The atlas is the first vertebrae of the neck or cervical spine (C1) and is firmly attached to the skull. The axis is the second vertebrae of the cervical spine (C2). Unlike all the other vertebrae that are firmly connected to each other by an intervertebral disk, the atlas (C1) and axis (C2) are connected by just a piece of bone called the dens and multiple ligaments. The advantage of this unique connection (articulation) is that it allows for a much greater range of motion of the head compared to a traditional connection, however the connection can fail and the two vertebrae will then slip or slide into each other (luxation). This occurs when the dens or ligaments never form or are too small (aplasia, hypoplasia) or simply break. The resulting luxation causes damage to the lining of the spinal cord, spinal cord and brainstem. Moderate to severe episodes of pain are the most common sign of an atlanto-axial (C1-C2) luxation. However, there can be a broad spectrum of persistent or episodic signs including weakness, head turn or tilt, and even seizure-like episodes. Radiographs made with the neck in different positions (neutral position, extended, slightly flexed) will demonstrate excess movement or luxation and indicate whether the dens has formed in a particular patient. MRI is also needed to assess spinal cord or brainstem damage and look for other common conditions that we see in small breed dogs such as Caudal Occipital Malformation Syndrome (COMS) and Meningoencphalitis (ME). A CT scan can be helpful to best assess the presence and location of the dens (which can be important if the fractured dens is going to be removed during surgery) and to assess for another potential problem at this location: displacement of C1 too far forward and compression of the brainstem. Lastly, a CSF analysis is occasionally performed to exclude inflammation or infection as the possible cause of the symptoms. Multiple tests are typically recommended because often there is more than one diagnosis that must be considered prior to treatment.
Six to eight weeks of crate rest while sometimes wearing a neck bandage that restricts movement of the C1-C2 joint can provide comfort and may allow torn ligaments to heal. The disadvantage of this treatment include: resolution of clinical signs about 50% of the time and is unlikely to work in dogs with severe signs (extreme weakness, severe pain), recurrence rates of about 40%, and the bandage is cumbersome by design in order to restrict movement. Furthermore, severe skin infections can develop under the bandage especially if they get wet, and bandage changes are required every 1-2 weeks. Surgery can also be performed where pins and bone cement are placed to prevent luxation. When surgery goes well it has about a 75% success rate, does not typically require a bandage and recurrence rates after surgery are low, however crate confinement for 6 weeks is still required. Unfortunately in the cases where the implants fail, the patient maydie from respiratory arrest and/or require a surgical revision. Due to the sensitive location of this procedure immediate post operative mortality can range from about 20-30%. Dogs that survive the first 24-48 hours after surgery tend to do well long term. As with any surgery, additional risks would include bleeding and infection but thankfully these are infrequent complications that occur <5% of the time. Other uncommon complications that may be encountered are coughing, laryngeal paralysis, and pneumonia. Pain medication with an NSAID and pain modulators are prescribed with either management technique.
Veterinary Neurology 