Cranial Anatomy - Arteriovenous Malformation

What is Arteriovenous Malformation?

Arteriovenous malformations (AVMs) are defects of the circulatory system that are generally believed to arise during embryonic or fetal development or soon after birth. Although AVMs can develop in many different sites, those located in the brain or spinal cord can have especially widespread effects on the body. Most people with neurological AVMs experience few, if any, significant symptoms. The malformations tend to be discovered only incidentally, usually either at autopsy or during treatment for an unrelated disorder. But for about 12 percent of the affected population these abnormalities cause symptoms that vary greatly in severity.

Normally, arteries carry blood containing oxygen from the heart to the brain, and veins carry blood with less oxygen away from the brain and back to the heart. When an arteriovenous malformation (AVM) occurs, a tangle of blood vessels in the brain or on its surface bypasses normal brain tissue and directly diverts blood from the arteries to the veins.

Normal Blood Vessels	Abnormal Connection of Blood Vessels

What symptoms can it cause?

Seizures and headaches are the most generalized symptoms. AVMs also can cause a wide range of more specific neurological symptoms that vary from person to person, depending primarily upon the location of the AVM. Such symptoms may include muscle weakness or paralysis, loss of coordination, difficulties carrying out tasks that require planning, dizziness, visual disturbances, problems using or understanding language, abnormal sensations (such as numbness, tingling, or spontaneous pain), memory deficits, mental confusion, hallucinations, or dementia.

Is there any treatment?

Medication can often alleviate general symptoms such as headache, back pain, and seizures caused by AVMs and other vascular lesions. However, the definitive treatment for AVMs is either surgery or focused irradiation therapy. The decision to perform surgery on any individual with an AVM requires a careful consideration of possible benefits versus risks. Because so many variables are involved in treating AVMs, doctors must assess the danger posed to individual patients largely on a case-by-case basis.

How is an AVM evaluated?

Physicians now use an array of traditional and new imaging technologies to uncover the presence of AVMs. Angiography provides the most accurate pictures of blood vessel structure in AVMs. The technique requires injecting a special water-soluble dye, called a contrast agent, into an artery. The dye highlights the structure of blood vessels so that it can be recorded on conventional X-rays. Although angiography can record fine details of vascular lesions, the procedure is somewhat invasive and carries a slight risk of causing a stroke. Its safety, however, has recently been improved through the development of more precise techniques for delivering dye to the site of an AVM. Superselective angiography involves inserting a thin, flexible tube called a catheter into an artery; a physician guides the tip of the catheter to the site of the lesion and then releases a small amount of contrast agent directly into the lesion.

Two of the most frequently employed noninvasive imaging technologies used to detect AVMs are computed axial tomography (CT) and magnetic resonance imaging (MRI) scans. CT scans use X-rays to create a series of cross-sectional images of the head, brain, or spinal cord and are especially useful in revealing the presence of hemorrhage. MRI imaging, however, offers superior diagnostic information by using magnetic fields to detect subtle changes in neurological tissues. 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 can provide three-dimensional representations of AVMs by taking images from multiple angles.

A detailed history-taking of the patient's symptoms and a physical examination are done first, followed by any of several tests, such as x-ray studies, Computerised Tomography (CT) scans, Magnetic Resonance Imaging (MRI), and angiograms. All findings are used to evaluate the patient's symptoms, determine the tumour's exact location, and provide the physician with a tentative diagnosis of the tumour type. During surgery, ultrasound imaging may be used to pinpoint the tumour's precise location and help the surgeon plan his approach for its removal. If an emergency craniotomy is required, an extensive workup may not be possible.