Normal Filum Terminale Size on Ultrasound and MRI
The filum terminale is a slender fibrous strand extending from the conus medullaris to the coccyx, anchoring the distal spinal cord. Accurate measurement of its diameter is essential in pediatric practice, where abnormal thickening may indicate tethered cord syndrome or an underlying fibrolipoma.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Longitudinal | <2 mm |
Clinical Significance
A filum terminale diameter of less than 2 mm is considered normal. Measurements at or exceeding this threshold are regarded as pathologically thickened and warrant clinical correlation, particularly in children presenting with lower-extremity weakness, bladder dysfunction, or cutaneous stigmata of spinal dysraphism such as a sacral dimple, hairy patch, or skin tag.
A thickened filum may harbor fatty infiltration (fibrólipoma of the filum), which appears as T1 hyperintense signal on MRI and echogenic on ultrasound. Even a filum measuring exactly 2 mm should be interpreted in clinical context, as symptomatic tethered cord can occur with borderline measurements. Surgical sectioning of a thickened or fatty filum is often curative when neurological deterioration is documented.
- Tethered cord syndrome
- Fibrolipoma of the filum terminale
- Open or closed spinal dysraphism
- Sacral agenesis with cord tethering
- Post-surgical re-tethering
Reference: Donnelly LF. Pediatric Imaging. Saunders. p. 252 (2009).
Imaging Notes
In neonates and young infants, spinal ultrasound through the posterior acoustic window of the incompletely ossified posterior elements allows real-time longitudinal assessment of the filum terminale. The filum is measured in the longitudinal plane at its widest point caudal to the conus; echogenicity greater than the surrounding nerve roots raises suspicion for fatty infiltration even when diameter is borderline.
On MRI, sagittal T1-weighted sequences provide the most reliable diameter measurement and are essential for detecting T1 hyperintense fat within the filum. Axial T1 images at the L5–S1 level confirm thickness and fat content. A low-lying conus (below the L2–L3 disc in infants beyond 2 months) frequently accompanies a thickened filum and should prompt careful measurement in both planes.
