Discussion:
In addition to trauma causes, other air sources may be found as epidural infection with gas-producing organisms, air introduction via lumbar puncture, or spinal surgery. Relatively rare, the presence of epidural gas in the degenerative spine was reported in a few cases of literature, secondary to vacuum phenomenon.
The vacuum phenomenon is a radiological finding defined as the presence of gaseous collection in the intervertebral disc space resulting from the movement of the spine, especially extension and traction [2]. It is a common finding that has been detected or reported in 46% of patients aged over 40 years [3].
The intradiscal gas diffuses from the extra-cellular fluid into areas of subatmospheric pressure and it cannot be reabsorbed because the degenerated disc is avascular. Some authors have discussed the role of disc aging in the genesis of the gas [2, 4]. This would be due to dehydration and the decrease in the proportion of proteins in the disc [5]. The gas is composed of 90-92% nitrogen along with oxygen, carbon dioxide, and other trace gases [2, 6].
The vacuum phenomenon has been mostly observed in L5-S1 disc with decreasing frequency at a higher lumbar level [7].
Whereas the vacuum phenomenon is a common finding, the presence of gaseous collection within the spinal canal is unusual. The gas formation is often associated with a disc herniation such as in our second case, but it can also be found free within the spinal canal as in the first case [8, 9].
In most previous cases, the gas-containing cyst has been noticed at the level of an intervertebral disc with a vacuum phenomenon [10]. This suggests that the gas originated from the intervertebral disc.
Interestingly, the gas can migrate to the spinal canal within a degenerated nucleus pulposus, resulting in a gas-containing disc herniation. Otherwise, the gas that fills the disc is expulsed in the spinal canal by a “valve-pump” mechanism through ruptures in the annulus fibrosus and collects under pressure within a fibrous capsule or a pseudo-capsule [6].
The histological study of a few cases of free gas noticed that the gas cyst was surrounded by the posterior longitudinal ligament (PLL). This suggests that air coming from the disc was trapped between the PLL and the bone. Furthermore, no recurrence has been observed after the PLL removal [11, 12]. Damierre et al. reported the case wherein gas aspiration was failed due to the persistence of pseudo capsules. Surgical removal of membrane proved identical PLL tissue of capsule and improved patient pain [11].
Mostly asymptomatic, only a few cases of intraspinal gas can cause compressive radiculopathy. The clinical features are similar to those of common sciatica. Moreover, we can notice an exacerbation of the pain at the change of positions reflecting the free movements of the air in the epidural space [13]. Furthermore, only a few cases reported severe neurological symptoms. These symptoms ranged from lower extremity paresthesia to paralysis. An acute cauda equina syndrome and cervical myelopathy have been reported [14, 15].
On standard spine x-rays, we often notice degenerative modifications of the spine and sometimes a vacuum phenomenon [13].
CT scan is currently the imaging tool of choice to diagnosis pneumorrachis. The typical finding includes degenerative modifications of the spine with a vacuum phenomenon and, at the same level, the intraspinal gas collection with a density from -200 to -900 Hounsfield units [16]. On MRI, gas is seen in low signal on the T1 and T2 weighted sequences [16].
For the management of intraspinal gas, most authors recommend conservative therapy including bed rest, analgesics, non-steroidal anti-inflammatory drugs, myorelaxant drugs, and epidural glucocorticoid injections [16]. Intraspinal gas may disappear spontaneously [17, 18]. In our cases, we noticed an improvement without using invasive procedures.
Some authors reported the efficacy of the CT guided aspiration of the gas cyst [19, 20]. However, in some cases, intraspinal gas recurred with a relapse of the radicular syndrome [10, 11]. Indeed, this procedure does not modify the local environment, and the origin of the air remains.
Surgery is the preferred treatment for patients who failed to respond to conservative therapy. The procedure combines gas evacuation with a disc curettage to prevent recurrence [12, 21–25].