Hypnic headache is one of several relatively uncommon headache syndromes that may occur either as a primary headache or as a headache secondary to potentially malignant processes. Careful evaluation for underlying causes is important for these uncommon types of headache.
This topic will review hypnic headache. Other types of uncommon primary headache disorders are discussed separately. (See "Primary stabbing headache" and "Primary cough headache" and "Exertional headache" and "Primary headache associated with sexual activity" and "Thunderclap headache" and "Nummular headache".)
The cause of hypnic headache is not understood. The apparent cyclicity, the similar duration, and response to lithium suggest a pathophysiologic connection with cluster headache and hypothalamic dysfunction . (See "Pathophysiology of the trigeminal autonomic cephalalgias", section on 'Hypothalamic activation'.)
To test this hypothesis, a voxel based morphometric magnetic resonance imaging (MRI) study compared 14 patients with hypnic headache and 14 matched healthy controls . Subjects with hypnic headache had reduced gray matter volume in the posterior hypothalamus. Ironically, an earlier report using similar methods found increased gray matter volume in a similar location among patients with cluster headache . Nevertheless, both studies implicate the posterior hypothalamus in headache disorders with a tendency toward cyclical and nocturnal occurrence. Although early reports suggested that hypnic headache might be a rapid eye movement (REM) sleep disorder, later studies showed that the majority of hypnic headaches attacks arise from non-REM sleep stages, mainly sleep stage N2 [1,4].
The onset of hypnic headache typically occurs after age 50 years, though a few cases younger than age 40 years have been reported, including five pediatric cases ranging from 8 to 11 years in age [1,5,6]. While hypnic headache is rare, the actual incidence and prevalence are unknown. In data from tertiary headache centers and clinics, the overall proportion of patients with hypnic headache ranged from 0.07 to 0.35 percent, while the proportion among geriatric patients with headache ranged from 1.4 to 1.7 percent . There is a female predominance of approximately 2:1 [1,6].