Atlantoaxial instability, clinically referred to as AAI, is defined by the excessive movement at the intersection of the atlas, the first cervical vertebra (C1) and the axis, the second cervical vertebra (C2). Although AAI’s exact cause remains unknown, acute trauma or congenital conditions have been cited as factors leading to atlantoaxial instability. In this paper, we will discuss AAI’s etiology, symptomatology, and treatment options.
Symptoms of Atlantoaxial Instability
Although the asymptomatic form of atlantoaxial instability has been observed in most cases, the symptomatic form has clearly-defined neurologic manifestations.
The signs of symptomatic AAI include:
Lack of coordination;
Limited neck mobility.;
In some instances, the condition progressed up to the point of hemiplegia, paraplegia, or quadriplegia. For more information about the clinical aspects of symptomatic atlantoaxial instability, which is a condition that can result in serious complications in both humans and dogs, consult an additional resource
Atlantoaxial Instability: Etiology
It has been noted that AAI’s primary causes are traced back to abnormalities in the bones or ligaments. However, further research based on necropsies, have revealed that congenital abnormalities, such as osteogenesis imperfecta (brittle bones disease), Larsen syndrome (bone developmental issues), Down syndrome, or metatropic dysplasia (dwarfism and skeletal abnormalities), could lead to atlantoaxial instability.
Another possible cause for the condition is an infection. The venous plexus and the arterial blood route facilitate the way to infectious sequelae. However, in adults, it has been observed that the causes are linked to either acute trauma (e.g. motor accidents or sports injuries) or degenerative changes mostly caused by rheumatoid arthritis.
Neurological symptoms ensue if the nerve roots or the spinal cord are affected.
In general, atlantoaxial instability has been noted when the odontoid process (i.e. a bony protuberance than ensure the stability of the Axis) is subjected to ossification. Odontoid fractures or tumors can stunt the normal development of this part, leading to atlantoaxial instability.
In patients with Down syndrome, clinicians have observed that an anomalous protein structure located in the connective tissue could cause the degradation of the ligaments. Another causes that may weaken the ligaments is inflammation.
Treatment Options for Atlantoaxial Instability
Unfortunately, drug regimens have proven to be futile against AAI. Chronologically speaking, the first pharmacological intervention ever attempted in traumatic atlantoaxial instability was based on corticosteroids. However, the American Academy of Neurological Surgeons (AANS) has stated that corticosteroids-based therapies have little to no effect in treating AAI since the condition is chronic in nature.
Furthermore, in acute trauma cases, gangliosides and corticosteroids could worsen the patient’s condition.
As far as non-surgical treatment options, the AANS generally recommends cervical spine stabilization and bed rest. Depending on the condition’s severity at the time of the consultation, the neurologist can make certain recommendations.
For instance, in treating atlantoaxial rotatory displacement (fixed rotation of the first and second cervical vertebrae due to facet dislocation and/or subluxation), the doctor could recommend a soft collar and one week of rest if the patient experienced symptoms for more than one week.
During follow-ups, if the doctor finds no changes in the patient’s condition, he will explore other treatment options. The most common is halter traction and drug regime consisting of muscle relaxants and analgesics followed by a three-week bed rest period.
If none of the treatment options work, the neurologist could recommend surgery. In general, surgery is required if other symptoms arise (e.g. spinal cord compression
) and it is usually preceded by a spinal cord stabilization procedure.
Depending on the condition’s severity, the surgeon can fixate the vertebrae after decompressing the spinal cord. The surgical instruments used for C1-C2 fusing are:
TASs (transarticular screws) – are the instruments used in the standard procedure. Transarticular screw fixation has high fusion rates (over 90 percent), and it’s the only know neurosurgical procedure capable of preventing nonunion (bones that are no longer capable of healing themselves due to trauma). TASs are placed along the posterior elements.
SRCs (screw-rod constructs), usually placed along the atlas’ latera mass and axis’ pedicle. This method has a 97.6 percent rate of fusion.
Posterior sublaminar wiring.
Halifax clamp (a surgical clamp which is attached to the adjoining laminae to stabilize the cervical region).
Atlantoaxial instability is a condition that lead to the excess movement of the joint connecting the first and the second cervical vertebrae. Some of the most common causes are acute traumatic injuries, congenital abnormalities, inflammatory processes in the ligaments, or protein anomalies in the connective tissue.
Clinicians have determined that, in most cases, the condition is asymptomatic or could recede without intervention. However, in symptomatic atlantoaxial instability, your neurologist might recommend bed rest and a drug regimen based on analgesics and muscle relaxants. Surgery is only recommended if the bony anomaly compresses the spinal cord.
Although the condition has been observed mostly in adults, atlantoaxial instability can also be diagnosed in children with Down syndrome and even pets. In most cases, AAI is chronic though it can become degenerative.
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