Dorsal thoracic arachnoid web and the "scalpel sign": a distinct clinical-radiologic entity

Abstract

Arachnoid webs are intradural extramedullary bands of arachnoid tissue that can extend to the pial surface of the spinal cord, causing a focal dorsal indentation of the cord. These webs tend to occur in the upper thoracic spine and may produce a characteristic deformity of the cord that we term the "scalpel sign." We describe 14 patients whose imaging studies demonstrated the scalpel sign. Ten of 13 patients who underwent MR imaging demonstrated T2WI cord signal-intensity changes, and 7 of these patients also demonstrated syringomyelia adjacent to the level of indentation. Seven patients underwent surgery, with 5 demonstrating an arachnoid web as the cause of the dorsal indentation demonstrated on preoperative imaging. Although the webs themselves are rarely demonstrated on imaging, we propose that the scalpel sign is a reliable indicator of their presence and should prompt consideration of surgical lysis, which is potentially curative.

Fig 1.

A, CT myelogram demonstrates the scalpel sign with the characteristic focal dorsal indentation of the upper thoracic spinal cord. An insert of a scalpel shows how the dorsal indentation relates to the pointed edge of the blade. B, Corresponding axial image above the level of dorsal indentation (level of the arrow in A). C, Corresponding axial image at the level of dorsal indentation (level of the arrowhead in A).

Fig 2.

Examples of the dorsal indentation demonstrated in the upper thoracic spine of several patients in the article. A and B, Examples of increased cord signal above the level of indentation. C, Dorsal indentation without cord signal changes. D, A case in which there is extensive cord signal change and syringomyelia above the level of indentation. E and F, CT myelograms. E, An example of the upside down scalpel sign, where cord expansion occurs inferior to the level of dorsal indentation. Note that previous laminectomies are present in E.

Fig 3.

A, T2WI MR image demonstrates a focal dorsal indentation in the upper thoracic spinal cord (arrow). Note prominent epidural fat (asterisk). B, T2WI MR image status post–posterior laminectomy and lysis of a dorsal arachnoid web demonstrates resolution of the dorsal indentation, improved cord signal, and resolution of the syringomyelia. C, Intraoperative image demonstrates the dorsal arachnoid web (between the white arrowheads) before lysis of the web.

Fig 4.

A, T2WI MR image demonstrates a dorsal indentation (arrow) and anterior displacement of the upper thoracic spinal cord. Increased cord signal and syringomyelia are present above the level of indentation. B, T2WI MR image status post laminectomy and resection of a posterior arachnoid web demonstrates resolution of the dorsal indentation, cord signal changes, and syringomyelia.

Fig 5.

A, Sagittal T2WI MR image demonstrates a focal dorsal indentation (arrow) in the thoracic spinal cord. B, Intraoperative image after opening of the dura demonstrates an intradural white arachnoid band (between arrowheads) passing transversely across and compressing the dorsal surface of the spinal cord. C, Intraoperative image of the cord after the band of arachnoid tissue has been cut. The position is marked by arrowheads.

Fig 6.

A, Sagittal T2WI MR image demonstrates a typical intraspinal extramedullary arachnoid cyst. Superior and inferior aspects of the cyst are delineated by the arrow and arrowhead, respectively. B and C, Sagittal and axial CT myelograms show how the arachnoid cyst fills more slowly than the CSF after intrathecal contrast injection.