AAI is congenital, but can be genetic or traumatic, genetic symptoms will appear within the first year, traumatic can appear at any age, from minor neck pain to total paralysis and death.
Conservative treatment with casts/braces may be tried in less severe cases, but many times reinjury may occur making surgery a better option.
I found this article from the ACVS (American College of Veterinary Surgeons)
ACVS - Atlantoaxial Instability
Here are some excerpts from the site:
Atlantoaxial instability is an uncommon condition of dogs in which there is abnormal movement or instability in the cervical spine or neck, between the atlas (first cervical vertebra) and axis (2nd vertebra). This instability allows abnormal bending between the two bones, which causes pressure or 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 fibrous attachments (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 fibrous attachments. 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.
Conservative Treatment
Treatment for atlantoaxial instability can be conservative or surgical. Conservative management is more likely to be recommended when clinical signs are mild, or surgical treatment is not possible due to other medical conditions of the animal. Patients are managed with strict cage rest (4-6 weeks) and a neck brace for 6 weeks. Steroids and pain medications may also be used, and the patient will need to be protected indefinitely from trauma. Stability relies long term on the formation of scar tissue. Complete recoveries have been reported, even for dogs initially with severe signs, but dogs can also either remain unchanged or deteriorate. One study on 16 dogs demonstrated 10/16 having a good long-term outcome (10). Dogs were more likely to have a good outcome with conservative treatment, when the duration of their clinical signs was less then 30 days, at the time of treatment, while dogs with chronic signs (>30 days) were less likely to have a successful outcome without surgery. (10) With continued instability, there is a risk for suddenly luxating the spine, with acute paralysis and death. Potential complications include continued instability, inadequate stabilization by the brace, chronic spinal cord compression, and failure to heal fractures of the dens or body of the Axis. Problems such as bandage sores and eye ulcers related to the splint/neck brace are frequently seen. (1,2,9-11)
Surgical Treatment
Surgery is usually recommended over conservative treatment because of the possibility of recurrence and further spinal cord damage. Goals of surgery are to relieve pressure on the spinal cord, and permanently stabilize the joint. The pressure is usually relieved by reducing the vertebrae into a normal position, without requiring any other decompression methods. If the dens is malformed and is deviated towards the cord, occasionally this will need to be removed to relieve the compression. In the past, the joint has been stabilized with many different techniques. It can be stabilized from the top (dorsal) or from the bottom (ventral). Most surgeons will now use a ventral technique, as approaches from the dorsal side do not usually result in fusion of the two bones, and long-term stability relies on scar tissue and lasting strength of the implants.
