Using multiple modalities for BAVM ascertainment, we report an overall BAVM detection rate of 1.42 (95% CI=1.29-1.57) per 100,000 person-years in this large multiethnic population during 1995 through 2004. Despite an increasing trend for neuroimaging utilization during this 10-year period, the overall detection rates did not uniformly increase for all brain vascular malformations. In particular, this is the first report noting a recent declining trend for detection of both unruptured and ruptured BAVMs.
Our BAVM detection rate was similar to previously published prospective,13-15
as well as retrospective population-based studies.9, 16
Al-Shahi et al reported a detection rate for first-in-a-lifetime diagnosis of BAVM of 1.12 (95% CI=0.90-1.37) per 100,000 person-years in their prospective study following the Scotland population for two years between 1999 and 2000.15
Hillman et al report a detection rate for de novo diagnosed BAVMs of 1.24 per 100,000 person years in Linköping, Sweden, between 1989 and 1999.14
Stapf et al reported a detection rate of 1.34 (95% CI=1.18-1.49) per 100,000 person-years in their prospective study of a defined population in the New York islands during 27 months from 2000 to 2002.13
Furthermore, detection rates were also similar to older retrospective studies based in Olmsted County, Minnesota from 1965 to 1992,9
and the Netherlands Antilles between 1980 and 1990.16
Despite different study populations and methods used, there was no significant heterogeneity between studies allowing us to combine estimates together with our study for a pooled BAVM detection rate of 1.31 (95% CI=1.21-1.41) per 100,000.
Detection rates for unruptured (0.70 per 100,000) and ruptured BAVMs (0.72 per 100,000) in our study were also similar but slightly higher than those reported in the New York Islands AVM Study (0.51; 95% CI=0.41-0.61),13
the Northern Manhattan Stroke Study (0.55; 95% CI=0.11-1.61),22
and the Scottish Intracranial Vascular Malformation Study (0.51; 95% CI=0.37-0.69). However, the 95% CI intervals overlap with estimates from our study.
In the KPMCP study population, we observed an overall decreasing trend for detection of BAVMs, an increasing trend for IAs, and no change in OVMs over the 10 year period. Interestingly, the rate of unruptured BAVM detection was decreasing at the same time the rate of unruptured aneurysms was increasing. This finding was surprising to us as we had expected the rate of all unruptured vascular malformations to increase given the improved neuroimaging resolution to detect smaller lesions (especially for aneurysms) and increasing rate of utilization for CT, MRI, and angiography in the KPMCP population over this time period. Our detection rate and trends for total aneurysms (10.3, 95% CI=9.9-10.7) and SAH (7.4 per 100,000) were consistent with other published studies. In a population-based study conducted in Olmsted County, Minnesota between 1965 to 1995, for example, the detection rate for total IA and SAH were 9.0 (95% CI=7.8-10.2) and 6.9 (95% CI=5.9-8.0), respectively.23
The declining detection rate of BAVM observed in our study population, despite increasing neuroimaging volume or age of cases, is more difficult to explain. The results may be due to a general decrease in the true incidence of BAVM, or some fundamental change in how newer lesions become symptomatic or are diagnosed, e.g., greater specificity with newer imaging technology. Perhaps previous BAVM cases diagnosed in the pre-MRI era would now be ruled out as a true case, resulting in fewer diagnosed BAVM cases. Another interpretation would be related to biases in case ascertainment and referral algorithms, although less likely because BAVM cases would be captured in the KPMCP databases even if diagnosed outside the Kaiser system, as long as the patient remained an active Kaiser member.
Our results should be interpreted in light of several study limitations. Even though KPMCP is a community-based system and we evaluated both outpatient and in-hospital medical records, true detection may be underestimated because of sudden death before medical evaluation was possible (e.g., survival bias). Results may not be generalizable to enrollees that permanently drop out of the KPMCP system. However, 8-year retention rates in KPMCP for members between 35-84 years are high (>70%), and morbidity and mortality from BAVM hemorrhage is significantly less than other forms of brain hemorrhage.24
By definition, all BAVM cases receive some form of diagnostic imaging; however, not all cases would necessarily receive confirmatory angiography in addition to MRI or CT. Thus, some of these cases classified as BAVM may have been misdiagnosed; detailed neuroimaging data on cases (e.g., type) was not available for analysis. Our estimates for IA and OVM cases may also be subject to misclassification because these cases were identified using ICD-9 codes only and not verified by chart review as with BAVMs. However, relative to one another, we nevertheless were able to detect increased detection of unruptured IA vs. decreased detection of unruptured BAVM.