cluster headache is a headache that causes intense, almost unendurable pain that is localized in just one part of the head.  They appear to occur when there is a malformation in the hypothalamus and are triggered when blood vessels dilate and impinge on the trigeminal nerve.  They usually have a very rapid onset and can last from fifteen minutes to three hours.  They are believed to be the most painful phenomena known to medicine - worse than childbirth, burns or broken bones.  They are also known for their almost unbelievable regularity - occurring at the same time every day or even at the same time once a week. This timeliness is apparently due to the role of the hypothalamus in setting daily sleep/wake rhythms.  

Remarkably, cluster headaches are non-life threatening and do not cause any other health complications, although patients often feel suicidal.

Cluster headaches are often misdiagnosed as a severe migraine, but do not respond to migrane treatment.  In addition, care must be taken to distinguish the condition from both chronic paroxysmal hemicrania and an ictal headache, both of which are treatable.  

Treatment concentrates on prophylactic measures.  Both calcium channel blockers and steroids can be effective in stopping an attack.  However, when attacks strike, ordinary analgesics are useless and even narcotics have a limited effect on the pain.  Patient's usually respond well to 100% oxygen and certain triptans can also relieve some of the symptoms.  

Cluster headache at Wikipedia

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