Of the headache syndromes, cluster headache is associated with the most excruciating pain. The pain of cluster headaches is so severe, that most patients cannot sit still and they often pace back and forth and hold their heads in agony. Some patients may even be driven to commit suicide during an attack. While the pain may be more severe than that of a migraine headache, the cluster headaches are typically much briefer in duration, typically lasting less than thirty minutes. They are called "cluster" headaches because the headaches occur in a series or "cluster." After a typical cluster of headaches, an individual may be symptom-free for months to years, only to have another series of attacks at a later time.

Criteria for diagnosing cluster headache include the following:

A. At least five headache attacks.

B. Severe pain that is one-sided, around or behind the eye. Each attack lasts 15-180 minutes.

C. Each headache is accompanied by at least one of the following signs on the same side as the pain:

1. Eye redness
2. Tearing
3. Nasal stuffiness
4. Runny nose
5. Sweating of the forehead and face
6. Smaller pupil
7. Drooping eyelid
8. Eyelid swelling

D. Frequency of attacks: from one every other day to eight per day.

E. The headaches are not secondary to specific brain problem such as a stroke or a brain tumor.