Ultrasound assessment of spinal dysraphism

This paper focusses on the role of ultrasound in assessing spinal dysraphism, a common feature in many spinal defects occurring in infancy. We also review the state of the art of ultrasound investigation of the spinal cord in the light of our experience and ongoing technical advances. Spinal cord investigation using ultrasound was first proposed in 1982 but only entered routine practice in the late eighties. The examination can be performed using electronically focussed 5, (7.5) and 10 MHz linear probes or devices fitted with annular array mechanical sector 7 (7.5), 10, 13 and 15 MHz small part transducers. In practice, the anatomical and technical drawbacks linked with small part probes have led to the predominant use of linear transducers which give better results with an improved signal/noise ratio and a wider field of view. The quality of ultrasound assessment is inversely proportional to the age of the infant and is optimal in the first three to four months of life. The spinal card cart also be evaluated thereafter in cases presenting an acoustic window due to spinal dysraphism or surgery. At ultrasound investigation the spinal cord is displayed as a tubular hypoechoic structure with hyperechoic walls. The central echogenic complex is visible in the middle surrounded by perimedullary structures The cauda appears as a set of echogenic lines surrounding the hyperechoic filum terminale. The main indication for spinal ultrasound investigation in the neonate is the search for spina bifida occulta suspected in babies with a lumbosacral cutaneous abnormality covered by skin or a dorsal dimple or sinus often associated with tethered cord. The general ultrasonographic features of tethered cord include:low position and rough conformation of the conus, thickened filum, terminale, dorsal shift of the cord, absent physiological pulsatility and cord movements on flexion-extension of the spine and possible identification of a spinal mass. Ultrasonographic findings and the method's potential vary depending on the type of defect responsible for spinal dysraphism. Lipomas, for example, are displayed as hyperchoic masses variably involving the cord and/or filum. Ultrasound only gives a rough idea of the extension of the lesion, but the thickness of the filum can be measured. Ultrasound also presents major drawbacks in assessing the cutaneous sinus since it offers no information on possible intradural extension and it is usually difficult to identify any associated (epi)dermoid cysts. Instead, ultrasonography offers excellent diagnostic images of diastomatomyelia. Ultrasound is of no help in the diagnosis of open spinal defects, but may be useful in searching for possible retethering. This can be determined by the persistence of physiological cord pulsatility when an extensive view of the region is not possible. We recently compared the presence or absence of spinal motility at ultrasound examination with clinical signs in patients who had undergone surgery for spinal dysraphism. Despite its limitations, ultrasonography of the spinal cord in infants is safe, easy to use and offers real time images. However, suspect spinal dysraphism requires further investigation using MR and CT.