Filum Terminale: A Comprehensive Review with Anatomical, Pathological, and Surgical Considerations

Histological Characteristics of the Filum Terminale in Occult Tethered Cord Syndrome and Filum Terminale Lipoma

BACKGROUND

Tethered spinal cord syndrome is a neurological disorder characterized by pathological tethering of the spinal cord, leading to neurological, urological, and orthopedic dysfunction. Occult tethered cord syndrome (OTCS) presents with similar clinical manifestations but lacks definitive radiological evidence, whereas filum terminale lipoma (FTL) involves adipose infiltration of the filum terminale, contributing to spinal traction. The histopathological characteristics of the filum terminale in these conditions remain incompletely understood.

OBJECTIVE

To characterize the histological features of the filum terminale in OTCS and FTL.

METHODS

Filum terminale specimens were obtained from 17 patients with OTCS and 22 patients with FTL undergoing surgical untethering (April 2020-April 2024) and from six cadavers as reference samples. The histopathological evaluation included hematoxylin-eosin, Masson trichrome, and Elastica van Gieson staining. Immunohistochemistry assessed collagen types I and III.

RESULTS

OTCS cases showed reduced collagen type I, sparse and irregularly looped elastic fibers, capillary dilation, and hyaline-like degeneration. FTL specimens exhibited adipose infiltration, densely packed and elongated elastic fibers, and intense collagen type I staining, although some showed reduced elasticity. Cadaveric samples displayed normal histology without degeneration.

CONCLUSIONS

The observed decrease in collagen type I and elastic fibers in OTCS suggests compromised filum terminale elasticity, potentially contributing to pathological tethering. In FTL, adipose infiltration is the primary factor driving spinal cord traction, although some cases also showed reduced elastic fiber integrity. These findings emphasize the importance of further research into the relationship between histopathological changes and clinical presentations. Better understanding of this correlation is crucial for developing standardized management strategies.

Morphological analysis of the filum terminale and detailed description of the distal filum terminale externum: a cadaveric study

This observational, descriptive anatomical cadaveric study aimed to identify, characterize, and analyze the morphometric parameters of the filum terminale (FT) and macroscopically describe the distal insertion of the FTE. The FT is a complex, three-dimensional, fibro-cellular structure of connective tissue and glial cells, extending from the conus medullaris (CM) to the dural sac (DS) and coccyx. It is divided into two segments: an intradural filum terminale internum (FTI) and extradural filum terminale externum (FTE). Few studies have comprehensively addressed its morphometric characteristics in the last decades. Thirty-eight embalmed (M = 16, F = 22) human cadavers were examined to determine the CM-FTI and DS-FTE vertebral levels and FT, FTI, and FTE lengths and widths. FTI and FTE segmental diameters, correlations, gross characteristics, tension, and mobility in situ and ex vivo were assessed. FTE distal insertion is thoroughly described. FT, FTI, and FTE mean lengths were 254.32 mm (±26.46), 152.75 mm (±22.02), and 106.64 mm (±12.21), respectively. The CM-FTI junction was observed at the L1-L2 disk space (32.1%), DS-FTE fusion in the upper third of S2 (39.3%), and FTI-DS fusion in the DS midline (46.4%). FT length and FTI, FTE lengths were directly correlated, as were all FTI diameters. FT gross characteristics were an irregular surface (71.4%), bright hue (57.1%), macroscopic FTI-CM contrast (64.3%), filiform shape (60.7%), and movement-resistance (53.6%). The FTE exhibited a flattened shape (64.3%), immobility (60.7%), distal insertion at Cx1 (67.8%) and one distal strand (60.7%). FTI segments ≥ 2 mm were uncommon (21.4%). The FTE distal insertion is variable, inserting as strands, with vascular tissue surrounding it. A distal coccygeal venous plexus and single or multiple strand-like insertions of the distal FTE are for the first time described in detail. Discrepancies in the morphometric parameters of the FT between studies highlight the need for standardized protocols, especially given the FT's anatomic-clinical importance and potential role as a neural progenitor niche. We provide a comprehensive basis for future standardized morphometric analyses, acknowledging the limitations of embalmed cadaveric studies.

Utility of Multimodal Intraoperative Neuromonitoring for Excision of Filum Terminale Ependymoma in Close Proximity to Conus

Myxopapillary ependymomas (MPEs) are well-circumscribed tumors arising mainly from the caudal neuraxis, i.e., conus medullaris (CM) and filum terminale (FT), commonly seen in adults with median age at presentation of 39 years.1 Owing to its partially aggressive clinical behavior involving cerebrospinal fluid dissemination and local recurrence, MPE is classified as grade 2 in the fifth edition of the World Health Organization Classification of Tumors of the Central Nervous System.2 Gross total resection without capsular violation is key, with subtotal resection being directly associated with local recurrence.3The FT has an intradural and extradural component. The intradural FT extends from the inferior tip of the CM to the coccyx.4 The intradural FT-CM junction is not demarcated, but rather a zone of transition, with neural tissue being incrementally replaced by fibrous tissue of filum, gradually converging to a pure non-neural FT.5 In intramedullary FT MPE in close proximity to the lower end of conus, achieving gross total resection presents a great challenge. Neuromonitoring is crucial to ensure preservation of vital CM functions. We present the case of a 33-year-old man with focal nocturnal back pain of 6 months' duration followed by bilateral lower limb deep boring pain. He had no neurological deficits. Preoperative magnetic resonance imaging revealed a T2 hyperintense, heterogeneous contrast-enhancing intradural extramedullary mass at L1. Video 1 highlights step-by-step en bloc excision of the FT MPE with technical nuances, including intraoperative neurophysiological monitoring. Triggered electromyography (EMG) was used to positively map the eloquent CM and identify the intradural FT-conus interface at the superior pole of the tumor, which was then carefully dissected under continuous bulbocavernosus reflex monitoring. Similarly, we confirmed non-neural intradural FT at the lower pole by negative mapping and resected the lesion en bloc with an adequate stump for clear margins. Free-run EMG monitored all the rootlets that adhered to or were in close proximity to the lesion, ensuring their integrity and an uneventful postoperative recovery. Figure 1 depicts the anatomical orientation of the lesion with surrounding neural structures. Histopathology confirmed MPE. En bloc resection with preservation of neurological function remains the mainstay of treatment for FT ependymoma. Understanding the transitional intradural FT-CM interface is essential, often precluding a clear filum stump superiorly while resecting MPE. Intraoperative neurophysiological monitoring is an indispensable adjunct to ensure safe en bloc resection. It is also theoretically possible to use tibial and pudendal sensory evoked potentials (SEPs) in this surgical procedure. However, the clinical utility of SEPs is limited in FT surgery compared with triggered EMG or transcranial motor evoked potentials because conventional SEPs from posterior tibial nerve of the lower extremity do not cover all the root levels at risk, and the change in SEPs cannot be immediately recognized (as SEPs are averaged responses, and there is always a time lag). We did not use pudendal SEPs in this study because SEPs may give information only on the sensory sacral pathway.6 Dermatomal SEPs may be helpful, but again, they provide only sensory information. Instead, we used triggered EMG for mapping the nerve roots and transcranial motor evoked potentials to monitor the motor tracts. Further, we used the bulbocavernosus reflex, an alternative and more precise technique to monitor both motor and sensory nervous pathways at the sacral root level. Moreover, SEPs are more difficult to monitor in very young children and are less relevant in guiding the surgical strategy. Thus, we used both mapping (triggered EMG) and monitoring (transcranial motor evoked potentials and bulbocavernosus reflex) techniques, which can preserve sensory and motor sacral roots in this surgical procedure.

The discrepancy in triggered electromyography responses between fatty filum and normal filum terminale

BACKGROUND

Functional role of filum terminale (FT) was not well studied though it contains structure basis for nerve impulse conduction. We aimed to explore the possible functions of the FT from the perspective of triggered electromyography (EMG) during surgery.

METHODS

We retrospectively reviewed intraoperative neurophysiological monitoring data from pediatric patients who underwent intradural surgeries at the lumbar level in Shanghai Children's. Hospital from January 2018 to March 2023. Altogether 168 cases with complete intraoperative neurophysiological recordings of the FT were selected for further analysis. Triggered EMG recordings of the filum originated from two main types of surgeries: selective dorsal rhizotomy (SDR) and fatty filum transection.

RESULTS

96 cases underwent SDR and 72 cases underwent fatty filum transection. Electrical stimulation of the FT with fatty infiltration did not elicit electromyographic activity in the monitored muscles with the maximum stimulus intensity of 4.0 mA, while the average threshold for FT with normal appearance was 0.68 mA, and 89 out of 91 FT could elicit electromyographic responses in monitored channels. The threshold ratio of filum to motor nerve roots at the same surgical segment was significantly higher in patients with fatty filum, and a cut-off point of 21.03 yielded an area under curve of 0.943, with 100% sensitivity and 85.71% specificity.

CONCLUSION

Filum with normal appearance can elicit electromyographic activity in the lower limbs/anal sphincter similar to the performance of the cauda equina nerve roots. The threshold of fatty filum is different from that of normal appearing FT. Triggered EMG plays an important role in untethering surgeries.

Tethered Cord Syndrome: Role of Imaging Findings in Surgical Decision-Making

For infants presenting with urinary problems or lower extremity weakness, imaging is ordered to investigate spinal pathology. Tethered cord syndrome (TCS) often manifests without conclusive anatomic evidence. In our case, a premature infant presented with urosepsis and was found to have an asymmetric gluteal crease and a sacral dimple. Renal ultrasound showed mild hydronephrosis, and a cystourethrogram revealed bilateral high-grade vesicoureteral reflux. Ultrasound and magnetic resonance imaging demonstrated a borderline low-lying spinal cord at the mid-L3 vertebral level. Urodynamic testing to confirm neurogenic bladder could not be completed on the first attempt due to urinary tract infection and on the second attempt due to instrument intolerance. Despite the lack of conclusive imaging evidence of a tethered cord, enough supportive clinical data was present to proceed with surgical intervention with the goal of preventing the progression of neurological dysfunction. Because TCS is ultimately a clinical diagnosis, appropriate management should not be discouraged by inconclusive or borderline imaging findings.