Atlantoaxial instability

Congenital atlantoaxial subluxation usually causes problems in immature animals, although signs can develop at any age. It is seen most often in small breeds of dog. Instability or subluxation are usually the result of a malformed or absent dens. The dens is a small, finger-like projection from the front of the second vertebra in the neck (Figure 24-1). A normal dens has strong ligaments that anchor the first and second vertebrae together; these ligaments may be weak or absent if the dens is malformed.

Figure 24-1: 3D reconstruction of CT scans; the dens is indicated by an asterix (*). A: A normal toy breed dog. B: A toy breed dog with atlantoaxial subluxation and a malformed dens, which occupies much of the vertebral canal. X-rays of these same dogs are shown in Figure 24.2.

Neck pain and sometimes weakness or even paralysis in all four legs are the most common clinical signs. Other conditions may coexist with atlantoaxial subluxation, such as hydrocephalus (which is an abnormal accumulation of fluid within the brain) or hepatic encephalopathy and see Schulz et. al., below.

Figure 24-2: X-rays from a one-year-old toy breed dog with congenital atlantoaxial instability. The X-ray on the left (A) is taken with the dogs neck in a normal, neutral position and the X-ray on the right shows how the first and second vertebrae separate abnormally when the dog’s neck is flexed (B). Note the marked increase in the gap between the first and second cervical vertebrae, shown by the distance between the arrowheads.

The diagnosis is usually made by taking X-rays of the neck to show a change in distance between the first and second vertebrae (Figure 2). A CT scan (Figure 1) is very useful for surgical planning and MRI is also very helpful, particularly as it is the only way to reveal syringohydromyelia.

Non-surgical treatment mainly comprises cage rest and the application of a neck brace (which can be actually be very challenging to apply to a toy breed dogs). Nevertheless, this can provide a very useful alternative for very young dogs, for dogs that cannot walk, or for other high-risk patients. It can be used as a temporary measure prior to surgery or it may work long-term. Surgical management is still generally recommended for most dogs with congenital lesions once the dog is large enough. Non-surgical treatment often gives excellent long-term outcomes for animals that fracture a normal atlantoaxial articulation.

Surgical treatment is indicated in most toy breed dogs with congenital lesions. Even dogs with profound neurological deficits are likely to benefit from stabilization. Ventral fusion is the treatment of choice using multiple implants and bone cement.

Figure 24-3: Postoperative reconstruction of a CT scan done following pins and cement fixation in a 1.5 kg Yorkshire terrier.

The prognosis for dogs with congenital lesions is good if the animal survives the perioperative period. The best predictor of a successful outcome is when the onset of signs is prior to two years of age. The final outcome also tends to be better if signs have been present for less than 10 months, if the dog can still walk, and if the reduction after surgery is good (Beaver et. al., below). Nevertheless, this is a serious condition and even using the most successful technique of multiple ventral implants and bone cement the failure rate has been reported to be 12%, the mortality rate 8% and 4% of dogs need a second surgery.

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