The blink reflex in cluster headache. To investigate the involvement of the trigeminal system in cluster headache, in twelve subjects the electrically-elicited blink reflex during a symptomatic period was examined. In eleven cases, the amplitude of the contralateral R2 response on the symptomatic side was significantly lower, at the same stimulus intensity, than on the asymptomatic side (p = 0.005). The blink reflex can be useful to evaluate biological and drug-induced phenomena in cluster headache. 