Vacuum phenomenon (also called Knutsson's sign) is a well-described radiological sign
of intervertebral disc degeneration. It was initially presumed that following fissuring
in the degenerated disc substance, a vacuum would develop when the walls of the fissure
cavity gaped,[1] which would appear lucent on plain radiographs. However, it is now accepted that
this radiological appearance is due to the accumulation of gas (mainly nitrogen) in
the disc space. It is speculated that the development of negative pressure within
the disc space following disc degeneration and resorption causes a pressure gradient
(negative within the disc space) and gases dissolved in the neighboring tissues enter
this space by diffusion.[2]
We encountered a 50-year-old woman with a history of pacemaker implantation who presented
with sudden onset low back pain and right lower limb radiculopathy. Neurological examination
was normal except for restricted straight leg rising and absent right ankle jerk.
As magnetic resonance imaging (MRI) could not be done due to the pacemaker, a computed
tomography (CT) scan of the lumbosacral spine was done and showed hypodensity in the
L5-S1 disc space with intraspinal hypodensity along the path of the exiting S1 root
on the right side ([Fig. 1A–C]). She was treated with medications and rest and her radiculopathy decreased. A repeat
CT scan done after 3 weeks showed no intraspinal hypodensity and decrease in the hypodensity
in the lumbar intervertebral disc space.
Fig. 1 Computed tomographic (CT) scan of the lumbosacral spine showing (A) vacuum phenomenon in the L5-S1 intradiscal space and gas pseudocyst in the canal
along the S1 root; (B) coronal images showing hypodensity in the L5-S1 disc space; and (C) axial images showing the gas in the lateral recess on the right side.
Intradiscal air is common in degenerative spinal disease and is reported to occur
in 46% of patients over the age of 40 on CT imaging.[3] Our case had the additional finding of extrusion of the gas into the intraspinal
space (called a “gas pseudocyst”) and this caused radiculopathy–an occurrence uncommonly
reported in the literature.[2]
[3]
[4]
[5] It has been speculated that the upper and lower vertebrae act “like pistons”[3] to push the trapped intradiscal gas into the spinal canal on weight bearing through
an annular rent.[2]
Surgery is indicated in cases where conservative treatment fails and in such cases
an epidural pseudocyst has been found intraoperatively, manipulation of which releases
the gas bubbles.[2]
[3] When surgery is indicated, some authors state that root decompression along with
a thorough discectomy and excision of the posterior longitudinal ligament[4] is adequate but others opine that a vacuum phenomenon in the disc is an indicator
of “vertical instability” and advocate fusion if these patients undergo operative
intervention.[5]
The primary imaging modality for a patient with sciatica is an MRI (following an initial
assessment with plain roentgenograms). However, with increasing longevity of patients
with implanted pacemakers or metallic cardiac valves, in whom MRI is not possible,
CT scans (with our without myelogram) will be done in a growing number of cases. Air
is better appreciated in CT scans and surgeons need to aware of this entity as they
are likely to encounter it more frequently.
