Retro-odontoid pseudotumors are mass-like lesions induced by damage of surrounding structures of the odontoid process including the anterior arch of the atlas and the transverse ligaments1. These pseudotumors occur in the setting of a variety of diseases including rheumatoid arthritis, os odontoideum, odontoid fracture, atlas hypoplasia, hemodialysis, and chronic cryptogenic atlantoaxial instability (AAI). Retro-odontoid pseudotumor formation induced by chronic AAI can compress the spinal cord and induce myelopathy or neurologic paralysis2-4. Atlantoaxial arthrodesis with posterior fixation is the standard method for treatment of this pathologic process5-7. When symptomatic, they usually require surgical treatment; thus, the natural history of untreated or nonoperatively treated pseudotumors is unknown. In this case report, we present a rare case of a patient who was managed nonoperatively due to multiple comorbidities. She was treated with external orthosis and showed remarkable, sustained regression of the retro-odontoid pseudotumor for 8 years as evident on magnetic resonance imaging (MRI).
The patient was informed that data concerning the case would be submitted for publication, and she provided consent.
An 87-year-old women with a medical history of a severe arrhythmia and hypertension was admitted to our institution complaining of a 6-month history of cervical pain along with bilateral stiffness of the paravertebral muscles. She progressively developed a spastic gait as well as a movement disorder of the upper extremities. Ten-second grasp and release test result was 8 times per side. Micturition problems were also described, with a postvoid residual urine volume of 700 mL. A mild tactile hyperalgesia was obvious on the trunk and all 4 limbs, and bilateral finger and lower leg numbness was also observed. Muscle strength was 5/5 on bilateral upper and lower extremities, but grip strength was relatively weak with 10 kg on the right and 5 kg on the left. Appendicular hyperreflexia of the deep tendon reflexes as well as bilateral Achilles clonus was noted. The patient's Japanese Orthopaedic Association (JOA) score at admission was 4 out of 17 points. On lateral plain X-ray images, the atlantodental interval was 8 mm on flexion and 4 mm on extension (Figs. 1-A and 1-B), with spondylotic change in middle-to-lower cervical segment. Coronal section on computed tomography showed osteoarthritic changes of the atlantoaxial joint with narrowing of the lateral atlantoaxial joint line (Fig. 2). MRI demonstrated a retro-odontoid pseudotumor with low signal intensity both in T1 and T2 weighted images, with severe compression of the spinal cord and a high T2 signal intensity inside the cord at the retro-odontoid level, with narrowing of the subarachnoid space at the lower level (Figs. 3-A and 3-B).
On lateral plain X-ray, atlantoaxial instability was noted. Atlantodental interval was 8 mm upon flexion (Fig. 1-A) and 4 mm upon extension (Fig. 1-B).
Figs. 3-A and 3-B Magnetic resonance imaging before treatment demonstrated a retro-odontoid pseudotumor with low signal intensity both in T1- and T2-weighted imaging, with severe compression of the spinal cord and a high T2 signal intensity inside the cord at the retro-odontoid level.
Four days after her first visit to our outpatient clinic, the patient was admitted to our hospital for preoperative evaluation and potential atlantoaxial fusion surgery. Bed rest was instituted, and a Philadelphia collar was applied until the scheduled day of surgery. Surprisingly, however, the neurological symptoms of micturition, extremity numbness, and urinary retention improved 1 day after admission and improved gradually day by day. Given the improvement as well as her perioperative risk factors including advanced age and comorbidities such as arrhythmia, surgery was cancelled. She began occupational and physical rehabilitation including walking under strict observation. Six weeks after admission, she was independent in activities of daily living (ADL) and was discharged with a Philadelphia collar and a front wheel walker. She continued occupational and physical rehabilitation once a week. The patient's JOA score at discharge was 11 of 17.
Four months after discharge, she was walking with a T-handle cane. Follow-up MRI was performed at 11 months after discharge and demonstrated that the size of pseudotumor was diminished as was the subarachnoid space surrounding the spinal cord improved (Fig. 4-A). Evaluating neurological symptoms and MRI findings together, compression of the spinal cord in lower cervical level was considered to relate less with her symptoms.
Magnetic resonance imaging was performed at several time points during follow-up. Compared with pretreatment, the size of the pseudotumor decreased as the subarachnoid space surrounding the spinal cord improved 11 months after starting treatment (Fig. 4-A). The pseudotumor continues to shrink at 2 (Fig. 4-B), 5 (Fig. 4-C), and 7 years (Fig. 4-D).
Follow-up MRI was repeated at 2, 5, and 7 years after initiation of nonoperative treatment (Figs. 4-B through 4-D). As the pseudotumor continued to shrink, the external support was changed to a plastic cervical collar 1 year after application of the Philadelphia collar and was stopped after 2 years. During the follow-up period in our outpatient clinic, we educated the patient in the use of the cervical orthosis strictly, resulting her remarkable adherence to its use.
Eight years after this intervention, the patient is still independent in ADL. The patient's JOA score at the final follow-up was 9 of 17.
Noninflammatory retro-odontoid pseudotumor is a reactive fibrocartilaginous mass formed posterior to the odontoid process1,8,9. Retro-odontoid pseudotumor is reported to be caused by chronic AAI. Currently, posterior fusion is the mainstay treatment and induces regression of the mass after posterior fusion with reduction of forward stress on the atlantoaxial junction3,10-12. Currently, it is standard of care to apply screw fixation when treating AAI so as to obtain greater cervical rigidity13,14.
On the other hand, some reports describe good radiological and clinical results in cases treated with decompression without fusion surgery. Suetsuna et al. reported 3 cases treated with C1 laminoplasty using hydroxyapatite spacers, and regression of the retro-odontoid pseudotumor was confirmed in all cases with excellent clinical results15. Takemoto et al. reported on 10 patients with a retro-odontoid mass treated with C1 laminectomy without fusion, and the retro-odontoid pseudotumor was smaller in 4 cases with a minimum follow-up of 12 months; there was neurologic improvement in all cases16. The detailed mechanism of regression of the mass without fusion remains unclear. In our case, before application of the external orthosis, osteoarthritic changes of the lateral atlantoaxial joint already existed (Fig. 2). These findings might have induced subsequent restabilization of the atlantoaxial junction during the follow-up period, the process of which has previously been reported on in the lumbar spine17. To support this hypothesis, we previously reported a case of pseudoarthrosis after posterior atlantoaxial fusion surgery with good clinical results, indicating that stabilization alone possibly causes regression of the pseudotumor in certain cases18.
In a previous study using MRI, Yonezawa et al. classified MRI findings of a retro-odontoid mass into 3 types according to signal intensity on T2-weighted MRI: pannus type (low T1 and high T2), pseudotumor type (low T1 and low T2), and mixed type (low T1 and mixed T2)19. Park et al. evaluated preoperative and postoperative MRI in 38 patients including three rheumatoid arthritis (RA) patients with retro-odontoid pseudotumors and showed regression of pseudotumors after fusion surgery in all cases, irrespective of pseudotumor T2 signal intensity on MRI. They speculated that the signal intensity can be variable and nonspecific depending on the proportion of the tissue components evaluated, and that immobilization of the atlantoaxial joint decreased mechanical stress, friction, and the inflammatory process in structures such as the synovial capsule and transverse ligament. In our case, signal intensity of the retro-odontoid pseudotumor was low T1 and low T2 (Fig. 3), which is common in non-RA patients in which intensity is based on the fibrous component8, suggesting that the pathology might not be the same compared with the other non-RA cases. Suetsuna et al. reported in 3 cases that pseudotumors began to diminish at 1 to 3 months after decompression surgery, and completely disappeared at 11 and 6 months in 2 cases. Unfortunately, in our case, the timing of the size decrease of the pseudotumor was unclear because follow-up MRI was performed 11 months after application of external orthosis. However, neurological improvement was noted only 1 day after Philadelphia collar application with institution of bed rest.
Clearly, surgical treatment should be chosen for severe neurological symptoms, such as myelopathy, which are not expected to recover within the short term. However, external orthosis is one option for the treatment of retro-odontoid pseudotumor induced by AAI, especially in patients with significant comorbidities for whom surgical procedures are relatively contraindicated.
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