Cluster headaches are excruciating attacks of pain around one eye or temple, due to widening of blood vessels in the brain.
Attacks begin suddenly, affect only one side of the head, and may be accompanied by watering of the eye, drooping of the eyelid, and a stuffy or runny nostril.
Individual episodes may last from minutes to hours and may occur several times a day. Attacks happen in clusters, with periods when attacks occur, typically every day for weeks, followed by attack-free periods, which may last months or years, before attacks recur.
The condition may be treated with medication, by breathing pure oxygen, or by nerve stimulation (using a hand-held device to stimulate a nerve in the neck).