Placebo response in cluster headache trials: a review.

Probably because of its relative rarity as primary headache, there are few well-controlled clinical trials on cluster headache (CH) patients. Due to the severity of the pain, the placebo response in CH has been considered to be small. During the eighties the first double-blind, placebo-controlled trials were reported, and placebo responses demonstrated. Here we review the placebo response in CH trials in order to assess its magnitude and consider how future studies can be optimized. Six trials were identified with a double-blind, placebo-controlled, cross-over design testing treatments of acute CH. For those with a primary endpoint set to no or mild headache the placebo responses varied from 7 to 42%. In five of seven prophylactic trials, using a double-blind, placebo-controlled, parallel-group design, the placebo was merely used to set a baseline for comparison. The placebo responses were reported in only two trials. Here the response varies from 14 to 43%, the lowest value was reported using the strict endpoint; cessation of headache attacks. We conclude that a placebo response exists in trials of drugs on CH patients. Furthermore, this placebo response is of the same magnitude as that seen in migraine studies. We recommend the use of IHS guidelines when designing new trials. The possibility of a genuine biological mechanism responsible for the placebo response is discussed.