What is Atlantoaxial Instability?
The atlantoaxial joint, composed of the atlas (C1) and axis (C2), is the most flexible joint in the body. It serves as a bridge between the skull and the neck. This joint has many essential nerves and blood vessels passing through it. It has the dual function of both supporting the skull and providing a broad range of movement and flexibility while staying stable.
Sometimes, this joint can become unstable. This instability often happens at birth but in adults, it could be a result of a sudden injury or wear-and-tear over time. This flexibility and potential for instability make it a critical area in terms of health and functioning.
What Causes Atlantoaxial Instability?
The atlas, also known as C1, is a ring-shaped structure that doesn’t have a body part, similar to other vertebrae. Instead, it is made up of front and back arches connected by two side masses. The sides of the atlas are made up of upper and lower parts that join with specific parts of the skull and the second vertebra, also known as the axis.
Directly behind the front part of the atlas, you find the odontoid process of the axis. This arrangement allows for lots of bending, straightening, and turning at the area where the skull meets the neck. A complex system of ligaments keeps this area stable to prevent severe nerve damage. The most crucial ligament here is the cruciate ligament.
The cruciate ligament is made up of two parts: one that runs horizontally between the sides of the atlas and one that runs vertically from the base of the skull to the body of the axis. There are also two other ligaments that extend from the tip of the odontoid process to the large opening in the skull and the base of the skull. This location helps provide more stability.
Lastly, there are several other ligaments that help hold the atlas, the base of the skull, and the axis together to form a strong connection between the head and the neck. There are a few reasons why this connection may not be as stable as it should be, including from injury, birth defects like Down syndrome or skeletal abnormalities, and inflammation from conditions like rheumatoid arthritis.
Risk Factors and Frequency for Atlantoaxial Instability
Atlantoaxial instability, a condition involving the top two bones in your neck, is extremely rare in people who don’t have certain risk factors. However, it is quite common in some groups. For example, up to 30% of people with Down Syndrome (DS) can have this on their X-Rays, but only 1% may experience symptoms. People with Rheumatoid Arthritis (RA) also show more occurences of this condition, with rates between 25% and 80%. Advancements in RA medications might reduce the occurrence of Atlantoaxial instability in these patients. This condition can happen to anyone regardless of their age and gender. However, higher risks are associated with people who have Down Syndrome and older adults who have Rheumatoid Arthritis.
- Atlantoaxial instability is very rare in people without risk factors.
- Up to 30% of people with Down Syndrome (DS) may show this on X-Rays, but only 1% have symptoms.
- In Rheumatoid Arthritis (RA) sufferers, the condition is more common, with rates of 25% to 80%.
- Newer RA medicines may lower the occurrence of this condition in these patients.
- Atlantoaxial instability can affect anyone, regardless of age or gender.
- However, those at highest risk include people with Down Syndrome and elderly people with Rheumatoid Arthritis.
Signs and Symptoms of Atlantoaxial Instability
Atlantoaxial instability is a medical condition that often doesn’t show symptoms. However, when symptoms do occur, they range from mild to severe. Here are some possible signs:
- Neck pain
- Limited neck movements
- Rough or jagged movements
- Nerve issues that can affect the face, throat, or neck
- Difficulties with breathing
- A tear in the main artery supplying the spine
- Paralysis in all four limbs
- Even death in extreme cases
This condition is usually hard to detect when doctors are checking for acute neck injuries. It’s important for healthcare professionals to get a full overview of a patient’s health history. This should include information about any existing or past neck injuries, head trauma, or serious falls. This is particularly crucial for children, as any previous injury to the spine may not heal correctly. This can cause instability in the spine or neurological symptoms in later life.
Testing for Atlantoaxial Instability
To identify different forms of atlantoaxial instability (AAI), which is an abnormal movement between the first and second neck bones, doctors use a number of imaging techniques. These include special x-ray views and computed tomography (CT) scans. The patient’s head position during imaging also plays a significant role in classifying the AAI variants.
The criteria or measurements used to diagnose AAI include:
- A gap of more than 5 mm between the first two neck bones
- The first neck bone overlapping the second one
- Less than 13 mm space left for the spinal cord
- Displacement of the tip of the second bone by more than 4 mm
- Violation of the “rule of thirds”: one-third spinal cord, one-third second bone, and one-third safe space
AAI can be classified into different types, depending on different systems:
- The Greenberg system breaks it down into reducible and irreducible forms
- The Fielding and Hawkins system divides it into anterior, posterior, lateral, and rotational AAI
- The Wang classification system sorts AAI into instability, reducible dislocation, irreducible dislocation, and bony dislocations
Another classification is based on the alignment of joints in the neck (facets) during neutral head positioning:
- Type 1: the joint of the first neck bone is displaced in front of the second one
- Type 2: the joint of the first neck bone is behind the second one alongside rotatory AAI
- Type 3: Even when joints seem aligned, the instability is confirmed by certain clinical and imaging signs
Types 2 and 3 are referred to as chronic or ‘central’ or ‘axial’ AAI.
Treatment Options for Atlantoaxial Instability
Non-operative treatment for cervical issues include wearing a cervical halter for traction and doing exercises that involve moving the neck. After that, the patient is encouraged to move around while doing active exercises that move the neck.
Surgery may be required for adults when the atlantodental interval (ADI), a measurement in the neck area, exceeds 5 millimeters, or when there are signs of instability or significant displacement shown on dynamic films (a type of X-ray).
In children, surgery is indicated if they experience neurological deficits, have a persistent ADI exceeding 4 millimeters, have lasting deformity for more than three months, or have repeated deformity despite six weeks of immobilization.
If the patient is to undergo surgery, it often starts with brief skeletal traction under general anesthesia. Depending on the case, doctors may use the posterior or anterior approach. The posterior approach gives most success, but if it doesn’t work, they could opt for the anterior approach which includes transoral odontoidectomy, transoral anterior release, and transoral anterior reduction plate. These procedures have shown positive results in aligning the structure and improving neurological outcomes.
Various methods of Posterior Arthrodesis (a procedure that connects vertebrae in spine) are available. They include Gallie or Sonntag or Brook’s methods, that use wiring and bone grafts. However, these may require long periods of immobilization post-surgery and have high risk of complications. Another method involves using implants to secure the affected vertebrae, which have higher fusion rates and improved results. They are applicable to children as young as 1.7 years old. The screws used in the process are often placed accurately using guided methods, which help avoid injury to the vertebral artery.
Occipitocervical fusion, which involves connecting the base of the skull to the cervical spine, is used in cases of instability, basilar invagination (a condition where the top of the spine pushes into the base of the skull), an anomaly in the C1 vertebrae, and failed previous fusions. This does restrict head movements.
Long-term complications of the posterior fusion include graft subsidence, instability of the subaxial spine, and cervical lordosis, where the neck curves in an abnormal way. Subaxial kyphosis, a condition where the neck curves forward, usually corrects itself over time according to Toyama remodeling theory.
What else can Atlantoaxial Instability be?
When identifying potential conditions in a patient, three possibilities to consider might include:
- Torticollis (a condition where the neck is twisted and causes the head to tilt)
- Atlantoaxial rotary fixation (an issue with the two top vertebrae in the neck that causes abnormal rotation)
- Odontoid fractures without atlantoaxial dislocation (breaking of a small bone in the spine near the neck, without any displacement of neck bones)
What to expect with Atlantoaxial Instability
If patients with symptoms get treated, their outlook is generally positive. Early decompression and fusion, a medical procedure, can stop and even reverse cord dysfunctions. Nevertheless, some patients might still experience pain as a common and possibly disabling symptom. They may also experience certain neurological complications, which can include:
- Limited neck movements
- Signs of damage to the nerve cells in the brain and spinal cord
- Nerve damage in the lower part of the brain
- Difficulty breathing
- Damage to the artery in the vertebra
- Quadriplegia, or paralysis of all four limbs
- In rare cases, death
A person’s ability to hold their breath for less than 10 seconds or count to less than 10 in a single breath, are significant factors that can affect the effects of their illness and their likelihood of survival when dealing with congenital AAI. Surgical precision, such as accurate screw positioning and avoiding injury to the vertebral artery, can be improved by spinal navigation as compared to using free-hand or fluoroscopic-guided methods.
Possible Complications When Diagnosed with Atlantoaxial Instability
The greatest danger of atlantoaxial instability, which is a condition where the first and second bones in your neck become unstable, is the potential for nerve compression. This compression can cause a range of issues, including:
- Limited neck movement
- Signs of damage to the central nervous system like difficulty walking or maintaining balance
- Weakness or numbness in lower facial muscles
- Difficulty in breathing, sometimes leading to respiratory failure
- Dissection or tearing in one of the key arteries in the neck
- Paralysis in all four limbs, known as quadriplegia
- In severe cases, death
Specific surgeries to treat this condition also come with their own set of risks. For instance:
- An anterior approach, which is when the surgery is done from the front of the neck, can lead to complications like throat wound separation, leakage of cerebral spinal fluid, meningitis, or even the need for a tracheostomy which is a procedure to create an opening in your windpipe to help you breathe.
- A posterior approach, which is when the surgery is done from the back, can result in complications like bleeding from a network of veins in your neck, an injury to the vertebral artery, complications related to the surgical implants, instability of the spine below the level of the neck, the sinking or settling of a surgical graft, failure of a graft to join with existing bone, complications at the site of the graft.
Recovery from Atlantoaxial Instability
It’s suggested that before starting any kind of physical exercise, a comprehensive nerve system check-up is done to identify any existing problems. Attempting to exercise too early could lead to severe conditions like paraplegia or quadriplegia.
Preventing Atlantoaxial Instability
The guidance given to patients will depend on how serious the injury is and the symptoms they’re experiencing. If the injury is really serious, surgery may be necessary. In this case, the patient or their parents should be fully informed about all the possible aftereffects and risks that can come from the procedure.
Less serious injuries can often be treated with physical therapy. But for this to be effective, it’s really important that patients follow all the instructions and do the exercises as they’re told.
After surgery, patients need to take it easy and cut back on their activities. This is especially true when it comes to sports. High-impact sports might not be suitable for a lot of patients with a condition called atlantoaxial instability, even after they’ve had surgery to correct it. Decisions about what activities are suitable should be made on a case-by-case basis, depending on the exact cause of the condition and the treatment received.
