TIPS FOR THE RADIOLOGIST Ultrasound evaluation of occult spinal dysraphism S Adronikou MB BCh, FCRsd(D), FRCR (Lond) B Smith MBChB Department of Paediatric Radiology University of Cape Town and Institute of Child Health Red Cross War Memorial Children's Hospital Cape Town Introduction Occult spinal dysraphic lesions include midline fusion defects that are skin covered and have no exposed neural tissue or visible cystic mass. A palpable mass mayor may not be present. I High frequency ultrasound pro- vides an ideal non-invasive technique for the evaluation of these lesions in children under 6 months of age. The aim of this article is to give practical advice on both the imaging technique used and the interpretation of abnor- mal findings. Ultrasound technique Ultrasound evaluation of the spinal cord and the surrounding con- tents of the spinal canal in children under 6 months makes use of the win- dow provided by the unossified poste- rior elements of the vertebral col- umn.' In our department we use the following technique: 1. The patient is placed prone with the hips and knees slightly flexed and a small cloth roll is placed under the pelvis to minimise the lumbar lordot- ic curve.' 2. A 7 MHz linear-array transduc- er is used to evaluate the lumbo-sacral region in both the longitudinal and transverse scan planes.The probe fre- quency is set at 7 MHz. 3. The distal spinal cord is identi- fied in the thoracolumbar region and is followed caudally to its termination at the conus. The level of the conus is recorded in the following manner: • By identifying the first sacral seg- ment, it is then possible to count up from this reference point to the vertebral body/disc space adjacent to the conus. The first sacral seg- ment is identified by the orienta- tion of its spinous process which is the first to be angled dorsally. • If it is not possible to identify this landmark, then a metal marker such as an unfolded paper clip can be placed under ultrasonic guid- ance on the skin overlying the site of the conus and then taped into position. Thereafter plain antero- posterior (AP) and lateral radi- ographs are taken with the marker in situ, and the level at which the conus terminates is assessed according to the vertebral level of the marker. 4. The morphology of the taper- 46 SA JOURNAL OF RADIOLOGY. October 2001 ing of the conus is recorded. The posi- tion of the cord in the spinal canal and the presence or absense of spinal cord pulsation are noted.' 5. Visualisation of the internal structure of the spinal cord and the overlying dura, cerebrospinal fluid (eSF), cauda equina, filum terminale, bony elements, paraspinal muscles, subcutaneous tissues and skin is pos- sible.' Normal ultra- sound anatomy The normal structures identified are depicted in Figs la and lb, and include the following: (l) a homoge- neously hypo echoic spinal cord with a central hyperechoic line representing the depth of the paramedian sulcus,' (il) the cord surrounded by the hyper- echoic pia mater and the hypoechoic eSF; (iii) eSF is usually seen only dor- sal to the cord as a normal untethered cord is dependent and lies in a ventral position against the posterior aspect of the vertebral bodies when the patient is prone; (iv) hyperechoic dura surrounding the eSF and the unossified bony elements dorsal to this are identifiable; (v) the paraspinal muscles are seen in a paramedian position covered by a continuous layer of subcutaneous fat and skin:' (Vl) the conus is recognised as a smooth taper- ing structure that ends at the lower end of L3 in a normal infant:' (Vil) the filum terminale is seen extending as a hyperechoic line from the tip of the conus in a caudal direction to the back of the first coccygeal segment and should never be more than 3 mm in diameter or have any structures with- in it;' and (viil) the fine echogenie strands extending from the caudal portion of the conus represent the cauda equina.' TIPS FOR THE RADIOLOGIST il -C1 ,. • Fig. 1. Structure of the normal cord on ultra· sound Fig.la. Linear ultrasound demonstration of the lower lumbar cord demonstrates the hypoechoic cord substance (white X), the echogenie line of the paramedian sulcus (curved arrow), the echogenie line of the pia mater lining the cord surface (straight arrow), the dorsal hypoechoic eSF (white dot) and the echo shadows originating at the spinous processes (white snow-heed). Fig. lb. The normal cord tapers to form the conus medullaris (thick arrow), and the nerve roots are seen as a mass of echogenie structures running caudally (thin arrow), The position of the conus is assessed as described by evaluating its verterbral level according to the echo shadowing of the spin· ous processes (white arrow·head). Interpretation of abnormal ultra- sound findings Tethered cord (Fig. 2) Diagnose a tethered cord if the fol- lowing findings are present: (I) a low- lying conus, i.e. a conus that termi- Fig. 2. Tethered cord. This scan shows that the cord does not taper to a normal conus and that it is low·lying. eSF is seen both dorsally and ventrally (thIn arrow) indicating that the cord is suspended due to tethering and cannot lie in a dependent post- tlon. An associated distal cord syrinx (thick arrow) is a common association. nates below the lower end of L3 or a non-tapering cord extending to the sacrum;' (it) a cord that is suspended in the spinal canal when the patient is lying prone, i.e. there is eSF both ven- tral and dorsal to the cord; (iii) absense of cord pulsation; (iv) a thick- ened filum terminale (> 3 mm in diameter);' (v) a lipoma within the filum;' (vi) other dysraphic anomalies such as lipomyelomeningocoele, mye- locystocoele, non-tapering of the cord; (viI) a distal cord syrinx;' and (viii) an intra-spinal lipoma, a der- moid or a sacral teratoma.' Lipomyelomeningocoele (Fig.3) Diagnose a lipomyelomeningo- coele if the following findings are present: (I) a subcutaneous lipoma that extends via a posterior bony defect and is attached to the cord, I (it) a low-lying conus or non-tapering cord (tethered cord); and (iii) a eSF- containing sac extending from the spinal canal into the adjacent soft tis- sue (meningocoele). The presence and size of the meningocoele are vari- able.' 47 SA JOURNAL OF RADIOLOGY. October 2001 Fig. 3. Llpomyelomenlngocoele. The cystic por· tlon of the IIpomyelomeningocoele is seen as an ecno-tre« area (white X) protruding from a dorsal bony defect and is covered by skin (curved arrow). There is an associated dorsal lipoma (thick straight arrow) and there are also cord elements protruding through the bony defect (white arrow·head), An associated distal cord syrinx is also present (thin straight arrow). Myelocystocoele Diagnose a myelocystocoele if the following findings are present: (i) a trumpet-shaped distal cord; and (il) a syrinx that herniates through a dorsal bony defect into the subcutaneous tis- sue - the eSF-containing mass can be seen just under the skin.I Diastomatomyelia Diagnose diastomatomyelia if the following findings are present: (i) par- tial or complete split of the cord with an intervening fibrous or bony sep- tum, and hemicords (often of unequal size);' (ii) features of cord tethering;' and (iiI) multiple clefts within the cord along its length with a normal cord in between these clefts.I Intraspinal lipoma Diagnose an intraspinal lipoma if the following findings are present: (I) an echogenie intraspinal mass that mayor may not extend through a bony defect into the subcutaneous tis- sue; (ii) the cord is attached to this either dorsally or at its terminal end; and (iii) no associated covered meningocoele is present. TIPS FOR THE RADIOLOGIST Dorsal dermal sinus Diagnose a dorsal dermal sinus if the following findings are present: (t) a tract that connects the skin to the intracanalicular space which may be median or paramedian; this may communicate with the subarachnoid space; 1 and (it) an associated lipoma, epidermoid or dermoid with the sinus tract. I Conclusion High-resolution spinal ultrasound is a valuable means of diagnosing sus- pected occult spinal dysraphic lesions in neonates and infants before the posterior spinal elements ossify. Ultrasound has numerous advan- tages: it is portable and available with high-resolution capabilities, it is safe and requires no sedation, and it is rel- atively inexpensive. When findings are confusing, abnormal or equivocal, then MR! must be performed to delineate the pathology more accu- rately. References 1. Korsvik H, KellerM. Sonography of occult dys- raphism in neonates and infants with MR imaging correlation. Radiographies 1992; 12: 297-306. 2. Nelson M jun. Ultrasonic evaluation of the tethered cord syndrome. In: Yamada S, ed. Tethered Cord Syndrome. American Association of Neurological Surgeons Publications Committee, 1996. 3. Hinshaw D jun., Engelhart J, Kaminsky C. Imaging of the tethered spinal cord. In: Yamada S, ed. Tethered Cord Syndrome. American Association of Neurological Surgeons Publications Committee, 1996. Hypertrophic pyloric stenosis •an overview H Grove Nst. Cert. Rsd. Ultrssound Department of Paediatric Radiology, Red Cross War Memorial Children's Hospital, University of Cape Town and Institute of Child Health Definition The term hypertrophic pyloric steno- sis (HPS) refers to hypertrophy of the circular muscle of the pylorus that can cause obstruction (HPO). Clinical findings Clinical findings include the fol- lowing: (i) non-bilious projectile vomiting; (it) peristaltic waves that can be seen travelling across the left upper quandrant to the right and ter- minating beyond the midline; I (iii) a palpable 'olive' (pseudotumour) over an empty stomach;' (iv) age typically 2 - 8 weeks; (v) male-to-female ratio 5:1; (vi) uncommon in black patients; (vit) often the male offspring of an affected mother; (viii) gastric residual > 10 ml;' and (ix) an association with oesophageal atresia.' Plain film findings Plain film findings include the fol- lowing: (i) gastric dilatation; (ii) paucity of small bowel and colonic air; (iii) frothy gastric contents; (iv) absence of an air-filled duodenal bulb; (v) gastric pneumatosis; and (vi) nor- mal appearance. 48 SA JOURNAL OF RADIOLOGY. October 2001 Ultrasound technique A high frequency transducer (7 MHz) is used, preferably a linear or vector probe (Acuson 128 XP/lO). With the patient in the supine posi- tion start off scanning in the longitu- dinal section until the gall bladder is located,' The 'olive' of the hypertro- phied musculature should be located medial to it. Visualisation is good when the 'olive' has a foreshortened appearance (Fig. I). The transducer now has to be rotated and angled so that it is aligned with the long axis of the channel (Fig. 2). On this view, the beak sign can be identified as on a contrast meal. If the stomach is too full, the chan- nel is distorted and accurate measure- ments won't be possible. In such a case a nasogastric tube can be passed to empty some of the contents. Once the long axis is obtained, one should note the position of the trans- ducer and turn it 90 degrees. This way the bull's eye of the pyloric channel can be identified end-on (Fig. 3).