Vertebral Basilar Insufficiency
Vertebrobasilar insufficiency (VBI) refers to a condition in which blood flow to the vertebral and basilar arteries is restricted, thereby providing transient insufficient blood flow to the posterior portions of the brain. The National Institute of Neurologic and Communicative Diseases defined the symptoms of VBI as: vertigo, ataxia, dizziness, syncope, drop attacks, visual disturbances and motor and sensory changes (sometimes bilateral). While VBI may be the result of a congenital abnormality, it is more commonly the result of atherosclerosis and resultant narrowing of the blood vessels. The characteristics of atherosclerotic plaque at the vertebral artery origin are unique from the morphological and pathological nature of plaque associated with the carotid. Vertebral artery plaques are often annular and concentric, hard and smooth, with minimal incidence of intramural hemorrhage or ulceration.
VBI may also cause a phenomenon known as subclavian steal, whereby blood is diverted from the vertebral-basilar system due to a subclavian stenosis. Subclavian steal takes place when a subclavian stenosis proximal to the origin of the vertebral artery leads to retrograde flow down the ipsilateral vertebral artery, thereby "stealing" blood from the circle of Willis, with the subsequent distal subclavian artery filling from the retrograding vertebral artery. When compensatory flow to the subclavian artery from the vertebral artery diverts too much flow toward the arm and away from intracranial structures, neurological deficits take place.
Doppler ultrasonography is insensitive to vertebral basilar insufficiency due to the presence of surrounding bone and because the most frequent site for occlusive disease is at the vessel origin, a nonsuperficial location. The proximal vertebral artery is often tortuous, making accurate diagnosis of lesions difficult using MRA because of spin dephasing caused by turbulent flow.
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