Incompetent circle of Willis and vertebrobasilar insufficiency.

An analysis of the vascular basis of vertebrobasilar insufficiency was derived from the examination of 135 subjects by magnetic resonance imaging and angiography. The data from this cohort were compared to those from normal, ie, asymptomatic, individuals. We concluded that there is a significant difference in the potential for intracerebral collateralization and regional perfusion due to structural defects such as stenosis or developmental absence of the posterior communicating arteries in this symptomatic group. In addition, an occlusion of one or more of the source vessels, the internal carotid and vertebral arteries, in conjunction with an incomplete circle of Willis increased the potential for hindbrain perfusion deficits. Clinical cases illustrate the relationship between failure of internal shunting and the myriad symptoms of hindbrain dysfunction.