[1] SGHMS, Dept Clin Neurosci, London, England
[2] SGHMS, Dept Clin Biochem, London, England
[3] St James Univ Hosp, Leeds LS9 7TF, W Yorkshire, England
[4] Stoke Mandeville Hosp, Aylesbury HP21 8AL, Bucks, England
[5] UCL Natl Hosp Neurol & Neurosurg, London WC1N 3BG, England
[6] Thames Valley Nuffield Hosp, Slough, Berks, England