1. Headache Group, Department of Neurology, University of California, San Francisco, San Francisco, CA 94158, USA.
2. Department of Oral and Maxillofacial Pathology, Radiology and Medicine, New York University College of Dentistry, New York, NY 10010, USA.
3. National Institute for Health Research–Wellcome Trust Clinical Research Facility, King’s College London, London SE5 9PJ, UK.