Stenosis/Occlusion of the Middle Cerebral Artery and Moyamoya The middle cerebral artery (MCA) provides blood supply to the large part of the cerebral hemisphere. Stenosis and/or occlusion of the MCA occurs due to various causes. In the majority of such cases, atherosclerosis is the leading cause among them. I have noticed some of the patients with MCA occlusion/MCA stenosis have no predisposing risk factors, such as hypertension, diabetes mellitus, hyperlipidemia. They are relatively young between 30 and 40 years of age, and even in their third decade. Interestingly enough, they do not develop large cerebral infarction or have no ischemic symptoms at all. Some of them was diagnosed incidentally due to other medical concern. There are unilateral lesions, but bilateral symmetrical (mirror image) lesions are also found. Under these conditions, the small arteries (perforation arteries supplying blood to the deep brain) may be enlarged or not. Problems in diagnosis is that differentiation of these MCA lesions from moyamoya disease is occasionally difficult. In pediatric moyamoya disease, moyamoya vessels are usually well developed, but in adults moyamoya disease, moyamoya vessels are poorly developed in most cases. Clinically, clear-cut differentiation between moyamoya and MCA occlusion is not straightforward. This is partly because vague definition of the entity of "moyamoya disease". I believe there are common etiological factors among these two clinical entities. Moyamoya vessels are found not only in moyamoya disease, but in other diseases. Moyamoya vessels are the enlarged perforating arteries in the deep brain observed on angiography. In the cases of steno-occlusive states of cerebral arteries in non-moyamoya disease, such moyamoya vessels are frequently observed. In this situation, it is called "moyamoya phenomenon". 2004.2.8