A patient was seen at our institution because of quadriparesis that had developed after a fall. Confusion with regard to the diagnosis occurred when a non-enhanced computerized axial tomography scan revealed a collection of gas centrally within the cervical spinal canal. Only after magnetic resonance imaging of the cervical spine was it apparent that the gas within the canal was enclosed by a large extruded herniated disc.
A collection of gas within a desiccated nucleus pulposus is termed a vacuum disc. Evidence of gas within an intervertebral disc and within the spinal canal has been described in both cervical and lumbar spines affected by a degenerative disorder3-5,10. Although a vacuum disc in the cervical or lumbar spine is not uncommon, its relationship to an intraspinal herniated disc may be overlooked if a disc fragment has not been noted on radiographic or computerized axial tomography studies.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
†Department of Orthopaedic Surgery, Thomas Jefferson Hospital, 850 Walnut Street, Philadelphia, Pennsylvania 19107.
‡Jefferson University Orthopaedics and Sports Medicine, 130 South 9th Street, Room 106, Philadelphia, Pennsylvania 19107.
A fifty-seven-year-old man was transferred to our institution for evaluation of quadriparesis that had developed after a fall from a standing position. The patient had a history of weakness in the right upper extremity with decreased grip strength, as well as occasional numbness and dysesthesias in the right lower extremity, for one month before the injury. On neurological examination, the motor strength of the biceps, wrist extensor, and triceps muscles was grade 3 of 5 bilaterally and the muscle strength of the hip flexor, quadriceps, and hallucis longus muscles was grade 4 of 5 bilaterally. These findings were consistent with an injury of the central aspect of the spinal cord at the fourth cervical level, according to the criteria of the American Spinal Injury Association.
Plain radiographs of the cervical, thoracic, and lumbar spine revealed diffuse spondylosis with one to two millimeters of degenerative anterolisthesis at the third and fourth, fourth and fifth, and fifth and sixth cervical levels (Fig. 1). A non-enhanced transaxial computerized axial tomography scan demonstrated a large collection of gas in the mid-portion of the spinal canal at the interspace between the third and fourth cervical levels (Fig. 2). Magnetic resonance imaging demonstrated a collection of gas within a large extruded disc at the interspace between the third and fourth cervical levels with severe compression of the anterior aspect of the thecal sac and spinal cord (Fig. 3).
The patient subsequently had an anterior discectomy at the interspace between the third and fourth cervical vertebrae, a subtotal vertebrectomy at the fourth cervical level, and decompression of the thecal sac and an arthrodesis with autogenous iliac-crest bone graft at the third, fourth, and fifth cervical levels. A large free disc fragment that had extruded through the posterior longitudinal ligament was removed.
At the six-month follow-up examination, the strength of the biceps, wrist extensor, and triceps muscles had improved to grade 4+ of 5 bilaterally and a solid fusion was evident on dynamic plain radiographs. The strength of the hip flexor, quadriceps, and extensor hallucis longus muscles remained grade 4 of 5 bilaterally. All other neurological findings were normal.
The differential diagnosis of gas within an intervertebral disc or the spinal canal includes vertebral spondylosis, vertebral osteomyelitis, Schmorl nodes, vertebral metastasis, sequelae of blunt trauma to the chest, synovial cyst, a previous operation, and vertebral collapse with osteonecrosis2,8,10,11,13,19,20. To our knowledge, the vacuum disc phenomenon was first reported in 1937 by Magnusson, who described the presence of "air" in the intervertebral soft tissues of the spine. Knutsson theorized that the pathophysiology of a vacuum disc is related to advanced arthrosis of the intervertebral disc. In the early stages of disc degeneration, the nucleus pulposus begins to desiccate, which results in fissuring of the fibrocartilage. These fissures can develop into spaces or clefts that fill with gas from the surrounding tissues14. Ford et al. analyzed gas obtained by aspiration directly from a vacuum disc and found it to consist of 92 per cent nitrogen. As the surrounding tissues have a high partial pressure of nitrogen, they theorized that nitrogen gas fills the clefts through diffusion, which results in radiolucent collections. In 1953, Raines demonstrated the dynamic nature of this gas by recording the presence of air within the intervertebral discs of the lumbar spine when the spine was in extension and the intradiscal pressure was low and then noting the disappearance of the gas when the spine was in flexion and the intradiscal pressure was high.
The importance of the vacuum disc phenomenon has become increasingly apparent as investigators have identified symptomatic herniated intervertebral discs with intradiscal radiolucent areas in both the cervical and the lumbar spine with use of computerized axial tomography8,12. Isu et al. used high-resolution computerized axial tomography for the evaluation of thirty-seven consecutive patients who had a degenerative disorder of the spine and found that eight (22 per cent) had evidence of a radiolucent area within an intervertebral disc space. Gebarski et al. noted that eight (6 per cent) of 127 vacuum discs in patients who had advanced spinal degeneration were extruded within the spinal canal, but only two of the patients had symptoms related to the level of the herniation. Several authors have reported considerable relief of symptomatic radiculopathy after operative removal of an extruded herniated disc containing gas11,16,20.
Although most reports of vacuum discs concern the lumbar spine, evidence of radiolucent areas within the cervical spine on plain radiographs is not rare in patients who have a degenerative spinal disorder. Isu et al. noted that the level of the cervical spine in which the vacuum phenomenon was the most common was the disc space between the fifth and sixth cervical vertebrae, which is also the most common level of involvement by degenerative spondylosis. As far as we know, Elster and Jensen, in 1984, were the first to report gas within a symptomatic herniated disc in the cervical canal. They concluded that evidence of gas within the spinal canal strongly suggests a herniated disc with enclosed gas whether or not the disc has the characteristic density on computerized axial tomography. In separate studies, Mortensen et al. and Pierpaolo et al. reported on five patients who had gas within a herniated lumbar disc that was identified with high-resolution computerized axial tomography and was confirmed at operation.
Gas within the spinal canal rarely is found in a patient who has an injury of the cervical spinal cord. When it is seen, the surgeon should have a high level of suspicion of an extruded herniated disc and therefore should consider magnetic resonance imaging for additional clarification of the pathoanatomy.