AVMs are congenital malformations that create a direct shunt between arterial and venous systems. AVMs were first recognized in the middle 19th century, and first surgically exposed by D. Giorano in 1890. From the surface, AVMs appear to be tortuous masses of arteries and veins with overlying meninges that are thickened and discolored from frequent minor hemorrhages. The tissue around the AVM is often underdeveloped and pale or discolored. AVMs are often cone-shaped, pointing inward with one or more large draining veins following the direction of the cone, and while half of AVMs are mostly superficial, they may occur anywhere in the brain or spinal cord.
An AVM on the surface of the left parietal lobe.
Web Path by Edward C. Klatt MD, Department of Pathology, University of Utah,
Salt Lake City, Utah, USA
Most AVMs occur in the cerebrum (85-90%), with a relatively rare occurrence (6-7%) of AVMs in the brain stem2. About 4% of AVMs are multiple and associated with other vascular abnormalities3. Because of the huge flow of blood in the AVM, the feeding and draining vessels often lose their elasticity and develop into aneurysms. Fifteen percent of AVMs occur in conjunction with an aneurysm in one of its feeder arteries1.
Developmental stages of AVMs
Malformations result from the enlargement and preservation primitive vascular channels in the embryo that shunt blood directly from the arterial system to the venous system. The lack of capillary beds between the two systems leads to decreased resistance in the arteriovenous shunt, which in turn leads to an increase in blood flow. The increase in blood flow causes enlargement in the feeder arteries and the AVM itself, which causes other vessels to connect to the AVM. For centimeters around the AVM, diverted arterial branches fail to properly nourish the surrounding neural tissue; instead they enlarge and feed the AVM. Many investigators believe that AVMs continue to enlarge throughout life.
As the arteries feeding the AVM increase in diameter to meet with the increased flow of blood, the stress on their walls causes them to either enlarge or deteriorate. The walls of the feeder vessels become irregular, ranging from extremely thin to thick enough to block the lumen. In addition, the strain on the vessels combined with their deterioration causes frequent atherosclerosis and thrombosis in AVMs. Back to top.
When AVMs are large, a phenomenon known as “blood steal” occurs. Because the resistance on blood going to the AVM so much lower than the resistance in the surrounding normal vessels, the volume of blood directed to the AVM increases. Over time, the AVM begins to draw blood away from normal vessels, which are deprived of flow and cannot sufficiently perfuse the surrounding parenchyma. A large AVM can increase cranial blood flow by 50-100% but tissue perfusion is still significantly reduced3.
The results of a “steal” are often insidious. Chronic ischemia (lack of blood flow) of the regions around an AVM often leads to seizures or neurological deficits. AVMs that divert a large portion of cerebral circulation during development can lead to the underdevelopment of that side of the skull and opposite side of the body. Perhaps worst of all is an AVM subtype, the vein of Galen malformation, usually found in infants. This disorder is characterized by a large aneurysm in the vein of Galen that is fed from both sides of the brain by the anterior, posterior and middle cerebral arteries. The aneurysm can cause obstructive hydrocephalus, and the huge volume of blood flow through the AVM causes over-taxation of the infant’s heart. The eventual result of this over-taxation is heart failure. Back to top.
1) (1997). Arteriovenous Malformations. Introduction. (2000, April 8).
2) Dembo, M. (1982). Arteriovenous malformations of the brain: A Review of the literature since 1960. Archives of Physical Medicine and Rehabilitation, 63(11), 565-568.
3) Toole, J.F. (1990). Cerebrovascular Disorders (4th ed.). New York: Raven Press.