1. Cephalea Headache Centre, Gothenburg
2. Institute of Neuroscience and Physiology, Sahlgrenska Academy, Göteborg University, Gothenburg, Sweden
3. Department of Clinical Neuroscience, Section of Neurology and Department of Community Medicine, Faculty of Medicine, Norwegian University of Science and Technology, Trondheim
4. Department of Neurology, Ullevål University Hospital, Oslo, Norway