(hypoplasia of the dens)
by Leanne Bertani

This is not a common disorder, but when it does occur, it can be a very difficult experience for the dog, owner and breeder.


Atlantoaxial Subluxation is an instability of the first and second vertebrae in the neck. It takes its name from the proper name of the first cervical vertebrae (the atlas) and the second vertebrae (the axis). The lack of stability causes compression of the spinal cord that results in pain or paralysis.


This condition can be caused by trauma in any dog, but is more often seen as either a congenital or developmental malformation in small breeds. It has even been called “unavoidable” in small breeds by some veterinary authors. Other authors believe it is most likely related to a genetic vascular insufficiency that allows at least part of the dens to be reabsorbed.


Most affected dogs exhibit symptoms prior to a year of age, although those with problems due to injury may be older. The first symptom is neck pain, made worse by flexion of the neck. It may be noticed that the Chin is reluctant to lower its head to the food bowl, or the Chin may let out sudden “yips” of pain, followed by trembling. Depending on the degree of spinal cord compression, other symptoms can include clumsiness or even paralysis and death. Diagnosis is generally made by xray.


To understand this instability, think of the first vertebrae (the atlas) as a ring in a ring-toss game. Then think of the second vertebrae as the platform and the post. The ring of the first vertebrae fits over the little post of the second vertebrae, and is held in place by a ligament. But if the little post (called the “dens” or “odontoid process”) is broken or malformed, the ring won’t stay properly on the base. Injury or laxity of the ligament may also allow enough movement of the vertebrae to traumatize the spinal cord. The dens forms at about the age of 9-12 weeks in a pup, and fuses to the axis at about 7-9 months of age. For this reason, if there are going to be symptoms due to an abnormality of the dens, they will usually become noticed prior to a year of age. Obviously, the dens is smaller and more fragile in toy dogs than it is in large dogs, so it is more easily malformed or broken.


Treatment is dependent on the severity of symptoms. If symptoms are mild, sometimes small dogs can be treated conservatively (non-surgically) with anti-inflammatories, crate rest (for several weeks), and possibly a neck brace, in order to allow time for fibrous tissue to form and stabilize the vertebrae. Dogs with more severe symptoms require surgical stabilization of the two vertebrae with wire, suture, pins, or bone grafts. Dogs with this condition must be handled with care until the condition is corrected. A sudden major dislocation of the vertebrae can cause paralysis or death.


Small dogs are more likely to respond to conservative treatment than large dogs, but dogs that are treated conservatively are more likely to have a recurrence of the problem than dogs that are treated surgically. Even surgical treatment is not fool-proof. In a study of 14 dogs that underwent the surgery for severe symptoms, two of them died and two required an additional surgery.