The Surgical Histopathology of the Filum Terminale: Findings from a Large Series of Patients with Tethered Cord Syndrome

Abstract

This study investigated the prevalence of embryonic and connective tissue elements in the filum terminale (FT) of patients with tethered cord syndrome (TCS), examining both typical and pathological histology. The FT specimens from 288 patients who underwent spinal cord detethering from 2013 to 2021 were analyzed. The histopathological examination involved routine hematoxylin and eosin staining and specific immunohistochemistry when needed. The patient details were extracted from electronic medical records. The study found that 97.6% of the FT specimens had peripheral nerves, and 70.8% had regular ependymal cell linings. Other findings included ependymal cysts and canals, ganglion cells, neuropil, and prominent vascular features. Notably, 41% showed fatty infiltration, and 7.6% had dystrophic calcification. Inflammatory infiltrates, an underreported finding, were observed in 3.8% of the specimens. The research highlights peripheral nerves and ganglion cells as natural components of the FT, with ependymal cell overgrowth and other tissues potentially linked to TCS. Enlarged vessels may suggest venous congestion due to altered FT mechanics. The presence of lymphocytic infiltrations and calcifications provides new insights into structural changes and mechanical stress in the FT, contributing to our understanding of TCS pathology.

Keywords: filum terminale; spinal cord disorders; tethered cord release; tethered cord syndrome.

Figure 1

Illustration of the anatomy of the filum terminale, and its transition from the filum terminale internum (red), to the filum terminale externum (blue) at S1-2.

Figure 2

MRI (A) and histologic (B,C) findings of the lumbar spine. (A) Sagittal T1 (left) and T2 (middle) sequences show the conus terminating at the L1-2 disc space, and an axial T1 sequence (right) reveals a hyperintense signal of the filum (arrow), suggestive of thickened FT. (A: Anterior, P: Posterior, RH: Right, LF: Left). (B) Hematoxylin and eosin (HE) stain shows FT with prominent lymphoid follicles and focal adipose tissue, ×40. (C) The HE stain reveals a lymphoid cell population with focal germinal centers, ×400. (D) CD20 immunohistochemical stain of FT demonstrates the predominance of B-cells within the lymphoid follicles most consistent with hematogones, ×100.

Figure 3

MRI (A) and histopathological examination of the FT (B,C). (A) Sagittal T1 (left), T2 (middle), and axial T1 sequence (right) reveal a hyperintense signal of the filum (arrow), suggestive of thickened FT. (A: Anterior, P: Posterior, R: Right, L: Left). (B) Fatty FT with several large, thick-walled, congested blood vessels, suggestive of a venous congestion. The H and E stain, ×40 (C) Low power of an elastic stain demonstrating a blood vessel with an internal elastic lamina, confirming its arterial origin. This suggests signs of venous congestion, ×40.

Figure 4

MRI (A) and histological examination (B–D) of the FT. (A) MRI of the lumbar spine with conus terminating at L1-2 disc level, and axial sequence reveals a T1 hyperintense signal suggestive of thickened filum (arrow). (AHR: Anterior. PFL: Posterior. RFA: Right. LHP: Left.) (B) Fatty filum terminale with focal, incidental ependymal cell proliferation. H&E stain, ×40. (C) Higher power of incidental ependymal cell proliferation. An ependymoma cannot be completely ruled out, but no pathognomonic features are present. H&E stain, ×200. (D) GFAP immunohistochemical stain demonstrating diffuse positivity within the ependymal cell proliferation. The proliferative index on Ki-67 immunostain is less than 1%, ×200.

Figure 5

MRI (A), intraoperative exploration (B,C), macroscopic (D), and microscopic (E) examination of the FT. (A) MRI of the lumbar spine showing the tip of conus medullaris at the L1-2 disc space, and axial T1 scans notable for the hyperintense signal at the conus-filum transition zone suggestive of thickened filum terminale (arrow). (AL: Anterior. PL: Posterior. RFP: Right. LHP: Left). (B) The intraoperative picture shows the surgical approach and dural opening after performing a single level laminotomy, with the calcified filum (arrow) lifted using a nerve hook instrument. (C) The intraoperative picture demonstrates the surgical approach with dural opening, filum isolation, and resection of a larger filum portion (arrow). (D) gross pathological specimen with evidence of large focal calcification (arrow). (E) Filum terminale with focal calcifications alongside thin-walled blood vessels. HE stain, ×40.