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Year : 2003  |  Volume : 13  |  Issue : 1  |  Page : 105-106
Images : Gas in the spinal canal as sign of prolapsed intervertebral disc (PID) on plain CT

National Scan Center, K.E.M. Hospital, Rasta Peth, Pune, 411011, India

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Keywords: gas in spinal canal, vacuum phenomenon, lumbar spine, disc prolapse

How to cite this article:
Rahalkar M D, Sawlani V. Images : Gas in the spinal canal as sign of prolapsed intervertebral disc (PID) on plain CT. Indian J Radiol Imaging 2003;13:105-6

How to cite this URL:
Rahalkar M D, Sawlani V. Images : Gas in the spinal canal as sign of prolapsed intervertebral disc (PID) on plain CT. Indian J Radiol Imaging [serial online] 2003 [cited 2021 Feb 28];13:105-6. Available from:

   Introduction Top

In the spine 'vacuum phenomenon' refers to the radiographic or CT appearance of gas in the inter-vertebral disc (IVD), usually in the lumbar region. It is produced by liberation of gas, particularly nitrogen, from surrounding tissues and its accumulation in the fissures produced in the degenerating nucleus pulposus of the IVD. When PID occurs, it may contain a pocket of gas, which may be seen within the spinal canal. This gas remains entrapped by the disc material and can be an important sign of PID on plain CT.

   Case Material Top

All plain CT scans of the lumbar spine, performed from 1985 to 1993, were scrutinized for this sign. 8 cases were shown to demonstrate gas-containing soft tissue density lesion due to PID, clinically presenting with corresponding radiculopathy. 5 cases are presented [Figure - 1][Figure - 2][Figure - 3][Figure - 4][Figure - 5][Figure - 6]. Case No. 4 was treated conservatively, and a repeat scan after 6 months also showed the identical lesion at the same extra-foraminal site. In case No.5, an additional scan in prone position showed that the gas did not change its position confirming its entrapment within the PID.

   Discussion Top

Presence of gas (so-called vacuum phenomenon) in the IVD has been described since 1980 [1]. Gas within the degenerated disc may reach the canal through the fissures or cracks that reach the annulus ligament complex, which can also get torn. This gas may get entrapped within the herniated disc material and can be regarded as an interesting sign of PID on plain CT of the lumbar spine.

Herniated disc material is generally slightly hyperdense and is easily identifiable as a focal, hyperdense bulge behind the disc margin. However, the displaced disc material sometimes produces a large fluid-filled pouch [2], easily distinguishable from the gas pocket in continuation with the gas in the disc. In such a case intra-spinal gas may be the only sign to suspect PID. This gas was even reported to be recognizable on the lateral scanogram by Kaiser [3]. Just as PID is classified as central, para-central, foraminal and extra-foraminal (far lateral), the location of the gas pocket would vary. Our cases No.1 and 2 illustrate the para-central type, while case No.3 shows an entrapped gas in the inter-vertebral foramen. Our case No.4 is particularly more important, because it beautifully demonstrates the PID with gas in it outside the foramen and compressing the nerve. This appearance was un-changed even after 6 months, as the patient was treated conservatively. This far lateral PID can completely be missed on myelography and only be shown on CT [4]. The entrapment can be confirmed by obtaining CT scan in prone position, as was done in case No.5. There was no change in the position of the entrapped gas.

Rarely herniated disc entrapping gas may be seen to dissect the dura and present as an intra-dural mass. This was first described by Dandy in 1942. Kaiser MC [5] and Anda, presented identical cases, in which CT and myelography had shown an intra-dural mass containing gas due to PID. As expected the gas was shown to be more posteriorly placed and the thecal outline was normal. Intra-dural PID is very rare and was not encountered by us.

Gas within the spinal canal has to be differentiated from inadvertent introduction of air from previous lumbar puncture and gas within a synovial cyst adjacent to the apophyseal joint deriving gas from the vacuum phenomenon in the facet joint.

What is the relevance of this finding? CT of the lumbar spine is still carried out at many places, outside or inside the teaching hospitals, an MRI may not be done due to non-availability, claustrophobia on the part of the patient, contra-indication due to metallic implants and high cost of MRI. Awareness of this sign to make a confident diagnosis of PID has not been appreciated in Indian radiological literature. Moreoer, MRI fails to identify the important signs like vacuum phenomenon in the disc or gas inside the spinal canal. In the former it will be hypointense like the degenerating disc itself and in the latter it will not be differentiated from an osteophyte, as both will show signal voids.

   References Top

1.Gulati AN, Weinstein ZR. Gas in the spinal canal in association with the lumbo-sacral vacuum phenomenon : CT findings. Neuroradiology 1980; 20: 191-192.  Back to cited text no. 1  [PUBMED]  
2.Millette PC. Classification, diagnostic imaging and imaging characterization of a lumbar herniated disk. RCNA Nov. 2000; 38:6 - 1267-91.  Back to cited text no. 2    
3.Kaiser MC, Capesius P, Viega-Pires JA, et al. Recognition of gas-containing disc herniation on lateral scout-view. Neuroradiology 1987; 29:98..  Back to cited text no. 3    
4.Anda S, Stovring RM. CT of extra-foraminal disc herniation with associated vacuum phenomenon. Neuroradiology 1988; 30-76-77.  Back to cited text no. 4    
5.Kaiser MC, Sandt G, Roilgen A, et al. Intra-dural Disk Herniation with CT Appearance of Gas Collection, AJNR 1985; 6:117-118.  Back to cited text no. 5    

Correspondence Address:
M D Rahalkar
X-Ray Clinic, 711 Narayan Peth, Laxmi Road, Pune 411030
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Source of Support: None, Conflict of Interest: None

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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6]


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