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Ann Indian Acad Neurol
Ann Indian Acad Neurol
AIAN
Ann Indian Acad Neurol
Annals of Indian Academy of Neurology
0972-2327
1998-3549
Wolters Kluwer - Medknow India

39196811
AIAN-27-416
10.4103/aian.aian_208_24
Images in Neurology
Histopathologic Emphasis on Subarachnoid Web Diagnosed on Neuroimaging
Gohri Jay
Garg Hanish 1
Kanwrani Vishal 2
Satish Suchitha 3
Department of Medicine, JSS Medical College, Mysore, Karnataka, India
1 Department of Medicine, Vardhaman Mahavir Medical College and Safdarjang Hospital, New Delhi, India
2 Department of Neurosurgery, JSS Medical College, Mysore, Karnataka, India
3 Department of Pathology, JSS Medical College, Mysore, Karnataka, India
Address for correspondence: Dr. Jay Gohri, 60/25, Karol Bagh, New Delhi - 110 005, India. E-mail: jay.gohri26@gmail.com
Jul-Aug 2024
21 8 2024
27 4 416418
20 3 2024
13 7 2024
18 7 2024
Copyright: © 2024 Annals of Indian Academy of Neurology
2024
https://creativecommons.org/licenses/by-nc-sa/4.0/ This is an open access journal, and articles are distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as appropriate credit is given and the new creations are licensed under the identical terms.
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pmcCLINICAL DESCRIPTION

Subarachnoid webs are variants of incomplete or collapsed subarachnoid cysts which manifest as bands of thickened arachnoid tissue that exerts intradural mass effect on the spinal cord. They are more commonly located within the dorsal spinal cord and present an extremely rare entity (with only 50 cases reported since 1997 until January 2022).[1]

Here, we present a case of a woman in her early 40s who presented with complaints of dull, aching lower back pain for 6 months, which had recently worsened. The pain had become sharp and stabbing in nature, exacerbated by sitting and walking. The patient also reported associated numbness in both legs. She had no complaints of claudication, radicular, or girdle pain, and no history of type 2 diabetes mellitus, tuberculosis, or cerebrovascular accidents. On examination, the patient exhibited tenderness at the T8–T10 level and restricted mobility due to intense pain. She had n o sensory or motor deficits and showed a positive Patrick’s test. The presentation was suggestive of myelopathy, given the presence of lower limb numbness but intact sphincter functions and normal reflexes. Although the patient had no neurologic deficits, the examination revealed 4/5 muscle strength in both lower limbs. Laboratory investigations were normal, and a spinal X-ray was unrevealing.

Further evaluation with magnetic resonance imaging (MRI) of her spine revealed the presence of an intradural, extramedullary mass at the T9–T10 level, showing the characteristic scalpel sign, which aided in making a provisional diagnosis of a subarachnoid web. The scalpel sign was not well delineated on other MRI sequences [Figure 1]. The patient subsequently underwent surgery, and the subarachnoid web was surgically removed via durotomy. The excised specimen was sent for histopathologic evaluation, which revealed a characteristic meningoepithelial and fibrous tissue pattern suggestive of a spinal subarachnoid web [Figure 2]. The patient experienced complete postoperative recovery and had no complaints upon follow-up. This case highlights the diagnostic challenges posed by such a rare lesion and how small clues can provide significant diagnostic insights.

Figure 1 (a) Sagittal T2W MRI dorsolumbar spine shows a well-defined intradural–extramedullary lesion of 16 × 3 mm, seen in the posterior aspect of the spinal canal at T9–T10 vertebral level with AP spinal canal compromise (canal diameter- 5 mm). The scalpel sign (arrow). (b) Axial MRI of the dorsolumbar spine shows anterior displacement of the cord with no associated herniation

MRI = magnetic resonance imaging, T2W = T2-weighted, AP = Anterior posterior

Figure 2 (a) Intraoperative images showing removal of SAW. (b) Gross specimen showing a single membranous fragment measuring 5 cm. (c,d) Histopathologic report: The microscopic image of the resected SAW shows a delicate fibrovascular core and meningoepithelial cells. The stain used is hematoxylin and eosin with 40× magnification

SAW = subarachnoid web

DISCUSSION

Most of the cases described in the available literature do not provide an image of such a large gross specimen of the excised subarachnoid web. One of the reasons this case is unusual is the preserved nature in which the subarachnoid web was extracted.

Theories on arachnoid web formation fall into traumatic and nontraumatic categories. The traumatic theory suggests a clear event disrupts an existing arachnoid cyst. Nontraumatic causes include a thickened ligamentum flavum or coalescence of rogue arachnoid strands in the thoracic posterior subarachnoid space. The hydrops meningeus theory proposes an infectious or inflammatory origin. Another hypothesis suggests septations within the septum posticum may form an arachnoid cyst, which, if incompletely developed or ruptured, could result in web formation. Some cases remain idiopathic, with no clear cause identified.[2]

Our case presented with a spinal arachnoid web (SAW) in the lower part of the thoracic spine (T9–T10), compared to the usual SAWs that have a predilection for upper thoracic spine. According to most recent guidelines, cardiac gated MRI is the investigation of choice for an SAW and not computed tomography (CT) myelography.[3]

The scalpel sign, a focal indentation of the spinal cord resembling a scalpel blade, is pathognomonic for SAW. It appears as anterior cord displacement with dorsal cerebrospinal fluid (CSF) accumulation, which is visible as increased signal above the indentation on sagittal MRI. Imaging is crucial in SAW management. MRI reveals the scalpel sign and may show associated syrinx, cord edema, or subarachnoid hemorrhage, though these were absent in our case. Intraoperative ultrasound guides surgery by determining web extent, documenting CSF flow, and showing immediate postoperative changes.[4]

Differentiating SAW from similar conditions is essential. Unlike SAW, arachnoid cysts have well-defined walls and cause cord scalloping on MRI. SAW can be distinguished from loculated CSF collections, racemose neurocysticercosis, epidermoid cysts, and cystic neurofibromas. Ventral cord herniation lacks CSF between the cord and the ventral thecal sac, as the cord abuts or bulges through an anterior defect, which is best visualized on CT myelography. Arachnoiditis ossificans presents with calcification, which was absent in our case. These distinctions are crucial for accurate diagnosis and appropriate treatment.[56]

Given the limited literature on SAW diagnosis, cine MRI is proposed as superior for assessing CSF flow dynamics in web formation. While CSF flow studies were not performed in our patient, existing literature reports findings ranging from inconclusive to obstructed or turbulent CSF flow, with asymmetric pulsatile dynamics above and below the web (venturi effect). Yamaguchi et al. visualized regional CSF flow using gentian violet dye, revealing variable diffusion patterns across the web. Obtaining both sagittal and axial MRI views is crucial for accurate lesion assessment. Notably, Choi et al. reported SAW diagnosis without the classical scalpel sign, highlighting the complexity of identification in some cases.[78910]

The presenting complaints of our patient are commonly encountered among patients with SAW, who can present with neuropathic back pain, symptoms of myelopathy, and/or radicular pain. According to cases reported by Pham et al.[2] and Wali et al., surgical removal of the web is of utmost necessity if the patient is symptomatic, even without the presence of progressive neurologic symptoms. In our patient, the presence of bilateral lower limb weakness, chronic back pain with recent aggravation, and local radiation made the patient a good candidate to undergo surgery.[11]

Management of SAW typically involves surgical removal, with laminectomy and durotomy being the most common approaches, in line with Voglis et al.[4] Alternative techniques include percutaneous fenestration or web lysis. Post-surgery, histopathologic examination confirms the diagnosis.

Microscopically, SAW presents a delicate fibrovascular core with meningoepithelial cells, indicating arachnoid origin. This is confirmed by immunohistochemical staining with epithelial membrane antigen. However, differentiating SAW from an incomplete arachnoid cyst on histopathology alone is challenging, necessitating correlation with neuroimaging for a definitive diagnosis.

MRI plays a crucial role in diagnosis and management guidance. A multidisciplinary approach involving neurosurgery, radiology, and pathology ensures optimal patient care. Although rare, SAW should be considered in the differential diagnosis for chronic low back pain.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Acknowledgements

We would like to thank Dr. B Gurumurthy, Associate Professor, Department of Orthopedics, JSSMC and Dr. (Col) Vassan TS, HOD and professor, Department of Neurosurgery, JSSMC for their support and contribution to the case. The case would have been incomplete without their guidance and work on the patient.
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