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Neurovascular surgeries are sophisticated procedures, and a thorough knowledge of the vessels before and during the operation is a necessity to prevent inadvertent damage and catastrophic results. Comprehensive monitoring especially real-time evaluation of cerebral blood flow is very helpful for surgery of cerebral aneurysms, cerebral arteriovenous malformations (AVMs), and extracranial-intracranial (EC-IC) bypass.

Arteriovenous malformations (AVM) are an abnormal connection between arteries and veins lacking intervening capillary network characterized by a complex, tangled web of abnormal vessels and believed to be congenital although not hereditary. This abnormal tangled web of vessels is called nidus (Latin word for nest), which has abnormally high blood flow resulting from the absence of dampening effect of capillary and has high tendency to rupture resulting in stroke. Lack of capillary network also induces enlargement of especially venous side with subsequent “growth” of AVM resulting in mass effect to surrounding brain parenchyma. AVM also disrupts normal metabolism of surrounding brain tissue by depriving them of adequate circulation (steal phenomena) causing neurological symptoms depending on its location whether superficial in cortex or deep within thalamus, basal ganglia, corpus callosum, insula/sylvian fissure, brainstem, or cerebellum. These mechanisms stated above underlie pathogenesis of symptoms and signs including hemorrhage as the most common presenting symptom in 50–52% of cases, followed by seizure in 27–30%, headache in 5–14%, and progressive neurological deficit in less than 5% [1, 2]. Hemorrhage is associated with high morbidity and mortality as much as 35% and 29%, respectively, that it becomes a primary concern in AVM management. Overall annual risk of hemorrhage on untreated AVM ranges between 2 and 4%, and although multiple studies implicated factors such as nidus size, complexity of AVM morphology, deep brain location or draining vein, associated aneurysm, and younger age to be related to hemorrhagic events, multivariate analysis indicated that only hemorrhage in initial presentation, deep AVM location or deep draining vein, and large nidus size is to be consistently predictive of higher risk of subsequent rupture up to 15% of annual rates [1, 2]. Figure 32.1 shows presentation of AVM hemorrhage on brain computed tomography (CT) scan and angiogram.