The atlas and axis are the first and second cervical vertebrae, respectively (i.e. just below the skull). The axis has an anatomical peculiarity: it has a bony projection called "dens of the axis." This 'tooth' articulates/meshes with the atlas, and if it is unstable may result in subluxation or dislocation of the joint between the atlas and the axis causing a compression of, and damage to, the spinal cord.

The two main causes of atlanto-axial instability are: 1) congenital; i.e. present at birth, caused by a malformation of the tooth during gestation, and: 2) due to trauma to the upper spine.

Which breeds are affected?

It affects mainly miniature dog breeds; usually these are under 2 years of age when signs are noticed, unless later trauma is involved:

  • Yorkshire Terrier,
  • Pomeranian,
  • miniature poodle,
  • Chihuahua,
  • Pekingese.

(Atlanto-axial instability should not be confused with 'Wobbler's syndrome', which generally affects large breed dogs and is caused by instability further down the cervical spine.)

What are the clinical signs?

Clinical signs can appear acutely, or of gradual onset. The most frequently encountered symptoms are pain, paresis (weakness or partial loss of voluntary movement), and ataxia (neurological signs consisting of lack of voluntary coordination of muscle movements, which may include gait abnormalities) on all four limbs.

How is an accurate diagnosis obtained?

In most cases, X-rays of the neck with or without an injection of contrast medium (in neutral, flexed and hyper-extended positions) are used to highlight the instability. Advanced imaging tests such as CT and MRI will allow better visualization of bony structures and the spinal cord itself, to assess damage.

What is the treatment?

Medical treatment with the administration of painkillers and anti-inflammatory drugs is usually only palliative, not curative. It also is very important to give the animal strict rest. A rigid bandage around his neck, that has the effect of minimizing movement of the head and neck, can also be utilized.

Surgical treatment is recommended when the pet shows signs of any advanced neurological symptoms, when the pain does not respond to medical treatment, and/or when continued deterioration of the neurological condition is evident. The surgery is major, and consists of spinal fusion, in order to stabilize the atlas and axis. Several techniques are possible, and this procedure is usually performed by a specialist orthopedist or neurosurgeon.

What is the prognosis?

The prognosis is considered to be good with successful early surgical treatment. The more time that elapses between the onset of clinical signs and surgery, the more likely there is to be permanent damage to the spinal cord. Also, delay is likely to extend post-operative recovery time, and permit a potentially worse future outlook.

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