Hypothalamic involvement and activation in cluster headache.

Cluster headache is an episodic form of primary neurovascular headache that is both severe and relatively rare. It is characterized by episodes of headache with cranial parasympathetic activation and sympathetic impairment that come in bouts, or clusters. Its pathophysiology can be divided into understanding the attack phenotype and the biotype of the periodicity. Acute attacks of cluster headache are marked by trigeminal nerve-mediated pain and with cranial autonomic activation, trigeminal-autonomic cephalalgia; an activation that characterizes the phenotype of a group of headaches. The signature feature of cluster headache is its periodicity, the daily cycle of attacks when the patient is in an active bout, or the circumannual, or other period, cycling that distinguishes the on period from the off period. Functional brain imaging with positron emission tomography and structural imaging with voxel-based morphometry have identified an area in the posterior hypothalamic gray as key in understanding cluster headache. This area is subtly enlarged in its gray matter volume, active during an acute cluster headache but inactive when patients are challenged between bouts. Cluster headache is likely to be a form of primary neurovascular pain whose phenotypic expression relies on the trigeminal-autonomic reflex, with a biotype determined by the brain area, the posterior hypothalamus, in which the lesion seems to be located. Understanding both the phenotypic expression and the biotype will, respectively, enable better acute attack treatments and better preventative management of this horrible form of headache.