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. Diagnosis
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 increase space between 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. |