Atlantoaxial instability can affect any breed or age of dog, but is most often seen in small and toy breed dogs. This is generally because of the common occurrence of a malformation of the dens, a bony projection of the axis which helps connect it to the atlas. This dens malformation can make the ligaments connecting the first two vertebrae fail, causing the dislocation, and consequently, the spinal compression. Trauma can exacerbate this failure, or can itself cause a fracture in the dens.

Compression of the spinal cord can occur when the first vertebrae in the spinal column, called the atlas, and the second vertebrae, called the axis, are dislocated from one another. This luxation is usually caused by trauma or birth defects, and can result in pain, weakness, and paralysis.

Symptoms of atlantoaxial luxation can be sudden or gradual, and can vary in degree of severity. Usually, symptoms are seen within the first two years because of the developmental problems associated with this condition. However, older dogs, particularly after a neck trauma, can develop symptoms later. These symptoms include:

There are two main causes of this condition in dogs:

Birth defects, like a malformed dens, are more common in small or toy breed dogs, and therefore these breeds are more susceptible to the luxation of the first two vertebrae. The most common breeds reported with the condition are:

When a diagnosis of atlantoaxial instability is suspected, factors such as the age and breed of your dog are taken into account. Let your veterinarian know if any trauma occurred before symptoms started. Though most common in younger, smaller breed dogs, this vertebrae instability can afflict other breeds and older dogs, so a complete medical history can be beneficial.

X-rays may be used to look at the connections and locations of the first two vertebrae to determine the presence of a dislocation. These must be done carefully so as to not cause any further misalignment. 

MRIs can be used to identify the presence of compression in the spinal cord, and to assess the alignment of the vertebrae. An MRI can also show soft tissue injuries, and can alert your veterinarian to problems such as meningoencephalitis, occipital malformation syndrome, and syringomyelia.

A CT scan can show fractures of the dens and displacement of the atlas. The severity of the condition will be dependent on the amount of pressure on the spinal cord and the duration of time your dog has had this condition.

There are two main treatments for atlantoaxial instability. 

Non-surgical management entails applying a splint, cast or brace to immobilize the neck and stabilize the spinal cord. A neck brace can use a splint, soft-padded bandage, or fiberglass, and is generally left on for 4 to 15 weeks to give the body time to grow new fibrous tissue around the atlanto-axial joint. It is important to keep your dog rested during this time. This is the preferred method for puppies, because they are still growing and forming their soft bones.

Immobilization has a 50% - 60% rate of success, with a condition reoccurrence rate at 40%. Complications can arise from applying a splint too tightly, so your dog must be closely monitored. These complications can include labored breathing, bluish skin due to poor circulation, skin infections, ulcers, ear infections, corneal ulcers, and even death. Checking the bandages periodically and changing them are recommended, especially if the bandage gets wet as it can tighten. There is a possibility of relapse once the splint is removed, wherein surgery may be recommended. 

Surgery is considered the best treatment for a long term recovery, and is recommended for dogs that are full grown, older than 8 months, or have severe symptoms, such as pain and difficulty walking. This procedure entails using surgical screws or pins and bone cement to fuse together the atlas and axis. Surgery has been reported to have a 90% success rate. If complications occur, they are generally severe, such as bone fracturing, spinal cord trauma, and bleeding.

For those who received successful surgery, recovery is good. Be watchful for any symptoms of complications, such as weakness of laryngeal muscles, or pain near the pin area signifying infection, migration or fracture of pins or bones. After surgery, some dogs have a hard time swallowing, but this should improve in 1-2 weeks.

If your dog has been fitted with a cast, splint or brace, constant monitoring and daily bandage checks are needed. Bandage changes can also be done at home. Your dog will need to be checked by the veterinarian once a week. Note any secondary conditions that need to be treated, such as ear or skin infections and difficulties in eating, drinking or breathing. Antibiotics and brace adjustments may be necessary. Be sure to keep your dog rested, in a crate if needed.