Dorsal arachnoid web: the ‘scalpel’ sign—a case report and differential diagnosis

Abstract

Abstract Background Arachnoid web of the spine is a not so common lesion of the spinal cord. It is a band-like arachnoid tissue in the intradural extramedullary compartment that extends to the surface of the spinal cord, with a tendency to occur in the upper dorsal spine causing focal dorsal indentation of the cord that has been coined the ‘scalpel sign’. Patients usually present with progressive weakening and numbing of bilateral lower limb. Case presentation A 58-year-old man presented with difficulty in walking and weakness of right leg for 1 year, aggravated over the previous 2 months. There was no history of trauma or fever. Magnetic resonance imaging (MRI) of the dorsal spine was done which revealed a small syrinx within the cord at D4 vertebral level on right side. There was sharp dorsal indentation of the spinal cord and widening of the arachnoid space below this level. This appearance was similar to a surgical scalpel. The anterior subarachnoid space was preserved. On post-contrast images, no parenchymal or meningeal enhancement was detected. The patient was taken up for surgery as the symptoms further progressed. Laminectomy was done from D3 to D7 under general anesthesia, with release of arachnoid web and adhesions. Histology of the resected specimen revealed epithelial cells and fibrous tissue. No inflammatory or neoplastic cells were observed. Conclusions Thoracic arachnoid web is a rare entity, which if not treated can have devastating effects on a patient’s neurological function. Careful evaluation of radiological findings is important to avoid delay in diagnosis. Surgery is the only curative treatment and is known to have a favorable outcome.