[1] Univ Calif San Francisco, Dept Neurol, Headache Grp, San Francisco, CA USA
[2] Univ London Imperial Coll Sci Technol & Med, Div Med, Anesthet Pain Med & Intens Care Sect, London, England
[3] Univ Basel Hosp, Dept Neurol, CH-4031 Basel, Switzerland
[4] Univ Basel Hosp, Div Neuroradiol, CH-4031 Basel, Switzerland