Atlantoaxial subluxation (AAS) is a condition in the upper neck where the first two vertebrae of the spine, the atlas (C1) and the axis (C2), become partially misaligned or unstable. This instability occurs at the joint between C1 and C2, which sits directly beneath the skull. Excessive movement between these bones can potentially lead to compression of the spinal cord or adjacent nerves.
Understanding the Anatomy and Mechanism
The upper neck’s unique anatomy allows for the head’s wide range of motion, but this mobility also creates susceptibility to instability. The atlas (C1) is a ring-shaped bone, while the axis (C2) has a vertical projection called the odontoid process, or dens. The dens acts as a pivot point for the atlas, enabling about 50% of the neck’s total rotation.
The joint’s stability is maintained by a complex network of ligaments, primarily the transverse ligament. This strong ligament holds the dens of the axis securely against the atlas. When this ligament or other supporting structures are damaged, stretched, or weakened, the C1 vertebra can shift abnormally relative to C2. This mechanical failure allows the bones to translate or rotate excessively, risking impingement upon the spinal cord.
Primary Causes and Risk Factors
Atlantoaxial subluxation is often a secondary complication of an underlying medical issue, though it can also result from acute injury.
Systemic Inflammatory Diseases
Systemic inflammatory diseases are a common cause, with Rheumatoid Arthritis (RA) being a frequent contributor. The chronic inflammation associated with RA can lead to the formation of an inflammatory tissue, or pannus, that erodes the bone and causes laxity of the transverse ligament. This process gradually weakens the joint’s restraints, leading to instability in up to 25% of patients with RA.
Congenital Anomalies
Congenital anomalies are another significant category of causes, particularly in children and young adults. Conditions such as Down Syndrome (Trisomy 21) are associated with generalized ligamentous laxity, making individuals more prone to AAS. Surveys indicate that between 10% and 25% of individuals with Down Syndrome have radiographic evidence of atlantoaxial instability. Other genetic conditions, including Morquio Syndrome and Marfan Syndrome, can also cause ligamentous weakness or malformation of the odontoid process, such as os odontoideum, which compromises joint integrity.
Trauma and Infection
Acute trauma is a major cause, typically involving severe hyperflexion, hyperextension, or a direct axial load to the head or neck. This type of injury can result in a fracture of the C1 or C2 vertebrae or a direct tear of the stabilizing ligaments, leading to immediate or delayed instability. Infections in the upper respiratory tract can also lead to temporary instability, sometimes known as Grisel Syndrome.
Recognising the Clinical Manifestations
The presentation of atlantoaxial subluxation can range from being completely asymptomatic to causing severe neurological deficits. Symptoms are generally separated into mechanical and neurological manifestations.
Mechanical Symptoms
Mechanical symptoms are often the earliest and most common complaints. These include vague neck pain that may radiate to the back of the head (occipital headache), limited range of motion, muscle stiffness, or a sensation of the head feeling heavy. Some individuals describe apprehension or a “clunking” noise with certain neck movements, particularly when flexing the head forward. These signs reflect the abnormal movement and strain on the muscles and joints compensating for the instability.
Neurological Symptoms
Neurological symptoms indicate that the misalignment is putting pressure on the spinal cord or nerve roots. These signs include numbness, tingling sensations (paresthesias), and progressive weakness in the arms and legs. Gait instability, characterized by an unsteady or clumsy walking pattern, is a sign of myelopathy or spinal cord dysfunction. In the most severe cases, significant spinal cord compression can lead to quadriplegia, respiratory distress, or sudden death.
Diagnostic Procedures and Treatment Approaches
Diagnosis of AAS begins with a physical examination and a review of the patient’s medical history, especially noting any underlying conditions or past trauma. Imaging studies are necessary to confirm the diagnosis and assess the degree of instability.
Diagnostic Imaging
Standard X-rays of the cervical spine are the first step, often including specialized flexion and extension views. These dynamic views are performed with the patient gently moving their neck to demonstrate the abnormal movement, or translation, between C1 and C2. The atlanto-dens interval (ADI) is used to quantify the distance between the C1 and C2 bones. An ADI greater than 3 mm in adults or 5 mm in children is generally considered a sign of instability. A Computed Tomography (CT) scan provides detailed bony images for detecting fractures or bony malformations. Magnetic Resonance Imaging (MRI) evaluates the soft tissues, allowing physicians to directly visualize the spinal cord and determine if it is being compressed or damaged by the subluxation.
Treatment Approaches
Treatment is guided by the severity of the subluxation and the presence of neurological symptoms. Mild or asymptomatic cases may be managed conservatively, involving cervical immobilization with a rigid collar or brace to rest the joint. Physical therapy may be used to strengthen the surrounding neck muscles for support. However, when instability is significant, symptoms are progressive, or there is evidence of spinal cord compression on imaging, surgical stabilization is the recommended approach. This typically involves a procedure called C1-C2 spinal fusion, where the two vertebrae are permanently joined using screws, rods, and bone grafts. The goal of the fusion is to eliminate the abnormal movement, stabilize the joint, and prevent further injury to the spinal cord.