Multi-modality imaging evaluation of the dorsal arachnoid web

Abstract

Purpose

Dorsal arachnoid web (DAW) is a rare intradural abnormality which is associated with progressive myelopathy. Our objective was to review multi-modality imaging techniques demonstrating the scalpel sign appearance in evaluation of DAW.

Methods

We retrospectively reviewed various imaging modalities of patients found to have DAW at our institution during January 2015 to February 2020. Five patients underwent surgical decompression with pathological correlation. The remaining patients were presumptively diagnosed based on the characteristic finding of scalpel sign. Clinical data were evaluated and correlated to imaging findings. All imaging modalities demonstrated the characteristic scalpel sign.

Results

Sixteen patients (10 females, and six males) with multi-imaging modalities were evaluated. Their mean age was 52 year (range 23–74 years). Fifteen patients underwent conventional spine MRI. Further high-resolution MR imaging techniques, e.g. 3D T2 myelographic sequence, were utilized with two patients. MRI spine CSF flow study was performed to evaluate the flow dynamic across the arachnoid web in one patient. Eight patients were evaluated with CT myelogram. Syrinx formation was discovered in seven (44%) patients; five (71%) of them underwent surgical resection and decompression. Two patients underwent successful catheter-directed fenestration of the web with clinical improvement. We found a statically significant positive correlation between the degree of cord displacement and compression with syrinx formation (r = 0.55 and 0.65 with p-value of 0.03 and 0.009, respectively).

Conclusion

DAW has characteristic scalpel sign independent of imaging modality. Multi-modality imaging evaluation of DAW is helpful for evaluation and surgical planning.

Introduction

Dorsal arachnoid web of the spine is an increasingly identified abnormality that presents with progressive myelopathy.¹ Arachnoid webs are a rare variant of arachnoid cysts in which one or multiple focal membranes of arachnoid tissue obstruct the subarachnoid space.² Although arachnoid webs are frequently intradural dorsal to the spinal cord, ventral arachnoid webs have also been described. Dorsal arachnoid webs most commonly occur in the upper thoracic segments, with slight female predilection.³

Anatomically, the spinal cord has multiple stabilizers during movement: paired denticulate ligaments, Hoffman ligament and septum posticum.¹,⁴ The denticulate ligaments run laterally and connect the pia to dura mater. Hoffman ligament is anteriorly located, whereas the septum posticum is present posteriorly along the posterior median sulcus.¹,⁵

The etiology of arachnoid web is not fully understood.^5–7 It has been hypothesized that arachnoid webs may be idiopathic, remnants of a previous arachnoid cyst that ruptured, or are precursors to arachnoid cysts that have not fully walled off.³,⁵,⁶,⁸ Arachnoid adhesions could also result secondary to trauma or prior surgery.⁹

Radiologically the characteristic appearance of the scalpel sign, first described by Reardon et al.,⁶ is potentially pathognomonic for dorsal thoracic arachnoid web. The scalpel sign means focal indentation of the thoracic spine with anterior displacement of the thoracic cord and widening of the dorsal subarachnoid space at the level of cord compression. This resembles the surgical scalpel. Randall et al.¹⁰ also found the scalpel sign in all patients with arachnoid web.⁸,¹⁰ Previous studies are summarized in Table 1. Computed tomography (CT) myelogram is the gold standard to diagnose dorsal arachnoid web, and magnetic resonance imaging (MRI) evaluation is imperative to properly evaluate for severity and complication, e.g. syrinx formation. Our goal was to describe spinal imaging findings in different radiologic modalities for an accurate diagnosis and evaluation of dorsal arachnoid webs.

Table 1.

Summary of previous studies on dorsal arachnoid web.

Study	Type of study	Number of cases included
Reardon et al., 20166	Case series on scalpel sign	5 cases + 6 cases previously reported in the literature
Sayal et al., 20169	Case report on syrinogomyelia with dorsal arachnoid web	2 cases
Zhang and Papavassiliou, 20172	Case series	3 cases
Randall et al., 201710	Clinical article	6 cases
Ruschel et al., 201818	Case report	1 case
Hirai et al., 20198	Research on clinical outcomes of surgical management of dorsal arachnoid web	5 cases
Nisson et al., 20195	Systematic review	41 cases

Methods

Patients

This retrospective study was conducted under an Institutional Review Board approved protocol, with a waiver of written consent. Using our institution McKesson PACS system, we searched for radiology reports which included the terms “arachnoid web” between January 2015 and February 2020 to review spine imaging of patients diagnosed with dorsal arachnoid web. Of 4043 thoracic spine MRIs on our PACS, we found 16 patients (10 females and six males) with the diagnosis of dorsal arachnoid web. The demographic data, clinical presentation and neurological signs of the patients were obtained from the electronic medical records. Fifteen patients (15/16) had MRI thoracic spine; two of them had 3D T2 myelographic sequence, while one patient only had MRI cerebrospinal fluid (CSF) flow study. Eight patients had CT myelogram; one of them was evaluated only with CT myelogram. Pathological evaluation was available for five patients who underwent surgical decompression. The remaining patients were presumably diagnosed based on the presence of characteristic scalpel sign. Three patients had conservative treatment with follow-up imaging, as they had mild symptoms with mild radiological findings. Two patients underwent catheter-directed fenestration of the web with clinical improvement and continued follow-up. Two patients had multiple comorbidities and the surgery has been postponed. Four patients sought another opinion from a different institute.

MRI protocol

MRI spine was conducted on 3 Tesla MRI machine (Magnetom Vida and Skyra, Siemens Healthineers, Malvern PA). Our routine MRI of the thoracic spine includes sagittal T2 and STIR, and axial T2 imaging. High-resolution T2 sampling perfection with application-optimized contrasts using different flip-angle evolution (SPACE) imaging sequence was also acquired in two patients. Post-processing images with reconstruction of multi-planar 2D and 3D myelographic images were routinely obtained for the latter. Evaluation of CSF flow dynamics was conducted with utilization of cardiac gated phase contrast MRI CSF flow study. The MRI acquisition protocol is summarized in Table 2.

Table 2.

MRI parameters for MRI thoracic spine with myelographic sequence and CSF flow study.

Sequence	TE (ms)	TR (ms)	TI (ms)	FOV (mm)	Flip angle	Slice thickness	Voxel Size	Matrix
Sag T2	106	3800		50	150°	3 mm	0.3 × 0.3 × 3.0 mm	640 × 640×16
Sag STIR	38	4000	160	50	150°	3 mm	0.3 × 0.3 × 3.0 mm	640 × 640×16
Sag T1	10	672		50	150°	3 mm	0.7 × 0.7 × 3.0 mm	320 × 320×16
Ax T2	101	4660		288	150°	5 mm	0.6 × 0.6 × 3.0 mm	240 × 320×16
Sag T2 SPACE	139	1500		60	120°	0.9 mm	0.3 × 0.3 × 0.6 mm	320 × 320×16
CSF flow study	8.45	50.12			10°	6 mm	0.9 × 0.9 × 5.0 mm	256 × 256×16

CT myelogram protocol

With the patient in the prone position on the fluoroscopy table under fluoroscopic guidance, an inter-laminar space was localized, typically at L2–L3. Approximately 3 ml of 1% lidocaine solution was administered for local anesthetic. Under fluoroscopic guidance, a 20-gauge 3¹/₂ inch spinal needle was carefully advanced till clear CSF returned back. Following confirmation of needle placement, 10 ml of iodinated contrast medium (Omnipaque 240, Guerbet, France) was instilled, under fluoroscopic guidance, into the thecal sac. The needle was removed with the stylet in place and multiple films were taken. Following this, the patient was taken to the CT scanner for additional post-myelographic CT examination of the thoracic spine. CT thoracic spine is acquired on 128 row-detector second-generation dual-source DECT system (Somatom Definition Flash, Siemens Healthineers, Malvern PA). Spiral acquisition of overlapping 1.25 mm images of the thoracic spine from C5 to L3 vertebral levels was performed. Sagittal and coronal 2D reformations with 2 mm slice thickness were done by the technologist. The CT scan was acquired using kV140, mA 277 and matrix of 512 × 512.

Imaging analysis

We reviewed all imaging modalities and evaluated for the presence of the scalpel sign in each imaging modality: this is a focal indentation of the thoracic spine with anterior thoracic cord displacement and widening of the dorsal subarachnoid space at the level of cord compression.⁶ The exact level of cord compression was determined, as the degree of cord compression was classified into two groups: less than half the cord circumference and more than half the cord circumference. We also annotated the length of the compression in cranio-caudal dimension and classified it into three categories: mild (less than or equal one vertebral body level), moderate (two vertebral body levels) and severe (more than two vertebral body levels).

We additionally evaluated the presence or absence of the syrinx, and extension and location of syrinx (determined by the corresponding vertebral body level, and classified into focal if less than one vertebral body level or large if more than one vertebral body level). The location of syrinx was classified into rostral or caudal to the level of cord compression.

Pathological evaluation

Five patients in our cohort underwent surgical decompression. The pathology results revealed fibrotic wall-membrane, reflective of dorsal arachnoid web. The remaining patients were presumably diagnosed based on the presence of the characteristic scalpel sign.

Statistical analysis

The data were analyzed using Microsoft Excel and SPSS software version 22 (IBM SPSS Statistics, USA). The Kolmogorov–Smirnov and Shapiro tests were used to test normality of our data. Our data were normal, so mean and standard deviation were used as descriptive measures.

Results

Our study included 16 patients with dorsal arachnoid web who underwent various spine imaging, e.g. MRI and CT myelography (Figure 1). The patients’ ages ranged from 23 to 74 years with a mean age of 52 years. The demographic distribution is illustrated in Table. 3. Back pain (11/16) was the most common clinical presentation in our cases and included low back pain, mid and upper thoracic pain. Other frequently encountered symptoms were lower extremity weakness (9/16), numbness and tingling sensation of the feet, unsteady gait (7/16), and frequent falls (4/16). Some patients presented with complications from frequent falls, e.g. cervical fracture (1/16), and right parietal hemorrhage (1/16). Few patients were presented with general symptoms such as headache (4/16) and memory problems (1/16). Complete details of clinical findings are shown in Table 3.

Table 3.

Patients’ demographic and clinical findings.

Variable	Number (Percentage)
Age	52±16
Sex
Male	6 (38%)
Female	10 (72%)
Clinical presentation:
Presenting symptoms:
Back pain	11 (69%)
Lower extremity weakness	9 (56%)
Muscle spasm and stiffness	2 (12%)
Unsteady gait	7 (44%)
History of:
Frequent falls	4 (25%)
Urinary or bowel dysfunction	1 (6%)
Prior back surgery	3 (19%)
Interventions;
Epidural injections	2 (13%)
Blood patch	1 (6%)
Other symptoms
Headache	4 (25%)
Memory loss	1 (6%)
Associations/complications:
Right parietal AVM	1 (6%)
Small right parietal hemorrhage	1 (6%)
Cervical spine fracture	1 (6%)
Left ACA aneurysm	1 (6%)

Imaging evaluation revealed the characteristic focal indentation of the thoracic cord with variable degrees of anterior displacement (scalpel sign) in all our patients (Table 4). The most common encountered levels were T3–T4, T4–T5, T5–T6 and T6–T7 levels, with three patients for each level. According to degree of thoracic cord indentation, we found (7/16) patients with more than one half of the cord circumference. Regarding the level of anterior displacement of the cord, six patients had severe cord displacement, three patients demonstrated moderate cord displacement, and seven patients demonstrated mild cord displacement. Seven patients in our cohort had a syrinx; two of them involved more than three vertebral levels. In most of cases (5/7, 71%), the syrinx was rostral to the arachnoid web. We found a statistically significant positive correlation between the degree of cord displacement and compression with syrinx formation (r = 0.55 and 0.65 with p-value of 0.03 and 0.009, respectively).

Table 4.

Detailed imaging parameters.

Variable	Number (Percentage)
Imaging modalities
MRI	15 (94%)
MRI CSF flow study	1 (6%)
3D T2 SPACE myelographic sequence	2 (13%)
CT myelogram	8 (50%)
Intra-operative US	2 (13%)
Level of arachnoid web
T1–T2	1 (6%)
T2–T3	2 (12%)
T3–T4	3 (18%)
T4–T5	3 (18%)
T5–T6	3 (18%)
T6–T7	3 (18%)
T8–T9	1 (6%)
Degree of dorsal cord indentation
≤ half the circumference	9 (56%)
≥ half the circumference	7 (44%)
Length of cord displacement
≤ one vertebral level	7 (44%)
Two vertebral levels	3 (19%)
≥ three vertebral levels	6 (38%)
Presence of syrinx
Small focal syrinx	5 (31%)
Large extensive	2 (13%)
No syrinx	8 (50%)
Not evaluated	1 (6%)

Discussion

Precise localization of arachnoid web with pre-operative imaging is usually difficult.¹¹ Conventional MRI spine (Figure 2) and CT myelogram (Figure 3) have limited ability to reliably recognize the thickened arachnoid bands membranes forming the arachnoid web.⁶,⁹,¹⁰,¹²,¹³ However, other secondary and indirect imaging signs such as the scalpel sign indicate the diagnosis of arachnoid web.⁶,¹³,¹⁴ The appearance of focal cord indentation with anterior cord displacement and widening of the dorsal subarachnoid space resembles the surgical scalpel.⁶,⁹,¹² Our study demonstrated the presence of that sign in all investigated patients. Reardon et al.⁶ first described it in 14 patients, with pathological confirmation of arachnoid web in five patients who underwent surgical lysis.⁶,¹⁵ Randall et al.¹⁰ also found the scalpel sign in all patients with arachnoid web.⁸,¹⁰ Other advanced MRI techniques have been used in evaluation of arachnoid web. Thin arachnoid bands webs could be identified on high-resolution 3D T2 myelographic sequences. These sequences were acquired in two patients and demonstrated a thin hypointense line within the dorsal arachnoid space, which pointed at the underlying anatomic abnormality (Figure 4). Cardiac gated cine phase contrast MRI technique, which evaluates the CSF flow dynamics, has been found to provide functional information that could correlate with location of the arachnoid web.⁶,¹¹,¹³ Disturbance of the CSF flow at the level of arachnoid web could also explain syrinx formation.⁶,¹¹,¹⁶ CSF flow artifacts, which indicate CSF dynamic flow impairment at the site of blockage, would potentially differentiate arachnoid web from arachnoid cyst (Figure 5).¹ Chang et al.¹¹ reported that quantitative analysis of CSF flow in the axial plane provides important information for evaluation of arachnoidopathy.¹¹ Mauer et al. also reported that cardiac gated phase contrast MRI with quantitative CSF flow analysis was superior to CT myelography for evaluation of arachnoid web.¹⁷ However, despite the high spatial resolution of MRI, MRI techniques are susceptible to motion artifacts which could mask underlying thin arachnoid web.⁹,¹³ Intra-operative ultrasonography is a helpful imaging technique for better intra-operative localization and direct visualization of the arachnoid web.¹⁸ Two patients in our cohort had intra-operative ultrasound which showed thick arachnoid band with cord expansion at the site of syrinx (Figure 6).

The exact etiology of arachnoid web is not fully understood.^5–7 Some authors believed that arachnoid adhesions could result secondary to trauma or prior surgery.⁹ We found six patients in our cohort with history of fall, prior back surgery or epidural intervention that could consolidate this assumption. Arachnoid webs could also result from disruption or inflammation of intermediate leptomeninges.²,⁸ Chang et al.¹¹ also suggested an underlying inflammatory etiology of arachnoid web due to the presence of CD3-positive cells in their cases.¹¹ Hypertrophy and thickening of the septum posticum has been described as a probable etiology of an idiopathic arachnoid web.⁹ Nisson et al.⁵ proposed that in certain circumstances the posterior posticum thickens and impedes the laminar CSF flow.¹,⁵,⁹ Then, tensile forces have been exerted on this arachnoid strand which act as growth-signal activation stimulus for further thickening and hypertrophy of the arachnoid strand.⁵

Most arachnoid webs are associated with syringomyelia, the precise mechanism of which is uncertain.²,¹⁹ Several theories have been discussed including forceful CSF flow resulting in arachnoid herniation into congenital, post-traumatic, post-infectious and post-operative dural defect.³ Heiss et al. explained formation of syringomyelia as caused by increased CSF pressure proximal to the syrinx.¹⁸ This theory could not explain why syrinx forms at a distance from the site of blockage.¹¹ Other authors proposed that obstruction of CSF flow creates a pressure gradient between the inside and outside of the spinal cord, which acts as a driving force for syrinx formation.¹¹ This is also called the intramedullary pulse pressure hypothesis, proposed by Greitz.²⁰ Systolic CSF pressure forces the CSF flow from high- into low-pressure areas. In the case of arachnoid web with CSF flow blockage, syrinx is formed as a result from transmission of CSF fluid into the spinal cord.²,²⁰ In non-obstructing lesions syrinx is usually formed as a result of the Venturi (suction) effect.²,²⁰ The Venturi effect would also explain syrinx formation rostral to the site of blockage.²¹ Zhang and Papavassiliou described an arachnoid web presented as multi-septated longitudinal membranes with partial obstruction of CSF flow. This derangement resulted in a one-way valve mechanism with progressive compressive myelopathy.² They also observed distension of the web rostrally with each systole, reflective of the Venturi effect.²

The management of dorsal arachnoid web depends on severity of clinical symptoms and radiological findings. Acute or sudden onset of neurological symptoms prompt surgical intervention.¹⁴ However, patients presenting with mild symptoms as pain without alarming radiological findings are usually treated conservatively with continued follow-up.⁸ The imaging evaluation should focus on detection of the scalpel sign, possible visualization of arachnoid web band, and complications such as severity of thoracic cord displacement–compression and syrinx formation.⁵ CT myelogram is helpful to determine the level of CSF obstruction and cord compression, and to detect the scalpel sign.⁹ In these circumstances, when dorsal arachnoid web is discovered on the patient’s CT myelogram, MRI should be performed for further evaluation of severity and complications, e.g. syrinx.¹⁹ A suggested diagnostic and therapeutic approach is provided (Figure 7).

Figure 7.

Flow diagram for a suggested diagnostic and therapeutic approach for dorsal arachnoid web.

Figure 1.

Flow chart illustrating included patients in our cohort.

Figure 2.

Dorsal arachnoid web in a 64-year-old female. Sagittal T2 (a), and STIR (b) images show focal indentation of thoracic cord at T3–T4 level (outlined arrow), with anterior displacement of the cord and widening of the dorsal subarachnoid space. Small faint central intramedullary area of T2/STIR hyperintense signal, more appreciated at STIR image (arrow), concerning of syrinx. Axial T2 (c) image at the level of CSF blockage demonstrates flattening of the dorsal cord aspect (black arrow) and anterior displacement of the cord with marked attenuation of the ventral CSF space.

Figure 3.

Dorsal arachnoid web in a 66-year-old female. Sagittal T2 MRI thoracic spine (a) and sagittal CT myelogram (b) images show characteristic focal indentation of dorsal aspect of the thoracic cord at T2–T3 level (arrows) with anterior displacement of thoracic cord widening of the dorsal subarachnoid space, suggestive of “scalpel sign.”

Figure 4.

Dorsal arachnoid web in a 30-year-old female. Sagittal T2 weighted image of thoracic spine (a) demonstrates scalpel sign at T4–T5 level (white arrow) with increased CSF pulsation artifacts. Sagittal T2 SPACE (b and c) images confirm the focal indentation of thoracic cord at T4–T5 level (white arrow) with visualization of faint hypointense membrane at this level (black arrow), suggestive of dorsal arachnoid web.

Figure 5.

Dorsal arachnoid web in a 66-year-old female. Sagittal T2 weighted image of the thoracic spine (a) reveals the scalpel sign at T2–T3 level (arrow) with increased CSF pulsation artifacts. Sagittal phase (b) and magnitude (c) images of CSF flow study show the disturbance of CSF dynamic flow with diminished CSF flow rostral to site of blockage (arrows).

Figure 6.

Dorsal arachnoid web in a 44-year-old male. Pre-operative sagittal T2 weighted image of the thoracic spine (a) demonstrates the scalpel sign (outlined arrow) with development of syrinx rostral to site of CSF blockage (arrow). Transverse (b) and sagittal (c) intra-operative ultrasound images demonstrates thickened band-like structure within the subarachnoid space, impressive of arachnoid web (outlined arrows). Post-operative sagittal T2 weighted image of the thoracic spine at the same level shows the interval resolution of CSF blockage and improvement of syrinx (arrow).

Surgical treatment of arachnoid web includes surgical intradural lysis, catheter-directed fenestration, and syringosubarachnoid and syringopleural shunts.¹⁰,¹³,¹⁶ Many studies showed that surgical lysis of the arachnoid adhesions and bands is a feasible curative procedure with rapid neurological recovery.¹⁵,¹⁸,²² Careful attention must be given to verify that there is satisfactory flow of CSF from the cranial and caudal ends of the dural opening.¹⁰

Conclusion

Direct visualization of arachnoid web membrane on imaging is not feasible in all instances. Secondary and indirect radiologic signs such as scalpel sign are suggestive of arachnoid web. Multi-imaging modalities could help in the evaluation of arachnoid web and increase confidence for such diagnosis. MRI is imperative for detection of associated syrinx formation and accurate depiction of severity. 3D high-resolution myelographic sequences can directly visualize thin arachnoid band membrane, reflective of dorsal arachnoid web. The cardiac gated cine phase contrast MRI technique demonstrates abnormalities in CSF flow dynamics and may be helpful to monitor treatment response in patients treated conservatively.