Extensive intradural arachnoid cyst of the lumbar spinal canal: Case report

Background: Spinal intradural arachnoid cysts are rare outpouchings of arachnoid lining occurring mainly in the thoracic and cervical regions. MRI is considered the diagnostic procedure of choice; however, some arachnoid cysts have been reported to elude diagnosis by MRI due to the similar signal intensity of the cyst and the subarachnoid space. Case Description: We present a case of a 41-year-old woman with an intradural arachnoid cyst of the lumbar spinal canal. Diagnostic studies demonstrated a herniated L4-5 disk, which led to two operations. Despite intractable pain postoperatively, the MRI failed to reveal further focal compression of the nerve root. A myelogram suggested compression of the thecal sac by a dorsal mass lesion. The patient underwent a midline durotomy that revealed two large arachnoid cysts with the nerve roots beneath them pushed ventrally and to the right. Removal of the arachnoid layer resulted in complete fenestration of the cyst and allowed the nerve roots to freely float in the spinal fluid. Conclusion: In retrospect, the abnormality could be appreciated on multiple prior scans performed at another institution, demonstrating that in the patient with low back pain, degenerative disc disease is not the only diagnosis to consider.