171 Not Just a Matter of Retained Embryonic Spinal Cord: Histopathology of the Filum Terminale in Tethered Cord Syndrome

INTRODUCTION: We present a large consecutive series of patients with tethered cord syndrome who underwent resection of the filum terminale (FT) and the histopathologic findings as analyzed by the neuropathologists. METHODS: FT specimens obtained from a consecutive series of 288 pediatric and adult patients that underwent spinal cord releases performed at a single academic institution from 2013-2021 were analyzed retrospectively (IRB #787945). Hematoxylin and eosin (HE) and immunohistochemistry staining was performed to study the spectrum of cells known to reside in connective tissues. RESULTS: The most common structures included peripheral nerves (97.6%), ependymal cells (70.8%), ganglion cells (38.5%), and neuropil (15.3%). Cases with excess ependymal cells were associated with ependymal canals (5.9%), ependymal cysts (3.1%), and one case with extensive ependymal “proliferation” (0.3%). With regards to the inflammatory cellular findings, eleven specimens (3.8%) contained scattered or accumulated lymphocytes, macrophages, mast cells, macrophage-derived giant cells, and a secondary lymph follicle in one patient. 36 patients (12.5%) had increased vascularity with enlarged capillaries. Nine patients (3.1%) had an associated type of vascular malformation. A group of specimens included dystrophic calcification within the FT (7.6%). CONCLUSIONS: Both vascular and inflammatory changes along with calcifications have been overlooked as possible associated histologic secondary to mechanical stress on the FT. Additionally, vascular anomalies could alter the filum’s mechanical properties due to venous congestion.