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].