INTRODUCTION
Syringomyelia is a neurological disorder characterized by the presence of a fluid-filled cavity within the spinal cord, containing fluid similar to cerebrospinal fluid (CSF) [1]. It is commonly associated with conditions such as Chiari malformation, spinal cord trauma, tumors, and posthemorrhagic or postinfectious arachnoiditis [2]. In contrast, idiopathic syringomyelia (IS) is a form of syringomyelia without these identifiable underlying causes, making its diagnosis and treatment particularly challenging [3,4]. The prevalence of syringomyelia varies significantly by region, with rates of approximately 8.2 per 100,000 in Western populations [5], compared to just 1.94 per 100,000 in East Asian populations, as reported in a nationwide epidemiological survey [6]. Despite its rarity, there is currently a lack of single-center clinical case series specifically focused on the diagnosis and treatment of IS, underscoring the need for further clinical investigation.
Arachnoid webs are intradural, extramedullary transverse bands that typically cause localized compression on the dorsal surface of the spinal cord, with a particular predilection for the upper spinal cord. IS with occult arachnoid webs (IS-OAW) refers to the development of fluid-filled syrinx within the spinal cord without an identified cause. The presence of subtle or undetectable arachnoid webs within the subarachnoid space (SAS) could disrupt normal CSF flow, alter spinal cord pressure dynamics, and potentially contribute to syrinx formation.
In some cases, IS-OAW may be discovered incidentally or remain asymptomatic, identified only through routine screening or incidental imaging findings. The exact etiology of these webs is often unclear, as they tend to be relatively occult. When symptomatic, patients might present with a wide range of neurological signs, including pain, sensory disturbances, spasticity, paralysis, and abnormal reflexes. Other potential symptoms include toe walking, constipation, and incontinence, which often prompt further evaluation with magnetic resonance imaging (MRI) [7]. For these patients, addressing the underlying structural causes of the syrinx, such as arachnoid adhesions, offers the best chance for lasting neurological improvement [7]. In particular, noninfectious and nontraumatic arachnoid adhesions, including occult webs or scarring within the spinal SAS, should be managed through intradural exploration and decompression to restore normal CSF circulation, a critical step in preventing syrinx progression [4].
However, the precise mechanisms driving CSF obstruction in cases of primary arachnoid webs remain poorly understood. In this study, we retrospectively analyzed a consecutive series of patients with IS from a single-center syringomyelia database, comparing them with cases of posttraumatic delayed syringomyelia (PTDS). By examining their clinical presentations, imaging findings, pathological features, and surgical outcomes, this study aims to provide valuable clinical insights into the diagnosis and management of IS-OAW, ultimately improving patient prognosis.
MATERIALS AND METHODS
1. Patient Selection
From December 2020 to March 2025, a retrospective analysis was conducted on a consecutive series of syringomyelia patients from a single-center database, which included 1,039 cases. Following a detailed screening process, 15 patients diagnosed with IS-OAW who underwent surgical treatment met the inclusion criteria for this study (Fig. 1).
This study was conducted by the principles of the Declaration of Helsinki. Ethical approval (No. 2018027001) for this study was provided by the Ethical Committee Xuanwu Hospital Capital Medical University, Beijing, China.
Inclusion criteria: (1) The patients with MRI demonstrating syringomyelia; (2) The patients who underwent surgery (posterior laminectomy and spinal arachnoid lysis) in our center.
Exclusion criteria: (1) Patients with identifiable etiologies, including Chiari malformation type I, spinal trauma, tumors, or infectious adhesive arachnoiditis; (2) patients with a history of spinal fusion or instrumentation surgery; (3) patients with basilar invagination, atlantoaxial dislocation, congenital vertebral anomalies, myelomeningocele, degenerative cervical spondylosis, or tethered cord; (4) patients with syringomyelia secondary to intracranial space-occupying lesions or hydrocephalus; (5) patients with insufficient clinical data or lack of postoperative follow-up.
Additionally, a validation cohort of 25 patients with PTDS who underwent surgical treatment at our center was included for comparative analysis. This study was conducted by the principles of the Declaration of Helsinki and was approved by the Ethics Committee of our center. Informed consent was obtained from all participants, and the work has been reported in line with the PROCESS criteria [8].
2. Clinical Evaluation
We collected general patient data, including age, sex, smoking history, and clinical symptoms like syrinx-related manifestations, duration of symptoms, and neurological signs. Both preoperative and postoperative neurological functions were evaluated. Clinical symptoms included paresthesia, muscle atrophy, urinary and fecal incontinence, and limb weakness. Notably, some patients presented with multiple concurrent symptoms.
3. Radiographic Evaluation
A routine MRI and myelogram were conducted within 1 week before surgery. After surgery, postoperative MRI was routinely performed on a 1-month, 3-month (±1 month), 6-month (±3 months), 1-year, 1.5-year basis, and then annually until evidence of significant syrinx resolution was found. The following indicators were measured at the median sagittal and transverse T2-weighted MRI positions: (1) syrinx tension index was evaluated with maximal syrinx/cord (S/C) ratio: the ratio of syrinx diameter to the spinal cord diameter at the same level; (2) syrinx resolution rate was estimated for the maximal S/C ratio using the formula below: (preoperative value-postoperative value)/preoperative value. RadiAnt DICOM Viewer software (ver. 4.6.9, Medixant, Poland) was applied to observe and measure images.
4. Surgical Procedure
Posterior laminectomy and spinal arachnoid lysis were performed in 15 patients with IS. Meticulous microsurgical dissection was undertaken to carefully detach the thickened dorsal arachnoid tissue adherent to the spinal cord. The procedure typically commenced with the removal of the fibrotic arachnoid layer over the dorsal midline and was progressively extended laterally toward both sides, approaching the region where the dentate ligament anchors to the dura. Blunt dissection techniques were utilized to develop a natural surgical plane between the arachnoid adhesions and adjacent anatomical elements, facilitating safe and effective release. Transection of the dentate ligament was followed by the reappearance of CSF circulation and rhythmic spinal cord pulsation, serving as intraoperative confirmation of adequate decompression.
For the 25 patients with PTDS, posterior laminar decompression or osteotomy, along with subdural exploration and arachnoid adhesion lysis, were undertaken. If these approaches proved ineffective, syringomyelia shunting, such as syringo-subarachnoid, syringo-peritoneal, or syringo-pleural shunts, was considered as a final treatment option.
5. Histological Analysis
For all patients who underwent laminectomy and arachnoid lysis, the detached arachnoid tissue was formalin-fixed and paraffin-embedded during the procedure. Tissue quality was assessed using hematoxylin and eosin staining, and additional immunohistochemical staining with specific biomarkers was performed as needed.
6. Syringomyelia Symptom Evaluation
Immediate postoperative outcomes were assessed at discharge. Long-term follow-up was defined as the most recent evaluation conducted at least 6 months after surgery. Improved syringomyelia symptom outcome was defined as a sustained alleviation of preoperative symptoms. Stabilization of preoperative symptoms was classified as “no change,” while postoperative neurological deterioration was considered “deteriorated.”
7. Variable Definition
Preoperative duration of symptoms: The interval from the initial onset of symptoms to the confirmed diagnosis of syringomyelia at our institution.
Postoperative syrinx resolution: significant improvement was defined as a resolution of more than 20% in the maximal S/C ratio on follow-up MRI, while complete resolution referred to the total disappearance of the syrinx after surgery [2,7].
Responsible segment: The spinal segment corresponding to the posterior median incision, identified through preoperative MRI and myelography.
8. Statistical Analysis
For statistical analysis, IBM SPSS Statistics ver. 22.0 (IBM Co., USA) was utilized. The mean values of continuous variables were shown with standard deviations. The differences between the 2 groups of continuous variables were analyzed using the Student t-test. For dichotomous categorical data, the chi-square or Fisher exact test was used, and for multiple categorical variables, the Kruskal-Wallis H-test was used. p<0.05 means statistically significant.
RESULTS
This study included a cohort of 1,039 syringomyelia patients from a single center (Fig. 1). After applying strict inclusion and exclusion criteria, 15 patients with IS-OAW, who underwent spinal arachnoid lysis, were identified. The average patient age was 50.1 years (range, 27–63 years), comprising 3 females and 12 males. Detailed patient characteristics, surgical types, followup status, and duration are summarized in Table 1. All patients experienced progressively worsening neurological function, presenting with symptoms such as walking difficulties, abnormal gait, and lower limb hyperreflexia. Most also had varying degrees of paresthesia in different regions, except for cases 2, 3, 15, and 16. We highlighted 2 representative cases (cases 13 and 14) to illustrate typical imaging findings and clinical presentations (Figs. 2A–I and 3A–O). Additionally, we included 25 patients with PTDS for comparison. These patients underwent both epidural decompression (laminar decompression or osteotomy) and subdural exploration (arachnoid adhesion lysis). Syringomyelia shunting was considered a final treatment option when these approaches were ineffective. 8 patients in the PTDS group underwent syringoperitoneal shunt surgery due to persistent or recurrent syrinx enlargement and worsening clinical symptoms despite decompression and arachnoid lysis. The clinical data, imaging features, and surgical outcomes of IS and PTDS patients were compared and summarized in Table 2.
1. Imaging Characteristics of IS Patients
All patients underwent comprehensive radiographic evaluations, including MRI and computed tomography myelography, to assess CSF circulation abnormalities. Among the 15 patients, 8 presented with the characteristic “scalpel sign” on MRI, seen as a focal dorsal indentation along the spinal cord, while the remaining 7 exhibited the “girdle sign,” characterized by a constricted ring caused by arachnoid compression (Fig. 4A–K). Preoperative myelography confirmed varying degrees of SAS obstruction, with the affected segmental levels ranging from T2 to T10, assisting in identifying the responsible segments. The mean preoperative maximal S/C ratio was 0.70±0.07 (range, 0.54–0.88), while the average syrinx shrinkage ratio at the final follow-up ranged from 12.2% to 100.0%, with a mean of 29.6% (Table 1).
2. Surgical Outcomes of IS Patients
Given definite neurologic abnormalities and the obstruction of CSF circulation, posterior laminectomy and spinal arachnoid lysis were performed in 15 patients. After the dura incision, dentate ligaments were thoroughly exposed and separated, and the arachnoid adhesions of these patients were carefully dissected under a microscope until the CSF flow was completely restored and the spinal cord pulse fully reappeared.
The follow-up periods ranged from 18 months to 42 months, and the overall postoperative prognosis was favorable. No severe neurologic complications occurred in our series following surgery, while one patient developed superficial wound infections. At the last follow-up, 13 patients had a favorable syringomyeliarelated symptom outcome, which means they have sustained improvement in preoperative symptoms. Radiographic followup results showed the collapse of the syringe in most cases.
3. Histological Results
Intraoperative pathological examination and histological analysis revealed fibrosis with some meningeal cells and psammoma body calcification within the removed subarachnoid samples in all patients with IS-OAW. The pathological results of PTDS patients showed: fibrous cyst wall-like tissue, partially covered by arachnoid endothelial cells, accompanied by arachnoid endothelial cell proliferation with hyaline degeneration of interstitial fibrous tissue and a small amount of lymphocyte infiltration. The staining results of different biomarkers suggested: cytokerati (+), Ki-67 (nuclear proliferation marker) (1%+), epithelial membrane antigen (+), and somatostatin receptor 2 (+).
4. IS Group Versus PTDS Group
There was no significant difference between the 2 groups of patients in terms of age, sex, duration of syringomyelia-related natural history, etc. Patients with PTDS had a relatively higher proportion of hypesthesia and a higher syrinx tension index. In addition, there were no significant differences between the 2 groups of patients in terms of syrinx length, deviation, and location. However, patients in the IS group had significantly better postoperative syrinx resolution and syringomyelia-related symptom outcomes than those in the PTDS group.
DISCUSSION
The clinical data for this study were drawn from a prospectively maintained database of 1,039 syringomyelia patients treated at our center between December 2020 and March 2025. Of these, 15 cases of IS were retrospectively analyzed to explore optimal management strategies for this rare condition (Supplementary Fig. 1). IS-OAW is a relatively uncommon form of the disease, characterized by syrinx formation in the absence of an identifiable cause, even after comprehensive clinical and routine radiological evaluations [3]. The precise pathophysiology underlying arachnoid web formation remains unclear, though some researchers suggest that occult arachnoiditis may play a significant role in the development of IS [10,12]. It is hypothesized that these webs obstruct CSF flow, leading to syrinx formation. Mallucci et al. [13] first identified and described occult arachnoid webs, pouches, and cysts as potential contributors to IS. Our intraoperative findings also revealed thickened arachnoid webs similar to those reported in previous studies. However, based on our direct surgical observations, these structures are more accurately described as hyperplastic transverse arachnoid septa, reflecting their distinct pathological characteristics (Fig. 3).
IS associated with primary occult arachnoiditis is distinguished by its unclear etiology, insidious onset, atypical clinical presentation, and complex diagnostic process, making it particularly challenging to treat [7]. In some cases, patients may remain asymptomatic, with the syrinx only identified incidentally during unrelated imaging studies [14]. In this series, patients with IS typically presented with gradual symptom onset and nonspecific findings, leading to frequent misdiagnosis or delayed diagnosis. As a result, many were only identified at mid-to-late stages of disease progression. Clinically, IS and PTDS share similar etiologies, pathogenesis, and treatment approaches, as both involve varying degrees of subarachnoid obstruction that disrupt CSF flow. However, PTDS often follows an acute spinal cord injury (SCI) phase before progressing chronically to syringomyelia, while IS tends to develop more insidiously. In both conditions, successful treatment requires complete restoration of CSF circulation to relieve the syrinx.
The diagnosis of syringomyelia caused by occult arachnoiditis primarily relies on a combination of preoperative MRI, myelography, and intraoperative pathology [15]. MRI remains the gold standard for syringomyelia diagnosis and is relatively sensitive for detecting arachnoiditis (Fig. 4). Preoperative myelography can further assist in localizing the site of arachnoid obstruction and determining the responsible spinal segment [16,17]. However, MRI alone may miss significant CSF pathway abnormalities, particularly in the absence of overt cord distortion (e.g., case 1). Even myelography can occasionally fail to accurately pinpoint the responsible segment, especially in cases of incomplete subarachnoid obstruction (e.g., case 2). This limitation arises because focal arachnoid lesions are often confined to the dorsal SAS, potentially allowing contrast to bypass the obstructed area through the unobstructed side of the SAS [4]. In patients with PTDS, preoperative MRI often shows abnormal CSF signal interruptions consistent with the obstructed segment identified through contrast imaging. Imaging findings also differ between IS and PTDS. IS typically presents with distinctive features like the “scalpel sign” or “girdle sign” on MRI, while PTDS often lacks such clear patterns. Intraoperatively, IS is usually associated with hyperplastic transverse arachnoid septa, while PTDS is characterized by patchy arachnoid thickening, reflecting different mechanisms of CSF circulation disruption.
Intraoperative pathology confirmed the histological characteristics of primary arachnoiditis, namely a nonspecific inflammatory lesion obstructing the SAS present in the majority of cases that were nonacute and noninfectious. At present, the inherent etiology of this intriguing occult arachnoiditis is still unclear and needs further study [11]. The pathological profiles of PTDS and IS exhibit notable differences (Fig. 5A–F). In PTDS, beyond arachnoid hyperplasia, there is evidence of nonspecific inflammatory exudation, likely resulting from neuroinflammatory responses triggered by acute SCI near the initial lesion site. In contrast, IS pathology is dominated by fibrosis with scattered arachnoid endothelial cells, reflecting a chronic fibrotic process with minimal inflammatory infiltration. Histologically, PTDS displays more pronounced changes, including proliferation of arachnoid endothelial cells, hyaline degeneration of the interstitial fibrous tissue, and lymphocytic infiltration-features indicative of an ongoing, active inflammatory and fibrotic response. These contrasting patterns underscore the distinct pathophysiological mechanisms of IS and PTDS: IS primarily reflects a chronic, quiescent fibrotic process, whereas PTDS represents an active phase of scarring accompanied by persistent inflammation.
The primary goal of surgical intervention in syringomyelia is to restore normal CSF circulation, either by reconstructing the SAS through decompression or by diverting CSF away from the syrinx [7]. Treatment options for syringomyelia related to arachnoiditis include conservative management, laminectomy, arachnoid lysis, posterior fossa and foramen magnum decompression, syrinx fenestration, and shunting procedures such as syringosubarachnoid, syringoperitoneal, or syringopleural shunts [18]. Conservative treatment might be appropriate when a patient with syringomyelia is asymptomatic or exhibits relatively mild symptoms [19]. However, for patients with progressive syrinx enlargement and worsening neurological function, surgical intervention is often necessary. It is crucial to address the underlying pathology, such as bony anomalies or arachnoid adhesions, to improve long-term outcomes. In cases of IS-OAW, the primary surgical goal should be complete decompression of the SAS and restoration of CSF flow through meticulous arachnoid lysis.
In the absence of any anatomical abnormality and progressive neurological dysfunction attributable to a syrinx, surgical exploration with arachnoid lysis is a reasonable option, with a shunt as a last resort [7]. Unlike CSF shunting, simple arachnoid lysis, as reported in the literature, has higher technical demands and carries a risk of postoperative readhesion [4]. Klekamp [4] reported that 34% of patients experienced neurological decline within 5 years following decompression, with this rate increasing to 40% within 10 years. However, if patients are appropriately selected, responsible segments are accurately identified, and SAS decompression is thorough, the prognosis can be significantly improved, avoiding the risks associated with foreign body implantation, such as tube blockage, infection, and shunt failure. Naito et al. [20] recommended CSF shunt as a safer treatment option since arachnoid lysis is more technically demanding and associated with a high risk of postoperative neurologic deterioration due to iatrogenic meningeal scarring.
However, shunting procedures themselves carry significant risks, including fistula recurrence, catheter tip displacement, and mechanical SCI. Moreover, shunts address only the symptoms of syrinx expansion, without correcting the underlying cause of the CSF circulation disorder, often necessitating frequent surgical revisions. In contrast, several studies [21], including ours, have demonstrated superior long-term outcomes for arachnoid lysis and duraplasty in treating arachnoid pathology compared to syrinx shunting [22]. Guillaumet et al. [23] similarly found that the reoperation rate for arachnoid lysis was significantly lower than for shunting procedures in their cohort.
Effective CSF circulation reconstruction requires meticulous separation of the dura mater and dentate ligament during surgery, with complete release of SAS adhesions, particularly near the denticulate ligaments. Our findings suggest that complete SAS decompression is achieved when both CSF flow is restored and intraoperative spinal cord pulsations reappear. In the early stages of our series (e.g., cases 7 and 11), our focus was primarily on restoring CSF flow. However, as our experience grew, we prioritized more comprehensive releases, ensuring spinal cord pulsations were fully restored alongside CSF recirculation. Despite overall favorable outcomes, some cases showed incomplete recovery. For instance, case 7 demonstrated persistent lower limb weakness postoperatively, and case 11 experienced progressive lower extremity weakness, with muscle strength in the distal right limb declining from grade 2 preoperatively to grade 1 postoperatively. For this case, recurrence was attributed to the formation of secondary arachnoid adhesions during the postoperative period, resulting in progressive enlargement of the syrinx. A second surgical exploration was performed, during which repeat arachnoid lysis was undertaken. Complete release of the adhesions was achieved, restoring spinal cord pulsation and re-establishing normal CSF circulation.
Comparative analysis of clinical outcomes between IS and PTDS patients revealed distinct differences in surgical approaches and prognoses. In IS, effective CSF recirculation can often be achieved through simple subdural exploration and arachnoid lysis, resulting in favorable syringomyelia resolution rates and symptom improvement. However, PTDS, characterized by an initial acute SCI phase and often accompanied by spinal fractures or deformities, presents a more challenging clinical scenario. First, the presence of vertebral fractures and spinal deformities in PTDS necessitates combined epidural bony decompression and subdural adhesion lysis to fully restore CSF pathways. Second, the localized neuroinflammatory response following acute SCI often leads to more severe and extensive arachnoid adhesions compared to IS, making complete decompression more challenging. Consequently, even with aggressive decompression, PTDS patients are more prone to long-term syrinx recurrence and require more frequent reoperations, including potential syringomyelia shunting, due to the chronic and progressive nature of their condition.
PTDS typically presents with a clear history of SCI, while IS often lacks an identifiable cause, necessitating careful differentiation from other spinal pathologies. Historically, the unclear etiology and poorly understood pathogenesis of IS have led to frequent misdiagnosis, often being mistaken for conditions like intramedullary tumors (Supplementary Fig. 2). The etiology, clinical manifestations, and treatment strategies for syringomyelia vary depending on the underlying cause, but all forms share the common feature of abnormal CSF circulation. However, the precise pathophysiology of syringomyelia remains poorly understood. It is widely accepted that the driving force behind syrinx formation is an increase in pulse pressure within the SAS, which forces CSF through the spinal cord into the syrinx cavity [24]. Hydrodynamic factors likely contribute to the delayed formation and gradual expansion of the syrinx after an initial cavity is established [25].
Analogous to intracranial compliance and hydrocephalus, spinal compliance might also reflect the compensatory ability of the spinal cord to the syringomyelia [26]. This physiological property likely plays a crucial role in the expansion of the syrinx and the subsequent development of clinical symptoms. Under normal conditions, spinal cord pulsations help maintain unobstructed CSF circulation [27]. However, when the CSF pathway is blocked, the increased pressure gradient within the spinal cord can weaken or eliminate these pulsations, leading to progressive fluid accumulation within the syrinx. This results in characteristic structural changes, including compression, expansion, and even morphological plasticity [28]. In our intraoperative observations, the initial spinal cord pulsations in many of these patients were significantly diminished or absent, emphasizing the importance of restoring spinal cord compliance as a critical goal in the surgical management of syringomyelia. Effective arachnoid lysis would aim not only to restore normal CSF flow but also to reestablish spinal cord pulsatility, thereby enhancing spinal autoregulation and facilitating syrinx collapse. This concept aligns with our earlier mechanistic studies on the pathophysiology of syringomyelia [29]. Nevertheless, further prospective studies and longer follow-up periods are needed to validate this hypothesis.
This study offers valuable clinical insights into the diagnosis and management of IS-OAW by integrating preoperative imaging, intraoperative techniques, histopathological findings, and postoperative assessments. Our findings highlight the importance of accurately identifying the responsible segment to achieve complete arachnoid lysis, which is critical for improving patient outcomes in IS. Additionally, we provide a comparative analysis of IS and PTDS, emphasizing the need for tailored surgical approaches based on the underlying etiology.
However, this study has several limitations. As a retrospective analysis, it relies on the quality and consistency of clinical documentation, which may introduce variability in data collection and accuracy. Furthermore, the small sample size of patients with IS-OAW limits the generalizability of our findings and precludes the possibility of conducting randomized controlled trials. Small sample size (n=15 for IS) inherently limits statistical power. Further research with larger cohorts and prospective designs is necessary to validate our observations and refine the treatment strategies for this challenging condition.
CONCLUSION
Although both IS and PTDS are forms of syringomyelia secondary to arachnoid adhesions, they differ significantly in their pathological features, treatment strategies, and clinical outcomes. For IS with occult arachnoid webs, thorough arachnoid lysis targeting the appropriately identified responsible segment is essential for restoring spinal cord pulsations and reestablishing CSF circulation, thereby providing a relatively favorable longterm prognosis.
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