Atlantoaxial Instability

Atlantoaxial instability is a relatively rare condition in dogs characterized by abnormal motion of the cervical spine between the atlas (the first cervical vertebra) and the axis (the second cervical vertebra). This instability permits excessive flexion between these two vertebrae, resulting in compression of the spinal cord. The extent of spinal cord injury depends on both the degree of compression and the length of time the condition has been present.

Under normal circumstances, the atlantoaxial joint is stabilized by a bony projection arising from the axis known as the dens, which fits securely into the atlas, along with several supporting ligaments connecting the two vertebrae. Instability of this joint arises from two primary causes: trauma and congenital abnormalities. Traumatic atlantoaxial instability typically follows forceful flexion of the neck and may result in fracture of the dens or other portions of the axis, as well as rupture of the stabilizing ligaments. This traumatic form may occur in dogs of any breed and at any age.

Congenital abnormalities can predispose dogs to atlantoaxial instability even with minimal trauma, such as jumping off furniture or being bumped by another dog. These defects include absence or malformation of the dens, or failure of normal ligamentous attachments between the atlas and axis. Absence or hypoplasia of the dens is the most common predisposing abnormality. This condition is observed predominantly in small-breed dogs, with Yorkshire Terriers, Chihuahuas, Miniature or Toy Poodles, Pomeranians, and Pekingese being the most frequently affected. Dogs with congenital forms typically exhibit clinical signs before one year of age.

Clinical signs vary widely in severity, and onset may be gradual or sudden. Neck pain is the most frequently observed sign and may be the only clinical manifestation, although the pain can be severe. Affected dogs may show varying degrees of ataxia, weakness, or paralysis extending from the neck downward.

In cases of complete paralysis of all four limbs, paralysis of the diaphragm may also occur, resulting in respiratory failure. These animals often die suddenly before veterinary care can be provided. Some dogs may exhibit episodes of intermittent collapse.

Diagnosis is based on a combination of signalment (breed and age), medical history, clinical signs, and radiographic examination.

Radiographs may reveal dorsal displacement or tilting of the axis, along with widening of the space between the atlas and axis (Figures 1 and 2). The dens may appear absent, abnormally small, or fractured. Specific radiographic views may be used to better demonstrate these abnormalities, including gentle flexion of the neck, angled projections, or open-mouth views. Extreme caution must be exercised during positioning to avoid excessive neck flexion.

CT scanning can also be a useful diagnostic tool, allowing detailed evaluation of the vertebrae for additional structural abnormalities.

Radiographic findings may include an increased angle between the first and second cervical vertebrae, bending of the vertebral canal, and resulting compression of the spinal cord. In some dogs, the dens is absent, with a rounded cranial margin where it would normally project.

Management of atlantoaxial instability may be either conservative or surgical. Conservative treatment is generally considered when clinical signs are mild or when surgery is contraindicated due to other medical conditions. Conservative management includes strict confinement and immobilization of the neck using a brace or splint for several weeks. Corticosteroids and analgesic medications may be administered, and lifelong protection from trauma is required.

Although complete recovery has been reported even in dogs with severe neurological deficits, some dogs may show no improvement or may worsen. Persistent instability carries a risk of sudden spinal dislocation, potentially leading to acute paralysis and death. Possible complications of conservative treatment include:

• Ongoing instability
• Inadequate immobilization with the neck brace
• Chronic compression of the spinal cord
• Failure of fractures of the dens or axis to heal
• Pressure sores and corneal ulcers associated with prolonged brace or splint use

Due to the risk of recurrence and progressive spinal cord injury, surgical intervention is generally preferred. The objectives of surgery are to decompress the spinal cord and provide permanent stabilization of the atlantoaxial joint. Decompression is typically achieved by realigning the vertebrae into their normal anatomical position. If the dens is malformed and deviated toward the spinal cord, surgical removal may be required to alleviate compression.

Stabilization of the joint can be achieved using a variety of techniques, either through a dorsal or ventral approach. Currently, ventral approaches are favored because dorsal techniques often fail to achieve bony fusion, relying instead on scar tissue and implant strength for long-term stability.

Ventral techniques allow removal of articular cartilage to promote fusion, placement of bone graft to stimulate bone healing, proper reduction and alignment, and removal of the dens when necessary. Common ventral stabilization methods include cross-pinning, transarticular screws, combinations of pins or screws with bone cement, or bone plate fixation (Figures 3a, 3b, 4a, and 4b). The choice of technique depends on surgeon preference and the size of the dog.

Potential surgical complications include:

• Death due to acute respiratory arrest resulting from spinal cord injury during surgery
• Failure of the stabilization construct due to implant migration or breakage
• Inadequate reduction or malalignment of the cervical spine, which may result in chronic pain or spinal cord compression and necessitate implant removal
  Improper implant placement is a recognized risk, particularly in small dogs, due to limited bone size and the close proximity of the spinal canal.

Following surgery, strict activity restriction is required for an additional 6 to 8 weeks. Neck braces may be used to provide supplemental support. Follow-up radiographs are commonly obtained at approximately 4 and 8 weeks postoperatively to assess implant stability and progression of healing.

Prognosis for dogs with atlantoaxial instability depends largely on the severity of spinal cord injury and the degree of neurological dysfunction present at diagnosis. Prognosis is generally good for dogs with mild clinical signs and guarded for dogs presenting with paralysis; however, substantial recovery is still possible when timely decompression and stabilization are achieved. Higher success rates are observed in younger dogs (less than 2 years of age), dogs with a shorter duration of clinical signs (less than 10 months), and dogs with milder neurological deficits.