[1] Univ Oxford, Dept Publ Hlth, Oxford OX3 7LF, England
[2] Minist Hlth Care & Nutr, Colombo, Sri Lanka
[3] Brigham & Womens Hosp, Div Headache & Pain, Dept Neurol, Boston, MA 02115 USA
[4] Hosp Luz, Headache Outpatient Clin, Lisbon, Portugal
[5] Glostrup Cty Hosp, Dept Neurol, Danish Headache Ctr, Glostrup, Denmark
[6] Univ Duisburg Essen, Essen, Germany
[7] Evangelic Hosp, Unna, Germany
[8] Norwegian Univ Sci & Technol, Dept Neurosci, N-7034 Trondheim, Norway
[9] Univ London Imperial Coll Sci Technol & Med, Dept Neurosci, London, England