Atlantoaxial instability

What it is

Atlantoaxial instability (atlantoaxial subluxation) is excessive flexion between the first two cervical vertebrae. When the joint is unstable, the cranial part of the axis rotates dorsally into the vertebral canal of the atlas and compresses the spinal cord. In most dogs it is congenital: the dens of the axis is hypoplastic, absent or not fused, or the ligaments that hold it are missing. A minor trauma, such as a jump or rough play, often triggers the first signs. In dogs of any breed it can also follow trauma that fractures the dens or tears the ligaments.

Who it affects

Young toy and small breeds, typically before 2 years of age: Yorkshire terrier, Chihuahua, toy poodle, Pomeranian, Maltese, Pekingese.

Anatomy involved

The atlas (C1) has no vertebral body and no real spinous process; its dorsal arch lies over the cord. The axis (C2) carries a long spinous process and, cranially, the dens (dens axis), which projects into the ventral part of the vertebral foramen of the atlas. The dens is held against the floor of the atlas by the transverse ligament of the atlas and is tied to the occipital bone by the apical and alar ligaments. A dorsal atlantoaxial ligament joins the dorsal arch of the atlas to the spinous process of the axis. Without a normal dens or these ligaments, flexion of the head lets the axis tilt up and the dens or the body of the axis presses on the cord.

Clinical signs

Neck pain, low head carriage and signs of a cranial cervical (C1–C5) lesion: ataxia and paresis of all four limbs, from mild to tetraplegia, with upper motor neuron reflexes. Signs may be acute or progressive and episodic. Severe compression can cause respiratory paralysis and sudden death.

Diagnosis

• Radiographs: a lateral view shows an increased space between the dorsal arch of the atlas and the spinous process of the axis, and the ventrodorsal or open-mouth view may show an abnormal dens. Flexed views are dangerous and should be avoided or taken with great care.
• CT shows the dens and the bony anatomy; MRI shows the cord compression and concurrent malformations of the craniocervical junction.
• Handle the neck carefully during examination and anaesthesia.

Treatment

Conservative: a rigid neck splint for several weeks with strict rest and analgesia; it can work in young dogs with a short history and mild signs, but relapses occur. Surgical stabilisation is preferred in most cases: ventral fixation (transarticular screws or pins, often with bone cement, plus arthrodesis of the atlantoaxial joints) or dorsal techniques. The surgery is technically demanding in very small dogs and carries a real perioperative risk, but most dogs that recover from it do well.