1) Description of Measurement

The filum terminale is a thin fibrous structure extending from the conus medullaris to the coccyx, anchoring the spinal cord. Abnormal thickening of the filum terminale is a key imaging feature of tethered cord syndrome (TCS) and may cause progressive neurologic deterioration due to traction on the spinal cord.

2) Instructions to Measure

• Identify the conus medullaris tip on mid-sagittal images.

• Follow the filum terminale inferiorly to the upper sacral canal (typically S1–S2 level).

• On the slice where the filum is best visualized, use an electronic caliper to measure the maximum transverse diameter of the filum terminale.

• Record the thickness in millimeters (mm).

3) Normal vs. Pathologic Ranges

• Normal thickness: ≤ 2 mm

• Suggestive of TCS: > 2 mm

• Highly suspicious for TCS: Fatty infiltration present 

4) Important References

Yamada S, Won DJ, Siddiqi J, Yamada SM. Tethered cord syndrome: overview of diagnosis and treatment. Neurol Res. 2004 Oct;26(7):719-21. doi: 10.1179/016164104225017947.

Hertzler DA 2nd, DePowell JJ, Stevenson CB, Mangano FT. Tethered cord syndrome: a review of the literature from embryology to adult presentation. Neurosurg Focus. 2010 Jul;29(1):E1. doi: 10.3171/2010.3.FOCUS1079.

Grimme JD, Castillo M. Congenital anomalies of the spine. Neuroimaging Clin N Am. 2007 Feb;17(1):1-16. doi: 10.1016/j.nic.2006.11.002.

5) Other info....

Filum thickness should always be interpreted alongside:

• Conus medullaris level

• Neurologic symptoms

• Associated dysraphic anomalies (lipoma, dermal sinus)

A thickened filum with a low-lying conus is highly diagnostic of tethered cord syndrome.