Atlantoaxial Instability (AAI)

Know the at-risk list - Rheumatoid arthritis, Down syndrome, achondroplasia, Morquio syndrome, os odontoideum, Klippel-Feil syndrome, osteogenesis imperfecta, neurofibromatosis, and ankylosing spondylitis all carry atlantoaxial instability. Rheumatoid arthritis is the most common inflammatory cause, and roughly 20% of rheumatoid patients also develop basilar invagination as the lateral masses of C1 erode and the skull settles down onto the dens.

Look for cord signs before you induce - Hyperreflexia, Hoffmann sign, Babinski, clonus, a spastic or broad-based gait, and loss of hand dexterity are the myelopathy findings that separate a radiographic curiosity from a neck that will not tolerate laryngoscopy. Ask about neck pain and torticollis as well.

Read the flexion-extension films - Lateral cervical radiographs taken in flexion and extension are what quantify the instability. An anterior atlanto-dens interval above 3 mm marks transverse ligament injury, translation greater than 5 mm is the threshold at which surgeons fuse, and a posterior atlanto-dens interval under 12 mm predicts paralysis regardless of the underlying cause.

Flexion is the movement that hurts them - Anterior subluxation of C1 on C2 happens in flexion, and that is when the canal narrows onto the cord. Keep the head neutral from the moment the patient reaches the table through transfer, positioning, and emergence. Once they are anesthetized there is no protective muscle tone left to stop a bad turn of the head.

Pick the airway technique that moves the neck least - There is no single best technique here; the goal is minimum neck movement. Direct laryngoscopy displaces the spine more than indirect visualization does, so video laryngoscopy, flexible fiberoptic intubation, or an intubating LMA are the better tools. Have a supraglottic airway open on the cart as the rescue.

Manual in-line stabilization for every intubation - A second provider takes the anterior collar off and cradles the occiput and mastoid processes, working from the head or the side of the bed. That is what keeps the segment from displacing while you are in the mouth.

Awake fiberoptic when you need the neurologic exam - It buys you a patient you can examine after positioning, but it needs cooperation, a well-topicalized airway with lidocaine jelly and spray, and a provider who does them regularly. This is not the neck on which to attempt your first one.

The vertebral arteries sit in the danger zone - They run through the C1 and C2 transverse foramina. Blunt vertebral artery injury tracks with a widened atlanto-dens interval and presents as basilar insufficiency, so a cranial nerve and posterior circulation exam belongs in the preoperative note.

Document a baseline and repeat it - Record the neurologic exam before induction and again on emergence. Without the baseline you cannot tell a new deficit from an old one, and a new deficit is the thing you most need to catch early.

The atlantoaxial joint is C1 (atlas) articulating with C2 (axis), and it is the most mobile segment in the spine — the atlanto-occipital and atlantoaxial articulations together supply about half of all cervical flexion and rotation. Stability comes from ligament rather than bone. The transverse ligament is the strongest ligamentous attachment in the cervical spine and is what holds the odontoid process against the anterior arch of C1. When that restraint fails — eroded by rheumatoid arthritis, congenitally lax or malformed as in Down syndrome, achondroplasia, Morquio syndrome and os odontoideum, or torn by trauma — C1 translates forward on C2 during flexion and the dens encroaches on the cord.

Most patients are asymptomatic and undiagnosed, which is the problem: minor trauma to an unrecognized unstable neck can produce catastrophic cord injury. Radiographic instability appears in up to 30% of patients with Down syndrome, though only about 1% are symptomatic.