Botulinum toxin type A for the prophylaxis of chronic daily headache: subgroup analysis of patients not receiving other prophylactic medications (a randomized double-blind, placebo-controlled study) - Dodick,D.W.; Mauskop,A.; Elkind,A.H.; DeGryse,R.; Brin,M.F.; Silberstein,S.D. (Neurology, 2005)