A congenital anomaly characterized by clusters of dilated hypertrophic veins surrounded by normal nervous system tissue. The blood vessels have thin walls composed of a single layer of fibromuscular tissue lined by epithelium. These lesions may occur in the brain, but are more frequent in the spinal cord and spinal meninges. The incidence of hemorrhage is relatively low, but when it occurs it may be associated with venous infarctions of the nervous system. (From Davis et al., Textbook of Neuropathology, 2nd ed, pp628-9).