Cerebral arteriovenous malformation

Overview
A cerebral arteriovenous malformation (AVM) is a congenital disorder of blood vessels within the brain, characterized by tangle(s) of veins and arteries. While an arteriovenous malformation can occur elsewhere in the body, this article discusses malformations found in the brain.

Pathophysiology
While the cause of AVMs remains unknown, the main risk is intracranial hemorrhage. This risk is difficult to quantify. Approximately 40% of cases with cerebral AVM are discovered through symptoms caused by sudden bleeding due to the fragility of abnormally-structured blood vessels in the brain. However, some patients may remain asymptomatic or have minor complaints due to the local effects of the tangle of vessels. If a rupture or bleeding incident occurs, the blood may penetrate either into the brain tissue (cerebral hemorrhage) or into the subarachnoid space. This space is located between the sheaths (meninges) surrounding the brain (subarachnoid hemorrhage).

Once an AVM bleeds, the probability of rebleeding may increase. However, as long as the AVM is unruptured, the risk of hemorrhage may be relatively low.

AVMs that do not bleed may cause symptoms such as epileptic seizures, headaches, or fluctuating neurological symptoms. Many of them may even remain asymptomatic.

Spetzler-Martin Grading System

 * Size: Measure the largest diameter of the nidus of the lesion on angiography.
 * Eloquence: Eloquent areas include sensorimotor, language, visual, thalamus, hypothalamus, internal capsule, brain stem, cerebellar peduncles, and deep cerebellar nuclei.
 * Venous drainage: The lesion is considered superficial only if all drainage is via the cortical drainage system.

Symptoms
The most frequently observed problems related to an AVM are headache and seizure. Moreover, AVMs in certain critical locations may stop the circulation of the cerebrospinal fluid, causing accumulation of the fluid within the skull and giving rise to a clinical condition called hydrocephalus.

Symptoms of bleeding within the brain (intracranial hemorrhage) include loss of consciousness, sudden and severe headache, nausea, vomiting, incontinence, and blurred vision. A stiff neck can occur as the result of increased pressure within the skull and irritation of the meninges. Impairments caused by local brain tissue damage on the bleed site are possible, including seizure, one-sided weakness hemiparesis, a loss of touch sensation on one side of the body, or deficits in language processing (aphasia). A variety of other symptoms can accompany this type of cerebrovascular accident.

Generally, intense headache, perhaps coincident with seizure or loss of bodily consciousness, is the first indication of a cerebral AVM. Estimates of the number of AVM-afflicted people in the United States range from 0.1% to 0.001%. of the population.

Diagnosis
An AVM diagnosis is established by neuroimaging studies. A computed tomography scan of the head (head CT) is usually performed; this can reveal the site of the bleed. More detailed pictures of the tangle of blood vessels that compose an AVM can be obtained by using radioactive reagents injected into the blood stream, then observed using a fluoroscope or Magnetic Resonance Imaging (MRI). A spinal tap (lumbar puncture) can be used to examine spinal fluid for red blood cells; this condition is indicative of leakage of blood from the bleeding vessels into the subarachnoid space. The best images of an AVM are obtained through cerebral angiography. This procedure involves using a catheter, threaded through an artery up to the head, to deliver a contrast agent into the AVM. As the contrast agent flows through the AVM structure, a sequence of X-ray images can be obtained to ascertain the size, shape and extent of that structure.

MRI
Images shown below are courtesy of RadsWiki

Treatment
The treatment in the case of sudden bleeding is focused on restoration of vital function. Anticonvulsant medications such as phenytoin are often used to control seizure; medications or procedures may be employed to relieve intracranial pressure. Eventually, curative treatment may be required to prevent recurrent hemorrhage. However, any type of intervention may also carry a risk of creating a neurological deficit.

In the U.S., surgical removal of the blood vessels involved (craniotomy) is the preferred curative treatment for most types of AVM. While this surgery results in an immediate, complete removal of the AVM, risks exist depending on the size and the location of the malformation.

Radiation treatment (radiosurgery) has been widely used on smaller AVMs with considerable success. The Gamma Knife, developed by Swedish physician Lars Leksell, is one apparatus used in radiosurgery to precisely apply a controlled radiation dosage to the volume of the brain occupied by the AVM. While this treatment is non-invasive, two to three years may pass before the complete effects are known. Complete occlusion of the AVM may or may not occur, and 8%-10% of patients develop long term neurological symptoms after radiation.

Embolization, that is, occlusion of blood vessels with coils or particles introduced by a radiographically guided catheter, is frequently used as an adjunct to either surgery or radiation treatment. However, embolization alone is rarely successful in completely blocking blood flow through the AVM.

The benefit of invasive treatment for unruptured AVMs has never been proven, as the risk of intervention may be as high as the spontaneous bleeding risk. An international study is currently under way to determine the best therapy for patients with unruptured AVMs (ARUBA - A Randomized trial of Unruptured Brain AVMs, www.arubastudy.org).

Histopathological Findings


