Robert F. Spetzler, MD, is Director, Barrow Neurological Institute, J. N. Harber Chairman and Professor of Neurological Surgery, Division of Neurological Surgery. Barrow Neurological Institute, Phoenix, Arizona, USA. M. Yashar S. Kalani, MD, PhD, is Assistant Professor, Division of Neurological Surgery, Barrow Neurological Institute, Phoenix, Arizona, USA. Peter Nakaji, MD, FACS, FAANS, is Director, Neurosurgery Residency Program, Director, Minimally Invasive Neurosurgery, Barrow Neurological Institute, Phoenix, Arizona, USA.