Syringomyelia is often associated with Chiari malformation, trauma, infection, and spinal cord tumor. Although they are relatively rare diseases, arachnoid cysts and its related pathology, "arachnoid web" can sometimes lead to syrinx formation at the thoracic vertebral level. However, syrinx formation caused by degenerative spinal disorders, particularly at the thoracic vertebral levels, has rarely been reported. Herein, we present a case of syringomyelia with thoracic ossified yellow ligament (OYL) in a 79-year-old man, who underwent initial posterior decompression followed by arachnoid web removal. Posterior decompression via laminectomy of thoracic vertebra 2 and removal of the OYL improved the syrinx partially, but dorsal indentation of the spinal cord and a remnant syrinx were observed in postoperative magnetic resonance images, subsequent to the second surgery's successful removal of the arachnoid web, which had dorsally compressed the spinal cord. After the second operation, the syrinx shrunk further, and the patient could walk independently at 5 months after the operations. In our case, both the OYL and arachnoid web were responsible for syrinx formation. Therefore, the coincidence of degenerative vertebral diseases with a syrinx might indicate the coexistence of an underlying lesion. Furthermore, the arachnoid web in this case might have formed due to the denaturation of the arachnoid cyst triggered by the OYL.

Atlantoaxial synovial cysts can very rarely penetrate the dura mater into the thecal sac and cause direct neural compression. Several case reports have been available on "intradural synovial cysts" (IDSCs). In this study, we report on a case with an atlantoaxial IDSC mimicking an extradural lesion. A 90-year-old man was diagnosed with a cystic lesion located laterally to the atlantoaxial joint adjacent to the retro-odontoid pseudotumor (ROP) causing cervical spinal cord compression. Thus, surgical removal was planned. On preoperative examination, the cyst, which had a two-layer structure showing a T2-isointense small mass inside a T2-hyperintense lesion, was thought to be located in the extradural region. However, operative findings showed that the cyst was located inside the dura mater. Histopathological examination suggested a synovial cyst. No recurrence of the cyst was observed until the latest follow-up after 3 years, and the ROP decreased in size. Almost all IDSCs reported previously were observed in the medial site of the atlantoaxial joint. In our case, however, the cyst was observed adjacent to the posteromedial site of the right atlantoaxial joint and the ROP, mimicking an extradural lesion. We had no knowledge regarding the IDSC before the surgery and assumed an extradural lesion. Albeit rare, the existence of such a condition should be considered.