Atlantoaxial instability is a condition in which the first two cervical (neck) vertebrae are not firmly attached. Normally, the atlas (the first cervical vertebra) and the axis (the second cervical vertebra) are attached by a group of ligaments. They are further stabilized by a prominence on the axis called the dens that protrudes into a hole in the atlas.
Dogs with congenital atlantoaxial instability are born without ligament support to their atlantoaxial joint, and may also be born without a dens. Trauma to the neck can also cause tearing of the ligaments or fracture of the dens, resulting in atlantoaxial instability. This causes the spinal cord to compress and results in pain or even debilitation for the pet.
The disorder is uncommon in older dogs and larger breeds of dogs. It is generally found in smaller, toy breeds, specially Chihuahuas, Pomeranians, Pekingese, toy poodles, and Yorkshire terriers. Any dog, young or old and of any breed, is at risk for atlantoaxial instability after a traumatic event, such as being hit by a car or being an unrestrained passenger in a car accident.
Onset of symptoms can be gradual, or can be very sudden. Symptoms associated with this disorder include back and neck pain, frequent collapses or even paralysis. Severity of these symptoms will depend on severity of spinal cord injury.
