Cluster headache is a condition that involves, as its most prominent feature, repeated attacks of excruciatingly severe pain that occur always on only one side of the head. (Unilaterally) Cluster headache belongs to a group of primary headache conditions, classified as Trigeminal Autonomic Cephalalgias or (TACs). There is currently no known cure for cluster headache.
Cluster headaches often occur periodically; spontaneous remissions may interrupt active periods of pain, though about 10-15% of chronic cluster headache sufferers never remit. The cause of the condition is not yet known. It affects approximately 0.1% of the population of middle-aged people and men are more commonly affected than women, by a ratio of 2.1:1.