The incidence and prevalence of HC is not known. It is regarded as a rare syndrome, but it may have been underdiagnosed (34,41).

HC has a female preponderance with a sex ratio of 2.4:1. The condition usually begins in adulthood, though the range of age of onset is 5 to 67 years (mean = 28 years) (34).

The pathophysiology of HC is poorly understood. Cranial vasculature involvement has been investigated with orbital phlebography in six patients but only one patient had an abnormality consisting of bilateral narrowing of the ophthalmic veins, which may well be a nonspecific finding with unilateral pain (1).

Pain pressure thresholds are reportedly reduced in patients with HC (6), similar to paroxysmal hemicrania (PH) and cluster headache (CH) (7).

Pupillometric studies have shown no clear abnormality in HC (5), and studies of facial sweating have shown modest changes similar to those seen in PH (2). Most recently, a positron emission tomography (PET) study indicated significant activation of the contralateral posterior hypothalamus and ipsilateral dorsal rostral pons in association with the headache of HC. These areas corresponded with those active in CH and migraine, respectively. In addition, there was activation of the ipsilateral ventrolateral midbrain, which extended over the red nucleus and the substantia nigra, and of the bilateral pontomedullary junction. No intracranial vessel dilation was obvious (22). The data suggest that HC, like migraine and CH, is fundamentally a brain disorder and further that its pathophysiology may be completely unique. This would fit with the clinical presentation that has similarities to other primary headaches but indeed seems unique.

HC is a unilateral, continuous headache, without side shift, though rare bilateral cases (18,33,40) and a patient with unilateral, side-alternating attacks (24) have been described. The pain is mostly in the “anterior” area of the head, but not infrequently in the auricular/occipital area, or other regions of the head or neck can be affected (11). The pain is, typically, mild to moderate in intensity. The quality of pain is described as dull, aching or pressing, and generally lacking associated features (11,17,25,31,38).

In the majority of patients, exacerbations of severe pain are superimposed on the continuous baseline pain. These exacerbations can last from 20 minutes to several days. In one-third of patients, the headaches are nocturnal and are mistaken for CH or hypnic headache.

During exacerbations, the pain may be accompanied by a variable combination of ipsilateral autonomic features, commonly lacrimation and conjunctival injection. Ipsilateral ocular discomfort and nasal stuffiness may also occur (17). When present, autonomic signs are not as prominent in HC as in CH or PH. Furthermore, photophobia, phonophobia, nausea, and vomiting are common
during exacerbations. In 26% to 41% of patients, primary stabbing headaches occur, predominantly during exacerbations (34). Jabs are frequent with HC, but they appear in other primary headaches as well.

There is a general paucity of factors that precipitate HC. Precipitants that are operative in other headaches generally seem to be of little relevance in HC; this includes stress, menses, alcohol, vasodilators, or exteroceptive stimuli acting on the tissues supplied by the trigeminal nerve. Furthermore, the headache is not aggravated by supine or other positions, Valsalva-like maneuvers, or physical activity. Neck movements do not trigger exacerbations, although occipital tenderness is present in 68% of patients (ipsilateral 44%, bilateral 24%) (25,34).

HC is continuous in the majority of patients, although some have an episodic or remitting form with distinct headache phases separated by pain-free remissions. HC is chronic from onset in 53%, chronic evolved from episodic in 35%, and episodic from onset in 12% (34). There is one case report of a patient who became episodic following a chronic onset (26).

International headache society criteria for HC (Revised International Classification of Headache Disorders—ICHD-II) (17):

A. Headache for >3 months fulfilling criteria B through D

B. All of the following characteristics:

1. Unilateral pain without side shift

2. Daily and continuous, without pain-free periods