Impaired circadian rhythmicity of nociceptive reflex threshold in cluster headache

Background. - Alteration of circadian rhythmicity involving several endocrinologic and autonomic parameters has been observed in cluster headache. Objectives. - To explore whether circadian failure of the pain control system may exist in cluster headache. Methods. - The nociceptive flexion reflex threshold was studied in 25 patients with episodic cluster headache (14 active, 11 in remission) and 6 patients with chronic cluster headache, along with 10 normal volunteers throughout a 24-hour period. The reflex response was evoked at the level of the biceps femoris by stimulating the sural nerve at the ankle. Single and population mean cosinor methods were used to detect the circadian rhythmicity. Results. - In the patients with episodic cluster headache, a significant reduction in the nociceptive flexion reflex threshold was observed in both the active subgroup and the subgroup in remission (P