Left untreated, atlantoaxial instability can lead to serious complications in both humans and dogs.
What is Atlantoaxial Instability?
The condition of atlantoaxial instability “is characterized by excessive movement at the junction between the atlas…and axis”. The atlas is the vertebra also known as C1 and the axis may sometimes be referred to as C2. These are cervical vertebrae at the top of the spine. AAI is caused by a “bony or ligamentous abnormality”, and this can lead to “neurologic symptoms…when the spinal cord or adjacent nerve roots are involved”. In adults, the causes of AAI are usually caused by trauma or degeneration “due to the inflammatory pannus of rheumatoid arthritis”. Infection can also trigger instability. There are also congenital causes of AAI, including Down syndrome, neurofibromatosis, osteogenesis imperfecta, Larsen syndrome, Kniest syndrome, spondyloepiphyseal dysplasia (SED), metatropic dysplasia, Morquio syndrome, and chondrodysplasia punctate. Atlantoaxial instability may present with no symptoms. Some patients experience pain, such as a “vague neck pain or a headache”. Because the vascular supply and spinal cord are nearby, vascular occlusion or myelopathy could also occur. There are neurologic symptoms as well. Atlantoaxial Instability symptoms in patients include a lack of coordination, abnormal gait, clumsiness, trouble walking, limited mobility, torticollis, and they might become easily fatigued. It is also possible that “paraplegia, hemiplegia, and quadriplegia” could occur. Patients with risk factors should be seen by a healthcare professional for “urgent radiographic and surgical evaluation”. Atlantoaxial instability can progress, leading to spinal cord compression and complications of not only myelopathy and neck pain, but spasticity and radicular symptoms as well.1
The purposes of treatments for AAI are to “protect the spinal cord, stabilize the spinal column, decompress neural tissue, and reduce any deformity”. Lack of treatment could lead to “severe consequences”. There are no medications for AAI. Corticosteroids are a controversial treatment. Without symptoms, AAI may not need treatment, but once symptoms arise, patients will need to have the cervical spine stabilized before undergoing “surgical stabilization”. Some will need a soft collar, while others will require halter traction “with analgesics and muscle relaxants”. Halo bracing may also become necessary. There are 4 types of AAI on the Fielding and Hawkins scale: Type I (“stable subluxations”, treated with a collar), Type II (“potentially unstable”, physician’s discretion about treatment), and Types III and IV (“unstable”, require surgery). Patients should be careful not to excessively extend or flex the neck. Any healthcare interventions that require “sedation or neck manipulation” should be performed only with “extreme caution”. The neck should be held in a neutral position if surgeries, such as otorhinolaryngologic procedures”, are undertaken. As for other activities, the Special Olympics currently require that children with Down syndrome undergo examinations to rule out AAI. The activities that are not allowed for patients with AAI include diving, gymnastics, pentathlon, high jump, soccer, and butterfly stroke in swimming.2
Atlantoaxial Instability vs. Subluxation
Atlantoaxial subluxation (AAS), like atlantoaxial instability, is a disorder of the C1 and C2, and it causes “impairment in rotation of the neck”. The subluxation can be antero-posterior, rotary, vertical, or lateral. The rotary subluxation has four subtypes describing how it can be rotated.3 There are two types of atlantoaxial subluxation: unstable (dynamic) and stable (fixed). Unstable AAS is actually a form of AAI, and it “can result in compression of the spinal cord or vertebral arteries”. The typical causes of AAS are congenital (such as from Down syndrome), rheumatological (rheumatoid arthritis, for example), and traumatic (fracture). The stable form of AAS “can involve significant widening of the joint space that does not change between the two views”, whereas the unstable form has a diameter that differs 2mm or more between extension and flexion radiographs. Without congenital, rheumatological, or traumatic conditions, it can be hard to detect the instabilities based on symptoms alone, as mild cases may look like other disorders. Headaches and neck pain are the milder signs. Yet early diagnosis is important.4 Atlantoaxial subluxation is relatively frequent “in patients with disorders of the upper cervical spine due to rheumatoid arthritis (RA)”.5 Traumatic AAS is rare.6
Atlantoaxial Instability is actually an uncommon condition in dogs, but when it occurs, there is “abnormal movement in the neck, between the atlas…and axis”. The abnormal bending that results compresses the spinal cord, and the extent of the cord injury determines how the condition presents. Normally, the atlantoaxial joint is stabilized by the dens, which is “a projection off the axis”. The dens “fits into the atlas (and) several ligaments between the two bones”. Birth abnormalities and trauma can cause this condition. Fracture of part of the axis or of the dens, ligament tears, or a forceful head flexion can trigger AAI. Birth defects in the dog can also mean that any small trauma could bring on the condition. Small breed dogs are most at risk. Typically, signs of the congenital abnormalities will show by the time the dog is one. Symptoms can range from mild to severe and be sudden or gradual. Neck pain, weakness, paralysis, and incoordination are other signs. Unfortunately, if the animal is paralyzed on all four limbs, the diaphragm will also become paralyzed, leaving the dog unable to breathe. Death often results suddenly, in these cases. Conservative treatments include a neck brace and cage rest, as well as pain medications and steroids. In addition, the dog must be “protected indefinitely from trauma”. While some dogs may recover, others deteriorate, and instability leads to a risk of “suddenly dislocating the spine, with acute paralysis and death”. The spinal cord could also become chronically compressed, and axis and dens fractures might not heal. Surgery is usually the recommended treatment to prevent further damage and recurrence. This kind of intervention is meant to “relieve pressure on the spinal cord, and permanently stabilize the joint”. The joint can be stabilized from the bottom or top, although the top (ventral) techniques are “currently preferred”. Doctors may use any of the following techniques: “cross-pinning, trans-articular screws, a combination of pins or screws and bone cement, or bone plates”.7
Atlantoaxial instability in Down syndrome is a possible issue that arises. Even though it is rare, it can lead to a “catastrophic event”. Both atlantoaxial instability (AAI) and atlanto-occipital instability (AOI) are potential disorders can occur with Down syndrome. In general, patients with Down syndrome should be tested around the age of 3, with a “c-spine x-ray” for AAI and AOI. The separation in the vertebrae (between C1 and C2 for AAI and between C1 and the occipital lobe for AOI) should be minimal. The separation between C1 and C2 should be less than 5mm and the separation between the C1 an occipital lobe should be about 1mm. About 10% of Down syndrome children have AAI and may not need any surgery; however, precautions must be taken with regard to doing certain activities, including trampolines, bounce houses, horseback riding, and somersaults. Jarring the neck is dangerous and the large gap may cause impingement upon the spine. Children who are not going to have surgery, and who do not take the necessary precautions, are at risk of paralysis or even death. Often, AAI has no symptoms; therefore that early x-ray is important. AOI is rarer than AAI.8
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