Discal cysts are extremely rare lesions, described as cysts with a direct connection with the corresponding intervertebral disc[2]. Discal cysts that contain gas are even more rare. The etiology and pathogenesis of discal cysts remains unknown but several hypotheses have been proposed[25]. The vascular theory hypothesizes that it is an organized epidural hematoma result of hemorrhage of the epidural venous plexus resulting from disc herniation or preceding discal injury, which develops acutely and later acquires a pseudomembrane[2]. Jeong[26] hypothesized that the formation of discal cysts was not a vascular phenomenon, but resulted from a change in a herniated disc. Some scholars support the theory that a discal cyst is due to focal degeneration or annular injury of an intervertebral disc producing a corresponding herniated disc with subsequent spilling of fluid from the herniated disc tissue that triggers an abacterial inflammatory response, resulting in the formation of a pseudomembrane and development of a discal cyst[27]. Based on our intraoperative findings, we agree that the underlying etiology and pathogenesis results from an annular injury or focal degeneration, leading to a herniated disc with a subsequent series of reactions resulting in the formation of a reactive pseudomembrane that finally becomes a discal cyst.
Chief complaints, symptoms and signs of discal cysts can be similar to those of patients with typical lumbar disc herniation[1]. The early stage of discal cyst formation is asymptomatic, and no treatment is necessary because the discal cyst puts only small pressure on the canalis spinalis. However, as the discal cyst grows, patients present with different symptoms, including backache and numbness. Other diseases, which can present with similar clinical symptoms including lower back pain and radiculopathy are perineural or Tarlov cysts, epidural hematomas, ligamentum flavum cysts, arachnoid cysts and synovial cysts[28].
Imaging examinations, including 3D-CT and MRI, are used for assessing discal cysts. Gassy discal cysts show low density shadows on 3D-CT. The typical findings for fluid discal cysts are round to oval extradural masses with low signal intensity on T1-weighted images and high signal intensity on T2-weighted images[29]. However, this signal depends on the contents of the fluid. When the discal cysts contain gas and no fluid, masses have low signal intensity on T1-weighted images and T2-weighted images.
According to cases in the literature, partial hemilaminectomy, microscopic excision or endoscopic excision are generally accepted as the definitive and effective treatment of choice for discal cysts. To our knowledge, the largest single center experience describing the surgical treatment of discal cysts was described by Wang[30]. This author reported the microscopic surgical outcomes of nine patients with symptomatic radiculopathy caused by discal cysts and believed that the operative indications for discal cysts are similar to those of lumbar disc herniation[30]. Although the majority of cases of discal cysts have been treated with surgical resection, computed tomography-guided aspiration has also been described[31, 32].Yoshimi Endo[28] described a lumbar discal cyst that was treated with computed tomography-guided aspiration and steroid injection. This author believed that corticosteroid injection into the cyst was important for minimizing the risk of recurrence [28]. However, Kang[33] performed similar aspirations without steroid injections, and no patients reported any recurrence of the cysts. Meanwhile, Cho HL[13] described a gas-filled intradural cyst that was treated with computed tomography-guided aspiration, Unfortunately, the patient’s symptoms recurred one month later, and the CT showed re-accumulation of gas in the intradural cyst. The patient underwent open intradural surgery via the posterior approach. Therefore, steroid injection for discal cysts is still controversial. In addition, Demaerel et al.[34] and Takeshima et al.[35] report cases of spontaneous regression of a discal cyst without intervention. In our study, discal cysts were treated effectively by TPED. However, in order to provide more definitive evidence of standard and effective treatment for discal cysts, more studies on diagnostic and therapeutic strategies for discal cysts are needed, and careful analysis and long-term follow-up are necessary.