**3.2 Cerebral hyperfusion syndrome (CHS)**

CHS was first reported by Uno et al. in a patient with MMD after extracranialintracranial bypass in 1998 [62]. Since 1998, CHS has been reported in both pediatric and adult MMD patients after direct revascularization surgery.

The incidence of cerebral hyperperfusion in patients with occlusive cerebrovascular disease is reported at 17% after bypass surgeries, and 0.4% to 20% after carotid endarterectomy [63]. In another study, the incidence of CHS in MMD patients after direct surgical revascularization was reported to be 21.5% [64]. Adult-onset, hemorrhagic presentation, and increased preoperative cerebral blood volume were the risk factors for developing CHS [63]. The most common symptoms are transient neurological deficits, followed by hemorrhage and seizure [65]. CHS after surgical revascularization is caused by an increase in the cerebral blood flow, control of blood pressure postoperatively is the direct way to prevent or treat CHS. Active treatment of CHS should be considered in a patient with postoperative neurologic deficits such as headache, seizure, or transient focal neurological deficits. The strict control of blood pressure at targets less than 120/80 mm Hg, and routine postoperative monitoring of cerebral blood flow using transcranial Doppler and dynamic imaging techniques (computerized tomography or magnetic resonance perfusion) to be effective in preventing permanent neurological sequelae secondary to cerebral hyperperfusion syndrome [63, 66].
