The blood supply to your brain arrives through four arteries, and they are not equally protected.
Two of them are the carotid arteries. They run up the front of your neck, tucked close to the midline, and there is nothing hard sitting in front of them. They are large, high-pressure vessels in a well-shielded position. It is genuinely uncommon for a misalignment to compress a carotid, and even in a field that works with these structures constantly, we rarely see it.
The other two are the **vertebral arteries**, and their routing is completely different.
Why Does Routing an Artery Through Bone Matter?
Instead of running up soft tissue, the vertebral arteries thread up through a series of small bony tunnels — one in the transverse process of each cervical vertebra. They pass through the atlas, the topmost vertebra, then make a sharp turn, pierce the membrane covering the spinal cord, and enter the skull.
Routing an artery through a stack of bones that rotate, tip, and glide against one another is efficient use of space. It also has a consequence: the flow becomes **position-dependent**. When those bones sit where they belong, the artery has the room it needs. When one of them is displaced, that space can narrow — and it narrows more in certain head positions than others.
That mechanism is documented. Rotational vertebral artery occlusion, sometimes called bow hunter's syndrome, describes flow in the vertebral artery being reduced or interrupted specifically by head rotation, producing posterior circulation symptoms (Jost & Dailey, 2015; Schulz et al., 2021).
Two things are worth stating carefully about that literature. The named syndrome is uncommon, and the reported cases generally involve both a compressive structure and inadequate flow from the artery on the other side. What those cases establish is not that every neck restricts flow — it's that the anatomy makes position-dependent flow possible in the first place.
What Do the Vertebral Arteries Actually Supply?
Where that blood goes determines which symptoms appear.
After entering the skull, the two vertebral arteries merge into a single vessel called the basilar artery, which travels up to the center of the brain. Along the way, branches supply the **brainstem** and the **cerebellum** — including a vessel called the posterior inferior cerebellar artery, which feeds the lower back portion of the cerebellum.
The cerebellum's primary job is coordination. The brainstem handles balance integration, eye movement control, and a long list of automatic functions. The vestibular nucleus, which [reconciles all your balance information](post-why-most-of-your-balance-comes-from-your-neck.html), sits in that same neighborhood.
So a restriction in vertebral artery flow doesn't produce symptoms scattered randomly across the body. It produces a specific and recognizable set. The posterior circulation literature lists the same cluster: vertigo and dizziness, imbalance and gait ataxia, visual disturbance, dysarthria, and in some cases difficulty swallowing (Nouh et al., 2014). In one series of 407 patients with vertebrobasilar stroke, dizziness was present in 47%, dysarthria in 31%, and gait ataxia in 31% (Carvalho & Cruz, 2020).
Those figures describe stroke, which is a far more severe event than what I'm describing. I cite them for the pattern rather than the severity — they show which functions depend on that particular blood supply.
Why Does the Front of Your Brain Usually Stay Fine?
There's an obvious question here. If blood supply is restricted, why aren't people's thinking, memory, and language affected the same way?
Part of the answer is a structure called the circle of Willis, and there's a good everyday comparison for it.
If you've seen a landscape irrigation system, you may have noticed how it waters a large bush or tree. Rather than a single line pointed at one spot, the tubing forms a ring around the plant, with small holes along the bottom. Water enters the ring at one or two points, pressurizes the entire loop, and then distributes evenly all the way around.
The circle of Willis is arranged the same way. Your carotid arteries and your basilar artery all feed into a ring at the base of the brain, and the cortex — the large outer surface responsible for thinking, memory, and language — is supplied from that ring. Because it's a loop with multiple inlets, losing pressure from one inlet does not necessarily starve the whole circle.
That said, the redundancy is not guaranteed and it is not universal. A complete, well-formed circle is present in fewer than half of people by most estimates, and how much collateral flow it can actually deliver varies considerably from person to person (Maguida & Shuaib, 2023). Some researchers argue its main role is pressure distribution rather than rerouting flow (Vrselja et al., 2014). So the ring is a real feature of the anatomy, and how much protection it provides depends on the individual.
What is consistent is the other half of the picture. The branches that come off the vertebral arteries **before** they reach that ring don't have a backup at all. The brainstem and the lower cerebellum are supplied directly, upstream of whatever redundancy exists. So an insufficiency in a vertebral artery tends to affect the back and the base of the brain specifically, while the front of the brain continues to receive what it needs.
That's why the symptom pattern is so consistent: coordination and balance degrade while thinking and language stay largely intact.
Why Does Turning Your Head Make It Worse?
This is the part patients recognize the moment it's described.
Rotating or extending your head is exactly the movement that changes the geometry of the upper neck most dramatically. Roughly half of all cervical rotation happens at the joint between the first and second vertebrae, and that is precisely where the vertebral artery makes its sharp turn.
So if a segment there is misaligned, turning or tipping your head can narrow the artery's path further and briefly reduce flow.
That means the [dizziness](condition-dizziness.html) that spikes when you back out of a driveway, check a blind spot, tip your head back at a sink, or reach up to a high shelf is not random and not psychological. It is a momentary reduction in blood supply to the part of your brain responsible for keeping you oriented and coordinated.
The symptoms come and go with position because the mechanism comes and goes with position.
Why Does the Scan Come Back Normal?
A standard brain MRI is very good at imaging brain tissue and identifying a bleed, a lesion, or a tumor.
None of those are what's happening here. The tissue is not damaged. It's intermittently underserved. A picture of healthy tissue, taken while you are lying still with your head in a neutral position, is an accurate picture — and it is answering a question about the tissue rather than about the supply line.
There's an additional detail worth knowing. Even when the vertebral arteries are imaged, the finding needs correct interpretation. A naturally smaller vertebral artery on one side is a common anatomical variant rather than an injury — in one imaging study of 934 patients it was present in roughly 16% (Thierfelder et al., 2014), and in a series of 742 asymptomatic subjects the figure was higher still depending on where the diameter cutoff was drawn (Gaigalaite et al., 2016).
The way to distinguish that variant from a traumatic restriction is to follow the vessel along its length. A congenitally small artery is narrow the whole way up. A traumatic restriction looks full and even until it reaches the point where something is compressing it, and then it isn't.
Same image, two very different conclusions, and the difference is where the narrowing begins.
What Would an Examination Matched to This Look Like?
If symptoms are position-dependent, the examination should be too. That means looking at the alignment and stability of the upper cervical spine directly, observing what happens to balance and coordination when the head is turned and held there rather than only in neutral, and — when it's warranted — imaging the structures and the flow rather than only the brain tissue.
A clean brain scan is genuinely good news about your brain. It is not the same thing as a clean bill of health for the route that keeps your brain supplied.
Unsteady or Clumsy With a Normal Brain Scan?
If your balance and coordination fail intermittently — especially when you turn or tip your head — and imaging of your brain came back normal, the supply line through your [upper neck](condition-balance-issues.html) is worth examining on its own terms.
Assessing where your neck hands blood and information up to your brain is the work we do at Cerebral. If you'd like a real evaluation, we're here.
References
- Jost GF, Dailey AT. Bow hunter's syndrome revisited: 2 new cases and literature review of 124 cases. *Neurosurgical Focus*. 2015;38(4):E7. https://pubmed.ncbi.nlm.nih.gov/25828501/
- Schulz R, Donoso R, Weissman K. Rotational vertebral artery occlusion (bow hunter syndrome). *European Spine Journal*. 2021;30(6):1440–1450. https://doi.org/10.1007/s00586-020-06680-5
- Nouh A, Remke J, Ruland S. Ischemic posterior circulation stroke: a review of anatomy, clinical presentations, diagnosis, and current management. *Frontiers in Neurology*. 2014;5:30. https://pubmed.ncbi.nlm.nih.gov/24778625/
- Carvalho V, Cruz VT. Clinical presentation of vertebrobasilar stroke. *Porto Biomedical Journal*. 2020;5(6):e096. https://pmc.ncbi.nlm.nih.gov/articles/PMC7710193/
- Maguida G, Shuaib A. Collateral circulation in ischemic stroke: an updated review. *Journal of Stroke*. 2023;25(2):179–198. https://doi.org/10.5853/jos.2022.02936
- Vrselja Z, Brkic H, Mrdenovic S, et al. Function of circle of Willis. *Journal of Cerebral Blood Flow & Metabolism*. 2014;34(4):578–584. https://doi.org/10.1038/jcbfm.2014.7
- Thierfelder KM, Baumann AB, Sommer WH, et al. Vertebral artery hypoplasia: frequency and effect on cerebellar blood flow characteristics. *Stroke*. 2014;45(5):1363–1368. https://pubmed.ncbi.nlm.nih.gov/24699051/
- Gaigalaite V, Vilimas A, Ozeraitiene V, et al. Association between vertebral artery hypoplasia and posterior circulation stroke. *BMC Neurology*. 2016;16:118. https://pmc.ncbi.nlm.nih.gov/articles/PMC4962415/