The detection of unruptured intracranial aneurysms (UIAs) has increased as the use of noninvasive cerebral angiography has becomes more widespread. It has been posited that UIAs in different locations may have different pathophysiological mechanisms and consequently, different risk factor profiles. Kang and Kim et al. conducted a cross-sectional study of 18,954 consecutively enrolled subjects who had magnetic resonance angiography (MRA) of the cerebral vessels as part of a routine health examination over an approximately ten year period.

MRA was performed with 1.5-T and 3-T scanners. Subjects with aneurysms less than 3 mm or having fusiform, mycotic, traumatic, or treated aneurysms were excluded. The location of aneurysms was classified as distal internal carotid artery (dICA), middle cerebral artery (MCA), MCA bifurcation, anterior cerebral artery (ACA), anterior communicating artery (Acom), posterior communicating artery (Pcom), or posterior circulation artery (including vertebral, basilar, posterior cerebral, and anterior and posterior inferior cerebellar arteries). Overall age, sex, and vascular risk factors were compared for those with and without an UIA and separately for each aneurysm location subgroup. UIA distribution in the study population was also compared with a group of 12,702 subjects who had MRA as part of an outpatient evaluation for headache at the same institution.

Compared to patients having an MRA for headache, those in the health screening group tended to be younger (57.2 ± 8.5 vs 63.1 ± 12.5) and less frequently women (39% vs 64.4%). The prevalence of UIA was higher in the headache evaluation group as compared to the health screening group (2.38% vs 1.94% p=0.007). The prevalence of UIA was similar in men and women in the two groups and similarly increased in both groups in women over age 50 years.

Patients in the health screening group with an UIA tended to be older (57.2 ± 8.5 vs 55.8 ±9.0 p=0.005) and more frequently women (50.7% vs 38.8% p<0.001) than those without aneurysms. Interestingly, coronary artery disease was less prevalent in the aneurysm group (10.1% vs 24.2% p<0.001). In multivariable analysis, advanced age, female sex, hypertension, smoking and CAD each had independent associations with UIAs.

In order of descending frequency, UIAs were most commonly found in the distal ICA (45.2%), followed by the MCA bifurcation (13.4%) and the Acom (13.4%). Pcom aneurysms were observed in 10.4% of patients, whereas MCA and ACA aneurysms were found in 6.5% and 5.7% of subjects, respectively. Female sex, hypertension, and CAD were independently associated (CAD was negatively associated) with dICA, MCA bifurcation and Pcom aneurysms, whereas MCA aneurysms were associated with advanced age, smoking and negatively associated with CAD. Posterior circulation aneurysms were only associated with hypertension.

The findings of this study are consistent with previous work finding associations between vascular risk factors and the presence of UIAs, particularly with respect to age, female sex and hypertension. The analysis of the relationship between risk factors and UIAs by location supports the possibility of varying pathophysiologic mechanisms for aneurysm formation depending on site. The negative association with CAD is particularly intriguing; it is somewhat counter intuitive to assume that one form of vascular degeneration could be protective against another. Additional research aimed at understanding the mechanical and molecular basis of IA formation would be helpful to further elucidate the issue.

The study does have limitations. The possibility for selection bias should be acknowledged. It is also possible that the presumed race-ethnic homogeneity of the subjects (recruited from sites in Korea) limits the generalizability. Additional studies in other sites using a similar protocol will be needed to validate these findings.