Atlantoaxial subluxation is a misalignment between the first two vertebrae in the neck: the atlas (C1) and the axis (C2). These bones form the atlantoaxial joint at the top of the spinal column, which is responsible for most of the head’s rotation. The atlas is a ring-shaped bone that the skull rests on, while the axis below it has a bony projection called the dens that fits into the atlas’s ring.
The joint’s stability depends on a network of ligaments connecting the two bones. When these ligaments are compromised, excessive movement can occur, leading to the misalignment known as subluxation. This instability can range from minor to severe. Due to the joint’s proximity to the brainstem and spinal cord, significant misalignment can have serious consequences.
Causes and Associated Conditions
Instability of the atlantoaxial joint stems from traumatic, inflammatory, or congenital causes. Traumatic injuries from car accidents, falls, or sports can disrupt the ligaments holding the C1 and C2 vertebrae in place by subjecting the neck to hyperflexion or hyperextension.
Inflammatory conditions like rheumatoid arthritis can also lead to instability, as persistent inflammation can erode the ligaments and bone of the C1-C2 joint. A rarer cause, seen in children, is Grisel’s syndrome, where subluxation occurs as a complication of infections in the head or neck.
Congenital conditions can also predispose individuals to this subluxation. Down syndrome often involves generalized ligamentous laxity, making the C1-C2 joint less stable. Genetic disorders affecting connective tissues, such as Ehlers-Danlos syndrome and Morquio syndrome, can also result in weakened ligaments. Structural bone abnormalities present from birth, like os odontoideum, create a point of weakness.
Recognizing the Symptoms
Symptoms of atlantoaxial subluxation often include persistent neck pain, stiffness, and headaches at the base of the skull. Individuals may also experience a noticeably limited ability to rotate their head.
If the misalignment compresses the spinal cord, a condition known as myelopathy, a range of neurological symptoms can appear. These signs indicate a more serious level of instability and require prompt medical evaluation.
- Numbness, tingling, or weakness in the arms and legs
- A decline in fine motor skills, making tasks like buttoning a shirt or writing difficult
- Changes to gait and balance, a condition called ataxia, leading to unsteady walking
- Loss of bowel or bladder control in advanced cases
Instability can also affect the vertebral arteries that pass through the C1-C2 complex. If these arteries are pinched during movement, it can cause vertebrobasilar insufficiency, leading to symptoms such as dizziness, vertigo, or fainting spells.
The Diagnostic Process
Diagnosing atlantoaxial subluxation involves a physical and neurological exam. A physician will assess the neck’s range of motion, check for tenderness, and perform a neurological evaluation to test reflexes, muscle strength, and sensation in the limbs.
Imaging studies are used to visualize the misalignment and assess its severity. The process begins with X-rays of the cervical spine. A specialized set of flexion-extension X-rays, taken while the patient carefully bends their neck, allows doctors to observe any abnormal movement between the C1 and C2 vertebrae. The atlantodental interval (ADI), the distance between the atlas and the dens, is measured from these images.
A computed tomography (CT) scan provides detailed images of the bones, making it excellent for identifying fractures or congenital abnormalities like os odontoideum. A magnetic resonance imaging (MRI) scan is used to assess soft tissues. An MRI can show ligament damage and visualize the spinal cord itself, allowing doctors to see if there is any compression, swelling, or injury from the instability.
Management and Treatment Options
The treatment for atlantoaxial subluxation is determined by the instability’s severity and the presence of neurological symptoms. For individuals with mild subluxation and no spinal cord compression, a conservative, non-surgical approach is used to prevent further movement and allow the area to stabilize.
Conservative treatment involves immobilizing the neck with a rigid cervical collar or, in more significant cases, a halo vest. Patients are advised to modify their activities to avoid stress on the upper neck, such as contact sports or high-impact exercise. Once the joint is considered stable, physical therapy may be recommended to help strengthen the surrounding neck muscles.
Surgical intervention is reserved for cases with significant instability, evidence of spinal cord compression, or progressive neurological deficits. The goals of surgery are to relieve any pressure on the spinal cord and to permanently stabilize the joint to prevent future injury.
The most common surgical procedure is a posterior C1-C2 spinal fusion. In this operation, a surgeon uses screws and rods to fasten the atlas and axis vertebrae together, effectively fusing them into a single, solid bone structure. This permanently eliminates movement at the atlantoaxial joint. While this procedure provides stability, it results in a loss of rotational movement in the upper neck.