What is Atlantoaxial Instability?

Atlantoaxial instability (AAI) is characterized by excessive movement at the junction between the atlas (C1) and axis (C2) as a result of either a bony or ligamentous abnormality. Neurologic symptoms can occur when the spinal cord or adjacent nerve roots are involved.

AAI can occur as a result of abnormalities or trauma associated with c1-c2 (top 2 vertebrae in the neck) articulation, causing an excessive and grossly abnormal range of movement around this joint.

The most common abnormalities involve the transverse ligament or odontoid process. The strong transverse ligament and the facet capsules maintain the integrity of the atlantoaxial articulation. The transverse ligament is the primary restraint against anterior translation of the C1 on C2, whereas the odontoid is the primary restraint against posterior translation.

AAI is defined as an atlantodental (or atlantodens or atlas-dens) interval (ADI) of greater than 3 mm in adults and of greater than 5 mm in children.  The ADI is the distance between the odontoid process and the posterior border of the anterior arch of the atlas.

Symptomatic AAI occurs when subluxation or dislocation causes the odontoid process, or posterior arch of the atlas, to impinge on the spinal cord and cause neurologic manifestations. In addition, motion of the C1-2 segment can cause compression of adjacent or exiting nerve roots.

As discussed in the craniocervical instability section often a pannus forms in a tumor like fashion. The associated inflammation of this can weaken the transverse ligaments, alar ligaments, and facet capsules early on increasing the instability.

The dangers of AAI-

AAI poses many risks to the patient.  With AAI the simple act of turning ones head can cause the the top 2 vertebrae in the neck to fully or partially dislocate. The spinal cord and nerve roots can become compressed, not only giving rise to debilitating symptoms but also risk of permanent damage to the spinal cord and nerve roots.  The vertebral artery which is the main artery supplying the brain can become kinked on rotation thus interrupting blood flow to the brain.  Again not only does this cause an array of symptoms but it can ultimately lead to stroke and worst case scenario death.

Diagnosing AAI-

Upright MRI or dynamic CT scanning are the imaging of choice when diagnosing AAI.  It is imperative this is undertaken in neutral, flexion and extension.

Symptoms of AAI-

  • Due to constant or intermittent vertebrobasliar insuffiency this can cause vertigo,dizziness, tinnitus, fainting and tingling/numbness may be present.
  • Neck pain and suboccipital headache.
  • Visual changes.
  • Nausea.
  • Facial pain.
  • Difficulty swallowing and choking.
  • Motor abnormalities- Weakness, abnormal gait, sensory difficulties have also been noted and much more.

Treatment of AAI

The first line of treatment should be a neck brace.  Physiotherapy is often tried in less extreme cases and avoidance of activities that could exacerbate symptoms.

The surgical treatment for stand alone AAI whereby CCI/Chiari is NOT present is fusion of C1-C2.

For further information on AAI you may find this research paper in the American journal of medical genetics of interest written by world renown neurosurgeon Dr Fraser Henderson.

Neurological and spinal manifestations of EDS

*I will discuss AAI further throughout my blog from my experience of living with it to testing, measurements, research and much more.  All posts on AAI will be located in the AAI category page along the sidebar*