Neurosurgery notes/Spine/Special diseases of the spine/Rheumatoid arthritis (RA)/Anterior atlantoaxial subluxation in RA

Anterior atlantoaxial subluxation in RA

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Rheumatoid arthritis (RA)

Numbers

  • the most common manifestation of RA in the cervical spine, found in up to 25% of patients with RA
  • Mean time between onset of RA symptoms to the diagnosis of AAS: 14 years.
  • Mean age at onset of AAS symptoms: 57 years.

Mech

  • Inflammatory involvement of the atlantoaxial synovial joints → erosive changes in the odontoid process (anteriorly at the synovial joint with the C1 arch, and posteriorly at the synovial joint with the transverse ligament) → decalcification and loosening of the insertion of the transverse ligament on the atlas → These changes lead to instability → a scissoring effect with anterior subluxation of C1 on C2.

Clinical

  • Usually slowly progressive.
  • Pain
    • Most common presentation
      • Local: 67%
        • Upper cervical and suboccipital regions, often from compression of C2 nerve root
      • Referred: 27%
        • To mastoid, occipital, temporal, or frontal regions
  • Vertebrobasilar insufficiency may occur from VA involvement
  • Sensory disturbance 20%
  • Neurogenic bladder
  • Myelopathy
    • Rarer
    • Due to
      • Mechanical neurovascular compression on the cervical spine and cervicomedullary junction.
    • Hyperreflexia 67%
      • The deep tendon reflex may not be elicited in RA because of appendicular joint destruction.
    • Spasticity 27%
    • Paresis 27%
    • Others
      • Weakness
      • Hyperreflexia
      • Positive Hoffmann
      • Babinski
      • Lhermitte signs
      • Clumsiness
    • Cruciate paralysis and even sudden death from respiratory arrest
    • Positive Sharp-Purser test is a clicking sensation in extension that results with spontaneous reduction of atlantoaxial subluxation
      • Ranawat classification of myelopathy
      Class
      Description
      I
      no neural deficit
      II
      subjective weakness + hyperreflexia + dysesthesia
      III
      objective weakness + long tract signs
      Ill A=ambulatory
      Ill B=quadriparetic & non ambulatory

Radiographic evaluation

  • Lateral C-spine X-ray
    • The magnitude of AAS usually increases with neck flexion.
    • Anterior atlantodental interval (ADI).
      • Gives information about the stability of the C1–2 joint.
      • The normal ADI in adults is < 3–4mm.
      • Widening of the ADI suggests possible incompetence of the transverse ligament.
      • does not correlate with the risk of neurologic injury
      • not predictive of progression from asymptomatic AAS to symptomatic AAS.
    • Posterior atlantodental interval (PADI)
      • The amount of room available for the spinal cord can vary for any given ADI depending on the AP diameter of the spinal canal and the thickness of any pannus.
      • The PADI and the AP diameter of the subaxial canal measured on a lateral C-spine Xray
        • correlates with the presence and severity of paralysis
        • predicts neurologic recovery following surgery.
          • Patients with paralysis from AAS showed no recovery if the pre-op PADI was < 10mm
        • PADI ≤ 14mm has been proposed as an indication for surgical stabilization.
    • Both the ADI and PADI (see above) are surrogate markers for instability and for spinal cord compression.
    • With the availability of MRI, the ability to directly assess spinal cord compression has diminished the usefulness of these measurements.
  • MRI
    • MRI is the optimal test to evaluate the source and magnitude of upper cord or medullar compression.
    • Demonstrates location of odontoid process, extent of pannus, and effects of subluxation .
  • CT scan
    • In addition to providing information that is invaluable for surgical planning, CT gives detailed information about the degree of bone destruction which can help in assessing stability. Bony erosion at the insertions sites of the transverse ligament on the C1 tubercles (▶Fig. 1.14) is a marker for potential instability due to incompetence of the ligament.

Natural history:

  • AAS in most patients progresses, with a small percentage either stabilizing or fusing spontaneously.
  • In one series with 4.5 years mean follow-up,
    • 45% of patients with 3.5–5mm subluxation progressed to 5–8mm,
    • 10% of these progressed to > 8mm
  • Once myelopathy occurs, it may be irreversible
    • The worse the myelopathy, the higher the risk for sudden death
    • the chances of finding myelopathy are significantly increased once the subluxation reaches ≥ 9mm10
  • Associated cranial settling further decreases the tolerance for AAS
  • The life expectancy of patients with RA is 10 years less than the general population
  • The morbidity and mortality of surgical treatment
  • Pannus often recedes after surgical fusion.
    • This may be enhanced with the use of tumor necrosis factor alpha (TNF-α) inhibitors such as etanercept (Enbrel) or adalimumab (Humira), or monoclonal antibodies against TNF-α, e.g. infliximab (Remicade)

Treatment

  • General information
    • Requires knowledge of the following information:
  • When to treat surgically?
    • Symptomatic patients with AAS:
      • Almost all require surgical treatment (C1–2 fusion in most cases).
      • Some surgeons do not operate if the maximal dens-C1 distance is < 6mm
    • Asymptomatic patients: controversial
      • Asymptomatic but ADI > 10 mm and SAC < 14 mm
      • Some authors feel surgical fusion is not necessary in asymptomatic patient if the ADI distance is below 8mm (an unvalidated delineation)
        • These patients are often placed in a rigid cervical collar, e.g. while outside the home, even though it is generally acknowledged that a collar probably does not provide significant support or protection
      • Some cases of sudden death in previously asymptomatic RA patients may be due to AAS and may then be erroneously attributed to cardiac arrhythmias, etc.
  • Surgical management
    • Options
      • C1-C2 instrumented fixation
        • Indication
          • Symptomatic instability
          • Reducible subluxation
            • Menezes assesses all subluxed patients for reducibility using MRI compatible halo cervical traction as follows:
              • Start with 5 lbs, and gradually increase over a period of a week.
              • Most cases reduce within 2–3 days.
              • If not reduced after 7 days then it is probably not reducible.
                • Only ≈ 20% of cases are not reducible (most of these have odontoid > 15mm above foramen magnum).
      • Occipitocervical fusion (O-C2)
        • Indication
          • Preop presence of basilar invagination or high risk of developing it
          • if anterior compression from pannus requires C1 posterior arch resection.
        • Posterior fusion
          • In RA, erosion and osteoporosis weakens the C1 arch, and extra care is needed to avoid fracturing it.
      • Anterior release of odontoid --> posterior fusion (C1-2 or C0-2)
        • Indication
          • Subluxation is not reducible
          • Anterior pannus compressing the upper cervical spine
        • Odontoidectomy
          • Through
            • transoral microscopic
            • transnasal endoscopic
          • The patient must be able to open the mouth greater than ≈ 25mm in order to perform transoral odontoidectomy without splitting the mandible.
    • Surgical morbidity and mortality
      • Because of the frequency of simultaneous involvement of other systems in RA, including pulmonary, cardiac, and endocrine, operative mortality ranges from 5–15%.
      • The non-fusion rate for C1–2 wiring and fusion has been reported as high as 50%, typical rates are lower (with 18% of patients in one series developing a fibrous union).
      • The most common site of failure of osseous fusion is the interface between the bone graft and the posterior arch of C1.
    • Postoperative care
      • The patient is usually mobilized almost immediately post-op in halo vest traction (some use an optional period of maintained traction before mobilization).
      • Impaired healing in RA dictates that the halo be worn until fusion is well-established, as seen on X-ray (usually 8–12 weeks).
      • Sonntag evaluates the patient with flexion-extension lateral C-spine X-rays by disconnecting the halo ring from the vest.