Atlantoaxial instability is an uncommon condition of dogs in which there is abnormal movement or instability in the cervical spine or neck, between the atlas (first cervical vertebra) and axis (2nd vertebra). This instability allows abnormal bending between the two bones, which causes pressure or compression of the spinal cord. The severity of the injury to the spinal cord depends both on the amount of pressure, and the duration of the condition.
The atlantoaxial joint is normally stabilized by a projection off the axis called the dens, which fits into the atlas, as well as several fibrous attachments (ligaments) between the two bones. There are two causes for the instability – trauma and birth abnormalities. Traumatic instability occurs after forceful flexion of the head, causing either fracture of the dens or other part of the axis, and/or tearing of the fibrous attachments. This form of the instability can occur in any breed, at any age. (1-4)
Figure 1: Increase in the angle between the 1st and 2nd vertebrae, with bending in the canal leading to compression of the spinal cord. The dens is absent, with a rounded off edge on the cranial aspect where the dens is normally projecting.
Certain birth defects can predispose this instability to occur with a very small amount of trauma (i.e. jumping off the couch, being jumped on by another dog). These abnormalities include a missing or malformed dens, or lack of normal attachments between the two vertebrae. An absent or smaller dens is the most common predisposing cause. This condition is seen mainly in small breed dogs, with Yorkshire terriers, Chihuahuas, Miniature or Toy Poodles, Pomeranians and Pekingese being the most frequently reported breeds. Dogs with congenital abnormalities usually show clinical signs at less than one year of age. (2,5,6)
Signs and Symptoms
Symptoms of atlantoaxial instability can vary in severity. Onset of symptoms can be gradual, or can be very sudden. The most common sign seen with atlantoaxial instability is neck pain. This may be the only symptom, however the degree of pain can be severe. More severe spinal trauma results in varying degrees of incoordination and weakness or paralysis from the neck down. With complete paralysis of all four limbs, paralysis of the diaphragm also occurs, and the animal is unable to breathe. Usually these dogs die very suddenly before they can receive medical attention. Some dogs may demonstrate intermittent collapse. Clinical signs may be more severe with an intact dens, as a normal but dorsally deviated dens causes a greater degree of spinal compression. (1-3,9)
Diagnosis is based on signalment (breed and age), history, symptoms and radiographs (x-rays). Dorsal deviation or tipping of the axis can be seen on radiographs, with increased space between the atlas and axis. The dens may be small or absent, or visible fractures may be present. Certain views may be used to highlight these changes, such as gently flexing the neck, or taking radiographs from an angle (lateral oblique) or with an open mouth view. Extreme care must be taken to avoid excessive flexion forces, especially if the dens is intact. A CT-scan (computed tomography can also be valuable to evaluate the vertebrae for a dens and other possible deformities. (1,3,6,9)
Figure 2: Note the angulation between the first and second vertebrae.
A dens is absent in this dog.
Treatment for atlantoaxial instability can be conservative or surgical. Conservative management is more likely to be recommended when clinical signs are mild, or surgical treatment is not possible due to other medical conditions of the animal. Patients are managed with strict cage rest (4-6 weeks) and a neck brace for 6 weeks. Steroids and pain medications may also be used, and the patient will need to be protected indefinitely from trauma. Stability relies long term on the formation of scar tissue. Complete recoveries have been reported, even for dogs initially with severe signs, but dogs can also either remain unchanged or deteriorate. One study on 16 dogs demonstrated 10/16 having a good long-term outcome (10). Dogs were more likely to have a good outcome with conservative treatment, when the duration of their clinical signs was less then 30 days, at the time of treatment, while dogs with chronic signs (>30 days) were less likely to have a successful outcome without surgery. (10) With continued instability, there is a risk for suddenly luxating the spine, with acute paralysis and death. Potential complications include continued instability, inadequate stabilization by the brace, chronic spinal cord compression, and failure to heal fractures of the dens or body of the Axis. Problems such as bandage sores and eye ulcers related to the splint/neck brace are frequently seen. (1,2,9-11)
Surgery is usually recommended over conservative treatment because of the possibility of recurrence and further spinal cord damage. Goals of surgery are to relieve pressure on the spinal cord, and permanently stabilize the joint. The pressure is usually relieved by reducing the vertebrae into a normal position, without requiring any other decompression methods. If the dens is malformed and is deviated towards the cord, occasionally this will need to be removed to relieve the compression. In the past, the joint has been stabilized with many different techniques. It can be stabilized from the top (dorsal) or from the bottom (ventral). Most surgeons will now use a ventral technique, as approaches from the dorsal side do not usually result in fusion of the two bones, and long-term stability relies on scar tissue and lasting strength of the implants. Find an ACVS Veterinary Surgeon.
Ventral techniques are currently preferred, as they allow removal of the joint cartilage to enable fusion, and placement of bone graft to stimulate this bony healing. This approach facilitates correct alignment and reduction, and removal of the dens if necessary. (1,6,13) Ventral techniques include cross-pinning, trans-articular screws, a combination of pins or screws and bone cement, or bone plates. (1,12,13,17,18) Plates are usually only used in larger dogs (ie. fractures), but have been reported in small breeds. (18) The choice of repair is often dependent on surgeon preference, as well as size of the dog – with no significant published difference in outcome between these techniques (5).
Post-operative radiographs are taken to assess reduction and alignment of the spine, and implant placement.
Figures 3a & 3b: Post-operative radiographs of two transarticular screws placed from the atlas to the axis from the ventral surface.
Figures 4a & 4b: Post-operative radiographs of stabilization using combination of screws and bone cement to form a bridge between the exposed screw heads.
Potential Complications of Surgery
The most severe potential complication of surgery is death – due to sudden respiratory arrest with injury to the spinal cord during surgery. Other possible complications include failure of the repair with migration or breakage of the implants, inadequate reduction or mal-alignment of the spine. Implants can be positioned incorrectly causing chronic pain or impingement of the spinal cord, and requiring removal. Improper positioning can be a problem due to the small amount of bone available to engage the pins or screws, and a very small target area in small dogs to avoid the spinal canal. Failure rates of 10-28% have been reported for ventral fixation techniques. (2,5,9,12,13,15,17)
Strict rest is still required for 6-8 weeks after surgery. Many surgeons will still place neck braces for additional support while healing is occurring. Repeat radiographs (x-rays) will often be repeated in 4 and 8 weeks to assess the repair, and healing.
Prognosis for atlantoaxial instability depends on the degree of spinal cord trauma and neurological deficits already present. It is considered good for dogs with mild clinical signs and guarded for dogs with paralysis, but significant recovery is possible, if decompressed and stabilized. Significantly greater success rates with surgery are seen in younger dogs (<2 years of age), dogs with more acute problems (<10 months of symptoms), and dogs with less severe neurological problems. (5,6)
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—Stephanie A. Lister, DVM, MS
Diplomate, Small Animal Surgery
Updated 6/22/2009 by Dr. Lister
Reviewed 10/1/2011 by Mitchell A. Robbins, DVM, Diplomate ACVS