Rebecca L. Carl, MD, MSCI
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Atlantoaxial instability (AAI) is the term for increased motion at the joint between the 1st and 2nd cervical vertebrae (the atlas and the axis).
Congenital, inflammatory, traumatic, or infectious conditions may weaken the structures supporting the C1-C2 joint leading to AAI.
Many patients who meet the radiographic definition of AAI are asymptomatic and at low risk for neurologic sequelae.
Synonym(s): atlantoaxial subluxation
The incidence of radiographic/asymptomatic AAI in individuals with Down syndrome is estimated between 10% and 20%.
The incidence of symptomatic AAI is much lower at ˜2.6%.
Incidence of AAI increases with age, and progression may occur during growth spurts.
For individuals with Down syndrome, males age > 10 yr are most likely to have progression of AAI.
Juvenile idiopathic arthritis
Many forms of dwarfism/skeletal dysplasias
Marfan syndrome: increased incidence of C1-C2 hypermobility, but symptoms are rare
Lateral cervical radiographs with flexion and extension views are used for screening. AAI is defined radiographically as an increased distance between the odontoid process and the anterior arch of the axis.
Obtaining screening x-rays for AAI in asymptomatic patients with Down syndrome is controversial:
Because of poor radiographic reproducibility and because many radiographic signs of AAI often resolve over time in asymptomatic patients, the American Academy of Pediatrics does not endorse routine screening (1)[A],(2)[C].
The Special Olympics requires a history and physical evaluation that includes screening for AAI. Individuals with symptoms that could represent AAI must have a comprehensive workup (3)[C].
Most individuals with AAI are asymptomatic.
AAI can be acute or chronic. However, acute AAI without a history of preceding symptoms is rare:
Patients with acute AAI often present after injury. Injury can be due to direct or indirect trauma. Mechanisms of injury include hyperextension, hyperflexion, and direct loading.
In patients who have an acute worsening of AAI, participation in organized sports is an uncommon triggering event.
Patients with chronic AAI often present with gait changes, progressive weakness, and fatigue. Neck pain, bowel and bladder incontinence, ataxia, spasticity, and quadriplegia can be present with longstanding, severe chronic AAI (4)[B].
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