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Engstrøm M, Hagen K, Bjørk M, Gravdahl GB, Sand T.Engstrøm M, et al. Among authors: hagen k.J Headache Pain. 2013 Aug 6;14(1):68. doi: 10.1186/1129-2377-14-68.J Headache Pain. 2013.PMID: 23919583Free PMC article.