Atlantoaxial Instability

What atlantoaxial instability is

The atlas (C1) is a ring of bone that holds up the skull. The axis (C2) has a peg of bone, the dens or odontoid, that points upward into the ring. This joint allows about half of the head's turning movement. A strong band, the transverse ligament, holds the dens against the front of the atlas so it cannot slide back into the spinal cord.

In atlantoaxial instability, this ligament is lax, damaged or the bones are malformed, so C1 can shift forward on C2. Doctors measure the gap between the front of the atlas and the dens, the atlantodental interval. In adults it is normally 3 mm or less, and in children up to about 4 to 5 mm. Larger gaps, especially with narrowing of the space for the spinal cord, indicate instability.

Causes and risk factors

• Down syndrome: ligament laxity and bone differences cause radiographic AAI in about 10 to 20 percent of people with Down syndrome, though only around 1 to 2 percent develop symptoms
• Rheumatoid arthritis: inflammation erodes the ligament and bone around C1 and C2, common in long-standing, severe disease
• Injury: fractures of the dens or tears of the transverse ligament after falls, car crashes or diving accidents
• Grisel syndrome: C1-C2 subluxation in children after a throat infection or ENT surgery, causing a painful, fixed head tilt
• Bone and connective tissue disorders: skeletal dysplasias, Morquio syndrome, Klippel-Feil syndrome, os odontoideum, and some people with Ehlers-Danlos syndrome
• Ankylosing spondylitis, psoriatic arthritis and juvenile idiopathic arthritis
• Tumors and infections affecting the upper cervical spine

Symptoms

Many people with atlantoaxial instability on X-ray never develop symptoms. When the spinal cord or nerves are compressed, symptoms can include:

• neck pain and stiffness, or pain at the back of the skull
• a head tilt or reduced ability to turn the head
• clumsiness, poor fine motor skills or dropping things
• changes in walking, stiffness of the legs, frequent falls or tiring easily when walking
• weakness, numbness or tingling in the arms or legs
• brisk reflexes and muscle spasms
• new loss of bladder or bowel control
• in rare severe cases, breathing problems or sudden paralysis after neck movement or injury

In people with Down syndrome or intellectual disability, symptoms may show only as changes in behavior, walking, toileting or a reluctance to move the neck, so caregivers' observations are important.

Diagnosis

1. Neurological examination: strength, sensation, reflexes, coordination and walking
2. Flexion-extension X-rays: pictures with the neck bent forward and backward, measured for movement between C1 and C2
3. CT scan: detailed bone anatomy and fractures
4. MRI: shows the spinal cord, any compression, swelling or damage, and soft tissue around the joint

Screening in Down syndrome

The American Academy of Pediatrics no longer recommends routine X-ray screening of children with Down syndrome who have no symptoms, because X-rays do not reliably predict who will develop problems. Instead, it advises regular checks for symptoms at health visits, education for parents about warning signs, and prompt imaging if symptoms appear. Some sports organizations have their own requirements, and anesthetists take precautions with neck positioning during surgery.

Treatment

Without symptoms

• Regular review for neurological symptoms
• Education about warning signs
• Avoiding or modifying activities with high risk of neck flexion or impact for some people, such as diving, trampolining, tumbling, heading the ball in soccer and contact sports, guided by a specialist
• Careful neck positioning during anesthesia and procedures

With symptoms or significant instability

• Collar or traction: for some acute injuries and for Grisel syndrome, along with antibiotics and anti-inflammatory treatment
• Surgery: posterior fusion of C1 and C2 with screws, rods or wires, and bone graft, so the joint heals solid. Surgery is recommended when there is spinal cord compression, neurological symptoms or marked instability
• Treating the underlying disease, such as controlling rheumatoid arthritis with disease-modifying medicines

After fusion, people lose some ability to turn the head, but most adapt well, and the operation prevents further spinal cord damage.

Outlook

Most people with radiographic AAI never develop problems. When symptoms develop, early treatment gives the best chance of recovery, because spinal cord damage present for a long time may not fully reverse. Long-term follow-up is advised after fusion surgery and in people with risk conditions.

Frequently asked questions

What is atlantoaxial instability?

It is excessive movement between the top two bones of the neck, the atlas (C1) and axis (C2), usually because the ligament that holds them together is loose or damaged. In severe cases the bones can press on the spinal cord.

What are the symptoms of atlantoaxial instability?

Many people have none. When the spinal cord is affected, symptoms include neck pain, head tilt, clumsiness, changes in walking, weakness, numbness and new bladder or bowel problems. Any of these in a person at risk needs prompt medical assessment.

Why is atlantoaxial instability common in Down syndrome?

People with Down syndrome often have looser ligaments and differences in the bones at the top of the neck. Around 1 in 10 to 1 in 5 show instability on X-ray, but only a small minority develop symptoms.

Should children with Down syndrome have neck X-rays?

Routine X-ray screening is no longer recommended for children without symptoms, because it does not predict who will develop problems well. Children should be checked for symptoms at each health visit, and imaged promptly if warning signs appear.

Can atlantoaxial instability be fixed?

Yes. When the spinal cord is at risk, surgery to fuse C1 and C2 stabilizes the joint and protects the cord. Some acute cases, such as Grisel syndrome, settle with a collar, traction and treatment of the infection.