Hemicrania continua is caused by abnormal activity deep in the brain, specifically in a region called the posterior hypothalamus, which disrupts normal pain regulation on one side of the head. Unlike most headaches, the pain doesn’t originate from inflamed tissues or blood vessels outside the skull. It is generated internally, making it a primary headache disorder with no single external trigger. The condition is rare, affecting roughly 7 people per 100,000, and it strikes women about 2.4 times more often than men.
The Role of the Hypothalamus
Brain imaging studies using PET scans and functional MRI have consistently shown activation in the posterior hypothalamus during hemicrania continua. This small brain structure normally helps regulate sleep cycles, hormone release, and body temperature, but it also modulates pain signals coming from the face and head. In hemicrania continua, the hypothalamus appears to create what researchers describe as a “central permissive state,” essentially leaving the door open for pain signals to persist rather than being filtered out the way they normally would be.
This hypothalamic involvement is a key reason hemicrania continua is now classified alongside cluster headaches and paroxysmal hemicrania in a family of disorders called trigeminal autonomic cephalalgias. All of these conditions share the same core mechanism: the hypothalamus loses its ability to properly regulate a pain pathway connecting the brain to the face and head.
How the Pain Pathway Works
The pain in hemicrania continua travels along a network called the trigeminovascular system. Under normal circumstances, sensory nerve fibers from the face and scalp send signals to a relay station in the upper spinal cord and brainstem (the trigeminocervical complex), which then passes them up to the thalamus and on to the brain’s pain-processing areas. In primary headache disorders like hemicrania continua, this system fires without a meaningful external cause. The brain essentially generates pain signals on its own.
The pain concentrates in the area supplied by the first branch of the trigeminal nerve, which is why it typically centers around the eye, forehead, and temple on one side. It stays strictly on that one side and never switches, which distinguishes it from migraine or tension-type headache.
Why It Produces Eye Watering and Nasal Congestion
Many people with hemicrania continua notice symptoms beyond the headache itself: a red or watery eye, a drooping eyelid, a runny or stuffy nose, facial sweating, or a constricted pupil, all on the same side as the pain. These happen because the hypothalamus directly influences a reflex arc that controls blood flow and gland activity in the face. When the hypothalamus loses its normal inhibitory control, parasympathetic nerve fibers running through a cluster of nerve cells behind the cheekbone (the sphenopalatine ganglion) become overactive, flooding one side of the face with signals that dilate blood vessels, trigger tear production, and swell nasal tissues.
Not everyone with hemicrania continua gets all of these autonomic symptoms, but at least one is required for diagnosis. Some people also feel a sense of restlessness or agitation during flare-ups, or find that physical movement makes the pain worse.
Two Patterns: Chronic and Remitting
Hemicrania continua can follow two distinct timelines. In the chronic form, the headache is present every day without interruption, though it fluctuates between a low-level background ache and sharper flare-ups of moderate to severe pain. In the remitting form, periods of daily headache alternate with headache-free stretches that can last weeks or months. The two forms can transition into each other. Some patients start with chronic daily pain, receive treatment that brings relief, and then find that when the headache returns it follows a remitting pattern instead. Both forms share the same underlying mechanism and respond to the same treatment.
Who Is Most Affected
Hemicrania continua peaks in prevalence between the ages of 50 and 65, though it can appear at virtually any age. A large U.S. electronic health records analysis found that prevalence rises steeply from about 0.3 per 100,000 in children and teenagers to 12.9 per 100,000 in the 50 to 65 age group, then gradually declines in older adults. The average age at diagnosis is around 55. Women account for roughly 70% of cases.
No clear genetic cause has been identified, and there are no known lifestyle or environmental triggers that reliably bring on the condition. It appears to arise from the brain’s internal wiring rather than from anything a person does or is exposed to.
The Indomethacin Mystery
One of the most distinctive features of hemicrania continua is that it responds completely to a specific anti-inflammatory drug, indomethacin. This response is so reliable that it is built into the formal diagnostic criteria: if the headache doesn’t resolve with indomethacin, the diagnosis is reconsidered. What makes this puzzling is that other anti-inflammatory drugs in the same class, including ibuprofen and naproxen, do not work. This points to indomethacin doing something beyond simply reducing inflammation, though the exact mechanism remains unknown.
One clue comes from studies showing that indomethacin lowers levels of certain signaling molecules involved in pain and blood vessel dilation. In patients with related headache disorders, treatment with indomethacin reduced levels of CGRP (a protein that dilates blood vessels in the brain and amplifies pain signals) and a compound that stimulates parasympathetic nerve activity. These reductions may help explain why both the pain and the autonomic symptoms resolve together, but researchers still cannot fully account for why only indomethacin produces this effect.
How It Differs From Migraine
Hemicrania continua is frequently misdiagnosed as chronic migraine because both involve one-sided head pain that can be severe. The key differences are continuity and side-locking. Migraine pain comes in discrete attacks separated by pain-free intervals and can switch sides. Hemicrania continua never fully goes away (in the chronic form) and never switches sides. The autonomic symptoms in hemicrania continua also tend to be more prominent and more clearly tied to the painful side than the mild congestion or tearing that sometimes accompanies migraine.
The most definitive differentiator is the indomethacin response. Migraine does not resolve completely with indomethacin. If a one-sided daily headache with autonomic features clears entirely on indomethacin, the diagnosis is hemicrania continua. Getting to that trial can take years, since many patients cycle through migraine treatments that provide little or no relief before the correct diagnosis is considered.