Neonatal Spinal Ultrasound in Occult Spinal Dysraphism: Conus Level and Terminal Filum
Indications and Purpose of Examination
Spinal sonography is performed primarily to assess the spinal canal in occult spinal dysraphic conditions, which are often associated with midline cutaneous changes in the lower back region. The most frequent objective is to exclude occult tethered cord.
Conus Medullaris Level
The most important parameter is the level of the apex of the tapered conus medullaris. In infants, the conus is normally located above the upper endplate of L3, with the spinal cord terminating at the mid-L2 level in the majority of cases. Since the conus migrates cranially with age, some variations in the normal lumbar level are acceptable.
The apex of the conus is identified in both longitudinal and transverse sonographic projections. If the conus apex is determined at the mid-lumbar level, correlation with radiography may be required to precisely establish the level.
Vertebral Level Counting
When using the panoramic function, counting can begin from S5—the last ossified bone before the coccyx, from the end of the dural sac at the S2 level, or from the lumbosacral junction (L5–S1). The nerve roots of the cauda equina are visualized as thin lines surrounding the terminal filum; stepwise ascent of the sacral vertebral elements is noted.
Terminal Filum
The normal terminal filum should measure <2 mm and is best assessed at the L4–L5 level, where it can be differentiated from the nerve roots of the cauda equina.
| Parameter | Normal (Infant) |
|---|---|
| Apex of conus medullaris | Above the upper endplate of L3; most commonly at mid-L2 |
| Terminal filum | <2 mm (assessment at L4–L5 level) |
| End of dural sac | S2 level (landmark for counting) |
Sonographic Signs of Pathology
In tethered spinal cord, the conus may be positioned too low and in some cases may be visualized from an anterior approach. With associated lipomas, displacement of the spinal cord to an abnormal dorsal position and its stretching are described, which is demonstrated in sagittal and transverse projections.
Frequently asked questions
At what level does the conus medullaris normally terminate in an infant?
Above the upper endplate of L3, with the spinal cord terminating at the mid-L2 level in the majority of cases. The conus migrates cranially with age.
What is the normal thickness of the terminal filum?
The terminal filum should measure <2 mm. The optimal level for assessment is L4–L5, where it can be distinguished from the nerve roots of the cauda equina.
From which landmark should vertebral level counting begin?
When using the panoramic function, counting can begin from S5 (the last ossified bone before the coccyx), from the end of the dural sac at the S2 level, or from the lumbosacral junction (L5–S1).
When is additional radiography required?
If the conus apex is determined at the mid-lumbar level, correlation with radiography may be required to precisely establish the level.
How does tethered spinal cord appear on ultrasound?
The conus is positioned too low and may be visualized from an anterior approach. With lipomas, stretching and displacement of the spinal cord to an abnormal dorsal position are noted.