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http://upload.orthobullets.com/topic/2043/images/Atlantoaxial lines xray_moved.jpg
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Introduction
  • Present in 90% of patients with RA
    • diagnosis often missed
  • Cervical rheumatoid spondylitis includes three main patterns of instability
    • atlantoaxial subluxation
      • most common form of instability
    • basilar invagination 
    • subaxial subluxation
Classification
 
Ranawat Classification
Class I Pain, no neurologic deficit
Class II Subjective weakness, hyperreflexia, dysesthesias
Class IIIA Objective weakness, long tract UMN signs, ambulatory
Class IIIB Objective weakness, long tract UMN signs, non-ambulatory. Do NOT operate
 
Presentation
  • Symptoms
    • symptoms and physical exam findings similar to cervical myelopathy 
    • neck pain
    • neck stiffness
    • occipital headaches
      • due to lesser occipital nerve, which is branch of the C2 nerve root
    • gradual onset of weakness and loss of sensation
  • Physical exam
    • hyperreflexia
    • upper and lower extremity weakness
    • ataxia (gait instability and loss of hand dexterity)
Imaging
  • Radiographs
    • flexion-extension xrays
      • always obtain before elective surgery
      • see subtopic for radiographic lines and measurements
  • CT scan
    • useful to better delineate bony anatomy and for surgical planning
  • MRI
    • study of choice to evaluate degree of spinal cord compression and identify myelomalacia
General Treatment
  • Nonoperative
    • pharmacologic therapy
      • pharmacologic treatment for RA has seen significant recent advances
        • has led to a decrease in surgical intervention 
  • Operative
    • spinal decompression and stabilization
      • indications 
        • goal is to prevent further neurologic progression and surgery may not reverse existing deficits
Atlantoaxial Subluxation
  • Introduction
    • present in 50-80% of patients with RA
    • most common to have anterior subluxation of C1 on C2 (can have lateral and posterior)
  • Mechanism
    • caused by pannus formation between dens and ring of C1 that leads to the destruction of transverse ligament and dens
  • Radiographs
    • controlled flexion-extension views to determine AADI and SAC/PADI
      • AADI (anterior atlanto-dens interval) 
        • instability defined as > 3.5 mm of motion between flexion and extension views 
          • instability alone is not an indication for surgery
        • > 7 mm of motion may indicate disruption of alar ligament
        • > 10 mm motion is indication for surgery
          • because of increased risk of neurologic injury
      • PADI / SAC (posterior atlanto-dens interval and space available for cord describe same thing) 
        • <14 mm is an indication for surgery
          • because of increased risk of neurologic injury 
        •  >13mm is the most important radiographic finding that may predict complete neural recovery after decompressive surgery  
  • Treatment
    • nonoperative
      • indicated in stable atlantoaxial subluxation
    • operative
      • posterior C1-C2 fusion
        • general indications for surgery
          • AADI > 10 mm (even if no neuro deficits)
          • SAC / PADI < 14 mm (even if no neuro deficits)
          • progressive myelopathy
        • indications for posterior C1-2 fusion
          • able to reduce C1 to C2 so no need to remove posterior arch of C1
        • technique
          • adding transarticular screws eliminated need for halo immobilization (obtain preoperative CT to identify location of vertebral arteries)
      • occiput-C2 fusion ± resection of posterior C1 arch
        • indications
          • when atlantoaxial subluxation is combined with basilar invagination
          • resection of C1 posterior arch for complete decompression
            • leads to indirect decompression of anterior cord compression by pannus
            • may be required if atlantoaxial subluxation is not reducible 
      • odontoidectomy
        • indications
          • rarely indicated
          • used as a secondary procedure when there is residual anterior cord compression due to pannus formation that fails to resolve with time following a posterior spinal fusion
            • pannus often resolves following posterior fusion alone due to decrease in instability
Basilar Invagination 
  • Introduction
    • also known as superior migration of odontoid (SMO)
      • tip of dens migrates above foramen magnum
    • present in 40% of RA patients
    • often seen in combination with fixed atlantoaxial subluxation
  • Mechanism
    • cranial migration of dens from erosion and bone loss between occiput and C1&C2
  • Imaging
    • radiographic lines
      • Ranawat C1-C2 index  
        • center of C2 pedicle to a line connecting the anterior and posterior C1 arches
        • normal measurement in men is 17 mm, whereas in women it is 15 mm
        • distance of < 13 mm is consistent with impaction
        • most reproducible measurement
      • McGregor's line  
        • line drawn from the posterior edge of the hard palate to the caudal posterior occiput curve
        • cranial settling is present when the tip of dens is more than 4.5 mm above this line
        • can be difficult when there is dens erosion
      • Chamberlain's line  
        • line from dorsal margin of hard palate->posterior edge of the foramen magnum
        • abnormal if tip of dens > 5 mm proximal Chamberlain's line
        • normal distance from tip of dens to basion of occiput is 4-5 mm
        • this line is often hard to visualize on standard radiographs
      • McRae's line
        • defines the opening of the foramen magnum
        • the tip of the dens may protrude slightly above this line, but if the dens is below this line then impaction is not present
    • MRI
      • cervicomedullary angle < 135° suggest impending neurologic impairment
  • Treatment
    • operative
      • C2 to occiput fusion
        • indications
          • progressive cranial migration (> 5 mm) 
          • neurologic compromise
          • cervicomedullary angle <135° on MRI
      • transoral or anterior retropharyngeal odontoid resection 
        • indications
          • brain stem compromise
Subaxial subluxation
  • Introduction
    • present in 20% with RA
    • often occurs at multiple levels
    • often combined with upper c-spine instability
    • lower spine involvement more common with
      • steroid use
      • males
      • seropositive RA
      • nodules present
      • severe RA
  • Pathophysiology
    • pannus formation and soft tissue instability of facet joints and Luschka joints
  • Radiographs
    • subaxial subluxation (of vertebral body) of >4mm or >20% indicates cord compression
    • cervical height index (body height/width) < 2.0 is almost 100% sensitive and specific for predicting neurologic compromise
  • Treatment
    • operative
      • posterior fusion and wiring
        • indications
          • > 4mm / >20% subaxial subluxation + intractable pain and neurologic symptoms
Operative Complications
  • Failure to improve symptoms
    • outcome less reliable in Ranawat Grade IIIB (objectively weak with UMN signs and nonambulatory) 
  • Pseudoarthrosis
    • 10-20% pseudoarthrosis rate
    • decreased by extension to occiput
  • Adjacent level degeneration
 

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