A 45-year-old woman presented with chronic bilateral proximal hand and wrist swelling and progressive lower extremity spasticity.
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Figure 1: Lateral cervical spine radiograph (A). Axial CT image (B). Post-myelography axial CT image (C).
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Answer to Radiologic Case Study
Atlantoaxial Instability Secondary to Rheumatoid Arthritis
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Figure 1: Lateral view of the cervical spine shows abnormal widening of the predental space to 6 mm (A). The superior margin of the odontoid process is ill defined. Axial computed tomographic image of the C1-C2 articulation confirms abnormal widening of the predental space and erosion of the odontoid process (B). Abnormal soft tissue consistent with pannus is seen posterior to the odontoid process (C). These findings are characteristic of rheumatoid arthritis with C1-C2 instability.
The images provided in the test case (Figure 1) represent atlantoaxial instability secondary to rheumatoid arthritis. Loss of stability has occurred at the anterior C1-C2 articulation secondary to pannus formation with odontoid erosion and ligamentous disruption.
Atlantoaxial instability is defined as loss of the stable configuration of the anterior C1-C2 synovial articulation resulting in abnormal movement at this site. The odontoid process or dens normally is combined to the anterior arch of C1, allowing little motion with extension or flexion. However, 50% of rotation in the cervical spine occurs at this level. The integrity of this articulation is primarily maintained by the strong inelastic fibers of the transverse ligament that course between the lateral masses of C1 posterior to the base of the odontoid process (Figure 2). The more flexible alar ligaments that extend from the junction of the C1 anterior arch and lateral mass to the apex of the odontoid process provide less reinforcement. The shape of the odontoid process also provides stability as the curved margin of the odontoid process fits into a reciprocal groove of the posterior aspect of the C1 anterior arch. When the odontoid process is not securely attached to C1, odontoid process movement can narrow the spinal canal and impinge on the adjacent spinal cord. Although the cervical spinal canal is widest at the cephalad aspect, the cephalad spinal cord is also relatively larger and at greater risk for impingement. Stability of this articulation is essential to maintaining normal spinal cord function.
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Figure 2: Axial CT image of cervical spine at level of odontoid process shows linear calcifications (arrows) defining the normal transverse cervical ligament posterior to odontoid process.
The clinical presentation of patients with atlantoaxial instability is highly variable, depending on the degree of cord impingement and the etiology of C1-C2 instability. Some patients have only mild signs and symptoms that do not progress, while others have progressive signs and symptoms with deterioration on imaging studies that lead to disabling neurologic outcomes.1 Neurologic manifestations of atlantoaxial instability occur when canal narrowing is sufficient to cause posterior impingement by the C1 posterior arch, or anterior impingement by the migrated odontoid process or pathologic soft tissue, such as pannus.
Normally, the area of the spinal canal safely accommodates the spinal cord. Steel2 described the ‘rule of thirds’; one-third of the diameter of the spinal canal at C1-C2 is occupied by the odontoid process, one-third by the cord, and the remaining one-third is unoccupied allowing space for the cord during movements of the head and neck. In general, an anterior atlantoaxial subluxation <10 mm is unlikely to cause neurologic symptoms and signs unless further narrowing occurs due to associated granulation tissue or proliferative synovitis in patients with inflammatory or crystalline arthritis.3
Neck pain radiating superiorly towards the occiput is one of the earliest and most common symptoms of atlantoaxial instability,4 and may be associated with early signs of myelopathy. Symptoms include general upper and/or lower extremity radiating pain, Lhermitte’s sign (electric shock sensation radiating down the spine), loss of fine motor control, and gait changes. Objective signs of chronic cord impingement include slowly progressive spasticity with upper motor neuron signs including hyper-reflexia, Hoffman’s sign, clonus, and a Babinski sign. Rarely, progression leads to paraplegia, hemiplegia, quadriplegia, or death.5
Patients may experience weakness of the diaphragm, and respiratory insufficiency can develop as patients become more reliant on accessory muscles of respiration. More advanced lesions extending to the medullary respiratory center are usually fatal. Miosis, ptosis, and anhidrosis (Horner’s syndrome) due to sympathetic involvement can accompany a cervical cord myelopathy at any level.
Due to the complex regional anatomy, ligamentous as well as bone pathology can contribute to atlantoaxial instability, and imaging may be key to identification and characterization of both.
Radiography in the neutral position remains the mainstay for primary imaging of patients with potential C1-C2 instability. On neutral lateral views, the atlantodental interval, measured from the posteroinferior margin of the anterior arch of the atlas to the anterior surface of the odontoid process, should not exceed 3 mm in adults, or 5 mm in very young children.3 In some cases, abnormal widening is not shown on a lateral view in the neutral position but is visible with flexion. The articulation occasionally is V-shaped, and measurement should then be taken at the narrowest aspect, which is inferior. On open mouth odontoid view, the odontoid process should be centered between the lateral masses of C1, and should be well formed and symmetric.
Traumatic instability may be the result of isolated ligamentous injury—usually a tear of the transverse cervical ligament, or a fracture of the base of the odontoid process. Isolated ligamentous, post-traumatic atlantoaxial subluxation is a rare injury and accounts for 1% of cervical injuries.3,6 Inferior odontoid fractures are more common, accounting for approximately 7% of cervical spine fractures.
Widening of the C1-C2 articulation may be apparent on the lateral view, and prevertebral soft-tissue swelling may occur. Anterior cortical irregularity due to odontoid fracture may be seen, and the odontoid process may be offset on anteroposterior open-mouth views. A transverse or oblique fracture of the base of odontoid may also be seen extending into the body of C2 on open mouth odontoid views (Figure 3). Although controversy exists regarding the exact etiology, an os odontoideum may be the sequela of a fracture of the base of the odontoid process with nonbony union. This, too, contributes to C1-C2 instability (Figure 4).
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Figure 3: In a 56-year-old man, lateral radiograph of cervical spine shows relative anterior subluxation of odontoid process and anterior arch of C1 secondary to fracture of base of odontoid process (A). Sagittal reformat image of cervical spine from axial CT images shows displaced fracture (arrow) to better advantage (B). Figure 4: In a 23-year-old patient, lateral view of cervical spine voluntarily positioned in flexion, shows abnormal motion at C1-C2 secondary to os odontoideum (arrow).
C1-C2 instability secondary to connective tissue disease, most often rheumatoid arthritis, is far more common than traumatic instability. As many as 32% of patients with rheumatoid arthritis have been reported to have some degree of atlantoaxial instability.7 The frequency rises with increasing severity of the disease.8 Rheumatoid pannus and associated inflammation of synovial tissue affect the apophyseal joints, the transverse ligament, and the alar ligaments—ultimately causing instability due to ligamentous attenuation or disruption. In addition, the odontoid process may be eroded. Other inflammatory causes of atlantoaxial instability include seronegative spondyloarthropathies such as ankylosing spondylitis. While the frequency of subluxation with ankylosing spondylitis is less than rheumatoid arthritis, similar synovial and ligamentous abnormalities are observed.9 Atlantoaxial subluxation secondary to ankylosing spondylitis is most often in the later stages of the disease. As with trauma, widening of the atlantoaxial interval may be apparent on the lateral view. Erosive changes of the odontoid are seen on both lateral and open-mouth AP views (Figures 5 and 6). The odontoid process usually is midline between the lateral masses of C1. Prevertebral soft-tissue swelling may or may not be visible, depending on the activity of the inflammatory process. Patients with ankylosing spondylitis also may have characteristic bony proliferation around the atlantoaxial junction that can actually provide stabilization of the subluxed spine.
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Figure 5: In a 41-year-old woman with rheumatoid arthritis, Fuch’s view of odontoid process shows abnormal erosions at base of odontoid process (A), and lateral tomogram of upper cervical spine confirms erosions (arrows) at base of odontoid process (B).
Atlantoaxial instability also can be the result of infection. Tuberculosis involving the atlantoaxial joint is rare, but has been reported in 2% of patients with tuberculous spondylitis and can result in atlantoaxial subluxation.10 In children, acute atlantoaxial subluxation is sometimes seen after oropharyngeal infection. This phenomenon, Grisel’s syndrome, is the result of hematogenous spread of infection from the oropharynx by way of a periodontoid vascular plexus that drains the superior pharyngeal region.11
Several congenital conditions that cause odontoid dysplasia or ligament incompetence can result in atlantoaxial instability. The most notable is Down’s syndrome, where an estimated 10%-20% of individuals show atlantoaxial instability radiographically.12 This instability is primarily attributed to congenital laxity of the transverse ligament, but as many as 26% of patients also have hypoplasia of the posterior arch of C1, which further narrows the spinal canal diameter and increases the risk of neurologic sequelae.13 Some patients with Down’s syndrome may not communicate their symptoms well, therefore routine radiographic screening has been advocated in this patient population.13
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Figure 6: In an 11-year-old girl, lateral radiograph of cervical spine shows erosions of superior odontoid process as well as widening of atlantoaxial interval due to ligamentous disruption. Figure 7: In a 16-year-old boy with os odontoideum, lateral view of cervical spine, voluntarily positioned in extension, shows near normal C1-C2 alignment, as well as os odontoideum (A), and lateral view, voluntarily positioned in flexion, shows markedly abnormal C1-C2 alignment (B).
Lateral views of the cervical spine with the patient voluntarily positioned in flexion and extension may unmask instability in the cervical spine. In particular, the flexion view may show widening of the space, which reduces to some degree on extension (Figure 7). 14 The overall degree of motion performed for these radiographs should be monitored to assure some degree of ligamentous challenge. In addition, for patient safety, the patient should perform the maneuver themselves because forced motion could worsen a partial ligamentous injury or result in impingement on the cord with clinical sequelae.
In some situations, however, flexion and extension views are indicated on a routine basis. For example, this is used as a screening measure in individuals with Down’s syndrome who wish to participate in contact sports, and for individuals with rheumatoid arthritis as preoperative screening of patients who will undergo general anesthesia with endotracheal intubation.
Computed tomography (CT) is well suited to evaluate the bony relationships and demonstrate cord impingement, and these findings correlate well with neurologic status.15 It should be emphasized, however, that the atlantoaxial interval as assessed with CT has limited sensitivity if the study is done in the extended or neutral position. Normal findings on CT do not exclude atlantoaxial instability unless the study is performed with cervical flexion. To accomplish this, axial images are acquired with the patient positioned in neck flexion, and alignment is assessed on sagittal reformat images. Computed tomography also is very sensitive for the diagnosis of fractures. A fracture at the base of dens often is associated with instability, even in the presence of a normal C1-C2 interval (Figure 8).
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Figure 8: In an 82-year-old man, sagittal reformat CT image in neutral position confirms fracture at base of odontoid process (arrow), but alignment appears anatomic (A). With voluntarily positioning in flexion, subluxation is seen with widening of C1-odontoid process space (B) With voluntarily positioning in extension, proximal cervical alignment is restored (C).
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) allows direct visualization of the C1-C2 articulation, including the synovial compartments, bone marrow, and supporting ligamentous structures. Soft-tissue distortion and inflammatory masses, including pannus in patients with rheumatoid arthritis (Figure 9) and ankylosing spondylitis, abscesses, and granulomas due to infection are well demonstrated.16 Further, the consequences of cord impingement are clearly shown, including mechanical impingement and development of abnormal signal within the cord, such as myelomalacic changes and syrinx (Figure 10).
Patients with chronic atlantoaxial instability may develop a pseudotumor mass at the craniovertebral junction as a result of chronic mechanical irritation. These masses are composed of fibrous and granulation tissue. In MRI, these masses have low signal intensity in T1-weighted spin echo images, intermediate or high signal in T2-weighted spin echo images, and enhancement following contrast infusion.17 Flexion and extension MRI is more sensitive than CT or radiographs for demonstrating details of atlantoaxial pathology and magnitude of cord compression.18 An advantage of MRI compared to CT is the ability to perform direct sagittal imaging. Again, positioning should be voluntarily performed by the patient.
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Figure 9: Sagittal T2-weighted spin echo image of the craniocervical junction shows severe whittling of odontoid process. High T2-signal material around odontoid process (arrows) represents pannus. Obliteration of cerebrospinal fluid interface occurs at this level. Figure 10: In a 59-year-old man with spinal cord atrophy, sagittal T1-weighted spin echo image following gadolinium infusion shows severely eroded odontoid process (arrow) impinging on spinal cord with resultant cord malacia/atrophy.
The main goals in treatment of atlantoaxial instability are decompressing neural elements and stabilizing the spine to prevent further injury to the spinal cord.19 Because minimal trauma of an unstable atlantoaxial joint can lead to serious neurological injury, surgical stabilization of the joint is indicated. Specific treatment recommendations depend on the etiology of the instability and the timing in the case of an acute injury. Instability secondary to a dens fracture (especially if it extends into the C2 vertebral body), or transverse ligament avulsion (with a bony fragment still intact), may be treated non-operatively with a halo vest or cervical collar. Posterior surgical fusion generally is recommended for patients with instability >5 mm in cases of dens anomaly, and for patients with signs of neurologic compromise.20 Several C1/C2 posterior fusion techniques are currently used. One of the most widely used is the technique of Magerl,21 which combines interspinous wiring with transarticular screw fixation. Not all patients are candidates for this technique, depending on the position of the vertebral artery. Most surgeons who have used this technique have reported successful atlantoaxial fusion rates in excess of 85%.22 Other techniques include C1-C2 wiring (if the posterior arch of C1 and C2 are intact) and C1-C2 screw-rod constructs. If instability is due to a dens fracture that is not amenable to conservative treatment, odontoid screws are sometimes recommended.
Multiple elements contribute to the stability of the anterior C1-C2 articulation, making this region subject to pathologies including trauma, inflammation, infection, and congenital deformities. C1-C2 instability places a patient at risk for significant neurologic compromise. Radiologic imaging plays a fundamental role in diagnosing atlantoaxial instability, indicating etiology, showing details of associated abnormalities, and providing information for planning treatment.
- Boden SD, Dodge LD, Bohlman HH, Rechtine GR. Rheumatoid arthritis of the cervical spine: a long-term analysis with predictors of paralysis and recovery. J Bone Joint Surg Am. 1993; 75:1282-1297.
- Steel HH. Anatomical and mechanical considerations of the atlanto-axial articulations. J Bone Joint Surg Am. 1968; 50: 1481-1490.
- Tucker SK, Taylor BA. Spinal canal capacity in simulated displacements of the atlantoaxial segment: a skeletal study. J Bone Joint Surg Br. 1998; 80:1073-1078.
- Stevens JC. Atlant-oaxial subluxation and cervical myelopathy in rheumatoid arthritis. QJM. 1971; 40:391-408.
- Marcos A. Atlantoaxial instability and ligamentous hyperlaxity in Down Syndrome. Acta Orthopédica Brasileira. 2005; 13:165-167.
- Bohlman HH. Acute fractures and dislocations of the cervical spine. An analysis of three hundred hospitalized patients and review of the literature. J Bone Joint Surg Am. 1979; 61:1119-1142.
- McRorie ER, McLoughlin P. Cervical spine surgery in patients with rheumatoid arthritis: an appraisal. Ann Rheum Dis. 1996; 55: 99-104.
- Bland JH. Rheumatoid subluxation of the cervical spine. J Rheumatol. 1990; 17:134-137.
- Sharp J, Purser DW. Spontaneous atlantoaxial dislocation in rheumatoid arthritis and ankylosing spondylitis. Ann Rheum Dis. 1961; 20:44-77.
- Levin MF, Vellet AD, Munk PL, Chait PG. Tuberculosis of the odontoid bone: a rare but treatable cause of quadriplegia. J Can Assoc Radiol. 1992; 43:199-202.
- Mathern GW, Batzorf U. Grisel’s syndrome. Cervical spine clinical, pathologic, and neurologic manifestations. Clin Orthop Relat Res. 1989; 244:131-146.
- Morton RE, Khan MA, Murray-Leslie C, Elliott S. Atlantoaxial instability in Down’s syndrome: a five-year follow-up study. Arch Dis Child. 1995; 72:115-118.
- Martich V, Ben-Ami T. Hypoplastic posterior arch of C-1 in children with Down syndrome: a double jeopardy. Radiology. 1992;183:125-128.
- Abel MS. Occult traumatic lesions of the cervical vertebrae. CRC Critical Reviews in Clinical Radiology and Nuclear Medicine. 1975; 6:469-553.
- Raskin RJ, Schnapf DJ, Wolf CR, Killian PJ, Lawless OJ. Computerized tomography in evaluation of atlantoaxial subluxation in rheumatoid arthritis. J Rheumatol. 1983; 10:33-41.
- Kawaida H, Sakou T, Morizono Y, Yoshikuni N. Magnetic resonance imaging of upper cervical disorders in rheumatoid arthritis. Spine. 1989; 14:1144-1148.
- Sze G, Brant-Zawadzki MN, Wilson CR, Norman D, Newton TH. Pseudotumor of the craniovertebral junction associated with chronic subluxation: MR imaging studies. Radiology. 1986; 161:391-394.
- Smoker WR, Keyes WD, Dunn VD, Menezes AH. MRI versus conventional radiologic examinations in the evaluation of the craniovertebral and cervicomedullary junction. Radiographics. 1986; 6:953-994.
- Proctor M. Spinal cord injury. Crit Care Med. 2002; 30:S489-S499.
- Moon MS, Choi WT, Moon YW, Moon JL, Kim SS. Brooks’ posterior stabilization surgery for atlantoaxial instability: review of 54 cases. J Orthop Surg (Hong Kong). 2002; 10:160-164.
- Magerl F, Seeman P. Stable posterior fusion at the atlas and axis by transarticular screw fixation. In: Kehr P, Weidman A, eds. Cervical Spine. New York, NY: Springer-Verlag; 1987:322-327.
- Madawi AA, Casey AT, Solanki GA, Tuite G, Veres R, Crockhard HA. Radiological and anatomical evaluation of the atlantoaxial transarticular screw fixation technique. J Neurosurgery. 1997; 86:961-968.
Mr Timpone is from the Stritch School of Medicine, Drs Lomasney, Demos, and Woods are from the Department of Radiology, and Dr Rinella is from the Department of Orthopedics, Loyola University Medical Center, Maywood, Ill.
Reprint requests: Terrence C. Demos, MD, Dept of Radiology, Building 103, Loyola University Medical Center, 2160 S First Ave, Maywood, IL 60153.