When to conduct the examination

Spinal cord ultrasound remains feasible as long as there is no interference between the bone and ligamentary development of the posterior arc of the spine and the visual field (up to 3 to 4 months).

It is ideally conducted during the first month of life when the visibility of spinal content is better. After 4 months, due to the progressive ossification of the posterior arcs, it provides only increasingly short segmented images through intervertebral spaces.

How to conduct the examination

Spinal cord exploration is performed using a high frequency linear probe (at least 7 MHz)

The infant placed in a prone position, a pad under the belly to reduce lumbar lordosis and the head slightly raised. If procubitus is not possible, the examination can be done in lateral decubitus ⌈Fig.1⌉

The examination is carried out posteriorly due to the superficial nature of the spinal cord

The entire spinal canal should be explored from the cervical region to the sacral region by axial and sagittal sections

The bulbo-medullary junction requires a flexion of the neck and will be analyzed by a convex probe adapted to physiological cervical lordosis. The medullary cone is sought and its level is specified.

At the end of the examination, a transfontanellar ultrasound (ETF) must be systematic as soon as there is any abnormality on the spinal cord ultrasound.

Landmarks

Clinical landmarks: The tip of the last rib corresponds to L2 and the apex of the iliac crest corresponds to L4.

Ultrasound landmarks: the first landmark is the follow-up of the twelfth rib to T12 and the second is the visualization of the renal pedicle located at the level of L2 and possibly locating the first sacral vertebra (in the absence of transitional anomaly).

Normal echo-anatomy

In ultrasound, the spinal cord appears hypoechoic bounded by two hyperechoic interfaces.

In the center there is a linear echo, called "central echo", which corresponds to the interface between the bottom of the anterior median fissure and the anterior white commissure. Its position is more anterior to the upper part of the cord.

The medulla has two enlargments, cervical (C3-D2) and lumbar (D9-L1) and in its prone position can be anterior or central, never posterior. ⌈Fig.2⌉

It thins at the end forming the conus medullaris, located between the T12 and L2-L3 disc. A cone under L3 should be considered pathological.⌈Fig.3⌉

The filum terminale is visualized on the midline, behind the nerve roots. Its thickness is normally equal to or less than 2 mm,

The cauda equina appears in the form of echogenic lines, arranged harmoniously around and below the terminal cone. 

The end of the dural sac, limited by the reflection of the dura mater and the arachnoid, is located in S2. The walls are visible, echogenic, anterior and posterior. The epidural space, located outside the dura mater, is hypoechoic, more developed behind, and contains the veins of the intrarachidian plexus. ⌈Fig.4⌉ 

Various movements can be observed: anteroposterior during cries, longitudinal during flexion extension of the neck. There are also vascular pulsatile movements, both at the spinal and radicular level.

The normality criteria are: conus medullaris in place, a fine filum terminale, movable cauda equina roots, the absence of echogenic mass of the dural sac

Variants of normal

The improvement in the quality of the ultrasound devices and probes has enabled the discovery of non-pathological atypical aspects which are relatively frequent (10%) and correspond to anatomical variants. Their knowledge is essential because no other additional imagery is required.

The most frequent are:

The fillum terminale cyst: arachnoid pseudocysts found on ultrasound: small cysts containing CSF and limited by a thin echogenic membrane within the roots of the cauda equina, independent or associated with a nerve root.⌈ Fig.5⌉

Transient dilatation of the central canal: It is discovered fortuitously in asymptomatic newborns. The appearance of the central echo is variable, can be discontinuous or split. It is most often seen at the terminal cone. In cross section, the section appears rounded or oval, however, it should not be mistaken for a syringomyelic cavity. ⌈Fig.6⌉

The terminal ventricle or 5th ventricle: It is a central cystic dilation of the distal spinal cord, due to incomplete regression of the embryonic terminal ventricle of the medullary cone with the appearance of ovoid cyst formation. ⌈Fig.7⌉

The differential diagnosis is made with the syringomyelic cavity of rare localization at the terminal cone and a cystic tumor. An MRI scan is necessary.

Pathological aspects

They are dominated by dysraphisms (open and occult):

The Meningocele is a hernia of the meninges through a posterior vertebral defect, producing a subcutaneous cystic mass secondary to an abnormality in the closing of the neural tube. The purpose of the examination is to locate and measure the defect, measure the mass and look for nerve structures there.⌈Fig.8⌉; ⌈Fig.10⌉

Myelomeningocele corresponds to a hernia of the meninges and nerve structures through a posterior bone defect. On ultrasound, it achieves the appearance of a compartmentalized hypoechoic formation. It is associated with a low attached spinal cord and the examination must be systematically supplemented by a brain examination in search of an Arnold-Chiari malformation. ⌈Fig.9⌉;⌈Fig.11⌉

The dermal sinus is a fine median pertuis extending from the skin to the spinal canal. Ultrasound shows a straight, hypoechoic fistulous path from the subcutaneous fat to the spinal canal. The medulla is often low and attached.⌈Fig.13⌉;⌈Fig.14⌉

Intraspinal lipoma can be located intra and / or extradural filum, is in the form of a thick hyperechoic ovoid filum and can be isolated or integrated as part of a lipomyelomeningocele. An attached cord is to be sought.⌈Fig.15⌉; ⌈Fig.12⌉

Caudal regression syndrome is rare and combines agenesis of the sacral and coccygeal vertebrae. In ultrasound, the termination of the cord can be blunt or square (normally conical) and the longitudinal analysis of the distal spine shows a sudden stop of the spinal cord above L1. ⌈Fig.16⌉;⌈Fig.19⌉

Diastematomyelia is the splitting of the spinal cord into two hemispheres. Ultrasound exploration allows the visualization of the bone or fibrous spur and the double spinal cord, to specify its location and its extension in height. A complement by MRI is essential. ⌈Fig.17⌉

The syringomyelic cavity corresponds to an accumulation of LCS in an intramedullary cavity. It is reflected in ultrasound by a centromedullary cavity with anechoic content sometimes difficult to distinguish from hydromyelia. Additional MRI is required. ⌈Fig.18⌉

Other indications

Congenital spinal tumors (rare), sacrococcygeal teratomas, neuroenteric cysts, dermoid cysts and anterior meningocele. In these cases MRI is the primary indication