Atlantoaxial Rotatory Displacement (AARD) - Spine

summary
- Atlantoaxial Rotatory Displacement is a pediatric cervical spine rotatory instability caused by C1-C2 subluxation or facet dislocation
- The most common presentation is a young child who presents with torticollis.
- Diagnosis is made by dynamic CT scan of the cervical spine.
- Treatment can be a soft collar, halter traction, halo traction, or surgical fusion depending on the chronicity of the condition.

Etiology
- Mechanism
  - spectrum of disease that ranges from mild subluxation to fixed facet dislocation
- Pathophysiology
  - common causes include
    
    infection (~35%)
    
    may have history of pharyngitis or otitis media
    
    Grisel's disease is the condition of AARD following a respiratory infection or retropharyngeal abscess
    
    thought to be linked to lymphatic edema in area of cervical spine
    
    trauma (~24%)
    
    recent head or neck surgery (~20%)
    
    idiopathic
    
    associated conditions
    
    Down's syndrome
    
    rheumatoid arthritis
    
    tumors
    
    congenital anomalies
  - pathoanatomy
    
    mechanism is thought to be related to ligamentous laxity
    
    transverse ligament integrity
    
    transverse ligament is intact
    
    spinal canal stenosis can only occur with severe rotation and facet dislocation
    
    transverse ligament is ruptured
    
    and there is a component of anterolithesis (> 5mm), then spinal canal stenosis can occur with less rotation (45 degrees)
    
    vertebral arteries may also be at risk

Anatomy
- Axis Osteology
  - axis has odontoid process (dens) and body
  - embryology
    
    develops from five ossification centers
    
    subdental (basilar) synchondrosis is an initial cartilagenous junction between the dens and vertebral body that does not fuse until ~6 years of age
    
    the secondary ossification center appears at ~ age 3 and fuses to the dens at ~ age 12
- Occipital-C1-C2 ligamentous stability
  - provided by the odontoid process and its supporting ligaments
    
    transverse ligament
    
    limits anterior translation of the atlas
    
    apical ligaments
    
    limit rotation of the upper cervical spine
    
    alar ligaments
    
    limit rotation of the upper cervical spine

                        Classification
                    
                                            Fielding Classification of AARD

                                            Type I
                                        
                                            • Unilateral facet subluxation with intact transverse ligament
• Odontoid acts as a pivot point with 1 facet subluxating anteriorly, 1 facet subluxating posteriorly.
•Most common and benign type
                                        
                                            Type II
                                        
                                            • Unilateral facet subluxation with 3 to 5 mm of anterior displacement.
• Injured Transverse ligament
• 1 facet acts as a pivot point and 1 lateral mass is displaced anteriorly
                                        
                                            Type III
                                        
                                            • Bilateral anterior facet displacement of > 5 mm.
• Rare with a higher risk of neurologic involvement or instantaneous death.
• Both lateral masses are displaced
                                        
                                            Type IV
                                        
                                            • Posterior displacement of the atlas (C1) (with odontoid fracture, or hypoplastic dens)
• Rare with a higher risk of neurologic involvement or instantaneous death.

Physical Exam
- Symptoms
  - tilted head
  - neck pain
  - headache
- Physical exam
  - ipsilateral rotation and contralateral tilt of the head in relation to the lateral mass of C1
    
    chin rotated to the side opposite the facet subluxation (e.g. right sided facet subluxation will have chin rotated to the left)
  - contra-lateral sternocleidomastoid may be spastic
    
    sternocleidomastoid (SCM) spasm occurs on the SAME side as the chin (e.g. right sided facet subluxation will have chin rotated to the left, and left SCM will be spastic)
    
    this protective spasticity occurs to reduce further subluxation
    
    C1-C2 subluxation (and resultant chin position) is primary, SCM spasm is secondary/reactive
    
    in contrast to congenital muscular torticollis where the SCM spasm occurs on the OPPOSITE side of the chin (e.g. left SCM spasm will rotate the head to the right, and chin will be on the right)
    
    SCM spasm is primary
  - reduced cervical rotation

Imaging
- Radiographs
  - recommended views
    
    AP, open-mouth odontoid
    
    look for variation in size and distance from midline of C2 lateral masses (reflects rotation)
    
    lateral
    
    facet joint appears anterior and wedge shaped instead of normal oval shape
    
    cervical flexion & extension views
    
    may be useful to exclude instability
    
    may be difficult due to position of head and resisted neck motion
- Dynamic CT
  - is diagnostic gold standard
  - take CT with head straight forward, and then in maximal rotation to right and left
    
    will see fixed rotation of C1 on C2 which does not change with dynamic rotation
- MRI
  - of little value unless neurologic symptoms

Treatment
- Nonoperative
  - soft collar, NSAIDs, exercise program
    
    indications
    
    subluxation present for < 1 week (traumatic or Grisel's disease)
    
    many patients probably reduce spontaneously before seeking medical attention
  - head halter traction, NSAIDs, benzodiazepines, then hard collar x 3 months
    
    indications
    
    subluxation persists > 1 week
    
    persistent torticollis in spite of soft collar (above) x 2 weeks
    
    technique
    
    small amount (5 lbs.) usually enough
    
    either in hospital or at home
    
    muscle relaxants and analgesics may be needed
  - halo traction, then halo vest x 3 months
    
    indications
    
    subluxation persists > 1 mos.
    
    failed halter traction x 2 weeks (above)
- Operative
  - posterior C1-C2 fusion
    
    indications
    
    subluxation persists > 3 mos
    
    neurologic deficits present
    
    failed halo traction x 2 weeks
    
    recurrent subluxation

Complications
- Missed diagnosis
  - diagnosis is often missed delayed

Fielding Classification of AARD
Type I	• Unilateral facet subluxation with intact transverse ligament • Odontoid acts as a pivot point with 1 facet subluxating anteriorly, 1 facet subluxating posteriorly. •Most common and benign type
Type II	• Unilateral facet subluxation with 3 to 5 mm of anterior displacement. • Injured Transverse ligament • 1 facet acts as a pivot point and 1 lateral mass is displaced anteriorly
Type III	• Bilateral anterior facet displacement of > 5 mm. • Rare with a higher risk of neurologic involvement or instantaneous death. • Both lateral masses are displaced
Type IV	• Posterior displacement of the atlas (C1) (with odontoid fracture, or hypoplastic dens) • Rare with a higher risk of neurologic involvement or instantaneous death.