Age / Sex : 68 / M

Chief complaint: Both lower extremity hypesthesia

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Courtesy: Wan Tae Kim, Veterans Health Service Medical Center

Dorsal arachnoid web

Discussion

Findings:

Sagittal T1WI and T2WI MR image demonstrates a focal dorsal indentation in the thoracic spinal cord.

Sagittal CT myelogram show a dorsal indentation with anterior displacement of the upper thoracic spinal cord.

Corresponding axial image at the level of dorsal indentation demonstrating the presence of a widened subarachnoid space and anterior displacement of the cord.

Differential Diagnosis:

 1. Dorsal arachnoid cyst

 2. Ventral spinal cord herniation

Diagnosis:  Dorsal arachoid web

Discussion:

Arachnoid webs are described as intradural extramedullary transverse bands of arachnoid tissue that extend to the dorsal surface of the spinal cord, causing mass effect and dorsal indentation, and they are often associated with syringomyelia.

Sridharan and Heilman suggested that the determining factor for syringomyelia formation above or below the level of an arachnoid web is related to the CSF pressure differential on either side of the web.

The exact etiology of arachnoid webs in uncertain. However, arachonid webs may represent the incomplete or disrupted formation of an arachnoid cyst.

Arachnoid webs are not easily visualized with conventional magnetic resonance imaging (MRI) or computed tomography (CT) myelography because of their comparatively thin width relative to other structures.

Sagittal CT myelogram and T2WI MR image show a pathognomonic dorsal indentation with anterior displacement of the upper thoracic spinal cord.

The finding resembles a scalpel with its blade directed posteriorly (the “scalpel” sign).

When the arachnoid web is suspected, neurosurgical intervention with resection of the band can be curative.

References:

1: Mukherjee S, Reardon MA, Raghavan P. Dorsal thoracic arachnoid web: another intradural entity with ventral cord displacement. Radiographics. 2015 Jan-Feb;35(1):297-8.

2: Sridharan A, Heilman CB. Transverse dorsal arachnoid web and syringomyelia: case report. Neurosurgery. 2009 Jul;65(1):E216-7; discussion E217.

3: Reardon MA, Raghavan P, Carpenter-Bailey K, Mukherjee S, Smith JS, Matsumoto JA, Yen CP, Shaffrey ME, Lee RR, Shaffrey CI, Wintermark M. Dorsal thoracic arachnoid web and the "scalpel sign": a distinct clinical-radiologic entity. AJNR Am J Neuroradiol. 2013 May;34(5):1104-10.