Onabotulinumtoxin A for treatment of chronic migraine: Results from the double-blind, randomized, placebo-controlled phase of the PREEMPT 1 trial - Aurora, S; Dodick, D; Turkel, C; Aurora, SK; Dodick, DW; Turkel, CC; DeGryse, RE; Silberstein, SD; Lipton, RB; Diener, HC; Brin, MF; PREEMPT 1 Chronic Migraine Study Group (Cephalalgia, 2010)