Basilar invagination and cervical spine instability

Ross Hauser, MD.

We do see patients whose 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 on the base of the skull. The closer the vertabrae gets to the base of the skull, the more brain stem and spinal cord compression occurs.

People with basilar invagination suffer from many symptoms

When someone first 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 your real-life challenges just as equally because patient history and patient quality of life is a test in itself.

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 blackout.

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 a 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 blackouts 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 gastroenterologist. 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 gastroenterologist 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. As 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 similar and familiar symptoms of dizziness, neck pain at the base of the skull, visual vertigo, VOR dysfunction, oscillopsia, tachycardia, and tinnitus. Their problems may have started with a car accident 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.

What are we seeing in this image?

The caption reads: Normal equilibrium requires input from three sensory systems: Vestibular (inner ear), visual (control of eye movement), and proprioception (actions of muscles, joints, and skin). If any of these give improper sensory input to the brain, body balance cannot be maintained and the person experiences imbalance or dizziness.


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 complaints can be caused, in some people, by cervical spine and neck instability. The same problems can lead to a diagnosis of basilar invagination.

Please refer back to these articles on our website:

Congenital and acquired causes of basilar invagination

The image above reads:

  • First described in 1790 it was felt from the 18th to the 21st century to be primarily from a congenital birth cause but has come full circle and is now believed to be from a mechanical etiology.
    • In a June 2021 update of the online publication STATPearls at the National Library of Medicine (1), doctors write: “Basilar invagination is an abnormality at the craniovertebral junction, either congenital or degenerative, resulting in the odontoid (the “peg” of the axis or second cervical vertebrae) prolapsing (points up) into the already limited space of the foramen magnum. While frequently seen in rheumatoid arthritis, it more commonly presents in a myriad of congenital conditions (i.e., Chiari malformation, syringomyelia, Klippel-Feil syndrome, and hydrocephalus).”
  • The basilar impression is the acquired form of basilar invagination but the neurological manifestations are the same.
    • In the above illustration the acquired forms are listed as:
      • Car accidents
      • Facedown lifestyle (text neck).
      • Hyperparathyroidism
      • Ricketts
      • Paget’s disease

In the image below:

Potential life-threatening consequences of Basilar Invagination

  • Obstruction of Cerebrospinal Fluid
  • Reduction of posterior cranial fossa volume
    • Reduction of posterior cranial fossa volume can be responsible for tonsillar herniation through the foramen magnum in patients with Chiari I malformation.
  • Compromised spinal cord nerve
  • Impaired Carotid and vertebrobasilar blood supply
  • Traction on the brain stem
  • Occipital assimilation of the atlas

In the video below is a case discussion.

Ross Hauser, MD demonstrates 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.

Blurry vision, eye pain, eye pressure, light sensitivity and other vision problems, along with symptoms above among the more troubling and disabling symptoms that are often due to cervical spine instability. An summary and explanatory notes of this video can be found here at: Blurry vision, light sensitivity, brain fog, increased ocular pressure and cervical Instability.

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 causing 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

  • At 3:30 the vein is represented by a balloon. When the balloon is stretched to 9 inches it has more space for the blood to flow than when it is stretched to 11 inches. A narrowing occurs. The more stretching of the vein the less space is contained within the vein for the blood flow and for drainage of fluids that need to be moved out of the brain.

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

  • At 4:15 It is common for patients to come in with brain fog there’s like fatigue that can’t get rid of. A possible cause of this is that normal cerebrospinal fluid flow is backed up. This causes the fluid to move slowly and take on a sludge-like consistency.

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 carotid 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 the 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 (2) describes the case of a 30-year-old female with Basilar invagination who developed recurrent ischemic stroke in the posterior circulation. What made this case curious, besides the 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 blackouts.”

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.

  • Brief explanatory note: pontine describes the pons, a portion of the brain stem where important nerves find their origin. The major nerve, the trigeminal nerve provides sensation in the face, helps control the muscles responsible for chewing and swallowing.
  • Another nerve that originates in the pons is the sixth cranial nerve (CN VI) which controls eye movements
  • The pons is also associated with disrupted sleep patterns and disruptions in natural breathing patterns.

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 the 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 a contralateral head turn. This case highlighted the necessity 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 (1) 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.”

Occipitocervical fusion – “fusion can cause significant morbidity”

Here is a November 2021 study in the European Spine Journal (3). In this paper, the researchers discussed: “Complications of occipitocervical fixation”

“Occipitocervical fusion is necessary for many pathologies of the craniocervical junction. The anatomy of the region is unique, and fusion can cause significant morbidity (suffering). This retrospective review aims to investigate the complication rates and outcomes of occipitocervical fixation.”

How the study was performed:

  • 128 patients with occipitocervical fixation operated between 1994 and 2020. The average follow-up is 63 months.
  • The indications of occipitocervical fixation were:
    • basilar invagination (53 patients; 41.4%),
    • trauma (25 patients; 19.5%),
    • tumor (23 patients; 18%),
    • instability due to rheumatoid arthritis (13 patients; 10.2%),
    • cervical deformity (7 patients; 5.5%) and
    • os odontoideum (7 patients; 5.5%).
  • There were six early postoperative (1st month) deaths.
  • (The researchers) observed complications in 67 patients (52%).
    • The most common complication was implant-related (32%),
    • followed by wound problems (23.4%),
    • systemic and other complications (11.7%),
    • neurologic complications (6.2%).
  • Implants are removed in 31 patients (24%) for different reasons:
    • deep wound infection (7 patients),
    • local pain and restriction of head movements (21 patients),
    • respiratory distress and swallowing problems (2 patients),
    • screw fracture and local pain (1 patient).


“Occipitocervical fixation has quite a large number of complications and significantly restricts head movements. With the advent of our biomechanical concepts, indications should be limited, and shorter cervical fixations should be preferred.”

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 history 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 (4) offers a summary of treatment and surgical options surrounding Atlas assimilation, atlantoaxial dislocation, Chiari malformation, and basilar invagination.

“Atlas assimilation may be associated with an 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 caused 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 help evaluate their basilar invagination. When the odontoid 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 occurring. 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 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 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. (5) Here a team of neurosurgeons discussed non-surgical and surgical approaches to basilar invagination. They 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 on 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 patients should be monitored closely for respiratory dysfunction.


Vertebrobasilar insufficiency (VBI) is poor blood flow to the back portion of the brain, or decreased circulation to the brainstem. This decreased circulation can be caused by compression on the vertebral and basilar arteries which supply blood to many vessels in in the back of the brain. This includes the basilar artery which is joined to the brainstem. The intracranial part of the
vertebral arteries and their branches which supply the medulla with blood. The vertebral arteries then join the basilar artery and the pons receives its blood supply from those and their branches.

The consequences of undiagnosed Vertebrobasilar insufficiency can be severe and we have discussed above. Occlusions of these arteries cause stroke and vertebrobasilar insufficiency can lead to an intermittently oxygen starved brain (transient ischemia). Symptoms of Vertebrobasilar insufficiency include dizziness, headaches, swallowing difficulties, vomiting, blindness, double
vision, dilated eyes, drooping eyelids, ataxia, weakness, and imbalance of legs.  Drop attacks (sudden fall due to loss of proper limb function), mental confusion, paresthesia, and tinnitus are also reported.

More than 60% of Vertebrobasilar insufficiency patients experience dizziness. Most patients experience multiple symptoms at once for minutes at a time due to transient ischemia. VBI should be suspected if positional dizziness is accompanied by neurological symptoms. Rotational occlusion of the vertebral artery may induce transitory ischemia. Sudden occlusions of the vertebral and basilar arteries can cause ischemic damage and severe clinical deficits. Instability at the craniocervical junction could cause vertebrobasilar insufficiency by causing these occlusions. Symptoms of Vertebrobasilar insufficiency were reproducible with rotational head movement; showing that rotational vertebral occlusion is an important cause.

Vertebrobasilar insufficiency usually occurs in the presence of atherosclerosis or cervical spondylosis, but symptoms can also arise when there is intermittent vertebral artery occlusion induced by extreme rotation or extension of the head. This mechanical compression of the vertebral arteries can occur along with other anomalies, including cervical osteophytes (bone spurs), fibrous bands, and osseous prominences (bone overgrowths). These anomalies are seen in many cases of vertebral artery injury after cervical manipulation, as reported in a recent review.



1 Donnally III CJ, Munakomi S, Varacallo M. Basilar Invagination. StatPearls [Internet]. 2020 Jan. [Google Scholar]
2 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]
3 Zileli M, Akıntürk N. Complications of occipitocervical fixation: retrospective review of 128 patients with 5-year mean follow-up. European Spine Journal. 2021 Nov 2:1-6. [Google Scholar]
4 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]
5 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]

This page was updated December 1, 2021


A November 2022 paper in the journal Spine (6) analyzed cervical spine lateral radiographs of 87 Basilar Invagination patients and 98 asymptomatic subjects. They were analyzed for cranial, cervical spine, and thoracic inlet parameters. The thoracic inlet is intersection between the neck and the chest. Many nerves go through this intersection. Here is the results of the study: “A significant difference was observed between Basilar Invagination patients and asymptomatic subjects. Basilar Invagination patients have craniums tilted forward and downward, smaller upper cervical lordosis, larger lower cervical lordosis, and smaller thoracic inlet angle. In Basilar Invagination patients, the SVA C2-C7 is an important parameter in cervical sagittal alignment. In both individuals with congenital anomalies of the craniovertebral junction and the asymptomatic population, cervical spine alignment is significantly associated with cranial alignment, particularly thoracic inlet alignment.”

Below is a visual demonstration of these angles. Spino-cranial angle (SCA)

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