Discal cyst — A rare cause of lumbar radiculopathy in the pediatric population
Intraspinal extradural cysts at the intervertebral disc level communicating with the disc space are termed as discal cysts. Other intraspinal extradural cysts arise from the facet joints, ligamentum flavum, or the posterior longitudinal ligament. Discal cysts are rare lesions, and only four such cases have been reported in pediatric patients., According to the available literature, discal cysts are commonly seen in Asian male patients and involve the lumbar spine at an earlier age when compared with a regular disc prolapse.,
The clinical presentation is indistinguishable from that of a herniated intervertebral disc. The exact natural course of the lesion is unknown, and long-term follow-up studies are not available. An optimal diagnostic modality (magnetic resonance imaging [MRI]/computed tomographic [CT] discography) and treatment strategy (excision of the cyst only or its combination with a micro-lumbar discectomy) has not yet been established for the entity. In this article, the relevant literature is reviewed, and an illustrative case is reported.
A 15-year-old boy presented with a 1-month history of low backache, with radicular pain involving the left leg and weakness of the left foot. The symptoms started acutely while he was playing kabaddi. Neurological examination revealed that his left ankle dorsiflexion was 3/5 (Medical Research Council [MRC] grade) and extensor hallucis longus was 2/5 (MRC) with loss of inversion. Sensory loss was present over the left L5 dermatome. The straight leg raising (SLR) test was positive on the left side at 30 degrees. The MRI of the lumbar spine revealed an epidural T1W and T2W hyperintense cystic lesion located adjacent to the L4–L5 intervertebral disc space compressing on the left L5 nerve root [Figure 1]. Partial hemilaminectomy and microscopic excision of the cyst were done. A grayish-white, thin walled cyst containing old blood-tinged fluid was found communicating with the L4–L5 disc space through a focal defect in the annulus. The histopathology correlated with a benign cystic lesion. The wall of the cyst comprised of fibrocollagenous vascular tissue with occasional chronic inflammatory cells [Figure 2].
There was no radicular pain postoperatively, and the clinical follow-up at 3 months showed complete resolution of symptoms. The follow-up MRI revealed no recurrence of the lesion [Figure 3].
Toyama et al., in 1997 first highlighted the existence of intraspinal cysts communicating with the intervertebral disc space. Kano et al., in 1999 described the imaging characteristics of discal cysts and suggested their pathogenesis. Chiba et al., described the clinical characteristics of discal cysts that included the following features: (1) The clinical symptoms relate to a unilateral single nerve root compression; (2) lesions occur at a slightly younger age and at a higher intervertebral disc level than the typical disc herniation; (3) there is minimal degeneration of the involved disc on imaging studies; (4) there is communication between the cyst and the corresponding intervertebral disc; (5) the cyst has a intralesional, bloody-to-clear serous fluid content; and, (6) there is absence of either disc material inside the cyst or of a specific lining cell layer on histological examination. All the aforementioned features were present in this case.
A discal cyst has often been described as a ganglion cyst of the posterior longitudinal ligament or as a synovial cyst. In pediatric patients, only four cases of a discal cyst have been reported so far. In the available literature, a total of 43 articles have been published describing approximately 105 cases of discal cysts. The lesion is most commonly seen in young individuals (age 30–40 years). There is an absence of significant disc degeneration with a predilection to occur in the Asian population. All the cases of discal cyst have been described in the lumbar spine, with the L4–L5 space being the most commonly involved site.,
Two theories have been proposed to explain its pathogenesis. The vascular theory hypothesizes that it is, in fact, an organized epidural hematoma, which develops acutely and later acquires a pseudomembrane., The presence of hemosiderin on the cyst wall supports this theory. The second theory hypothesizes that the origin is from the disc tissue acutely under high pressure. The documentation of the communication with the intervertebral space, accompanied by a disruption of the annulus, supports the second theory.
The second theory offers a more likely explanation for the pathogenesis of the discal cyst in our patient, as playing Kabaddi involves extreme flexion and lateral-bending movements of the lumbosacral spine, which might have led to increased pressure zone in the posterolateral annulus precipitating disruption and extrusion of the disc fluid. Absence of any lining epithelium and fibrocollagenous tissue within the cyst wall also substantiates this theory.
The most common clinical presentation is radicular pain involving a single nerve root followed by back pain. Neurological deficits correspond to the involved nerve root. Nerve root stretch tests are usually positive. Spinal radiographs are normal without any evidence of disc space narrowing or segmental instability.
MRI is diagnostic and usually reveals a small T1-hypointense and T2-hyperintense cystic lesion adjacent to the disc space, with compression on the nerve root. However, the MRI appearance may differ. T1-isointense as well as hyperintense lesions are also seen. CT discography  has also been described to be helpful in the diagnosis of the lesion but usually, the MRI appearance is characteristic and sufficient to diagnose and plan treatment.
These cysts are excised safely either by a partial hemilaminectomy and microscopic excision, or an endoscopic excision.,,,, Intraoperative demonstration of a communication of the discal cyst with the intervertebral space, associated with focal disruption of the annulus, clinches the diagnosis.
The simultaneous need for a discectomy is controversial. Nabeta et al., stated that although they excised the associated disc in all their five cases, this was far from being the thumb rule. Aydin et al., also excised the corresponding disc in all their five cases, but questioned the need for this additional procedure. Certo et al., in their review, discussed the controversy surrounding this issue of excision of the corresponding disc and concluded that the procedure cannot be advocated for all the patients. The goal of surgery is to decompress the involved nerve root. Depending on the extent of the tear in the annulus and the degeneration of the disc, decisions may, therefore, be individualized. As these cysts are mostly seen in young patients and the discs are relatively normal, only cyst excision is the favored procedure. The latter procedure has negligible morbidity and mortality and the patients have a good clinical outcome with a low recurrence rate following the procedure.
Few cases of spontaneous resolution of the discal cysts with conservative treatment have been described., CT-guided percutaneous aspiration has also been advocated as a treatment modality. Successful treatment with steroid injection  and laser ablation  have also been reported. However, as far as the management of patients with motor deficits due to nerve root compression is concerned, most of the articles are in favor of microscopic or endoscopic excision of the cyst. Complete resolution of symptoms is seen following excision of the cyst, with an early return to work. The procedure is usually not associated with any morbidity, has lesser overall cost, and is associated with a low recurrence rate.
To conclude, discal cysts, although rare, should be considered in the differential diagnosis of lumbar radiculopathy in young patients. MRI is the diagnostic modality of choice. Microscopic or endoscopic cyst excision is a simple and effective method for the treatment of patients with neurological deficits.
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[Figure 1], [Figure 2], [Figure 3]