Atlantoaxial instability

Atlantoaxial instability or Atlanto-axial instability (AAI) is characterized by excessive movement at the junction between the atlas (C1) and axis (C2) as a result of either a bone or ligament abnormality.

Causes
Atlantoaxial instability can be caused by hereditary conditions including Marfan Syndrome, neurofibromatosis, connective tissue conditions like rheumatoid arthritis, or Ehlers-Danlos Syndromes (EDS), as a result of physical trauma, or infection. It has been associated with Down syndrome, Morquio syndrome, Marfan syndrome, and Ehlers-Danlos syndrome.

AAI often co-occurs with craniocervical instability (CCI).

In individuals without predisposing conditions, it is thought to be extremely rare.

Symptoms

 * Neck pain
 * Neck stiffness, torticollis
 * Spasticity
 * Radicular symptoms
 * Lack of coordination
 * Clumsiness
 * Gait changes, difficulty with gait
 * Sensory deficits
 * Neurogenic bladder
 * Clonus, hyperreflexia
 * Paraplegia, quadriplegia
 * Muscle weakness, which is not a constant feature
 * Blurred vision
 * Occipital headache

Rheumatoid arthritis
Chronic inflammation can cause laxity and stretching of the transverse ligament, the formation of a pannus, as well as bone erosion.

Grisel’s syndrome
Grisel’s syndrome is AAI that occurs following inflammation of the soft tissue as a consequence of surgery or infection, frequently an upper respiratory infection. It is primarily seen in patients ages 5-12 but can also be seen in adults. It usually presents with torticollis, neck pain, neck tilt, and stiffness. It can often be treated with conservative therapies such as physical therapy, traction, immobilization, anti-inflammatories and treatment of any underlying infection.

A handful of cases of concomitant non-traumatic CCI and AAI following upper respiratory infections have also been documented.

Co-morbid conditions
AAI can cause vertebrobasilar insufficiency.

Diagnosis
AAI diagnosis is based on a neurological exam, reported symptoms, and radiological measurements. An atlantodental (or atlantodens or atlas-dens) interval (ADI) of greater than 3 mm in adults and of greater than 5 mm in children as measured on plain radiography is considered indicative of AAI. The ADI is the distance between the odontoid process and the posterior border of the anterior arch of the atlas. An abnormal degree of rotation of the atlas (C1) on C2 has also been determined to indicate instability.