A vascular malformation, or arteriovenous malformation (AVM), extending from the meninges near the falx into the cerebral cortex is seen to consist of tortuous channels in the angiogram (uppermost image). Below this the next image shows dilated vascular channels that are seen to form a mass in the T2 weighted MRI scan. After surgical removal, the histologic image at the bottom shows the thin-walled venous channels separated by gliotic brain tissue. The incidence of AVM is about 1 per 1000, with a mean age at diagnosis of 31 years. They are typically located in the cerebral hemispheres. About half of AVM's are complicated by hemorrhage. The principle clinical finding is seizures in about 1/3 of cases, though some patients have chronic headache or neurologic deficits.