The neck is made of seven vertebral bones, or cervical vertebrae, which are connected by ligaments. Each vertebra fits precisely with its neighbor to form a joint. The spinal cord is located in a tunnel within the vertebrae, where it is protected from injury.
The first cervical vertebra is called the atlas; it holds up the head like Atlas held up the world. The second cervical vertebra is called the axis. The atlantoaxial joint is held in place by several ligaments and by a bony prominence of the axis called the dens, which fits in a hole in the atlas.
If the ligaments or the dens do not develop correctly or are injured, the atlantoaxial joint becomes unstable. This places the delicate spinal cord at risk for injury. Symptoms of spinal cord injury vary with the severity of the injury. Dogs with mild cases may only show neck pain. Those with more severe cases can have an unstable, wobbly, drunken gait. And those with very severe cases can be completely paralyzed in all four legs, or suddenly die. Death is often due to injury to the respiratory center in the spinal cord, making the animal stop breathing.
Congenital atlantoaxial instability is usually seen in toy breeds less than one year of age. Breeds most often seen for this problem include the Chihuahua, Pomeranian, Pekingese, toy poodle, and Yorkshire terrier.
Dogs with congenital atlantoaxial instability are born without proper ligament support of their atlantoaxial joint, or are born without a dens, or both. The instability that is present can predispose the dog to a major spinal cord injury with only a minor incident, such as jumping off a piece of furniture.
Atlantoaxial instability can also develop in any dog after a major traumatic event, such as getting hit by a car or being an unrestrained passenger in a car accident.
Diagnosis History. Your veterinarian will ask you many questions regarding the development and progression of the problem. Your pet's age is important to know, as well as any history of trauma to the pet. If other veterinarians have done any X-rays, then you should bring these results to your veterinarian's attention. If you have tried any treatments for this problem, it's helpful to tell your veterinarian about them and whether they have had any effect or not.
Physical exam. Your veterinarian will perform a complete physical exam. He/she will watch your pet as he walks (if he can walk) to identify any abnormalities in his gait. Then the veterinarian will perform a complete neurological examination to determine the severity of the injury and to localize the level of the spinal cord that is affected. He/she will also listen to your pet's heart and lungs, take his temperature, and palpate your pet's abdomen to be sure there are no other problems.
Blood and urine tests. If your pet will be undergoing anesthesia and surgery, blood and urine tests are run to be sure that he is not anemic and that he doesn't have any underlying liver or kidney problems that might make anesthesia risky.
Radiographs (X-rays). X-rays are taken of the neck to identify the abnormal positioning of the atlantoaxial joint, and possibly to identify the absence of a dens or any fractures of the bones. These X-rays are often taken under general anesthesia to obtain the best position of the neck to identify the structures of interest. The doctors and technicians must be very careful when moving the neck of these patients, since further damage can be done to the spinal cord if sudden, extreme movements are made.
The goal of conservative management is to allow scar tissue to form around the atlantoaxial joint to stabilize it. Motion of the joint prevents scar tissue from forming, so motion is prevented by placing a neck brace on the dog and by confining it to a cage for several weeks.
The injury to the spinal cord results in inflammation. This can be reduced by the short-term use of steroids.
Most cases managed in this way have a recurrence of symptoms, since the scar tissue is not strong enough to hold the joint stable.
Surgery is usually recommended for this problem due to the high recurrence rate with conservative management. The goal of surgery is to stabilize the joint using internal devices. These can include wires, pins combined with an epoxy-like cement, or heavy-duty suture material. The joint can also be fused, using screws and bone grafts collected from the dog's shoulder.
Surgery is very difficult due to the small size of the patient and delicate nature of the bones. After surgery, cage rest, neck braces, and steroids are often used as you would for conservative management.
There is less of a recurrence rate when surgery is performed, but recurrences can still happen due to failure of the implants placed in the bones or failure to follow post-operative instructions. The type of surgery depends on the surgeon's preference and the specific injury.
The prognosis depends on the severity of the neurological signs present before surgery. The more severe the signs, the more guarded the prognosis. The spinal cord is an unpredictable organ. Accurate predictions are difficult to make as to how much function will recover as well as how long it will take to recover.
Atlantoaxial instability is a serious problem that requires dedicated and observant owners. Optimal treatment for your pet requires a combination of home and professional veterinary care. Follow-up can be critical, especially if your pet does not rapidly improve. Administer all prescribed medications as directed. Alert your veterinarian if you are experiencing problems treating your pet.
Follow your veterinarian's instructions for exercise restriction. The tissues must heal strongly to allow the joint to become stable. Excessive activity can lead to tearing of healing tissues and cause a major setback.