1. From the Department of Neurosurgery (D.M.H., K.B.M.), Carver College of Medicine, University of Iowa, Iowa City, IA; the Departments of Neurology (R.D.B., I.M.), Neurosurgery (D.G.P.), and Radiology (J.H.), Mayo Clinic, Rochester, MN; and the Department of Epidemiology (A.W.C., J.C.T.), College of Public Health, University of Iowa, Iowa City, IA.