Epidural gas-containing pseudocysts (GCPs) are uncommon causes of lumbar radiculopathy and it has been reported that GCPs are often associated with the intervertebral vacuum phenomenon (1–3). Most lumbar GCPs locate in the spinal canal and coexist with a disk fragment (4, 5). While the extraforaminal GCPs causing nerve root compression are extremely rare (3–5).

The treatment of lumbar GCPs including conservative management, percutaneous needle aspiration, percutaneous endoscopy and open surgery (6, 7). As a new endoscopic technique, unilateral biportal endoscopic spinal surgery (BESS) increases surgical movement of instruments with the independent visualization and working portals, provides good and wide field of visualization to unrestricted access contralateral and foraminal and extraforaminal areas (3). BESS has been applied to treat lumbar disc herniation and lumbar spinal stenosis (4). In this report, we present a symptomatic extraforaminal GCP, which was removed by BESS.

A 62-year-old female presented with low back pain and radiation to anteromedial aspect of right thigh and anterior aspect of right calf. The VAS score was 6 and the pain had persisted for 3 months. Physical examination revealed a positive straight leg raising (SLR) sign and lasegue test on the right side. Magnetic resonance imaging (MRI) and computed tomography (CT) of lumbar spine showed a right extraforaminal GCP in L4–5 level, which compressing L4 exiting nerve root (Figure 1).

Before BESS, we performed L4 exiting nerve root blocking in the extraforaminal area. The patient reported obvious pain relief after this blocking, which indicated that the responsible level was L4/5, and the target was the right extraforaminal GCP in L4–5 level.

We performed BESS through a paraspinal approach. The patient was placed in prone position on a radiolucent table and under general anesthesia. After the target level was confirmed under fluoroscopic guidance, the skin entry point was 1.5 cm above and 1.5 cm below the inferior margin of L4 pedicle, and 2 cm lateral to external margin of L4 pedicle (Figure 2). Two portals (walking portal and viewing portal) were created according to the skin entry point.

Serial dilators were used to dissect the back muscle and acquire operative space. The trocar of the scope was introduced into the viewing portal and a round, smooth periosteal elevator was inserted into the working portal. After triangulation occurs between scope and the periosteal elevator, minor bleeding was controlled and remnant soft tissues were removed by radiofrequency probe and shaver, to expose L5 superior articular process, L4 inferior articular process, L4 pedicle isthmus and the base of L4 transverse. Then, part of L4 transverse and pedicle isthmus, the tip of L5 superior articular process and the exterior part of L4 inferior articular process were removed with a 3 mm drill or arthroscopic burr and Kerrison punch (Figure 3). After that, the ligament flavum in foramen was exposed and flavectomy was performed. Maneuvering of the scope, extraforaminal GCP, exiting nerve root and foramen would be directly visual­ized (Figure 3). Through the working portal, pituitary forceps were used to remove the extraforaminal GCP, and the L4 exiting nerve root was successfully decompressed (Figure 3). The successful removal of extraforaminal GCP was confirmed on postoperative MRI and CT (Figure 4). A bone tunnel could be seen on a CT scan (Figure 4). The patient achieved a good outcome, VAS score improved to 3 on the first postoperative day, and improved to 1 on 7 days after operation.

In the spine, the presence of gas or vacuum phenomenon is a relatively common radiological finding, and generally occurs in the intervertebral disc spaces (8). Compared with intervertebral gas, epidural GCP is much rare (7). Kuh et al. assessed the reported that 48.0% epidural GCP were found in the canal, 32.0% were found in the foramen, and 20.0% were found in extraforaminal area (9). We reviewed the articles about epidural GCPs, and found that 20 articles reported 43 cases of epidural GCPs (Table 1). Among them, only 9 cases (20.9%) of epidural GCPs located in extraforaminal area (4, 6, 9).

In those reported cases, most epidural GCPs were found because of neurological symptoms, such as sciatica, lower extremity paresthesia and paralysis (6). The clinical features are very similar to common lumbar disc herniation. So CT and MRI are very helpful to diagnose and assess the epidural GCPs. Epidural GCPs can be identified with density from - 200 to - 900 Hounsfield units in CT scan (10), or with low signal on T1- and T2-weighted images of MRI (9). Sometimes, calcification has the same low signal intensity in MRI, making it difficult to distinguish between gas and calcification. So CT scan is the most useful radiological method for identifying epidural GCPs.

There are various therapeutic strategies to treat symptomatic epidural GCPs, including medications, percutaneous needle aspiration, surgical removal (endoscopic spinal surgery or open surgery) (5). The conservative management (such as medications and rest) and percutaneous needle aspiration are common treatments for epidural GCPs, especially for those patients with surgery contraindication. However, some authors reported that epidural GCPs recurred with a relapse of the radicular syndrome after conservative management or percutaneous needle aspiration (11, 12). Open surgery has good clinical outcomes by totally removing the epidural GCPs and herniated disk fragment. However, open surgery has large operative injury, as its protocol includes discectomy, decompressive laminectomy facetectomy and interbody fusion. So open surgery may be more suitable for those patients with concomitant spinal stenosis or segmental instability (9). Compared with open surgery, endoscopic spinal surgery has many surgical advantages, including less invasive procedure, faster postoperative recovery and lower cost (9). So for those patients without severe spinal stenosis or segmental instability, endoscopic spinal surgery may be a better choice. Percutaneous endoscopic surgery (interlaminar approach or transforaminal approach) is the most classical minimally invasive surgery for lumbar degenerative diseases. In classical percutaneous endoscopic surgery, a single portal was used, with multiple channels for irrigation, instrumentation, visualization, and a light source. So the single portal limited the motion of the instruments and obscures visualization of the operating field. Compared with classical percutaneous endoscopic surgery, BESS increases surgical movement of instruments with the independent visualization and working portals, provides good and wide field of visualization to unrestricted access contralateral and foraminal areas (13). So for those epidural GCPs located in lateral recess, foraminal or extraforaminal areas, BESS may be a better choice. In this case, we successfully removed extraforaminal gas-containing pseudocyst using BESS, and completely relieved the radiating pain.

Lumbar GCPs in extraforaminal area is rare and can cause lumbar radiculopathy. Paraspinal approach BESS is an alternative method to treat extraforaminal gas-containing pseudocyst and can provide good outcome.