Finland than in the reference region. In Finnish studies, the proportions of patients in whom the diagnosis of SAH was confirmed by CT were low (varying between 0% and 60%). If we apply the rate ratio for proportion investigated by CT on incidence found in the previous version of the review, and if we assume a hypothetical 100% proportion of patients investigated by CT, the incidence of SAH would be 10.6 (95% CI 8.9 to 12.5) in Finland, which is still higher than the incidence in the reference region. Thus the low proportions of CT in Finnish studies do not entirely explain the higher incidences found. Case finding methods in Finnish studies were not more exhaustive compared with other studies, thereby not increasing the incidence found. Other explanations for the high incidence in Finland include high prevalence of smoking and hypertension,68
and heavy episodic alcohol abuse.69
The low incidences in South and Central America can perhaps be explained in part by the relatively young mean age of people in these regions. Reported mean ages in the study populations varied between 25 and 35 years, whereas for the reference population this mean age was 37 years. However, the age adjusted incidence given in one study was also lower than in the reference region.9
Thus other factors are likely to be involved in the lower incidence in this region. No differences in case finding methods were noted, but access to hospitals in these regions may be less than in other regions. Another explanation might be racial differences, although in some studies the incidence of SAH in black populations was higher in comparison with white populations.70
In summary, none of these explanations can completely explain the regional differences, and other factors are likely to be involved.
The higher incidence of SAH in women was found in the previous version of our review but the age dependent gender difference is a new finding. While previous literature describes a peak incidence in the sixth decade,71
some recent studies found a continuous increase with age, or an age dependent gender difference.6
The current review confirms these observations from some individual studies. The reasons for the overall higher incidence in women are not clear, but hormonal factors (including use of hormone replacement therapy) are a possible explanation.72,73
Our finding that the preponderance of women starts only after the sixth decade further supports this suggestion.
Although several studies have reported a statistically significant decline in stroke of approximately 2% per year over the past two decades,12,74,75
it is still uncertain if the reduction in cardiovascular risk factors has also translated into a reduction in the incidence of SAH. Our study found a decrease in incidence of 0.6% per year, which is modest compared with the decline in stroke in general. In our analysis, the influences of region, age, gender and improved diagnostic criteria by CT were taken into account. In our previous review, we found that the apparent decline in the incidence of SAH until 1990 was entirely explained by the increasing proportion of patients investigated by CT.4
In this update, we found that in studies performed after 1990, the proportion of patients investigated by means of CT was no longer significantly related to incidence in any region. The most likely explanation is that after 1990, almost all hospitalised patients were investigated using CT. Thus the contrast between studies with small proportions investigated by CT (with over reporting of SAH)76
and studies with large proportions investigated by CT has disappeared. The time trend found in our study is therefore not explained by percentages of CT use for confirmation of diagnosis of SAH. The small magnitude of the decline in incidence of SAH may in part be explained by the stronger influence of genetic factors in SAH than in stroke in general.77
However, genetic factors explain only 10% of SAH, and most cases are attributed to smoking, hypertension and excessive use of alcohol.77
Perhaps the reduction in risk factors is more effective in older people (where most stokes in general occur) than in younger people (who are most at risk of SAH), but we have no data to support this hypothesis.
It seems contradictory that the incidence of SAH decreased over time, although the overall incidence in our update was higher than the incidence found in our previous review. However, by updating the review, we included five new studies in the reference region published after
1993 presenting data from before
1990. These five studies had a combined incidence of 10.4 per 100
000 person‐years, which is higher than the overall incidence from the studies that had been included in the previous version of the review. The net result is that the incidence of all studies (including the newly found ones) for the observation period from the previous review (1972–1990) has increased compared with that in the previous review. This effect in part explains the paradox of higher incidence in the current review despite declining incidence over time. Furthermore, we found the decrease in incidence only after adjustment for gender and age. Thus the increased incidence in the updated version of the review may be explained in part by inclusion of study populations with higher ages in the more recent years.
The number of population based studies (51) and number of person‐years (45
896) included in this review was large and therefore overall estimates are precise. Subgroup analyses according to region, age, gender and time trend were based on smaller numbers of studies and person‐years. Nevertheless, even for these analyses, CI values were narrow. This current review also included data from additional parts of the world compared with the previous version; only African, South Asian and Chinese populations were not represented.
Our study shows that the incidence of SAH has declined over the past decades, although to a lesser extent than that of stroke in general. Moreover, incidence continues to increase until older age, is higher in women than in men only after the fifth decade and varies considerably per region. Further studies should address the reasons for the relative moderate decline in incidence of SAH, the higher incidence in women only after the fifth decade and the regional differences in SAH incidence. The answers to these questions will probably provide further clues to the aetiology of SAH.