1. Danish Headache Centre, Department of Neurology, University of Copenhagen, Glostrup Hospital, Copenhagen
2. Department of Clinical Pharmacology, Gentofte Hospital, Gentofte, Denmark
3. Gothenborg Headache Clinic and Institute of Neuroscience and Physiology, Sahlgrenska University Hospital, Gothenborg, Sweden
4. Pain Research and Nuffield Department of Anaesthetics, University of Oxford, Oxford, UK
5. Department of Neurology, Brigham and Women's/Faulkner Hospital, Harvard Medical School, Boston, MA, USA
6. Department of Neurology, Lariboisière Hospital, Paris, France