[1] Salford Royal NHS Fdn Trust, Dept Neurosurg, Manchester M6 8HD, Lancs, England
[2] Univ Manchester, Salford Royal NHS Fdn Trust, Inst Populat Hlth, Ctr Biostat, Manchester M6 8HD, Lancs, England
[3] Univ Manchester, Salford Royal NHS Fdn Trust, Inst Cardiovasc Sci, Manchester M6 8HD, Lancs, England