Atlantoaxial Luxation

Atlantoaxial luxation typically affects young, small breed dogs such as the Yorkie, Toy Poodle, and Chihuahua but any breed or age can be affected. 

Clinical signs may include neck pain, abnormal head carriage, weakness in all four legs, or inability to use the legs. Atlantoaxial luxation occurs when there is instability between the first and second cervical vertebrae (bones of the neck). These two bones are normally firmly attached to each other with a series of ligaments. Diagnosis is based on radiographs that demonstrate the instability, however, MRI and CT are useful in identifying concurrent problems and in surgical planning. Surgery offers the greatest chance of long-term resolution of signs.

Note the gap between C1 and C2

 Cause

The atlas is the first bone in the neck (C1). The axis is the second bone in the neck (C2). The atlas is unique from the other bones of the neck in that it has no dorsal spinous process and two large lateral processes (wings). The axis has a process, called the dens, that extends cranially. The atlas and the axis are connected to each other by a series of ligaments. In several toy breeds, the dens is malformed. With minimal or no trauma, the ligaments can fail and instability between the atlas and axis ensues. This leads to spinal cord compression.

Clinical Signs

Clinical signs of atlantoaxial luxation may include neck pain, abnormal head positions (head tilt, holding the head down, etc), wobbliness of all four legs, weakness of all four legs, inability to stand, or inability to move the legs.

Diagnostics

Plain radiographs in a neutral and gently flexed positions can demonstrate instability in the C1-C2 joint. Myelography is contraindicated for several reasons: flexing the cervical spine excessively can exacerbate the compression, and myelography may cause seizures that can lead to serious complications. Computed tomography (CT) can show bony abnormalities such as fracture of the dens and cranial displacement of the atlas into the foramen magnum. Magnetic resonance imaging (MRI) is useful in identifying injuries to soft tissues and finding concurrent problems such as caudal occipital malformation syndrome, syringomyelia, and meningoencephalitis.

Treatment

Surgical stabilization of the C1-C2 joint is considered the best long-term treatment. Surgery involves fusing the atlas to the axis with surgical screws and bone cement. Surgery is successful about 90% of the time, however, when complications occur, they are typically severe. Nonsurgical management involving six to eight weeks of crate rest and placement of a body splint has about a 50% success rate. Unfortunately, recurrence rates are high (40%), and complications from the bandage are possible.

Prognosis

Prognosis is typically very good for patients that are treated successfully with surgery.

Pre-Treatment

Post-Treatment

Note the gap between C1 and C2

CT reconstruct of AA instability demonstrating malformation and dorsal deviation of dens

MRI demonstrating spinal cord changes