1. Department of Neurosurgery, University Medical Centre Groningen, University of Groningen, 9713 GZ Groningen, The Netherlands
2. Department of Anaesthesiology, University Medical Centre Groningen, University of Groningen, 9713 GZ Groningen, The Netherlands
3. Department of Epidemiology, University Medical Centre Groningen, University of Groningen, 9713 GZ Groningen, The Netherlands
4. Department of Neurology, University Medical Centre Groningen, University of Groningen, 9713 GZ Groningen, The Netherlands
5. Department of Orthopaedics, University Medical Centre Groningen, University of Groningen, 9713 GZ Groningen, The Netherlands
6. Department of Intraoperative Neurophysiology Mount Sinai West Hospital, New York, NY 10029, USA