NEUROSURGERY ARTICLES

MICROSURGERY FOR ARTERIO-VENOUS MALFORMATIONS

by Chiedozie Nwagwu, M.D.

Arteriovenous malformations (AVMs) of the brain consist of abnormal connections between arteries and veins. Normally, the connections between arteries and veins are through a network of minute blood vessels called capillaries, which enable the delivery of oxygen and nutrients into the brain tissue. In AVMs, the arteries and veins have a direct connection, bypassing the capillary network formed during embryonic development. An AVM is structurally a tangle of dilated blood vessels that disrupts normal blood flow in the brain by directly shunting blood within its dense center or nidus into veins. This effect deprives neighboring capillaries of vital oxygen and nutrients and may cause neurologic symptoms. These abnormal vessels may enlarge over time. The arteries and veins are tortuous, dilated, and are at increased risk of rupturing due to increased pressure within their channels. AVMs derive blood supply from one or a combination of vessels that normally supply the brain.

In the United States, about one in ten thousand people harbor an AVM in the brain. AVMs are the leading cause of stroke in young people. Often "silent" for many years, AVMs cause problems for patients between 10 and 30 years of age. They may experience severe headaches, seizures, or suffer a stroke from bleeding in the brain. Roughly 4% of AVMs bleed every year. Common symptoms of an AVM hemorrhage include loss of consciousness, sudden and severe headache, nausea, vomiting, urinary incontinence, and blurred vision. A stiff neck can occur as the result of increased pressure within the skull and because of irritation of the linings of the brain. These strokes can cause specific neurological problems such as paralysis, decreased sensation to touch, speech or vision problems, or they can cause a generalized decrease in consciousness and coma. Often symptoms can be due to problems related to the pressure exerted by the AVM on the normal surrounding brain tissue as well as decreased blood flow due to the shunting of blood away from its natural target tissue. AVMs in certain locations may impair the circulation of spinal fluid within the brain, causing high pressure in the brain, a condition known as hydrocephalus.

AVMs can be diagnosed using several imaging techniques. Angiography provides the most accurate pictures of blood vessel structure in AVMs. This invasive technique requires injecting a special water-soluble liquid, called a contrast agent, into an artery. The agent highlights the structure of blood vessels so that it can be recorded on conventional x-rays. Two of the most frequently employed noninvasive imaging technologies used to detect AVMs are computed axial tomography (CT) and magnetic resonance imaging (MRI) scans. A recently developed application of MRI technology-magnetic resonance angiography (MRA)-can record the pattern and velocity of blood flow through vascular lesions as well as the flow of cerebrospinal fluid throughout the brain and spinal cord. CT, MRI, and MRA provide three-dimensional representations of AVMs by taking images from multiple angles.

The goals of treatment include reduction of the risk of fatal or debilitating hemorrhagic stroke, or improving blood flow to the brain restoring normal neurologic function. Twenty years ago, many patients were told their AVMs were inoperable because the risks of treatment were considered too high. Today, advanced techniques make the treatment of AVMs safer and highly effective. A multidisciplinary team of neurosurgeons, neuroradiologists, and neurologists work together to formulate a treatment plan. Current acceptable treatment usually involves a number of techniques. The first, embolization; involves the delivery of various substances through the arteries which block blood flow within the AVM. Traditional open skull surgery with microsurgical techniques may then be used to remove the AVM. Finally, radiosurgery, which destroys the AVM tissue by focused xray, is utilized in those AVMs that cannot be resected. Benefits from treating AVMs using radiosurgery are delayed, up to 18 months, and in the intervening period, the patient continues to be at risk of AVM rupture.

Microsurgical procedures are carried out under general anesthesia, using an operating microscope. Using precise microsurgical tools and meticulous dissection, the neurosurgeon removes the AVM. In some instances, the entire AVM cannot be removed, and in such cases, the residual AVM may be effectively treated with radiosurgery. The length of stay in the hospital usually varies between 5-7 days with some shortterm rehabilitation. The option of surgery depends on the general health of the patient and the nature of the AVM. The advantage of surgical treatment is that cure is immediate if all of the AVM tissue is removed. Risks include damage to nearby brain tissue and stroke to other areas of the brain.

Irrespective of the treatment methods, it is important for the treating team to have all of these options in a state-of-the-art facility housing dedicated angiography suites, microsurgery equipped operating rooms, and radiosurgery.

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