Contribution of the circle of Willis to the subclavian steal syndrome.

The arteriograms and other findings in 42 patients with the subclavian steal syndrome were statistically evaluated to determine which factors predisposed to vertebrobasilar ischemia. Evidence of disconnection of the circle of Willis between the territories of the carotid and vertebral arteries was found in a significantly higher proportion of the patients with vertebrobasilar insufficiency. This finding was interpreted to mean that in these patients one of the major routes normally available for compensating for the basilar ischemia produced by the steal was not present. It is suggested that this or similar mechanisms may be necessary before the subclavian steal produces symptoms in individual patients.