Arteriovenous Malformations of the Central Nervous System

Excerpt

An arteriovenous malformation (AVM) is an abnormal connection between arterial and venous vessels without intervening capillary structures. The lack of capillary components leads to arterialized veins in a high-flow, low-resistance shunt system. Cerebral AVMs convey a 1% annual risk of epilepsy and a 3% annual risk of hemorrhage. The risk of hemorrhage comprises a 2% annual risk for unruptured AVMs and a 4.5% annual risk for previously ruptured AVMs.

Intracranial AVMs vary significantly in size, location, and vascular flow dynamics. AVMs are most commonly identified within the cerebral cortex, the brainstem (pons and midbrain), and the cerebellum. AVMs are also seen in the spinal cord. The Spetzler-Martin grading scale is a commonly utilized classification system for intracranial AVMs. Refer to "Spetzler-Martin Arteriovenous Malformation Grading System" in the Evaluation section for more information on this classification system. As the Spetzler-Martin grade increases, so does the surgical morbidity and mortality risk.

Spinal AVMs are subclassified as intramedullary, perimedullary, or both. Spinal AVMs may be direct or true nidal fistulas and high- or low-flow. A direct fistula is a direct connection between an artery or vein, most commonly at the dural sinuses. In contrast, a nidal fistula connects the artery and vein through a network of small noncapillary vessels. The Kim-Spetzler classification system describes 4 types of spinal AVMs. Type I is an intradural dorsal arteriovenous fistula, type II is an intramedullary AVM, type III is an extradural-intradural AVM, and type IV is an intradural ventral arteriovenous fistula. More recent classification systems describe a fifth type of lesion within the conus medullaris.

Direct fistulas comprise 20% of spinal AVMs and are commonly located on the surface of the spinal cord. True nidal spinal AVMs are more common and constitute 80% of cases; the nidus is typically intramedullary. Pediatric patients more commonly present with arteriovenous fistula pathophysiology, and adults more commonly present with true nidal spinal AVMs. Spinal AVMs often present with chronic progressive neurologic deficits or hemorrhage leading to acute back pain with lower extremity pain, paresthesias, motor weakness, or bowel or bladder dysfunction. Approximately 50% of spinal AVMs are located within the thoracic spine; another 30% are within the cervical spine.