Factors influencing the development of Moyamoya phenomenon.

Both the Moyamoya Phenomenon and occlusion of the internal carotid fork are essential radiological findings in true Moyamoya Disease of unknown aetiology. However, the Moyamoya Phenomenon is often observed in occlusive diseases of the internal carotid bifurcation of known aetiology. The authors recently observed acute development of the unilateral Moyamoya Phenomenon following severe vasospasm of the anterior and middle cerebral arteries due to rupture of an anterior communicating aneurysm. The following four factors have been suspected of contributing to development of the Moyamoya Phenomenon: 1. The chronology of arterial occlusion. 2. Extent and location of occlusion. 3. The cause of occlusion. 4. Anatomical and functional disposition of the basal circulation. As regards the chronology, chronic or slowly progressive arterial stenosis has been thought to be a mandatory factor in development of a Moyamoya network, which plays an important role in the form of collateral channels. However, based on the findings outlined in this paper, the congenital factor may be the most important of the four factors.