[1] Univ Oslo, Inst Clin Med, Oslo, Norway
[2] Oslo Univ Hosp, Dept Neurol, Oslo, Norway
[3] Oslo Univ Hosp, FORMI, Oslo, Norway
[4] Oslo Univ Hosp, Dept Biostat, Oslo, Norway
[5] Norwegian Univ Sci & Technol, Fac Med, Dept Neurosci, N-7034 Trondheim, Norway
[6] St Olavs Hosp, Neurol Sect, Norwegian Natl Headache Ctr, Trondheim, Norway