1 Division of Neurosurgery, Department of Clinical Neurosciences, Addenbrooke's Hospital and University of Cambridge, Cambridge CB2 0QQ, UK. Electronic address: ak721@cam.ac.uk.
2 Department of Neurosurgery, National Hospital for Neurology and Neurosurgery, London, UK.
3 Department of Neurosurgery, Leiden University Medical Center, Leiden and Haaglanden Medical Center, The Hague, Netherlands.
4 Division of Neurosurgery, Department of Clinical Neurosciences, Addenbrooke's Hospital and University of Cambridge, Cambridge CB2 0QQ, UK.
5 Nuffield Department of Surgical Sciences, University of Oxford, Oxford, UK.