Radicular compression by lumbar intraspinal epidural gas pseudocyst in association with lateral disc herniation. Role of the posterior longitudinal ligament.

Among unusual abnormalities of the lumbar spine reported since the introduction of Computed Tomography (CT), the presence of gas lucency in the spinal canal, known as vacuum phenomenon, is often demonstrated. On the contrary, epidural gas pseudocyst compressing a nerve root in patients with a lateral disc herniation has rarely been reported. We report a case of a 44-year-old man who experienced violent low back pain and monolateral sciatica, exacerbated by orthostatic position, one week before admission. A lumbosacral spine CT showed the presence of vacuum phenomenon associated with a degenerated disc material and a capsulated epidural gas collection with evidence of root compression. A microsurgical interlaminar approach was carried out and, before the posterior longitudinal ligament was entered, a spherical "bubble" compressing the nerve roots was observed. The capsulated pseudocyst was dissected out, peeled off and excised en bloc. A large part of the posterior longitudinal ligament and the lateral disc herniation were removed. Postoperatively the patient was completely free of symptoms. The mechanism of exacerbation of pain was probably due to the increased radicular compression in the upright posture and, besides the presence of a lateral disc herniation, could be related to a pneumatic squeezing of gas from the intervertebral space into the well capsulated sac by the solicitated L4-L5 motion segment. Histological study of the wall of the pseudocyst showed the presence of fibrous tissue identical to the ligament. We conclude that, in case of a lumbar disc herniation, it is recommended to perform a complete microdiscectomy and an accurate removal of the involved portion of posterior longitudinal ligament in order to prevent pseudocystic formations.