Cavernomas are blood filled clusters of vessels that look like dilated veins (caverns). They characteristically leak blood around themselves. The rate of occurrence is approximately 0.5% of the population. Most cavernomas are solitary. A familial form exists. This is characterized by multiple lesions and is inherited in a dominant fashion. The size of the cavernomas can vary from a few millimeters to a few centimeters. They can be associated with other venous abnormalities. Cavernomas can be located in the brain, the brainstem, and the spinal cord.