[1] Univ Melbourne, Royal Melbourne Hosp, Dept Surg, Melbourne, Vic 3010, Australia
[2] Royal Melbourne Hosp, Dept Neurosurg, Parkville, Vic 3050, Australia
[3] Univ Melbourne, Royal Melbourne Hosp, Dept Med, Melbourne, Vic 3010, Australia
[4] Univ Aberdeen, Inst Med Sci, Aberdeen AB25 2ZD, Scotland