Intracranial atherosclerotic disease associated with moyamoya collateral formation: histopathological findings.

Atherosclerotic disease has been suspected as a cause of moyamoya disease in some patients but has not, to the authors' knowledge, been confirmed by pathological studies. The authors present the histopathological findings in a patient with moyamoya collateral formation associated with atherosclerotic occlusive disease of the distal internal carotid artery (ICA). Typical atheromatous changes were evident in the distal ICA and proximal middle cerebral artery. In addition, intimal thickening, fibrosis, and abnormal internal elastic lamina were present in these vessels. These findings are common in moyamoya but not in atherosclerotic disease. Proliferation and enlargement of the lenticulostriate arteries in the basal ganglia was also identified. Moyamoya phenomenon secondary to atherosclerotic disease has similar histopathological features to idiopathic moyamoya phenomenon, both in the affected large basal arteries and lenticulostriate collaterals. These findings support the hypothesis advanced by Peerless that moyamoya is a 2-step process involving an obliterative vasculopathy of the terminal ICA and a secondary proliferative response