Posterior circulation cerebral hyperperfusion syndrome after high flow external carotid artery to middle cerebral artery bypass.

We present the first report, to our knowledge, in which revascularization of the middle cerebral artery (MCA) with a high flow extracranial-intracranial procedure resulted in symptomatic hyperemia of the posterior circulation. Cerebral hyperperfusion syndrome (CHS) is a poorly understood phenomenon that is classically seen in the distribution of a revascularized artery. A 37-year-old woman presented with a 3 month history of cognitive and speech difficulties, persistent headaches, weakness, numbness, and paresthesia which was worse in the right extremities and face. She was found to have bilateral watershed infarcts worse in the left cerebral hemisphere, severe bilateral stenosis of the supraclinoid internal carotid artery, and a small left superior hypophyseal aneurysm. The patient underwent left cerebral hemisphere revascularization with a high flow external carotid artery to MCA bypass with aneurysm trapping. During skin closure, significant changes were seen in her bilateral upper extremity motor-evoked potentials. The patient's postoperative exam was noted for an intermittent inability to follow commands, bilateral upper extremity weakness, vertical nystagmus, and alogia that all dramatically improved with strict blood pressure control. Postoperative perfusion imaging revealed posterior circulation hyperemia. This patient highlights the potential for hyperemic complications outside the revascularized arterial territory. Strict blood pressure control is recommended in order to prevent and manage hyperemia-associated symptoms. Improving our understanding of CHS may assist in identifying at risk patients and at risk arterial territories in order to optimize CHS prevention and management strategies.