​Atlantoaxial instability (AAI) is a neck (cervical) spinal condition where the first two neck vertebrae — atlas (C1) and axis (C2) — become unstable or misaligned.

 

This instability allows the dens (odontoid process) of C2 to press on the spinal cord, causing neck pain, weakness, or paralysis.

 

It’s most common in toy and small breeds (Yorkshire Terriers, Chihuahuas, Pomeranians, Toy Poodles) and usually results from a congenital (born-with) defect in bone or ligament structure.

 

It can also occur after trauma in any breed. Surgery is the best option for most dogs with neurologic deficits.

 

Medical management may be possible for mild or high-risk cases but carries a greater chance of relapse.