Cerebral arteriovenous malformations (AVM) are congenital lesions consisting of a nidus of tortuous vascular channels that are supplied by hypertrophied arterial branches and drained by enlarged cortical or deep veins. The nidus typically contains no normal brain. Gliosis and evidence of hemorrhage are often present at gross examination. High flow through these lesions may produce a variety of vasculopathic change. Aneurysms often develop within the nidus or in the feeding circulation. Stenoses of the feeding arteries and draining veins are also frequent findings. Ninety eight percent of cerebral AVMs are solitary, although multiple lesions may occur in hereditary hemorrhagic telangectasia (Osler-Weber-Rendu syndrome) or in the Wyburn-Mason syndrome.
   Most lesions become symptomatic, usually during the third and fourth decades. Approximately 50% will present with hemorrhage, 25% with seizures and 25% with focal neurologic deficits. Spontaneous occlusion is rare.
   At CT, cerebral AVMs appear as iso- or hyperdense serpentine structures with vigorous enhancement after contrast administration. Calcification or the sequella of hemorrhage may be present.
   MR likewise demonstrates a tangle of flow voids at the AVM nidus that enhance. Blood product of varying age and gliosis may be identified. MRA delineates flow to the lesion but provides inadequate detail.
   Digital subtraction angiography provides excellent detail of feeding, nidal and draining vessels and demonstrates arteriovenous shunting within the lesion with early filling of the draining veins.
   The Spetzler-Martin grading system for AVMs is based on size, location (eloquent or ineloquent) and the presence of deep venous drainage. Higher scores correlate with a higher surgical risk. Therapy may consist of surgical resection (often with pre-operative embolization), endovascular occlusion alone, or stereotactic radiation.

REFERENCES:
1.   Osborn, Diagnostic Neuroradiology, pp 284-308.
2.   Osborn, Cerebral Vascular Malformations. In Radiologic Pathology, American Registry of Pathology, Armed Forces Institutes of Pathology, pp 864-868.