Atlantooccipital Dislocation  

= ATLANTOOCCIPITAL DISTRACTION INJURY = ATLANTO-OCCIPITAL DISSOCIATION (AOD)

= disruption of tectorial membrane + paired alar ligaments resulting in grossly unstable injury

• Diagnosis difficult to make and easy to overlook at whole-body CT!

Cause: rapid deceleration with either hyperextension or hyperflexion → in up to ⅓ of high-velocity injuries

Age: in adults much less common than in children ← larger size of head relative to body, increased laxity of ligaments, shallow horizontally oriented occipitoatlantoaxial joint, hypoplastic occipital condyles

May be associated with: occipital condyle fracture

• neurologic symptoms: range from respiratory arrest with quadriplegia to normal neurologic exam
• discomfort, stiffness

Direction of dislocation / subluxation: anteriorly, posteriorly, superiorly (vertical = life-threatening)

Lateral radiograph:

• Powers ratio (assessment for anterior subluxation) = BC÷OA ratio >1 = ratio of distance between basion + spinolaminar line of C1 and distance between posterior cortex of anterior tubercle of C1 + opisthion (74% sensitive)
• basion-dens interval (BD) >12 mm on X-ray / 9.5 mm on CT without traction placed on head / neck
• basion-axial interval>12 mm anterior / >4 mm posterior to posterior axial line (PAL)
• atlanto-dental interval>3 mm (man) / >2.5 mm (woman):
  • “V sign” = cranially divergent predental lines ← transverse ligament injury
• atlanto-occipital interval >4 mm
• atlanto-axial interval >2.6–4 mm
• >10 mm soft-tissue swelling anterior to C2 + pathologic convexity of soft tissues (80%)

CT (≤1.25 mm thick sections, superior to radiographs):

A basion-dens distance >10 mm is highly suggestive of dissociation. The alar ligaments + tectorial membrane are the most important stabilizing ligaments given the little inherent osseous stability.

• blood in region of tectorial membrane + alar ligaments
• condylar fracture ± fracture extension through hypoglossal canal (for cranial nerve XII)
• widening / incongruity of articulation between occipital condyles + lateral masses of C1

MR:

Indications for MRI:

1. Detection of soft-tissue injury + spinal cord injury
2. Treatment planning of unstable cervical spine
3. Patients with neurologic deficits
4. Suspected ligamentous injury
5. Patients who cannot be clinically evaluated for >48 hours ← altered level of consciousness

• fluid in articular capsules, nuchal ligament, interspinous ligament

Cx:

1. Injury to caudal cranial nn. + upper 3 cervical nerves
2. Epidural hematoma with brainstem compression + upper spinal cord injury
3. Vasospasm / dissection of internal carotid and vertebral arteries

Prognosis: usually fatal; more survivable in skeletally immature pediatric trauma patients