By Drew Hall, D.C., Upper Cervical Chiropractor
The word migraine descends from hemicrania — half the head. One-sidedness has been considered the signature of the condition for as long as it has had a name, and the International Classification of Headache Disorders still lists unilateral location among its diagnostic criteria.
But one-sidedness is more informative than most people realize, and one particular pattern within it is a warning sign.
Sides that switch versus sides that do not
This is the single most useful distinction in this article.
Alternating unilateral headache — one-sided during any given attack, but the side varies between attacks — is characteristic of migraine and is reassuring.
Side-locked headache — always the same side, every attack, without exception — is different. Persistent strict unilaterality raises the probability of a structural or secondary cause and is treated with more suspicion by headache specialists. It does not mean something sinister is present; it means the threshold for investigation should be lower.
Two of the conditions most associated with genuine side-locking are cervicogenic headache and cluster headache.
Why the upper cervical spine produces one-sided pain
Cervical structures generate strictly one-sided head pain for a straightforward anatomical reason: their nerve supply is unilateral, and the convergence that refers their pain into the head occurs on one side of the brainstem.
Sensory fibers from the upper three cervical nerve roots enter the spinal cord and terminate in the same region as descending trigeminal fibers — the trigeminal nucleus caudalis, extending down to the C2 and C3 segments. This is the trigeminocervical complex. Second-order neurons there receive input from both systems and cannot reliably distinguish the source, so upper cervical irritation is perceived as head pain.
The referral has been mapped experimentally. Dreyfuss, Michaelsen, and Fletcher distended the atlanto-occipital and lateral atlanto-axial joints in asymptomatic volunteers in 1994 and charted where the resulting pain was felt — the suboccipital region and up over the head. Aprill, Axinn, and Bogduk later demonstrated that the lateral atlanto-axial (C1–2) joint is a source of occipital headache, confirmed with controlled diagnostic blocks (Cephalalgia 2002;22:15–22).
Sjaastad’s diagnostic criteria for cervicogenic headache place unilaterality without side-shift at the center of the picture, alongside pain starting in the neck, reduced cervical range of motion, and symptoms provoked by neck movement or sustained posture.
The functional relationship runs both ways
What makes the cervical contribution more than a curiosity is that the convergence is not passive.
Bartsch and Goadsby demonstrated that stimulating the greater occipital nerve increased central excitability to dural afferent input (Brain 2002;125:1496–1509), and in a companion study showed that noxious dural stimulation sensitized second-order neurons receiving cervical input, recording from 29 nociceptive neurons in the rat C2 dorsal horn (Brain 2003;126:1801–1813).
Each side of the system can raise the sensitivity of the other. Sustained input from an irritated upper cervical joint plausibly lowers the threshold of the very neurons that carry migraine pain — on that side, in that territory.
Mechanisms worth labeling accurately
You may encounter the myodural bridge in discussions of one-sided headache, and it is genuine anatomy. Hack and colleagues described in 1995 the fibrous continuity between rectus capitis posterior minor and the cervical spinal dura, and a systematic review of 26 anatomical studies confirmed dural connections from rectus capitis posterior minor, rectus capitis posterior major, and obliquus capitis inferior (Spine 2017;42:49–54). Because these connections are paired and can be asymmetrically loaded, they offer a plausible route by which one-sided cervical tension reaches a pain-sensitive membrane.
You will also encounter the dentate ligament-cord distortion hypothesis (Grostic, Chiropractic Research Journal, 1988) and Flanagan’s craniocervical choke point proposal. Both are single-source hypotheses published outside the indexed literature and never validated in controlled research. I mention them because you will hear them, and you should know their standing.
When one-sided headache warrants urgent evaluation
Some of these are emergencies.
New one-sided headache after age 50, particularly with scalp tenderness, jaw pain when chewing, or any visual change, requires same-day evaluation for giant cell arteritis. Untreated, it can cause irreversible blindness within days.
Thunderclap onset — maximum intensity within a minute — requires emergency assessment regardless of side.
One-sided headache with neck pain following trauma, chiropractic manipulation, heavy lifting, or even vigorous head turning, especially with a droopy eyelid, constricted pupil, or visual change, raises the possibility of cervical artery dissection and is a medical emergency. This is an uncommon condition, and it is one I disclose openly.
Headache with fever, neck stiffness, confusion, weakness, or new seizures requires emergency care.
Progressively worsening headache, headache that is worse in the morning or when lying flat, or headache with vomiting and no other migraine features warrants prompt imaging.
Severe one-sided eye pain with a red eye, blurred vision, halos, and nausea may be acute angle-closure glaucoma — an emergency that can destroy vision.
The upper cervical approach
Nothing is assumed and nothing is imaged by default. Specific objective testing comes first, establishing whether a misalignment exists; cone beam CT follows only that finding, giving three-dimensional detail of the atlas and axis — relevant here because C1-2 is a demonstrated source of one-sided occipital pain and its anatomy differs between individuals.
Delivery is low-force: no popping, no twisting, no yanking, which matters given that end-range rotational force is the mechanism behind the dissection risk described above. The correction is judged by whether it holds, since one that holds needs no repeating.
Healing takes time. An irritated joint regaining motion and a sensitized brainstem pathway settling are both processes measured in months.
The honest limit: upper cervical misalignment has not been proven to cause one-sided headache. The referral patterns are experimentally demonstrated and the convergence physiology is documented; the causal claim is not.
This article is for educational purposes and is not medical advice. It does not establish a doctor-patient relationship. Thunderclap headache, new headache after 50 with scalp tenderness or visual change, headache with fever or neck stiffness, headache after neck trauma with eyelid droop or visual change, or severe eye pain with a red eye requires emergency medical care.