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Arachnoid web is a rare, under-recognized spinal pathology involving thin, intradural bands of arachnoid tissue, typically found in the thoracic spine, that cause focal spinal cord indentation and compression, often resulting in neurological symptoms.
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It may mimic or be confused with arachnoid cysts or spinal cord tumors on imaging.
Illustrations depicting spinal cord herniation (SCH) (A) and Dorsal arachnoid webs (DAW) (B). A: Note the protrusion of the ventral spinal cord through a dural defect, resulting in interruption of the ventral subarachnoid space (arrows) and the C-shaped deformity on the dorsal cord surface. B: A DAW is depicted with a scalpel-sign deformity of the dorsal cord surface (asterisk). Note the preserved ventral subarachnoid space.
Copyright Aaron Wessell. Schultz, Randall, Jr., Andrew Steven, Aaron Wessell, Nancy Fischbein, Charles A. Sansur, Dheeraj Gandhi, David Ibrahimi, and Prashant Raghavan. "Differentiation of idiopathic spinal cord herniation from dorsal arachnoid webs on MRI and CT myelography". Journal of Neurosurgery: Spine SPI 26.6 (2017): 754-759. https://doi.org/10.3171/2016.11.SPINE16696
| Feature | Description |
|---|---|
| Origin | Focal thickening or fibrous band of the arachnoid membrane within the dorsal subarachnoid space |
| Location | Most often at the upper thoracic spine (T3–T7), dorsal to the spinal cord |
| Effect | Causes a ventral displacement and compression of the spinal cord, altering CSF flow |
| Pathogenesis | Possibly congenital, post-traumatic, post-infectious, or due to prior hemorrhage or inflammation |
| Feature | Description |
|---|---|
| Symptoms | Vague back pain, sensory disturbances, spastic paraparesis, gait ataxia, or bladder dysfunction |
| Progression | Often slow and progressive, mimicking compressive myelopathy |
| Misdiagnosis risk | Can be misdiagnosed as spinal cord tumor, syrinx, or arachnoid cyst |
| Modality | Imaging features |
|---|---|
| MR | • T2: ‣ - Sharp, focal dorsal indentation of spinal cord resembling a scalpel blade |
| • Axial T2: Ventral cord displacement, flattened dorsal cord without a mass | |
| • CSF flow artifact: May show CSF flow void dorsal to cord (not seen in cysts) | |
| • No mass lesion: Differentiates from arachnoid cyst or tumor | |
| Cine MR | • Shows disrupted CSF flow dorsal to the spinal cord at the level of the web |
| • Helps confirm diagnosis and plan surgery | |
| CT myelography | • Block in contrast flow dorsal to the cord |
| • May show focal indentation without a filling defect |
Dorsal arachnoid web. Sagittal T2-weighted image (WI) (A), T1-WI (B), and sagittal (C) and axial (D) 3-dimensional T2-weighted Sampling Perfection with Application optimized Contrasts using different flip angle Evolution (SPACE) reconstructions showing dorsal subarachnoid space widening with anterior deviation of the spinal cord. Zoomed-in sagittal T2-weighted SPACE reconstruction (E) depicting the scalpel sign (white line) and the presumed level of the web (black dots)
Cunha B, Rodrigues A, Gonçalves J, Conceição C. Imaging of intraspinal cystic lesions: A review. Journal of Neuroimaging. 2022;32(6):1044-1061. doi:10.1111/jon.13037
Preoperative MR images and CT myelograms obtained in 2 patients with AWS. A–D: Case 1. E–H: Case 2. Sagittal MR images (A and E) and CT myelograms (C and G) demonstrating the “scalpel sign” at the T4 and T5 levels, characterized by an indentation in the dorsal aspect of the spinal cord without an obvious intradural mass. Axial MR images (B and F) and CT myelograms (D and H) demonstrating ventral displacement of the cord due to the arachnoid web, again without an appreciable dorsal mass. The dorsal collection is isointense with CSF.
Nisson, Peyton L., Ibrahim Hussain, Roger Härtl, Samuel Kim, and Ali A. Baaj. "Arachnoid web of the spine: a systematic literature review". Journal of Neurosurgery: Spine SPI 31.2 (2019): 175-184. https://doi.org/10.3171/2019.1.SPINE181371 Web.