Atlantoaxial Instability

We can consider that there is atlantoaxial instability or atlantoaxial subluxation (AA subluxation) in cases where there is principally incompetence of the ligamentous elements of the atlantoaxial (C1-C2) joint, which allow a significant increase in the mobility of this area thus considered pathological mobility.

This increased mobility causes headache and cervical pain as well as signs of compression of adjacent neural elements that form cervicomedullary syndrome. Atlantoaxial subluxation frequently occurs in ligamentous and articular hypermobility syndromes such as Ehler Danlos syndrome.

The atlantoaxial subluxation may exist in patient’s neutral position (without neck movement) or may occur in relation to neck rotation movements (when the patient moves the neck to the right and left).

The diagnosis can be made by means of an Upright MRI (magnetic Resonance Imaging) or with a cervical CT scan with 3D reconstruction. Both tests should evaluate the movements of the occipitoatlantoid and atlantoaxial joints. Flexion-extension and cervical rotation on both sides should be evaluated. Lateral cervical x-ray and flexion-extension views can give us complementary information in regards to atlantoaxial instability, although it does not seem indicated as the first choice method of diagnosis. Another diagnostic method used is cervical cineradiology, which records joint(s) movement of the entire occipitocervical, atlantoaxial and subaxial joint system.

In these cases, the direct signs and indirect signs of atlantoaxial subluxation must be objectified. The most commonly used measures in the radiological evaluation of craniocervical instability and atlantoaxial instability are CXA, Grabb, BDI, BAI, ADI. As always, it is important to do a clinical radiological correlation to make an accurate assessment. The atlantoaxial subluxation can occur isolated or can be found in cases in which there is also craniocervical instability. In cases of hyperlaxity, It is not uncommon to find subaxial cervical alterations (levels below C3 to C7 . Must be carefully evaluated and correlated with the patient’s symptoms). In patients with Ehler Danlos syndrome, instability is present frequently in several segments, generally C0-C1-C2 (from occipital to axis).

The natural anatomic C1-C2 movement is basically rotation and approximately implies 50% of neck’s total rotation movement.

Treatment, depending on the neurological symptoms and related pain, may be surgery. It mainly consists of the posterior fusion of the affected vertebrae, in this case, the atlas (C1) and the axis (C2). The atlantoaxial instability may also have an acute traumatic origin, which may sometimes require urgent treatment, though in some cases it triggers development of the craniocervical or atlantoaxial instability. It should be stressed that C1-C2 fusion, indicated by symptomatology, results in the natural cancellation of C1 over C2 movement so it results in approximately a deficit of 50% of the rotation of the neck. Therefore before proposing surgery, the evaluation of each case must be done really carefully. More information about surgical treatment.

Abbreviations: BDI: basion dens interval, CXA: clivo axial angle, BAI: basion-axial interval, ADI: Atlantoaxial interval

Sources:

Dr. Vicenç Gilete, MD, Neurosurgeon & Spine Surgeon.

A review of the diagnosis and treatment of atlantoaxial dislocations. Yang SY, Boniello AJ, Poorman CE, Chang AL, Wang S, Passias PG. Global Spine J. 2014 Aug;4(3):197-210.

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