Atlantoaxial Luxation

Atlantoaxial instability is an uncommon disorder in dogs in which abnormal motion occurs in the neck between the atlas (the first cervical vertebra) and the axis (the second cervical vertebra). This excessive motion allows abnormal bending between these two bones and can compress the spinal cord. How severe the spinal cord injury becomes depends on both the amount of compression and how long the instability has been present.

Normally, the atlantoaxial joint is stabilized by a projection from the axis called the dens, which fits into the atlas, along with several ligaments connecting the two vertebrae. Instability arises from two general causes: trauma and congenital (birth) abnormalities. Traumatic instability occurs after forceful flexion of the head, leading to fracture of the dens or another part of the axis and/or tearing of stabilizing ligaments. This traumatic form can occur in any breed and at any age.

Certain congenital defects can allow instability to occur with only minimal trauma (for example, jumping off a couch or being jumped on by another dog). These defects include an absent or malformed dens, or inadequate normal attachments between the two vertebrae. The most common predisposing abnormality is a dens that is missing or abnormally small. This condition is primarily seen in small-breed dogs, most often reported in Yorkshire Terriers, Chihuahuas, Miniature or Toy Poodles, Pomeranians, and Pekingese. Dogs with congenital abnormalities typically develop clinical signs before one year of age.

Severity of signs can range widely. Symptoms may develop slowly over time or appear abruptly. Neck pain is the most frequent sign and may be the only one, though it can be intense. Dogs may also show varying degrees of incoordination and weakness or paralysis affecting the body from the neck downward. If all four limbs become completely paralyzed, the diaphragm can also be paralyzed, and the animal cannot breathe. These dogs often die suddenly before veterinary care can be provided. Some dogs may exhibit intermittent episodes of collapse.

Diagnosis is based on signalment (age and breed), medical history, clinical signs, and radiographs (x-rays).

• Radiographs may show dorsal displacement or “tipping” of the axis, along with increased spacing between the atlas and axis (Figures 1 and 2).
• The dens may be absent or small, or fractures may be visible.
• Certain radiographic views can better demonstrate changes, such as gentle neck flexion, angled views, or an open-mouth view. Extreme caution is required to avoid excessive flexion forces.
• A CT scan can also be helpful for assessing the vertebrae for additional deformities.

Radiographic interpretation may include findings such as an increased angle between the first and second vertebrae, bending of the spinal canal with resulting spinal cord compression, and absence of the dens with a rounded cranial edge where the dens would normally project.

Note the angulation between the first and second vertebrae. A dens is absent in this dog.

Treatment may be conservative or surgical. Conservative management is more commonly selected when signs are mild or surgery is not feasible due to other medical conditions. This approach involves strict confinement/cage rest and use of a neck brace for several weeks. Steroids and analgesics may also be administered, and the pet must be protected indefinitely from trauma. Complete recovery has been reported, even in dogs with severe signs, but some dogs may fail to improve or may worsen. With persistent instability, there is a risk of sudden spinal dislocation causing acute paralysis and death.

Potential complications of conservative care include:

• ongoing instability
• inadequate stabilization from the brace
• chronic spinal cord compression
• failure of dens or axis body fractures to heal
• bandage sores and corneal ulcers associated with the splint/neck brace (frequently observed)

Surgery is often recommended over conservative therapy due to the risk of recurrence and additional spinal cord injury. Surgical goals are to relieve spinal cord compression and provide permanent joint stabilization. Compression is typically alleviated by reducing the vertebrae to a more normal position. If the dens is malformed and deviated toward the spinal cord, it may occasionally need to be removed to resolve compression.

Stabilization can be achieved using multiple techniques, either from a dorsal (top) or ventral (bottom) approach. Most surgeons currently favor the ventral approach, because dorsal repairs typically do not achieve fusion of the two vertebrae, leaving long-term stability dependent on scar tissue and durability of the implants.

Ventral techniques are preferred because they allow removal of joint cartilage to promote fusion and placement of a bone graft to stimulate bone healing. This approach also supports proper alignment and reduction and permits dens removal when needed. Ventral methods include cross-pinning, transarticular screw fixation, combinations of pins or screws with bone cement, or bone plates (Figures 3a and 3b, 4a and 4b). Technique selection often depends on surgeon preference and the dog’s size.

Potential surgical complications include:

• death from sudden respiratory arrest caused by spinal cord injury during surgery
• failure of the repair with implant migration or breakage
• inadequate reduction or malalignment of the spine; incorrectly positioned implants can cause chronic pain or spinal cord impingement and may require removal. Improper placement can occur because there is limited bone for pin or screw purchase, and in small dogs the margin for avoiding the spinal canal is extremely narrow.

Figure 3b

Figures 4a & 4b: Postoperative radiographs showing stabilization using screws combined with bone cement, forming a bridge between exposed screw heads.

Figure 4b

After surgery, strict rest remains necessary for 6–8 weeks. A neck brace may be used for additional support. Follow-up radiographs are commonly obtained at 4 weeks and 8 weeks to evaluate the repair and bone healing.

Prognosis depends on how much spinal cord trauma has occurred and what neurologic deficits are already present. Prognosis is generally good for dogs with mild signs and more guarded for dogs with paralysis, although substantial recovery may still be possible if decompression and stabilization are achieved. Surgical outcomes are notably better in younger dogs (under 2 years of age), dogs with more acute disease (less than 10 months of symptoms), and dogs with less severe neurologic impairment.