Resection of an incidentally discovered spinal arachnoid web: illustrative case

Abstract

BACKGROUND

Spinal arachnoid webs (SAWs) are rare pathologies of the spinal meninges often associated with syringomyelia and the radiographic “scalpel sign.” Patients can experience pain, numbness, gait disturbances, or no symptoms at all. They are typically diagnosed via magnetic resonance imaging and treated with laminectomy and excision.

OBSERVATIONS

A 61-year-old male presented after a mechanical fall and had an incidentally discovered SAW on imaging. He was initially asymptomatic and was therefore conservatively managed. Several years later, however, the patient experienced new-onset back pain, paresthesia, and balance problems, with interval imaging demonstrating worsening of the edema surrounding his SAW. The patient subsequently underwent resection of the SAW, which led to significant resolution of his symptoms.

LESSONS

An SAW can be asymptomatic or can manifest with a wide variety of symptoms. When this condition is incidentally discovered in asymptomatic patients, neurosurgeons should guide these patients to follow-up urgently if they develop any neurological symptoms. At that time, further imaging can be performed to determine if surgical treatment is indicated. Although SAW is rare, clinicians should be aware of the signs and symptoms, because prompt surgical intervention can significantly improve neurological symptoms.

Spinal arachnoid web (SAW) is an uncommon condition characterized by a thickened band of arachnoid tissue that can cause localized pressure on the spinal cord.¹ The presence of arachnoid webs can obstruct the flow of cerebrospinal fluid (CSF), potentially resulting in syringomyelia. The disruption of CSF flow and subsequent spinal cord displacement create a distinct diagnostic indicator on magnetic resonance imaging (MRI) known as the “scalpel sign.” This sign refers to SAW’s characteristic appearance in which the spinal cord is displaced ventrally and compressed, resembling the shape of a surgical scalpel.^2,3

Patients with SAW can present with a variety of clinical symptoms, ranging from mild weakness and numbness to gait disturbance and incontinence. The most frequently observed symptoms are neuropathic pain, motor weakness, and sensory impairment, primarily affecting the lower limbs.^4–6 Although SAWs can present with these symptoms, they can also be discovered incidentally on MRI in asymptomatic patients.

In this report, we present an incidentally identified an SAW in a patient who presented to the emergency department after a mechanical fall. Although he was initially asymptomatic, he subsequently developed neurological symptoms prompting surgical intervention. Additionally, we provide a systematic review of the current literature describing and characterizing 212 SAW cases.

Illustrative Case

A 61-year-old male with a medical history of deep vein thrombosis for which he was taking rivaroxaban and reactive arthritis for which he was taking methotrexate presented to the emergency department after a mechanical fall at home. He reported no loss of consciousness during the incident but subsequently developed pain in his left shoulder and tenderness in his thoracic spine. On physical examination, he demonstrated 5/5 strength in both upper and lower extremities. His neurological examination did not show any abnormalities, with intact fine touch sensation throughout and no upper motor neuron signs.

Initial imaging with a computed tomography (CT) showed a T12 compression fracture with no retropulsion as well as T11 and T12 spinous process fractures. MRI without contrast was ordered to confirm that the fracture was limited to the vertebral body and demonstrated a comminuted fracture of the T12 body with mild anterior displacement of bone fragments. The MRI also revealed an incidental finding of a well-defined hyperintense T2 and short tau inversion recovery (STIR) signal in the central aspect of the spinal cord with cord expansion at T1–2. This could have been consistent with an inflammatory or demyelinating process, syringomyelia, or mass lesion. Subsequently, MRI with contrast was performed, which identified a band of tissue within the dorsal arachnoid space at T2–3 causing compression and anterior displacement of the cord, suggestive of an SAW (Fig. 1A and B). A CT myelogram further confirmed the presence of the SAW, demonstrating septations, anterior displacement, posterior cord contour deformity at T2, and differential contrast density above and below the T2 level with additional septations at T4 (Fig. 1C).

FIG. 1.

Initial STIR (A) and T2-weighted (B) MRI studies demonstrating a nonenhancing T2-hyperintense lesion with anterior displacement of the spinal cord. CT myelogram (C) demonstrating septations and posterior contour deformity at the T2 level, with an arrow indicating the scalpel sign.

Because the patient was asymptomatic and neurologically intact, he was offered conservative treatment with physical therapy and a thoracolumbosacral orthosis brace for his thoracic fractures. The patient was discharged home with a follow-up appointment with repeat imaging. The patient’s back pain improved during the follow-up period, and the 3-month follow-up MRI and CT were stable.

The patient presented 5 years later with 1 month of acute left-sided, low-back pain with worsening neurological symptoms, including paresthesias in both feet, leg weakness, and balance problems. MRI showed lumbar spondylosis with foraminal stenosis and an increase in the spinal cord expansion and edema. In addition, the originally detected intramedullary abnormality had grown from 3.9 × 0.8 × 0.7 cm to 4.3 × 1.0 × 1.1 cm, which likely contributed to the development of the patient’s symptoms (Fig. 2A and B). With these imaging findings and the patient’s worsening symptoms, surgery was recommended.

FIG. 2.

Preoperative STIR (A) and T2-weighted (B) MRI demonstrating increased spinal cord expansion and edema. Postoperative STIR (C) and T2-weighted (D) MRI demonstrating resolution of the cord edema.

The patient subsequently underwent a T2–3 laminoplasty with intradural exploration and resection of the SAW. After localization with intraoperative radiographs, the T2–3 levels were exposed in standard fashion. At that point a confirming localizing radiograph was taken and a T2–3 laminoplasty was performed. The dura was opened in a standard fashion and held opened with tacking sutures. Thickened arachnoid membranes were visualized from the dura to the pia over the spinal cord (Fig. 3C). The arachnoid adhesions were isolated with microdissectors and carefully resected with microscissors. A sample piece of the arachnoid web was sent for permanent pathology (Fig. 3F). The dentate ligament at T3 was incised bilaterally to further free the spinal cord. Ultrasound was used to document the degree of spinal cord tethering before and after the arachnoid web resection (Fig. 3A, B, D, and E). After hemostasis was confirmed, the dura was reapproximated in a standard fashion. The bone was reapproximated with small titanium plates and screws. Neuromonitoring showed no signal changes throughout the procedure. The arachnoid web was confirmed by histology, which demonstrated fibrous connective tissue composed of meningothelial cells and densely packed collagen fibers (Fig. 4).

FIG. 3.

Intraoperative sagittal (A) and axial (B) ultrasound images demonstrating the arachnoid web (arrows) and compressed spinal cord (asterisks). Intraoperative microscope image (C) after dural opening revealing the thickened arachnoid membranes from the dura to the pia over the spinal cord. Intraoperative sagittal (D) and axial (E) ultrasound images after removal of the arachnoid web. Sample of the extracted arachnoid web (F) that was sent for pathology.

FIG. 4.

Representative microscopic images of the SAW with hematoxylin and eosin staining.

After 2 days, the patient experienced rapid postoperative resolution of his symptoms and was discharged from the hospital. During the 6-month follow-up period, the patient’s overall condition improved. He exhibited excellent balance and no leg weakness, although he continued having lumbar back pain. On follow-up MRI, there was near complete resolution of the previous cord edema (Fig. 2C and D). The contour deformity at the T2–3 levels showed significant improvement, with the cord exhibiting near-normal caliber and signal.

Patient Informed Consent

The necessary patient informed consent was obtained in this study.

Discussion

Observations

SAWs are rare, severe pathologies that can cause significant pain, weakness, sensory changes, incontinence, and other neurological symptoms for patients of various age groups. To our knowledge, fewer than 300 cases have been reported in the literature, with a large proportion described within the last 5 years.⁶

To provide an update since the 2019 systematic review by Nisson et al.,⁶ we conducted a search of the literature on May 7, 2023, using the PubMed, Embase, Cochrane, Web of Science, and Scopus databases. The search strategy for each database is shown in Supplemental Table 1. A total of 5,003 records were identified and uploaded into the Covidence software, with the removal of duplicates yielding 2,196 unique records. Titles and abstracts were independently screened (V.V., S.A., C.W.L.) to exclude review articles, video articles, and articles studying pathologies other than SAW. The remaining 73 records underwent full-text screening for the exclusion of articles published prior to 2018 or included in the Nisson et al. review, not available as full English-language articles, and focusing on pathologies other than SAW. Supplemental Fig. 1 summarizes the record identification, screening, and inclusion process in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines.

Data were extracted from 33 full-text articles studying human patients with SAW. Recorded study variables were author(s), year of publication, and number of included patients. Patient demographic variables consisted of age, sex, history of spine surgery, and history of spine trauma. Clinical variables included the presence and location of weakness, pain, paresthesia, sensory loss, and upper motor neuron (UMN) signs at admission, as well as the presence of incontinence and gait disturbance, the progressive onset of symptoms, and a duration of symptoms greater or less than one year. The characteristics of SAW in each patient were also noted, including the spinal level(s) affected, dorsal/ventral orientation, presence of an associated syrinx, spinal level(s) affected by the syrinx, and presence of the scalpel sign on imaging. Finally, treatment and outcome variables were intervention, duration of follow-up, and final neurological outcome relative to symptoms at admission. Findings from 18 case reports and 15 case series are outlined in Supplemental Table 2. Further bias assessment was not conducted, because all included studies were case reports or case series.

Our literature search yielded a sample size of 212 patients who had been admitted for SAW evaluation between 2018 and 2023. Table 1 summarizes data extracted in this systematic review. Patient ages ranged from 23 to 91 years old, with a mean age of 57.2 years. There was a slightly higher prevalence of male patients as compared to female patients (51.4% vs 48.1%). Patients with SAW rarely presented with a history of prior spine surgery (13.2%) or spine trauma (5.7%).

TABLE 1.

Summary of data from 212 patients extracted from a full-text article review

Variable	Value	Number Not Reported
Demographics
Age in yrs, range (mean)	23–91 (57.2)	0 (0.0%)
Sex: M/F	109 (51.4%)/102 (48.1%)	1 (0.5%)
Prior spine surgery: yes/no	28 (13.2%)/151 (71.2%)	33 (15.6%)
Prior spine trauma: yes/no	12 (5.7%)/81 (38.2%)	119 (56.1%)
Symptoms
Pain: yes/no	143 (67.5%)/69 (32.5%)	0 (0.0%)
Weakness: yes/no	100 (47.2%)/112 (52.8%)	0 (0.0%)
Gait disturbance: yes/no	91 (42.9%)/121 (57.1%)	0 (0.0%)
Sensory loss: yes/no	67 (31.6%)/44 (20.8%)	101 (47.6%)
UMN signs: yes/no	52 (24.5%)/75 (35.4%)	85 (40.1%)
Paresthesia: yes/no	44 (20.8%)/114 (53.8%)	54 (25.5%)
Incontinence: yes/no	35 (16.5%)/177 (83.5%)	0 (0.0%)
Progressive: yes/no	30 (14.2%)/9 (4.2%)	173 (81.6%)
Duration in mos: range (mean)	0.1–360 (38.9)	133 (62.7%)
Web orientation: dorsal/ventral	212 (100.0%)/0 (0.0%)	0 (0.0%)
Syrinx present: yes/no	81 (38.2%)/123 (58.0%)	8 (3.8%)
Syrinx location: cervical/cervicothoracic/thoracic	2 (0.9%)/12 (5.7%)/ 25 (11.8%)	43 (20.3%)
Scalpel sign present: yes/no	121 (57.1%)/3 (1.4%)	88 (41.5%)
Intervention		3 (1.4%)
Laminectomy + excision	116 (55.2%)
Conservative management	77 (36.3%)
None	7 (3.3%)
Catheter-guided fenestration	3 (1.4%)
Laminectomy + fusion	2 (0.9%)
Laminoplasty + excision	1 (0.5%)
Laminectomy + shunt	1 (0.5%)
Hemilaminectomy + excision	1 (0.5%)
Intercostal injection	1 (0.5%)
FU period in mos, range (mean)	1–80 (16.5)	99 (46.7%)
Neurological outcome: improved/unchanged	71 (33.5%)/23 (10.8%)	118 (55.7%)

FU = follow-up.

The most reported symptom upon presentation with SAW was pain (n = 143; 67.5%), followed by weakness (n = 100; 47.2%) and gait disturbance (n = 91; 42.9%). Reported symptom duration before treatment ranged from 3 days to 20 years, with a mean duration of 38.9 months. All 212 SAWs presented on the dorsal aspect of the spinal cord and were most commonly localized at the T4 (n = 30), T6 (n = 28), T5 (n = 26), and T7 (n = 25) levels. Notably, only one SAW was reported in a cervical spinal level by Yamamoto et al.⁷ The presence or absence of a concomitant spinal cord syrinx was reported in 204 cases, and 39 patients had their exact syrinx location described. A syrinx was observed in 81 cases (38.2%), most frequently within the thoracic spinal cord (n = 25; 11.8%), whereas a concomitant syrinx was not observed in 123 cases (58.0%). On clinical imaging, an SAW was reported despite the explicit absence of the scalpel sign in only 3 patients (1.4%): the patient had presented in Nagashima and colleagues’ the 2022 case report by Nagashima et al.⁸ and two patients had presented in the 2021 case series by Laxpati et al.⁹

Most patients (n = 125; 59.0%) underwent procedural management of their SAW. Laminectomy with intradural excision was the most common procedural intervention (n = 116; 55.2%). Conversely, 77 patients (36.3%) underwent conservative SAW management. The 113 patients with recorded follow-up visits were seen at an average of 16.5 months after admission. Seventy-one patients (33.5%) were reported to have symptomatic improvement, 23 patients (10.8%) had unchanged symptoms, and 118 patients (55.7%) had no neurological outcome reported.

Limitations

The rare and poorly understood nature of SAW caused unavoidable limitations in our study. Most included articles had patient sample sizes of only one or two cases. Moreover, all studies in this review were retrospective. These factors limit our understanding of the diverse pathophysiology of SAW as well as our ability to form definitive conclusions about the efficacy of intervention options. Another notable limitation was inconsistent reporting of variables of interest during full-text review. Although some variables such as patient age and dorsal-ventral web orientation were reported for all 212 patients, other variables had upwards of 60% to 80% of cases missing explicit information we intended to use in our study.

Limitations may also arise from our literature search strategy. We excluded abstracts, preprint articles, articles in languages other than English, literature reviews, video articles, articles that studied nonhuman subjects, and articles that primarily studied pathologies other than SAW, even if SAW was mentioned in these study types. This may have led to the exclusion of some cases that could have contributed data to our study. Finally, we decided to exclude all relevant articles published prior to 2018 to focus on a modern series and since Nisson et al. conducted their literature search in late 2018.

Lessons

Our study further develops knowledge on SAW by identifying cases and summarizing data reported since the systematic review by Nisson et al.⁶ Compared to that study of 43 cases, we identified a larger (n = 212), older patient population (mean 57.2 vs 52 years) that was more evenly distributed in terms of patient sex (males: 51.4% vs 72%). A concomitant spinal cord syrinx was present in approximately one-third of our cases, which differs from the two-thirds of cases reported by Nisson et al.⁶ Notably, our methodology included patients with SAW who were treated both surgically and nonsurgically. This, as well as the rare nature of the pathology, may account for some differences between our findings and those of the prior review. Laminectomy with intradural excision was the most common surgical option and was generally associated with symptomatic improvement in the reported cases,^6,10 which aligns with the findings of Nisson et al.⁶ (Table 1). A case report by Qureshi et al.¹¹ further reported that catheter-guided fenestration of SAW produced stable neurological improvement as well. Among patients who had been treated conservatively in our sample (n = 77), there appeared to be lower rates of reporting on follow-up duration and neurological outcome. For example, the cohort of Elkadi et al.¹² included 59 patients who had been treated conservatively, but outcomes were only reported for 26 surgical patients. This may limit the ability to draw conclusions about the efficacy of a conservative SAW treatment approach, and future studies should explore outcomes for these patients.

Despite increasing research in recent years, the etiology of SAW is still unclear. Some researchers posit that webs form from disrupted arachnoid cyst formation, from the rupture of an arachnoid cyst,^13,14 or as a sequela of spinal trauma or surgery. Similar to Nisson et al.,⁶ we found a low prevalence of previous spinal surgery or trauma among our sample, indicating that additional research into the causes of SAW is warranted.⁶ Multiple studies have further suggested that SAW is underdiagnosed and underreported, perhaps due to slow symptom progression as described in our sample (mean symptom duration >3 years) or to the potential for asymptomatic presentation.¹³ The heterogeneity of symptoms experienced by patients with SAW may also complicate diagnosis. Patients in our sample presented with various combinations of symptoms ranging from pain to incontinence and gait disturbance. These affected various regions of the trunk and extremities, and even the patient’s face in one case,¹⁰ and symptoms were not always stereotypical of SAW presentation at a specific spinal level. Although the two patients in the case series by Inoue et al.¹⁵ had SAW at adjacent spinal levels and neither had a syrinx, one patient had presented with gait disturbance without pain, whereas the other had presented with only trunk and lower-extremity pain. Despite being explicitly noted in only one-third of patients in our sample, the presence or absence of an associated spinal cord syrinx can also affect SAW presentation. Brasil et al.¹⁶ reported two patients who were demographically similar and had SAW at the same spinal level, but the patient with a syrinx reported progressive gait disturbance and upper extremity paresthesia, whereas the patient without a syrinx reported only trunk pain.¹⁶

Early identification and intervention for SAW may reduce the disease burden and improve quality of life for patients.³ In our sample, despite the various clinical presentations, some radiographic signs were common across nearly all cases. Of the SAWs reported in our review and in the review by Nisson et al.,⁶ only one was present outside of the thoracic spine, at C7.⁷ All SAWs (n = 212) in our review, as well as all but two cases in the previous review, were located on the dorsal spinal cord. Finally, although the scalpel sign is present in other rare spinal pathologies and is not necessary for a diagnosis of SAW,³ our review contained only three cases of SAW with the explicit absence of this radiographic marker.^8,9 Neuroradiologists aware of these traits may increase the diagnosis and early detection of SAW. Spine surgeons are encouraged to correlate these imaging findings with clinical presentation to determine adequate treatment since successful outcomes can be achieved with the resection of the SAW.

Author Contributions

Conception and design: Bydon, Vattipally, Ahmadi, Khalifeh, Rincon-Torroella. Acquisition of data: Bydon, Vattipally, Ahmadi, Liu, Khalifeh. Analysis and interpretation of data: Vattipally, Weber-Levine, Rincon-Torroella. Drafting the article: Vattipally, Ahmadi, Weber-Levine, Rincon-Torroella. Critically revising the article: Bydon, Vattipally, Weber-Levine, Khalifeh, Rincon-Torroella. Reviewed submitted version of manuscript: Bydon, Vattipally, Weber-Levine, Abu-Bonsrah, Khalifeh, Rincon-Torroella. Approved the final version of the manuscript on behalf of all authors: Bydon. Statistical analysis: Vattipally. Study supervision: Rincon-Torroella.

Supplemental Information

Online Only Content

Supplemental material is available with the online version of the article.

• Supplemental Figure and Tables. https://thejns.org/doi/suppl/10.3171/CASE23701.