[1] UCL Inst Neurol, Sobell Dept Motor Neurosci, London WC1N 3BG, England
[2] Natl Hosp Neurol & Neurosurg, London WC1N 3BG, England
[3] Univ Coll Hosp NHS Trust, Dept Gastroenterol, London NW1 2BU, England
[4] Reta Lila Weston Inst, London WC1N 1PJ, England