Overview
Definition:
Atlantoaxial instability (AAI) refers to excessive movement between the first two cervical vertebrae (atlas and axis), potentially leading to spinal cord compression
In Down syndrome (DS), this is due to ligamentous laxity and structural abnormalities of the craniovertebral junction.
Epidemiology:
AAI is found in approximately 10-30% of individuals with Down syndrome, although symptomatic AAI is much less common, occurring in about 1-2%
The prevalence increases with age and is more commonly associated with specific vertebral anomalies.
Clinical Significance:
Untreated symptomatic AAI can lead to serious neurological deficits, including myelopathy, quadriplegia, and even death
Early identification and management are crucial for preventing irreversible neurological damage and ensuring optimal quality of life for individuals with DS.
Clinical Presentation
Symptoms:
Many individuals with AAI are asymptomatic
When symptoms occur, they can be subtle and progressive: Neck pain or stiffness
Occipital headache
Changes in gait, such as spasticity or ataxia
Upper extremity weakness or paresthesias
Lower extremity spasticity
Changes in bowel or bladder function (rare in early stages)
Torticollis (head tilt).
Signs:
Neurological examination may reveal: Spasticity in the lower extremities
Hyperreflexia
Clonus
Positive Babinski sign
Impaired proprioception
Limited neck range of motion
Tenderness to palpation over the upper cervical spine.
Diagnostic Criteria:
There are no universally agreed-upon diagnostic criteria for asymptomatic AAI screening
However, instability is generally defined by radiographic measurements: Increased atlanto-dental interval (ADI) >5 mm in children and >3 mm in adults
Dynamic radiographic views (flexion-extension) showing >5 mm of displacement
Other parameters include posterior displacement of the atlas and sagittal malalignment of the craniocervical junction.
Screening Controversies
Current Recommendations:
Historically, routine screening with radiographs was recommended for all children with DS
However, recent guidelines from organizations like the American Academy of Pediatrics (AAP) and the Centers for Disease Control and Prevention (CDC) have shifted away from universal routine screening due to low yield of symptomatic cases and potential radiation exposure.
Arguments For Screening:
Proponents argue that identifying asymptomatic instability allows for proactive management and prevention of neurological injury
Early detection can prevent irreversible damage
Some advocate for screening at specific ages or in children with concerning symptoms.
Arguments Against Screening:
Opponents highlight the low incidence of symptomatic AAI, the risk of radiation from X-rays, and the high rate of false positives
They emphasize that most individuals with DS and AAI remain asymptomatic throughout life
The focus is shifting towards clinical monitoring and selective screening.
Risk Stratification:
Current approaches often involve risk stratification, identifying individuals with DS who may benefit more from screening based on clinical signs or specific vertebral anomalies
This includes children with symptoms of neurological compromise or those with known structural abnormalities.
Diagnostic Approach
History Taking:
Detailed history focusing on neurological symptoms: gait disturbances, neck pain, headache, motor skill regression, changes in tone, bowel/bladder issues
Family history of AAI or neurological conditions
Previous injuries.
Physical Examination:
Thorough neurological exam: assess gait, tone, reflexes, coordination, cranial nerves, and sensory pathways
Assess neck ROM and palpate for tenderness
Look for signs of torticollis.
Investigations:
Radiographs of the cervical spine in neutral, flexion, and extension views are standard for assessing ADI and dynamic instability
MRI of the craniovertebral junction is preferred for detailed evaluation of spinal cord compression, ligamentous integrity, and bony abnormalities, especially if neurological symptoms are present or suspicion is high
CT may be used for detailed bony assessment.
Differential Diagnosis:
Other causes of neurological symptoms in children with DS: congenital heart disease, hearing loss, thyroid dysfunction, developmental delays, peripheral neuropathy, atlanto-occipital dislocation (rare)
Neurological symptoms can also be nonspecific and related to other conditions.
Management
Initial Management:
For asymptomatic AAI: conservative management with activity modification (avoiding contact sports, high-impact activities, and hyperflexion/hyperextension of the neck)
Regular clinical monitoring and periodic radiographic assessment may be considered based on risk factors and clinical judgment.
Medical Management:
No specific medical management for the instability itself
Focus is on symptom relief if present (e.g., pain management)
Bracing is generally not effective for AAI in DS.
Surgical Management:
Surgery is indicated for symptomatic AAI or asymptomatic AAI with significant radiological findings (e.g., marked instability or evidence of cord compression)
Surgical goals include decompression and stabilization (e.g., posterior fixation)
Indications are carefully individualized.
Supportive Care:
Education of parents and caregivers regarding activity restrictions
Regular follow-up with specialists (pediatrician, neurologist, orthopedist)
Early referral to physical and occupational therapy if neurological deficits are present.
Prognosis
Factors Affecting Prognosis:
The prognosis for individuals with AAI in DS depends on whether it becomes symptomatic and the extent of neurological compromise
Early diagnosis and appropriate management, including surgical intervention when necessary, lead to better outcomes
Aggressive activity can worsen prognosis.
Outcomes:
Most individuals with asymptomatic AAI remain so throughout life
Those who develop symptoms and are treated appropriately can have stable neurological function
Severe myelopathy can lead to permanent disability, but this is uncommon with vigilant care.
Follow Up:
Long-term follow-up is essential, especially for those with known AAI or a history of instability
This includes regular clinical assessments and potentially periodic imaging, tailored to the individual's risk profile and symptoms
Lifelong awareness is important.
Key Points
Exam Focus:
Understand the controversy surrounding routine screening vs
selective screening
Know the typical radiographic findings (ADI >5mm in children)
Recognize that symptomatic AAI is less common than radiographic AAI
Management is primarily conservative for asymptomatic cases, with surgery for symptomatic or severe instability.
Clinical Pearls:
Always consider AAI in the differential diagnosis of new neurological signs or symptoms in a child with Down syndrome
Emphasize activity modification for children with known AAI, even if asymptomatic
Recognize that "normal" ADI varies with age and flexion/extension.
Common Mistakes:
Over-reliance on routine radiography without considering clinical signs
Undertreating symptomatic AAI due to fear of surgery
Recommending aggressive sports for individuals with known AAI
Neglecting to re-evaluate symptoms or radiographic findings over time.