Atlanto-occipital instability, also known as craniovertebral instability or atlantoaxial instability, is a condition where there is abnormal movement or excessive mobility between the atlas (C1) and the axis (C2) vertebrae in the upper cervical spine, or between the occipital bone and the atlas.

The atlanto-occipital joint is responsible for the flexion and extension of the head, and any instability in this region can lead to serious complications such as compression of the spinal cord, nerve damage, or even paralysis.

This condition can be congenital, meaning it is present at birth, or acquired due to trauma, infection, or other underlying medical conditions such as rheumatoid arthritis, Down syndrome, or connective tissue disorders.

Symptoms of atlanto-occipital instability may include neck pain, headaches, difficulty with balance and coordination, numbness or tingling in the arms or legs, and even breathing difficulties. Treatment depends on the underlying cause and severity of the instability and may include immobilization with a neck brace, physical therapy, medication, or surgery in more severe cases.