Giant fusiform aneurysm of the basilar artery: consideration of its pathogenesis

We tried to determine the pathogenesis of a fusiform aneurysm of the basilar artery based on the findings of two patients who had pontine infarctions due to thrombosis within the aneurysm.

The patients were female, aged 75 and 62 years. At autopsy of the first case, the dilated basilar artery was filled with fresh and old thrombus. The wall was extremely thin on the left side, where a fresh red thrombus was evident, and thick on the right side, where an old white thrombus appeared. The thick wall had a thickened and hyalinized intima, and a deposition of atheromatous plaque disrupted both the internal elastic lamina and muscle layer. The left vertebral artery was atherosclerotic and its lumen was severely compromised, but the right vertebral artery was hypoplastic. On angiogram of the second case, the dilated basilar artery presumably was filled with thrombus on the left side, contralateral to the dilated and tortuous vertebral artery. The left vertebral artery was hypoplastic.

Atherosclerosis may be the essential factor in the pathogenesis of a fusiform aneurysm of the basilar artery in elderly patients. The disrupted internal elastic lamina and muscle layer may be susceptible to mechanical injury by hemodynamic strain, causing progressive attenuation of the arterial wall. Stenosis of the vertebral artery on the dominant side probably produces a jet stream within the basilar artery on the stenotic side and a stagnant zone on the opposite side, promoting the initial thrombus formation.