Chronic cluster headache: a review.

Cluster headache (CH) is a rare but severe headache disorder characterised by repeated unilateral head pain attacks accompanied by ipsilateral autonomic features. In episodic CH, there are periods of headache attacks with pain-free intervals of weeks, months or years in between. A minority of patients have the chronic form, without pain-free intervals between the headache attacks. Chronic CH can occur as primary or secondary chronic CH; the rarest form is episodic CH arising from chronic CH. In this article, we give a review of the chronic forms of CH and focus on demographics, clinical manifestations, social habits, predictive factors, head injury, genetics, neuroimaging and therapy. It is remarkable that little is known about risk factors that make CH chronic.