[1] Leiden Univ, Med Ctr, Dept Neurol, NL-2300 RC Leiden, Netherlands
[2] Albert Einstein Coll Med, Dept Neurol, New York, NY USA
[3] Albert Einstein Coll Med, Dept Epidemiol, New York, NY USA
[4] Albert Einstein Coll Med, Dept Social Med, New York, NY USA
[5] Montefiore Headache Unit, New York, NY USA
[6] Innovat Med Res Inc, Stamford, CT USA
[7] Natl Hosp Neurol & Neurosurg, Inst Neurol, London WC1N 3BG, England