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.