Although AVMs are often associated with the brain and spinal cord, they can develop in other parts of the body.Normally, the arteries in the vascular system carry oxygen-rich blood, except in the case of the pulmonary artery. Structurally, arteries divide and sub-divide repeatedly, eventually forming a sponge-like capillary bed. Blood moves through the capillaries, giving up oxygen and taking up waste products, including CO2, from the surrounding cells. Capillaries in turn successively join to form veins that carry blood away. The heart acts to pump blood through arteries and uptake the venous blood.As an AVM lacks the dampening effect of capillaries on the blood flow, the AVM can get progressively larger over time as the amount of blood flowing through it increases, forcing the heart to work harder to keep up with the extra blood flow. It also causes the surrounding area to be deprived of the functions of the capillaries—removal of CO2 and delivery of nutrients to the cells. The resulting tangle of blood vessels, often called a nidus (Latin for "nest"), has no capillaries. It can be extremely fragile and prone to bleeding because of the abnormally direct connections between high-pressure arteries and low-pressure veins. The resultant sign, audible via stethoscope, is a rhythmic, whooshing sound caused by excessively rapid blood flow through the arteries and veins. It has been given the term "bruit", French for noise.
On some occasions, a patient with a brain AVM may become aware of the noise, which can compromise hearing and interfere with sleep in addition to causing psychological distress. Diagnosis
AVMs are diagnosed primarily by the following imaging methods:
Computerized tomography (CT) scan is a noninvasive X-ray to view the anatomical structures within the brain to detect blood in or around the brain. A newer technology called CT angiography involves the injection of contrast into the blood stream to view the arteries of the brain. This type of test provides the best pictures of blood vessels through angiography and soft tissues through CT. Magnetic resonance imaging (MRI) scan is a noninvasive test, which uses a magnetic field and radio-frequency waves to give a detailed view of the soft tissues of the brain. Magnetic resonance angiography (MRA) – scans created using magnetic resonance imaging to specifically image the blood vessels and structures of the brain. A magnetic resonance angiogram can be an invasive procedure, involving the introduction of contrast dyes (e.g., gadolinium MR contrast agents) into the vasculature of a patient using a catheter inserted into an artery and passed through the blood vessels to the brain. Once the catheter is in place, the contrast dye is injected into the bloodstream and the MR images are taken.