Pupillary response to topical naloxone in cluster headache

Eye drops of naloxone were applied and pupillary diameters recorded at standard time intervals in 18 healthy controls and in 20 patients with episodic cluster headache during pain free intervals by means of a pupillometer. In the basal condition, the mean symptomatic side pupil in the cluster headache group was significantly smaller than the pupil of controls. Patients in the cluster period had a significantly smaller symptomatic-side pupil when compared to patients outside the bout. After naloxone, the pupils on the symptomatic as well as on the non-symptomatic side were significantly smaller than those in controls when expressed in mm. The difference between patients and controls persisted, and was, grossly speaking, of a not widely varying magnitude during the drug test. This study thus does not render any definite evidence for a role of opioids in the mechanism underlying the miosis of cluster headache.