1. From the University of Toronto Brain Vascular Malformation Study Group (P.J.P., K.G.tB., W.M. R.A.W., M.C.W.), Toronto, Ontario, and the Departments of Surgery (Neurosurgery) (M.A.S., P.J.P., M.C.W) and Radiology (Neuroradiology) (P.J.P., K.G.tB., W.M., R.A.W.), University of Toronto, Toronto, Ontario, Canada; and Faculty of Medicine (M.A.S.), Federal University of Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil.