Epidemiology of fixed unilateral headaches.

A fixed location unilateral headache suggests involvement of a precise nervous structure, and neuroimaging investigations are essential to seek to identify it. Nevertheless, side-locked primary headaches also occur, although they are rare. Side-locked primary headaches are more frequently found in the group of the short-lasting (</= 4 hours) headaches but long-lasting headache forms may also present with the pain always on the same side, including migraine, tension-type headache, new daily persistent headache and cervicogenic headache. Future studies should address the issue whether patients with side-locked headache form differ from those with non-side-locked form both in terms of natural history and biological markers. Among 63 consecutive chronic cluster headache patients seen by us from 1999 to 2007, 32 (51%) had side shift. We also found that the duration of the chronic condition was significantly longer in those with side shift than those without. The high frequency of side shift in chronic cluster headache should be considered when proposing surgical treatment for severe intractable forms of the disease.