Atlantoaxial Instability

Introduction

• Atlantoaxial instability (AAI) is a condition in which excessive mobility occurs at the articulation between the atlas (C1) and axis (C2), potentially resulting in spinal cord compression. AAI is most clinically relevant during anesthesia because airway interventions and patient positioning can exacerbate instability.
• An understanding of the underlying anatomy, etiologic mechanisms, and perioperative anesthetic considerations is essential to prevent iatrogenic neurologic injury.¹

Anatomy of the Atlantoaxial Joint

• The atlantoaxial complex comprises the atlas (C1), axis (C2), and the odontoid process (dens). It is the most mobile joint in the body, contributing to the full range of neck rotation, producing the “yes” and “no” motions, and working with the occipitoatlantal joint to support the occiput.
• Its high degree of mobility also makes it particularly vulnerable to instability. Because the joint surfaces are relatively flat and rounded, features that permit movement in multiple directions, they also predispose the joint to become unstable. Based on clinical experience, atlantoaxial instability appears to be a frequently overlooked and inadequately managed condition.
• Stability is provided by bony anatomy and key ligamentous structures, including:
  • Transverse ligament of the atlas (primary restraint against anterior translation)
  • Alar ligaments (limit rotation and lateral flexion)
  • Tectorial membrane
• Injury or laxity of these ligaments predisposes patients to instability.¹

Clinical Significance

• Given that the normal canal diameter is only ~17-20mm, small increases in C1–C2 motion can significantly narrow the spinal canal, which contains important neurological structures, including the spinal cord.
• Neurologic injury can occur from direct cord compression or compromised blood flow through the vertebral arteries.¹

Etiology

AAI may be congenital, inflammatory, or traumatic.

Congenital Causes

• Down syndrome (Trisomy 21)
  • Radiographic AAI (often defined as atlantodens interval ≥4.5–5 mm) is seen in roughly 8–30% of patients with Down syndrome, while only about 1–2% are symptomatic.²
  • Pathogenesis involves intrinsic collagen abnormalities that cause generalized ligamentous laxity (especially of the transverse ligament), odontoid hypoplasia, and shallow or malformed C1–C2 facets, all of which increase translational motion at the atlantoaxial joint.²
• Morquio syndrome (mucopolysaccharidosis IV)
  • Morquio syndrome is associated with progressive odontoid hypoplasia and ligamentous laxity; with growth, this leads to severe craniovertebral junction stenosis and high risk of myelopathy.
  • Periodic flexion–extension cervical imaging is recommended after age 5.
  • Other mucopolysaccharidoses can also cause C1–C2 instability through glycosaminoglycan deposition in bone and ligaments, resulting in hypoplastic vertebral bodies and thickened soft tissues.³
• Os odontoideum
  • The dens is separated from the C2 body as an independent ossicle, and it predisposes to marked sagittal and rotational instability.⁴
  • It can be congenital or represent an unrecognized childhood fracture nonunion.
• Congenital C1–C2 malformations associated with pediatric rheumatoid like arthropathies and other skeletal dysplasias (e.g., occipitalization of the atlas, segmentation defects, and hypoplastic posterior arches) can similarly compromise stability at the atlantoaxial level.⁴

Inflammatory Causes

• Rheumatoid arthritis (RA)
  • Chronic synovitis of the atlantoaxial joint and transverse ligament leads to pannus formation, erosive changes of the dens, and progressive attenuation or rupture of the transverse and alar ligaments.
  • AAI is reported in roughly 60–65% of RA patients with cervical spine involvement, and anterior subluxation is the most frequent pattern.⁵
• Ankylosing spondylitis and juvenile idiopathic arthritis
  • It can produce cervical synovitis and enthesitis, leading to ligamentous weakening, erosions, and, ultimately, instability or fixed deformity at C1–C2.
  • Inflammatory destruction tends to be insidious, so flexion–extension cervical radiographs and, when indicated, MRI are used in long‑standing disease or pre‑intubation assessment to detect occult AAI.⁵
• Infectious processes
  • Grisel syndrome
    • A nontraumatic atlantoaxial rotatory subluxation typically occurring in children after upper respiratory tract infections or ENT procedures.
    • The proposed mechanism is an inflammatory hyperemia of the paravertebral venous plexus spreading from the pharynx to the C1–C2 ligaments, leading to painful torticollis and rotatory subluxation without fracture.
  • Osteomyelitis or septic arthritis
    • It can be pyogenic or tuberculous, can destroy the odontoid process, lateral masses, or stabilizing ligaments, resulting in painful, often rapidly progressive AAI.
    • In tuberculous disease of the craniovertebral junction, paradental granulation tissue and bony destruction may mimic pannus radiologically and can cause severe cord compression even with modest subluxation.¹

Traumatic Causes

• Fracture of C1, C2, or odontoid process
  • High-energy trauma can produce Jefferson fractures of C1, odontoid fractures (Anderson–D’Alonzo types II and III), or hangman-type injuries of C2, all of which may disrupt the ring integrity of C1–C2 and permit pathologic translation (Figure 3).
• Ligamentous disruptions following blunt trauma
  • Even when fractures are not radiographically obvious, flexion–extension mechanisms can rupture the transverse or alar ligaments, leading to purely ligamentous AAI after blunt trauma.

Pathophysiology

Instability at C1-C2 leads to increased anterior and posterior translation, excessive rotation, or vertical subluxation. Consequences include:

Spinal Cord Compression

• Anterior displacement of the atlas or dens can compress the cervical cord
• Symptoms: gait disturbance, clonus, hyperreflexia, loss of proprioception, or, in severe cases, quadriparesis

Compromised Vertebral Artery Flow

• Excessive rotation narrows the vertebral arteries, potentially causing vertebrobasilar insufficiency.
• Symptoms: syncope, vertigo, vision changes

Progressive Myelopathy

• Chronic compression (e.g., RA pannus formation) may cause irreversible neurologic deficits¹

Anesthetic Considerations

Preoperative Evaluation

• A focused evaluation for atlantoaxial instability includes documenting neurologic symptoms such as weakness, sensory changes, gait issues, incontinence, and dysphagia, while also reviewing risk factors like rheumatoid arthritis, Down syndrome, and prior cervical procedures.
• The physical exam should include a thorough neurologic assessment of motor function, sensation, reflexes, and pathologic signs, and a careful evaluation of cervical mobility with minimal manipulation to avoid exacerbating instability.^5,6

Imaging

Imaging may be obtained based on history, exam findings, or high-risk diagnoses.

• Flexion–extension cervical spine radiographs
  • The atlas-dens interval is measured as the distance from the posterior cortex of the anterior arch of C1 to the anterior surface of the odontoid; values greater than 3 mm in adults or 4–5 mm in children suggest instability⁵ (Figure 4).
  • Space available for the cord is measured on the lateral view from the posterior margin of the odontoid (or posterior vertebral body) to the anterior aspect of the posterior arch or lamina; a value less than 13 mm suggests clinically significant instability.⁶

• Computed tomography
  • Best for osseous abnormalities.
• Magnetic resonance imaging (MRI)
  • Recommended when neurologic symptoms are present.
  • Useful for evaluating spinal cord compression, pannus, or ligamentous injury.

Routine screening radiographs for asymptomatic patients with Down syndrome are no longer universally recommended, but anesthesia planning may still warrant imaging.^5,6

Airway Management Considerations

• The highest risk of neurologic injury occurs during airway manipulation, particularly during neck extension or rotation. Excessive motion can convert subclinical instability into acute spinal cord compression. The goal is to maintain neutral cervical alignment and minimize movement at the atlantoaxial joint.⁷

Techniques to Minimize Cervical Spine Motion

• MILS
  • MILS is achieved by removing the anterior collar; an assistant then stabilizes the occiput and mastoid processes to counteract forces generated by laryngoscopy while the patient remains in neutral alignment.
  • It reduces gross cervical motion but significantly worsens glottic visualization with direct laryngoscopy, increasing the incidence of Cormack–Lehane grade III views and the need for adjuncts such as a bougie or gum elastic introducer.⁷
• Awake Fiberoptic Intubation (AFOI) – gold standard
  • The least cervical movement occurs with AFOI because the airway is entered under direct endoscopic vision with the head maintained strictly neutral and without forceful jaw elevation.
  • It allows for documentation of neurologic status before and after intubation and positioning, but requires patient cooperation, adequate topicalization and sedation, and operator expertise; heavy secretions or blood can compromise success.^5,7
• Videolaryngoscopy – most frequently used
  • Videolaryngoscopy allows an indirect view of the glottis with less need to align oral, pharyngeal, and laryngeal axes, thereby reducing upper cervical extension compared with Macintosh blades.
  • Randomized data show devices such as the C MAC D Blade can reduce occiput–C1 motion by about 40–50% relative to direct laryngoscopy in simulated cervical immobilization, and observational series suggest videolaryngoscopy is now the most frequently used technique in unstable cervical injury.^5,7
• Supraglottic airway placement
  • Second-generation supraglottic airways can be placed with minimal mouth opening and only modest head elevation, providing a temporizing ventilatory route in “cannot intubate” or “bridge to fiberoptic” scenarios.
  • However, biomechanical and cadaveric studies indicate that laryngeal mask airways (LMAs) may exert higher posterior forces on the cervical vertebrae and can cause small degrees of posterior displacement, so they are generally reserved for rescue or controlled use rather than first line in known gross instability.^8,9
  • Supraglottic airways permit blind, fiberoptic-guided, or bougie-assisted tracheal intubation through the device. At the same time, the neck remains in a near-neutral position and has been successfully used in patients with cervical pathology undergoing spine surgery.
  • Cinefluoroscopic studies show that LMA-guided intubation primarily causes lower cervical flexion, with relatively limited movement at C1–C2, compared with direct laryngoscopy; however, it still generates measurable vertebral pressure, so its use remains controversial and should be individualized and accompanied by MILS when feasible.⁷