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Filum terminale internum

The filum terminale internum is a delicate fibrous extension of the pia mater that anchors the conus medullaris to the dural sac. It represents the upper portion of the filum terminale and lies within the lumbar cistern surrounded by the cauda equina. It provides longitudinal stability to the spinal cord and is clinically important in conditions such as tethered cord syndrome and intradural tumors.

Synonyms

  • Upper filum terminale

  • Intradural filum terminale

  • Pial filum terminale

Origin, Course, and Termination

  • Origin:

    • Arises from the conus medullaris, typically at the L1–L2 vertebral level

  • Course:

    • Descends within the thecal sac in the lumbar cistern, surrounded by cauda equina nerve roots

    • Appears as a thin fibrous cord, often containing fat or small vascular elements

  • Termination:

    • Continues down to approximately the level of S2, where it blends with the filum terminale externum and attaches to the dural sac

Relations

  • Anteriorly: Posterior surfaces of vertebral bodies and intervertebral discs

  • Posteriorly: Laminae and ligamentum flavum

  • Laterally: Cauda equina nerve roots within CSF

  • Superiorly: Conus medullaris

  • Inferiorly: Filum terminale externum and dural attachment at S2

Function

  • Provides longitudinal stabilization of the spinal cord within the thecal sac

  • Prevents excessive movement of the conus medullaris during body motion

  • Serves as a landmark in neurosurgical procedures involving cauda equina or intradural lesions

Clinical Significance

  • Tethered cord syndrome: Thickened or fatty filum may restrict movement of the conus, causing neurological deficits

  • Lipomas/dermoids: Fat within filum terminale may be seen in spinal dysraphism

  • Surgical relevance: Recognized during detethering procedures

  • Tumors: Rare site of intradural neoplasms (ependymomas, paragangliomas)

MRI Appearance

T1-weighted images:

  • Filum appears as a thin low-signal cord

  • Fatty filum or lipomas appear bright

T2-weighted images:

  • Filum appears as a thin hypointense linear structure within the bright CSF

  • May be difficult to distinguish from nerve roots unless thickened or fatty

T1 Fat-Sat Post-Contrast:

  • Normal filum does not enhance

  • Pathological thickened filum or tumor (ependymoma, paraganglioma) may show contrast enhancement

3D T2 SPACE / CISS:

  • Filum appears as a thin hypointense linear band against very bright CSF

  • Provides excellent contrast for differentiation from nerve roots of the cauda equina

  • Best sequence for detecting thickened filum or tethering pathology

MRI image

filum terminale internum MRI coronal anatomy image-img-00000-00000

MRI image

filum terminale internum mri axial image