**1. Introduction**

Moyamoya disease [MMD] is a form of chronic cerebrovascular occlusion characterized by occlusion of terminal internal carotid artery [ICA] along with a network of collateral vessels at the base of the brain. The disease was first brought to light by Takeuchi and Shimizu, where they described a young man with bilateral occlusion of ICA which was found to be due to congenital hypoplasia rather than atherosclerotic lesion [1]. Similar cases have been described in Japanese literature. After that, the condition came to be known by various names and the term 'spontaneous occlusion of the circle of Willis' by Kudo gained popularity [2]. The disease was finally coined '*moyamoya'* by Suzuki and Takaku based on the abnormal vascular network at the base of the brain that resembles 'vague or hazy puff of smoke' which is called *moyamoya* in Japanese [3].

This cerebral angiopathy is broadly termed 'moyamoya phenomenon' comprising of two nosological entities. The cerebrovascular syndrome is called 'Moyamoya syndrome' [MMS] when it is associated with neurological and extra neurological diseases like Neurofibromatosis 1 [NF1], Down syndrome, thyroid disease, cranial irradiation, sickle cell anemia, among other pathological conditions [4]. The Guidelines of the Research Committee on the Pathology and Treatment of Spontaneous Occlusion of Circle of Willis defined isolated moyamoya angiopathy as being idiopathic and called it 'Moyamoya disease' [5].

MMD is more common in Asian ethnicities as compared to the Western population [6]. The increased prevalence in Japan, Korea and other East Asian countries raised genetic predisposition to this condition. Subsequently, Kamada*et al.* identified a susceptibility gene, Ringin Protein 213 [RNF 213], and it was seen that this gene was positively associated with familial MMD [7]. Symptomatology of MMD varies according to the age group. Cerebral ischemia is more common in the pediatric than in the adult age group, whereas, the hemorrhagic type is more common in >40 years of age. Only one type predominates in a particular patient, but the symptoms are often recurrent. The disease diagnosis often rests upon angiography, and revascularization procedures are performed to avoid recurrence of symptoms. The lack of understanding of the exact pathophysiologic mechanism of MMD limits the development of treatments to prevent vascular damage.
