Atlantoaxial instability is an uncommon condition of dogs in which there is abnormal movement in the neck, between the atlas (first cervical vertebra) and axis (second vertebra). This instability allows abnormal bending between the two bones, which causes compression of the spinal cord. The severity of the injury to the spinal cord depends both on the amount of pressure, and the duration of the condition.
The atlantoaxial joint is normally stabilized by a projection off the axis called the dens, which fits into the atlas, as well as several ligaments between the two bones. There are two causes for the instability, trauma and birth abnormalities. Traumatic instability occurs after forceful flexion of the head, causing either fracture of the dens or other part of the axis, and/or tearing of the ligaments. This form of the instability can occur in any breed, at any age.
Certain birth defects can predispose this instability to occur with a very small amount of trauma (i.e. jumping off the couch, being jumped on by another dog). These abnormalities include a missing or malformed dens, or lack of normal attachments between the two vertebrae. An absent or smaller dens is the most common predisposing cause. This condition is seen mainly in small breed dogs, with Yorkshire terriers, Chihuahuas, Miniature or Toy Poodles, Pomeranians and Pekingese being the most frequently reported breeds. Dogs with congenital abnormalities usually show clinical signs at less than one year of age.
Symptoms can vary in severity. Onset of symptoms can be gradual, or can be very sudden. Neck pain is most common. This may be the only symptom; however, the degree of pain can be severe. Varying degrees of incoordination and weakness or paralysis from the neck down. With complete paralysis of all four limbs, paralysis of the diaphragm also occurs, and the animal is unable to breathe. Usually these dogs die very suddenly before they can receive medical attention. Some dogs may demonstrate intermittent collapse.
Diagnosis is based on signalment (breed and age), history, symptoms, and radiographs (x-rays).
- Dorsal deviation or tipping of the axis can be seen on radiographs, with increased space between the atlas and axis (Figures 1 and 2).
- The dens may be small or absent, or visible fractures may be present.
- Certain views may be used to highlight these changes, such as gently flexing the neck, or taking radiographs from an angle or with an open mouth view. Extreme care must be taken to avoid excessive flexion forces.
- A CT scan can also be valuable to evaluate the vertebrae for other possible deformities.
Strict rest is still required for 6–8 weeks after surgery. Neck braces for additional support may be used. Repeat radiographs (x-rays) will often be repeated in 4 and 8 weeks to assess the repair, and healing.
The prognosis for atlantoaxial instability depends on the degree of spinal cord trauma and neurological deficits already present. Good for dogs with mild clinical signs and guarded for dogs with paralysis, but significant recovery is possible, if decompressed and stabilized. Significantly greater success rates with surgery are seen in younger dogs (<2 years of age), dogs with more acute problems (<10 months of symptoms), and dogs with less severe neurological problems.