Arachnoiditis Ossificans in the Thoracic Spine With Associated Cyst and Syringomyelia: A Rare, Intraoperative Finding Complicating Dural Opening.

Arachnoiditis ossificans (AO) is a rare spinal pathology that develops because of bony metaplasia secondary to chronic inflammation. AO may present with debilitating myelopathy secondary to nerve root compression, making it distinct from spinal calcification commonly seen with aging. AO is extremely rare, having been reported less than 100 times, most commonly in the thoracic spine. Even rarer still, AO has been associated with syringomyelia and arachnoid cyst because of associated cerebrospinal fluid (CSF) flow disruption. In this report, we describe a case of AO that presented with right shoulder pain, right-hand numbness, and bilateral lower extremity fatigue who had syringomyelia and arachnoid cyst discovered on MRI imaging. When brought to the operating room for syrinx shunting and cyst fenestration the dural opening was complicated by severe calcification and a diagnosis of AO was made. The patient was treated with partial resection of the calcified plaques. Syringomyelia shunting was abandoned due to low volume. Post-operatively, the patient had improvement in their myelopathy though syrinx was still visualized on follow-up imaging. This report reviews the pathology, clinical and radiographic diagnosis, and treatment strategies for arachnoiditis ossificans.