Migraine and circle of Willis anomalies.

Several mechanisms are currently thought to contribute to migraine pathogenesis, including interictal neuronal hyperexcitability, cortical spreading depression underlying the symptom of aura, and trigeminal nerve activation at a peripheral and central level. However, these mechanistic concepts incompletely explain migraine susceptibility in individual patients and do not fully account for the well documented association between migraine and ischemic cerebrovascular disease, including increased risk of both clinical stroke and subclinical brain lesions in migraine patients. The circle of Willis is a major source of collateral blood flow supply in the human brain, and developmental morphologic variants of the circle of Willis are extremely frequent. Altered cerebral blood flow (CBF) has been demonstrated in regions supplied by variant circle of Willis vessels. Our central hypothesis is that circle of Willis anomalies correlate with alterations in cerebral hemodynamics and contribute to migraine susceptibility and ischemic complications of migraine. Dysregulation of CBF may allow relative ischemia to develop in the setting of increased metabolic demand related to neuronal hyperexcitability, may trigger cortical spreading depression, and may predispose individuals with migraine to ischemic lesions and stroke. Identification of structural alterations in the cerebral vasculature in migraine patients would have several important pathophysiological and clinical implications. First, it would provide a developmental mechanism for migraine susceptibility that may lead to further insights into genetic predisposition to migraine. Second, it would expand understanding of potential mechanisms underlying migraine aura and linking migraine with both clinical and subclinical cerebral infarction. Third, it could help to identify the subpopulation of patients at risk of progressive cerebral ischemia so as to target preventative therapies appropriately. Fourth, it would suggest a role for further diagnostic evaluation to determine migraine mechanism in individual patients, analogous to the current paradigm in ischemic stroke in which determination of stroke mechanism is critical to therapeutic decision-making.