Thoracic Spinal Arachnoid Web With Syringomyelia: A Case Report.

Spinal arachnoid webs are rare intradural extramedullary lesions that most commonly occur in the thoracic spine and may cause focal dorsal spinal cord compression, cerebrospinal fluid flow obstruction, syringomyelia, and progressive myelopathy. Diagnosis can be delayed because symptoms are often nonspecific and the arachnoid web itself may be difficult to directly visualize on standard magnetic resonance imaging. The characteristic "scalpel sign," defined by focal dorsal indentation of the spinal cord, is a key imaging feature. We present a case of a 47-year-old male patient with progressive mid-thoracic pain, left thoracic sensory disturbance, and bilateral foot numbness. Magnetic resonance imaging demonstrated a thoracic syrinx and focal dorsal indentation of the spinal cord at T8-T9 consistent with the scalpel sign. High-resolution constructive interference in steady-state imaging demonstrated a dorsal intradural arachnoid band, supporting the diagnosis of thoracic spinal arachnoid web. The patient underwent T8-T9 laminectomy, durotomy, and microsurgical excision of the arachnoid web. Postoperatively, he experienced resolution of sensory symptoms within two weeks, and follow-up imaging demonstrated interval reduction of syringomyelia. This case highlights the importance of recognizing thoracic arachnoid webs as an underdiagnosed cause of myelopathy and syringomyelia. Early recognition of characteristic imaging findings and definitive microsurgical treatment can result in favorable clinical and radiographic outcomes.