Arteriovenous malformations are serious disorders that can
have dire consequences if left untreated.
AVMs are a major cause of intracerebral and subarachnoid hemorrhage and
epilepsy. AVMs also divert large
amounts of blood away from healthy tissues leading to chronic ischemia of
surrounding regions. This “blood steal”
phenomenon can cause developmental abnormalities, neurological deficits
including dementia, severe headaches, and heart failure in newborns. AVMs are diagnosed with the aid of CT scans, MRI, and angiography. Several new imaging techniques also show
promising results for the future. Treatment for AVMs has traditionally been
accomplished through embolization combined with surgical resection. New radiosurgical techniques are being
introduced, but their efficacy and long-term success rate is still
debated.
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.
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An AVM on the surface of the left parietal lobe. Borrowed
from: 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.
Blood
Steal
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.
References
1) (1997). Arteriovenous Malformations. Introduction. http://mcns10.med.nyu.edu/vascular/avm.html (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.