1. Department of Neurology
2. University of Essen, Essen, Germany; The Kings Headache Service
3. Kings College Hospital, London, UK; Department of Neurology
4. Leiden University Hospital, Leiden, The Netherlands; Headache Center
5. University of Pavia, IRCCS C. Mondino, Pavia, Italy; Glostrup Hospital, Glostrup, Denmark