Ultrasound of the Newborn Spine: Conus, Filum, and Dysraphism

Clinical Objective of the Study

Ultrasound of the newborn spine is a primary imaging method for the contents of the spinal canal when occult spinal dysraphism, tethered cord, dermal sinus, filum terminale lipoma, meningocele/myelomeningocele, and other congenital anomalies are suspected. The practical purpose of the study is not to 'look at the coccyx' but to protocol answers to several questions: where does the conus end, is the terminal filum thickened, is there a fatty inclusion, are the cauda equina roots moving freely, and is there a connection between a skin tract and the canal.

The current profile document is the AIUM Practice Parameter for the Performance of an Ultrasound Examination of the Neonatal and Infant Spine, 2021. It sets a protocol approach: indications, scanning technique, mandatory description elements, and signs of pathology.

Age Window and Method Limitations

Ultrasound is possible because the posterior elements of the vertebrae in a newborn are not fully ossified and serve as an acoustic window. According to AIUM, the study is typically applicable in newborns and infants up to about 6 months; after this, ossification progressively limits the view of the spinal canal.

Even at the optimal age, the method should not replace MRI if the ultrasound image is incomplete or significant changes are detected. Ultrasound answers screening and protocol questions, but with a low conus, thickened filum, intraspinal mass, suspicion of a dermal sinus, or complex malformation, further neuroimaging and neurosurgical routing are required.

Indications According to AIUM

AIUM includes as indications cutaneous stigmata and masses in the lumbosacral region, suspicion of congenital anomalies of the spine and spinal cord, neurological or orthopedic signs of possible dysraphism, as well as the assessment of known or postoperative changes. The ultrasound diagnostician should not only scan the canal but also describe the soft tissues over the sacrum and lumbar region if a skin marker prompted the referral.

• midline or paramedian subcutaneous masses;
• areas of pigmentation, hemangiomas, hypertrichosis, skin appendages;
• skin dimpling, pits, suspicion of a sinus tract;
• congenital anomalies associated with spinal defects;
• suspicion of a tethered spinal cord or malformation of the posterior elements.

Technique: How to Achieve a Reproducible Protocol

The study is performed in longitudinal and transverse planes through the interlaminar spaces. In the longitudinal plane, the spinal canal is assessed from the lower thoracic/lumbar region to the sacrum: the position of the conus, the course of the terminal filum, the configuration of the dural sac, the pulsation of the spinal cord, and the movement of the roots. In the transverse plane, the symmetry of the roots, the presence of a septum, lipoma, dilation of the central canal, cystic or solid masses are clarified.

A critical step is determining the vertebral level. Practically, counting is done from the lumbosacral junction, from the sacrum upwards, or from the last rib downwards; in segmentation anomalies, the level may be erroneous. If the conus level is borderline or there is a transitional vertebra, it is better to directly indicate the method's limitation and the need for correlation with radiography or MRI in the conclusion.

Norms: Conus, Filum Terminale, Mobility

The main numerical landmarks according to AIUM are the level of the conus and the thickness of the filum terminale. The conus is normally located above the L2–L3 intervertebral disc. The filum terminale is normally thin, without fatty infiltration, and less than 2 mm thick. An additional normal sign is the free pulsation of the conus and the movement of the cauda equina roots in the cerebrospinal fluid space.

Ultrasound Markers of Tethered Cord

Tethering is suggested by the combination of a low conus position, thickened or lipomatous filum terminale, and limited mobility of the spinal cord/roots. A single sign requires cautious interpretation: for example, incorrect vertebral numbering may create a false impression of a low conus, and a technically weak window may mimic the absence of pulsation.

It is advisable to avoid vague formulations in the protocol. Instead of 'signs of tethered cord?', it is better to state facts: 'the conus is visualized at the level of ...', 'filum terminale thickness is ... mm', 'filum echogenicity is increased/fatty component is suggested', 'root movement is reduced/preserved'. The final phrase may be: 'Ultrasound signs suspicious for spinal cord tethering; MRI and neurosurgical consultation recommended'.

Dermal Sinus and Skin Tracts

In the presence of a skin dimple or opening, the ultrasound task is to determine whether the tract ends blindly in the soft tissues or extends towards the spinal canal. A dermal sinus is clinically important because it can communicate with the dural sac and be a pathway for infection, as well as be associated with intraspinal inclusions.

Describe the localization of the skin entry, the direction of the tract, depth, connection with the fascia, posterior elements, dural sac, or intraspinal structure. If the tract is not visualized throughout or approaches the canal, a negative ultrasound conclusion is insufficient: MRI is required.

Other Forms of Dysraphism Visible on Ultrasound

Ultrasound can detect lipoma, split spinal cord, meningocele, myelomeningocele, dilation of the central canal, caudal regression, and postoperative changes. In the transverse section, attention is paid to the separation of canal contents, asymmetry of roots, bony or fibrous septum, mass, connection of the mass with the skin and dural sac.

In open defects, ultrasound is performed only within clinically permissible tactics and with tissue safety in mind. In complex malformations, the conclusion should not be limited to naming: it is important to indicate the anatomy of the dural sac, the position of neural structures, the presence of a CSF component, and the associated low conus position.

Minimal Template for Conclusion

A practical protocol should be short but verifiable. Recommended points: quality of visualization; method of level determination; level of conus termination; thickness and echogenicity of the filum terminale; mobility of the conus and roots; condition of the dural sac; presence or absence of intraspinal masses; assessment of soft tissues and skin marker; limitations and recommendations.

Example of a normal conclusion: 'Acoustic window satisfactory. The conus medullaris ends above the L2–L3 disc. Filum terminale is thin, less than 2 mm, with no signs of fatty inclusion. Cauda equina roots are mobile. No masses or connection of the skin tract with the spinal canal detected'. In any uncertainty, it is better to describe it explicitly than to give a categorical norm.