Cause, symptoms, diagnose and treatment
Atlantoaxial Instability (AAI) is an incompetence of the atlantoaxial joint, causing an increased mobility of the area. The condition is common amongst patients suffering from hypermobility syndromes such as EDS and Marfans. Common symptoms are headache and cervical pain. Atlantoaxial Instability may appear along with CCI (Craniocervical Instability). Due to anatomical differences and concomitant diseases, treatment must be customized to each patient and performed by a neurosurgical specialist in this area.
Where there is incompetence of the ligamentous elements of the atlantoaxial (C1-C2) joint, a significant increase in the mobility of this joint occurs. This is referred to as atlantoaxial instability or atlantoaxial subluxation (AA subluxation). It frequently occurs in ligamentous and articular hypermobility syndromes such as Ehlers-Danlos syndrome and Marfans.
The increased mobility of the atlantoaxial joint may cause the following symptoms:
The atlantoaxial subluxation may exist either in the patient’s neutral position (without neck movement) or may also occur in relation to neck rotation movements (when the patient moves the neck to the right and left).
Atlantoaxial Instability diagnosis is confirmed by obtaining rotational imaging from either an Upright MRI (Magnetic Resonance Imaging) or cone-beam CT scanner (CBCT) offered here in Barcelona. A rotational cervical CT scan with 3D reconstruction can also be used. Either tests should evaluate the movements of the occipitoatlantoid and atlantoaxial joints.
The direct or indirect signs of atlantoaxial subluxation must be objectified. The most commonly used measures in the radiological evaluation of Atlantoaxial Instability are CXA, Grabb, BDI, BAI and ADI (please see below). As always, it is important to do a clinical radiological correlation to make an accurate assessment. The Atlantoaxial subluxation can occur either isolated or concurrently with a craniocervical instability. In cases of hyperlaxity, it is not uncommon to find subaxial cervical alterations at levels below C3 to C7. These must be carefully evaluated and correlated with the patient’s symptoms. In patients with Ehler Danlos syndrome, instability is presented frequently in several segments, generally C0-C1-C2 (from occipital to axis).
The personalized evaluation of each case is of utmost importance since abnormalities of the vertebral artery anatomy must be ruled out. This includes the possible anatomical differences regarding the layout and dimensions of the vertebral pedicles, lateral masses and other bone elements.
After the preoperative analysis of the upright imaging of each patient, a thin sliced preoperative CT oriented towards neuronavigation is obtained. Using this, the surgical planning of the intraoperative neuronavigation is performed to confirm the trajectories of screws and special anatomical dispositions of structures. It is later carried out during surgery.
Another important aspect to take into consideration and to know is each patients’ concomitant disease or comorbidity. This is frequent in patients affected by Ehler Danlos Syndrome. This includes POTS, Mast Activation Syndrome, cardiac abnormalities etc. Knowing this beforehand allows us to calculate and prevent any possible problem in the postoperative period.
Treatment, depending on the neurological symptoms and related pain, may be surgery. The surgical treatment for Atlantoaxial instability, when it manifests alone without Occipitocervical instability, mainly consists of a posterior fusion of the first cervical vertebra (C1 or Atlas) and the second cervical vertebra (C2 or Axis). The atlantoaxial instability may also have an acute traumatic origin, which may sometimes require urgent treatment, though in some cases it only triggers the development of the craniocervical or atlantoaxial instability.
It should be stressed that a C1-C2 fusion, indicated by symptomatology, results in the natural and complete cancellation of C1 over C2 movement. The natural anatomic C1-C2 movement is mainly rotation. Approximately 50% of the neck’s total rotation movement is therefore dependent of it. Before proposing surgery, the evaluation of each case must therefore be throughout.
The pedicle screws are usually placed in the Axis. Placement of isthmic screws may however be considered of the patient´s anatomy requires it. Atlas’ screws are generally placed in the lateral masses. Atlas and axis screws are joined in on each side by lateral bars that unify the instrumented fusion system. In most cases it is convenient to place bone graft, usually autologous, taken from the iliac crest or the patient’s own rib. In the cases where it is not possible to obtain autologous bone graft, heterologous graft (artificial bone) may also be used.
When Atlantoaxial instability occurs along with craniocervical instability, also known as occipitocervical instability (ie instability present between skull and first cervical vertebra or Atlas), the fusion performed should add a fixation to the cranial bone through occipital or condylar screws. This presents us as a whole C0 -C1-C2 posterior fusion.
Once in the Operating Room, surgery is performed under general anaesthesia, with Neurophysiological monitoring (SSEP – somatosensory evoked potentials), neuronavigational guidance and intraoperative fluoroscopy guidance. We can therefore control the spinal cord as well as the cranial and cervical nerves in order to avoid damage to these important structures. Neuronavigational assistance guides us all through the surgery, thus it diminishes (though it does not eliminate) risks while the screws for the fusion are placed. Both neurophysiological monitoring and neuronavigational guidance are performed as safety measures for the patient.
Postoperatively, the patient stays at the ICU (Intensive Care Unit) two days after which the patient is transferred to the Neurosurgical Ward. Postoperative hospital stay is approximately eight days. Waking up and walking begins the second day after surgery. After hospital discharge, the well-being and progress of the patient is monitored approximately once a week after hospital discharge on an outpatient basis. This to make sure everything is as it should be before flying back home. We therefore recommend our patients to stay in Barcelona for 10-15 days after discharge.
Abbreviations:
BDI: basion dens interval
CXA: clivo axial angle
BAI: basion-axial interval
ADI: Atlantoaxial interval
Sources:
-Dr. Vicenç Gilete, MD, Neurosurgeon & Spine Surgeon.
-Mummaneni PV, Haid RW. Atlantoaxial fixation: overview of all techniques. Neurol India. 2005 Dec;53(4):408-15. Review.
-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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