[1] Kings Coll London, Fac Dent Oral & Craniofacial Sci, London, England
[2] St Georges Univ London, Populat Hlth Res Inst, London, England
[3] Sheffield Univ Teaching Hosp NHS Fdn Trust, Charles Clifford Dent Inst, Sheffield, England
[4] Natl Univ Malaysia, Kuala Lumpur, Malaysia
[5] Guys & St Thomas NHS Fdn Trust, Headache Serv, Pain Management & Neuromodulat Ctr, London, England