Basilar invagination and cervical spine instability

Ross Hauser, MD.

We do see patients who MRI or Digital Motion X-ray has revealed that they suffer from a condition of basilar invagination. A vertebrae has rotated or crept out of position and is now approaching the base of the skull and with this approach to the base of the skull, brain stem and spinal cord compression has occurred.

When someone firsts contacts us, they will usually report to us how far in millimeters their occipital bone, atlas or other structures have separated. They will talk to us initially in terms of their McGregor line or Chamberlain line measurements that their doctors used to evaluate their basilar invagination. They will email us all their reports. At some point we have to remind these people that it is very important for us to know simply, how do they feel today? as well. While we use a lot of technology in getting to the cause of your problems, we want to know about what your real life challenges just as equally because patient history and patient quality of life is a test in itself.

People with basilar invagination suffer from many symptoms

Like the great majority of patients we see with cervical spine problems, people with basilar invagination have a long medical history and how they feel today is usually not good. They suffer from many neurologic-like, vascular-like, and physiological-like symptoms and neck pain. Rarely is the problem of basilar invagination a problem all by itself. Please see this article Treating Vertebrobasilar insufficiency – Bow hunter’s syndrome, for more on the various symptoms and co-factors involved in this problem in people who when they turn their head to one side, faint or black out.

Another challenge we see in these people is how confused they are. This confusion is not limited to a diagnosis of basilar invagination, it expands into trying to understand the varying symptoms that these people suffer from and why nothing up until this point has seemingly helped them in the long-term.

A straight neck and a humpback, loss of hearing and tinnitus and a long time before they understood about arterial compression

Often people will tell us that they lost their lordosis and have a humpback that goes from their cervical spine into their thoracic spine. They may even have head tilt that they are fixing with a chiropractor who specializes in upper cervical spine adjustments. They have hearing problems and tinnitus and they have black outs and other neurological type symptoms that they have had countless blood tests for. It may have been the chiropractor who told them about atlantoaxial instability after years of chasing diagnosis with other health care providers.

I have neck pain, I get dizzy, I lose my balance, I have tinnitus, I can’t eat

We get many emails from people who have spent years chasing a wrong diagnosis or having the symptoms treated as if they were isolated problems instead of part of a greater collection of problems this person is suffering from.

They will have dizziness and vertigo and that will be treated on its own. Please refer back to our article: Cervical Vertigo and Cervicogenic Dizziness, Vestibular migraine and spontaneous vertigo – Migraine Associated Vertigo. In these articles not only do we cover treatments and diagnostic possibilities but also we discuss when people come into our offices with a problem of vestibular migraine, spontaneous vertigo, or dizziness they talk to us about a very long medical journey they have taken that no one can quite understand.

Sometimes people will come in with their long history under the care of a gastroenterolgist. They will have had a barium swallow test, an endoscopy, a colonoscopy and other tests. They will complain of constipation, digestive difficulties, acid reflux, nausea and vomiting and other complaints. They will also tell us that their gastroenterolgist reports are rarely if ever discussed in conjunction with their neck pain and one is treated independently of another. Gastrointestinal or GI complaints can be caused by many problems.

The problem of dizziness and tinnitus VOR (vestibular ocular reflex) and Basilar invagination

The vestibular system is the body’s sensory system that regulates balance and spatial orientation (the understanding of where you are in your environment). It sits in the inner ear and works by adjusting fluid levels that act as the balance mechanism. In human beings, we set our awareness of our place in space by using the ground as the constant place of orientation. We can keep our balance when we walk because we understand the ground is the constant and our vestibular system makes constant involuntary adjustments to “keep things steady,” to prevent motion from creating dizziness or sway.

People will report the similar and familiar symptoms of dizziness, neck pain at the base of skull, visual vertigo, VOR dysfunction, oscillopsia, tachycardia, and tinnitus. Their problems may have started with a car accident, a or a series of concussions. They have been advised to alter their diet, try “hit and miss approaches” to medications, and upper cervical chiropractic. Some even say that because they had a “normal” neck MRI that they were told they have Multiple Sclerosis. But their brain MRIs did not show anything either.

The vestibular system is the body’s sensory system that regulates balance and spatial orientation (the understanding of where you are in your environment). It sits in the inner ear and works by adjusting fluid levels that act as the balance mechanism. In human beings, we set our awareness of our place in space by using the ground as the constant place of orientation. We can keep our balance when we walk because we understand the ground is the constant and our vestibular system makes constant involuntary adjustments to “keep things steady," to prevent motion from creating dizziness or sway.


We present the evidence that these complains can be caused, in some people, by cervical spine and neck instability. The same problems that can lead to a diagnosis of basilar invagination.

Please refer back to these articles on our website:

In the video below, Ross Hauser, MD and Brian Hutcheson, D.C., offer a case discussion.

Ross Hauser, MD, and Brian Hutcheson, DC discuss a case of basilar invagination and chronic headaches where Prolotherapy and curve correction may provide a more conservative option than surgery.

C1-C2 compression – basilar invagination

What are we seeing in this image? A possible cause of basilar invagination – cervical ligament laxity and damage. When cervical ligament damage is severe, the dens migrates upwards to compress structures in the foramen magnum.

Cervical Spine Instability, Vein blockage, fluid build-up and intracranial hypertension.

Ross Hauser, MD and Brian Hutcheson, DC explain the problems of blurry vision, brain fog, and other symptoms seen in patients with intracranial hypertension and treatments to alter the cervical spine dynamics to relieve these symptoms.

Below the video is a summary of the transcription.

How does venous obstruction occur in a neck? 

1:17 of video: We are finding and doing outcome research on is the problem of what’s causing the cerebrospinal fluid flow to accumulate and cause pressure inside the head to be elevated is a venous obstruction (vein blockage) so how does venous obstruction occur in a neck?

1:52 of video: Elongating the amount of space that the vein has to travel in order to get to the head and back to the body.

One reason is that the vein is getting stretched out in the neck. How? One way is that the patient’s head is moving forward on their shoulders. When the head is in this position, the veins get pulled on and stretched out. This narrows the veins. A narrowed vein has less room for blood and fluid to flow in, this narrowing caused by cervical spine instability, which leads the head forward is characteristic of the problems faced with stenosis.

A balloon demonstration of a stretched vein

A “sludge” buildup in the brain caused by malrotation at the C1

Relationship of the internal carotid artery to the upper cervical vertebrae. The internal cartoid artery sits just in front of the transverse process of the atlas (C1) and the axis (C2). Even its blood flow can be constricted or blocked by atlanto-axial (upper) cervical instability.

Relationship of the internal carotid artery to the upper cervical vertebrae. The internal cartoid artery sits just in front of the transverse process of the atlas (C1) and the axis (C2). Even its blood flow can be constricted or blocked by atlanto-axial (upper) cervical instability.

The main danger of brain venous congestion is that it increases intracranial pressure, this pressure is then transmitted to the brain’s arteries, which then increase blood flow to ensure adequate oxygenation of the brain. If the blood vessels cannot respond because of their obstruction in the neck, then brain ischemia can ensue.

The brain’s blood vessels may initially be able to respond via autoregulation (increases in blood vessel diameter in the brain) for a time, but if the cervical/brain venous congestion continues because of cervical instability, the increased intracranial pressure will eventually damage the brain neurons, and ultimately, the brain tissue itself.

While the most common cause of arterial or venous obstruction in patients seen at Caring Medical is narrowing caused by cervical instability, it can also be from autonomic nervous dysfunction. Autonomic nervous dysfunction or dysautonomia can cause detrimental changes in the arterial blood flow to the brain or venous blood flow out.

Loss of blood flow to the brain

In our article: How cervical spine instability disrupts blood flow into the brain, we discuss how cervical spine instability pinches on arteries and disrupts, impedes, and retards blood flow into the brain. This is one of the great challenges that face cervical spine or cervical neck instability patients. In our office, almost all the people who have upper cervical spine instability, who come in for our non-surgical treatments, have an amazing amount of brain fog, the inability to concentrate, anxiety, and depression. These are not the typical things that doctors look for in the neck. For more information, please continue with that article: How cervical spine instability disrupts blood flow into the brain

When the patient turned her head, she had a stroke.

A published case history in the British journal of neurosurgery (1) describes the case of a 30-year-old female with Basilar invagination who developed recurrent ischemic stroke in posterior circulation. What made this case curious, besides that fact that a 30 year old woman was suffering from strokes, was the confusion her doctors had in understanding what was happening to her.

This is a description from the case history: “Before the onset of ischemic stroke, she didn’t present neck pain or clinical signs of lower cranial nerve dysfunction, brainstem compression or transient ischemic attack. At first she suffered from sudden onset of left-sided hemidysesthesia (the stroke symptoms were on one side of her body). Magnetic resonance imaging from a local hospital revealed an acute infarction (circulation blockage) in the right thalamus.” Blockage in the thalamus can cause motor disturbances, change in sensation such as temperature and touch, coma or black outs.

Unfortunately it was ignored by the clinicians and the radiologists.

What doctors found was “the tip of the odontoid process had protruded into the foramen magnum and could be observed at the level of the lower medulla, but unfortunately it was ignored by the clinicians and the radiologists. She was given antiplatelet therapy and the sensory disturbance disappeared gradually. However she experienced a recurrence in the pontine and midbrain region 2 months later.

Returning to the case of the woman with the odontoid process protruding into the foramen magnum. The doctors of this case study performed a reconstructed computed tomography of cervical spine which demonstrated basilar invagination, atlanto-axial dislocation, and atlanto-occipital assimilation. Computed tomographic angiography revealed a dominant right vertebral artery and a redundant loop (twisted or torturous artery)  in its third segment. Dynamic cerebral angiogram demonstrated that the patient had a Bow Hunter’s type phenomenon, with dynamic occlusion of the right dominant vertebral artery during contralateral head turn. This case highlighted the necessary of hemodynamic evaluation in asymptomatic basilar invagination. (Doctors should check for blood flow blockage).

Are all my problems from cervical spine instability? Occipitalization of the atlas – atlanto-occipital assimilation

People will report to us a diagnosis of occipitalization of the atlas – atlanto-occipital assimilation. If you have received this diagnosis you know that your body is creating its own bone fusion to try to stabilize your cervical spine because your cervical vertebrae (the C2)  is wandering out of position and possibly pressing against the brain stem.

You may have also been told that this occipitalization of the atlas may be causing your atlantoaxial dislocation and basilar invagination.

Many people see us because they have been given a surgical recommendation. Depending on each case, some may be able to avoid surgery with conservative care and regenerative medicine options, some will have no choice but to get the surgery. For many people surgery will have very good outcomes. We see the people who are not good surgical candidates because their doctors are not sure how successful their surgery will be or people who will only explore surgery as a last resort for various reasons.

Basilar invagination surgery. Are there options?

The main treatment for Basilar invagination is surgery and pre-surgical traction. But in some cases surgery is considered controversial. Updated information in the National Center for Biotechnology Information publication STAT PEARLS (2) offers this explanation.

“Determining the need for operative intervention is controversial in asymptomatic patients, but those at risk of neurologic compromise could require preoperative cervical traction, and posterior-anterior decompression and fusion.”

What are we seeing in this image?

Instability below a massive cervical fusion. After two surgeries, this patient, who happened to be 15 years old still had severe and significant pain. The pain moved from the segmented section of the spine to the no-fused or adjacent segments at C6-C7 by creating ligament injury at this point. The C1-C2 is now the only joint that allows cervical rotation and as unfortunate histiry demonstrates, this segment will not be able to hand the load and more surgery will be needed.

What are the neurologists and neurosurgeons saying?

A July 2021 paper in the journal World neurosurgery (3) offers a summary of treatment and surgical options surrounding Atlas assimilation, atlantoaxial dislocation, Chiari malformation, and basilar invagination.

“Atlas assimilation may be associated with atlantoaxial dislocation, Chiari malformation, and basilar invagination. The importance of Atlas assimilation in the context of craniocervical junction anomalies is unclear. Considering this context, this study’s objective is to discuss the role of Atlas assimilation in the management of craniocervical junction anomalies, especially in Chiari malformation.

(Our note: craniocervical junction anomalies can be: a) Basilar invagination (the odontoid process of C2 has protruded through the foramen magnum and causing compression). b) a natural fusion of the atlas (C1) and occipital bone. c) Atlantoaxial subluxation or dislocation causing spinal cord compression. (Most people we see have a condition of chronic spinal cord compression).

What are we seeing in this image?

The Chamberlain line measurements that help evaluate their basilar invagination. When the odontid process of the C2 goes above this line (A Chamberlain line violation) a diagnosis of Basilar invagination can be offered.


The Chamberlain line measurements that help evaluate their basilar invagination. When the odontid process of the C2 goes above this line (A Chamberlain line violation) a diagnosis of Basilar invagination can be offered.

Returning to the July 2021 study we are exploring.

“Atlas assimilation is a proatlas segmentation anomaly that may be complete or incomplete. It may be totally asymptomatic or symptomatic as the result of transferred shifted forces onto the C1-2 joints, leading to clear instability (atlantoaxial dislocation) or mild C1-2 instability.”

Our note: The doctors of the study show the great complexity of Atlas assimilation. A congenital or acquired natural fusion is occuring. The fusion can be “in process” or complete. It can be demonstrated on an MRI but is maybe “totally asymptomatic or symptomatic.” It may lead to severe or mild C1-2 instability

“Cautious surgical planning may be required due to associated vertebral artery anomalies. Atlas assimilation with concomitant C2-C3 segmentation failure is highly associated with late C1-C2 instability.”

Our note: The doctors again note something that we see very often with a “natural” or “spontaneous” fusion in the neck and in people who had a spinal fusion surgery. C2-C3 segment failure is associated with fusion or late stage or advanced C1-C2 instability

“Craniocervical junction decompression failure was reported in patients with Chiari malformation and a low clivus canal angle (<130-135 degrees). (As many of you are aware the clivo-axial angle (CXA) measures the angle between the clivus, a bony part of the base of the skull, and the cervical spine. This is a measurement to help neurosurgeons try to document a diagnosis of craniocervical instability.)”

Patients with assimilated anterior C1 arches usually have evident Atlantoaxial dislocation. Chiari malformation patients with Atlas assimilation generally have type 1 BI (basilar invagination caused by craniocervical instability where the odontoid process invaginates inside foramen magnum or Type 2 Basilar Invagination (which is not associated with instability but with flattened base cranium) and are reported with higher rates of craniocervical junction instabilities when compared with those “pure” Chiari malformation.

Dynamic examinations may provide additional evidence of atlantoaxial instability. Although Atlas assimilation per se is not considered an unstable configuration, further and detailed evaluations of patients with Chiari malformation associated with Atlas assimilation are necessary. Some associated unstable configurations required concomitant craniocervical junction fixation.”

Finally, again this is something we see in many people. Atlas assimilation is or has occurred. The patient has many MRI images of the problem and their doctors are trying to decide the best route based on what they are seeing on the MRI. But there may be a need for additional evidence of atlantoaxial instability to plot a proper treatment course. A course that may not need a surgery and a course where surgery is recommended to fuse the area because of excessive craniocervical junction instabilities.

The problem of pre and post-surgical apnea

A November 2019 case history was presented in the journal Case Reports World Neurosurgery.(4) Here a team of neurosurgeons discussed non-surgical and surgical approaches to basilar invagination. The describe that non-invasive approaches for basilar invagination and moreover, cervical traction to reduce odontoid invagination, has not been thoroughly described in the literature. In their case presentation a 15-year-old boy with a 6-month history of progressive cervical myelopathy signs and symptoms had a pre-operative closed cervical traction followed by occipitocervical fusion planned. However, the patient developed 3 episodes of apnea on sleeping when on traction. Labeled as central hypoventilation, he was operated by foramen magnum decompression and occipitocervical fusion.

In this case the traction, which the doctors called an effective pre-operative treatment, caused issues of sleep apnea and that patients should be monitored closely for respiratory dysfunction.


1 Yang H, Zhong S, Hu Y, Bao Z. Rotational vertebral artery occlusion in a patient with basilar invagination. British journal of neurosurgery. 2019 Jan 10:1-3. [Google Scholar]
2 Donnally III CJ, Munakomi S, Varacallo M. Basilar Invagination. StatPearls [Internet]. 2020 Jan. [Google Scholar]
3 Joaquim AF, Barcelos AC, Daniel JW. The role of Atlas assimilation in the context of craniocervical junction anomalies. World Neurosurgery. 2021 May 20. [Google Scholar]
4 Mallepally AR, Karthik Y, Ansari N, Chhabra HS, Goel SA. Reversible Central Hypoventilation Syndrome in Basilar Invagination. World neurosurgery. 2019 Nov 1;131:120-5. [Google Scholar]


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