What is cluster headache?

The basics

Cluster headache gets its name from the pattern of attacks, which come in "clusters" - periods during which attacks occur daily, often multiple times per day, for weeks or months at a time. These cluster periods are typically separated by remission periods during which there are no attacks.

The pain is almost always one-sided, centred around or behind the eye on the affected side. It is often described as a burning, stabbing or drilling sensation - an ice pick driven into the eye. Accompanying symptoms on the same side as the pain typically include a red or watering eye, a drooping eyelid, a constricted pupil, nasal congestion or runny nose and facial flushing or sweating.

Unlike migraine, where patients typically want to lie down in a dark room, cluster headache patients are usually agitated during an attack - pacing, rocking, unable to stay still. The pain is simply too intense.

How painful is it?

Cluster headache is consistently rated at or near the top of pain scales. It has been described in the medical literature as one of the most painful conditions a human being can experience. The nickname "suicide headache" is used because the headache is so severe and distressing, and the level of pain and distress is a clue to diagnosis. Once this headache is identified, effective treatment can be started.

Patients who have given birth, had kidney stones, broken bones or undergone major surgery frequently report that cluster headache exceeds all of those experiences in pain intensity.

What causes cluster headache?

The exact cause is not fully understood. Current thinking centres on dysregulation of the hypothalamus - the part of the brain that controls circadian rhythms and acts as a "body clock." This explains several characteristic features of cluster headache:

• Attacks often occur at the same time each day, frequently waking patients from sleep at the same time every night.
• Cluster periods often start at the same time each year, commonly around the spring or autumn equinox.
• The trigeminal nerve and the parasympathetic nervous system are activated during attacks, producing the eye and nasal symptoms.

Genetics plays a role - cluster headache is more common in first-degree relatives of affected individuals, though it is not straightforwardly inherited.

Who gets cluster headache?

Cluster headache affects approximately 0.1% of the population - roughly 1 in 1,000 people. In New Zealand, this translates to approximately 5,000 people.

It is more common in men, with a male-to-female ratio of approximately 3:1, though this gap has been narrowing in more recent studies, possibly reflecting improved diagnosis in women. The condition most commonly begins in the 20s to 40s, though it can start at any age.

Smoking and alcohol use are associated with cluster headache. Alcohol reliably triggers attacks during active cluster periods in most patients, though not during remissions.

Triggers during a cluster period

Several triggers reliably provoke attacks during an active cluster period (though not during remission):

• Alcohol - the most reliable trigger. Even small amounts can trigger an attack within 30–60 minutes during a cluster period.
• Nitroglycerin and other vasodilating drugs - for example glyceryl trinitrate (GTN) sprays or tablets used for angina, and medicines such as sildenafil (Viagra)
• Strong smells - petrol, paint, solvents, some perfumes
• Bright light or heat - for some patients
• Sleep - many patients have attacks that wake them from sleep, often at the same time each night (the "alarm clock headache")
• Altitude changes - flying and high-altitude environments can trigger attacks in some patients

Is cluster headache the same as migraine?

No. Cluster headache and migraine are both primary headache disorders, meaning they are not caused by another underlying condition. Beyond that, they are distinctly different in their pattern, symptoms and treatment.

Key differences include the duration of attacks (migraine lasts 4–72 hours; cluster headache lasts 15 minutes to 3 hours), the behaviour during attacks (migraine patients rest; cluster patients pace), the presence of the characteristic eye and nasal symptoms in cluster headache and the clustered pattern of attacks. Most migraine treatments are not effective for cluster headache, and most cluster headache treatments are not used for migraine.

One of the reasons cluster headache is so frequently misdiagnosed is that GPs and patients often assume severe head pain must be migraine. It is not. Read more on the cluster headache vs migraine page.