Prenatal Diagnosis of Tethered Spinal Cord

PICTORIAL ESSAY CME ARTICLE

Prenatal Diagnosis of Tethered Spinal Cord Roya Sohaey, MD,* Karen Y. Oh, MD,* Anne M. Kennedy, MD,Þ Jonathon R. Ameli, BA,þ and Nathan R. Selden, MD, PhD* Abstract: Tethered spinal cord is associated with closed and open neural tube defects. With prenatal screening, spinal defects are consistently diagnosed during fetal life. We show that the conus medullaris can be seen well with prenatal ultrasound, and the diagnosis of tethered spinal cord can be made during fetal life. In this pictorial essay, we show examples of tethered cord in a variety of fetuses with spine anomalies. Key Words: fetal spine, tethered spinal cord, fetal spinal cord, conus medullaris (Ultrasound Quarterly 2009;25:83Y87)

LEARNING OBJECTIVES Upon finishing this paper, the reader should be able to: 1. Describe the appearance of the normal fetal spinal cord, including the level of the conus medullaris. 2. List fetal anomalies associated with tethered spinal cord. 3. Identify the imaging features of tethered cord on ultrasound and magnetic resonance imaging.

T

he fetal spine is routinely assessed at the time of the fetal anatomic survey. Although the spinal cord is easily identified within the canal, it is rarely evaluated, even when bony spine anomalies are seen. However, in neonates with spinal dysraphism or skin-covered back masses, sonography of the neonatal spinal cord is routinely performed, mostly, to rule out the presence of a tethered cord or other congenital anomaly.1Y3 Tethered cord typically refers to the attachment of the conus Received for publication November 2, 2008; accepted February 7, 2009. *Professor of Diagnostic Radiology, Professor of Obstetrics and Gynecology (Sohaey), Assistant Professor of Diagnostic Radiology (Oh), Associate Professor of Division of Pediatric Neurosurgery (Selden), Oregon Health & Science University, Portland, OR; †Professor of Diagnostic Radiology, Adjunct Professor of Obstetrics and Gynecology (Kennedy), University of Utah Medical Center, Salt Lake City, UT; and ‡Premedical Student (Ameli), University of Southern California, Los Angeles, CA. The authors have disclosed that they have no interests in or significant relationships with any commercial companies pertaining to this educational activity. All staff in a position to control the content of this CME activity have disclosed that they have no financial relationships with, or financial interests in, any commercial companies pertaining to this educational activity. Lippincott CME Institute, Inc, has identified and resolved all faculty and staff conflicts of interest regarding this educational activity. Reprints: Roya Sohaey, MD, Diagnostic Radiology and Obstetrics and Gynecology, Oregon Health & Science University, Mail Code L340, 3181 SW Sam Jackson Park Rd, Portland, OR 97239 (e-mail: sohaeyr@ ohsu.edu). Copyright * 2009 by Lippincott Williams & Wilkins

Ultrasound Quarterly

& Volume 25, Number 2, June 2009

medullaris (CM) to the nonneural structures in the distal spinal canal, resulting in a low-lying cord, although tethering may occur anywhere along the neural axis. Tethering may result from the thickening and fat infiltration of the filum terminale or a host of other dysraphic elements, such as lipomyelomeningocele or diastematomyelia. Failure to identify and surgically release a tethered cord may result in neurological impairment and voiding dysfunction.4 Tethered cord is seen in association with both closed and open congenital spinal dysraphic anomalies. The prenatal identification of specific spinal dysraphic conditions associated with tethered cord aids in accurate prenatal counseling.5 In this pictorial essay, we will show that presence of a tethered spinal cord can be identified in utero and should be considered in high-risk cases. We will review the sonographic appearance of the normal fetal spinal cord and discuss common spinal and cutaneous anomalies associated with tethered spinal cord.

NORMAL SPINAL CORD The spinal cord is hypoechoic and contains a linear central echo complex (Fig. 1). The distal portion of which, the ventriculus terminalis, normally contains fluid and should not be confused with a spinal syrinx.1 The central echo complex may contain a small amount of fluid and have a Btram-track[ appearance. The spinal cord eventually tapers inferior to the caudal margin of the central echo complex.1,6,7 The CM is the inferior tip of the spinal cord, and its ascent during fetal life can be seen with ultrasound.6,7 Between 13- and 18-week menstrual age, the CM is most often situated lower than the fourth lumbar (L4) vertebral level. Between 19 and 36 weeks, the normal CM is at the L3 level or higher. After 36 weeks, the CM level is most often higher than L2. The diagnosis of fetal tethered cord can be made if the CM is seen lower than the L3-L4 levels after 18 weeks.6 The vertebral level can be determined by finding the 12th rib and the associated 12th thoracic vertebrae, then counting the lumbar vertebrae inferiorly. The level can be confirmed by counting up from the lumbar sacral junction, assuming the presence of 5 normal lumbar vertebrae. Threedimensional (3D) ultrasound with multiplanar imaging can enable the identification of the CM and vertebral levels with a single volume sweep.8,9

VERTEBRAL BODY ANOMALIES Congenital malformations of the vertebrae may produce unbalanced growth of the spine and scoliosis.3 Vertebral anomaly morphology is variable and includes hemivertebra and segmentation bars. Scoliosis may be a later manifestation; www.ultrasound-quarterly.com |

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FIGURE 1. Normal. A, Spinal canal ultrasound of an 18-week-old fetus shows a normally tapered CM (arrow) ending at the L3 vertebral level. B, The central echo complex (arrows) is the echogenic line in the center of the hypoechoic spinal cord. C, Three-dimensional ultrasound with coronal reformatting and bone rendering can be used to identify vertebral body levels. In this case, the T12 and S1 levels of a normal 16-week-old spine are identified.

therefore, vertebral segmentation abnormalities can be seen in utero without discrete kyphosis or scoliosis. On prenatal ultrasound, vertebral bony dysmorphism is seen best on sagittal and coronal views of the spine and with 3D ultrasound. The Bjumbled-spine[ appearance suggests the diagnosis of vertebral body segmentation anomalies. Intraspinal abnormalities are present in 38% of neonates with congenital scoliosis, with tethered cord as the most common (Fig. 2).10 In addition, renal anomalies include unilateral renal agenesis, duplicated kidney, and ureteral obstruction and are seen in 30% of neonates with vertebral anomalies.11,12 Congenital heart defects are present in 10% of all neonates with vertebral anomalies.13 The VACTERL association should also be considered whenever vertebral anomalies are identified (Fig. 3). The

VACTERL (or VATER) association is an acronym for concurrent abnormalities seen: V for vertebral anomalies, A for anal atresia, C for cardiac defects, TE for tracheoesophageal fistula, R for renal anomalies, and L for limb anomalies (often radial defects).

CLOSED NEURAL TUBE DEFECTS Congenital spinal dysraphism can be further subclassified into those with or without a soft tissue back mass and with or without skin covering.14 Skin-covered lesions rarely demonstrate hindbrain malformation, hydrocephalus, or other findings of Chiari 2 malformation and are therefore generally more subtle and more difficult to diagnose in utero.15,16 The

FIGURE 2. Vertebral segmentation anomalies. A, Multiple jumbled vertebral bodies (arrows) are seen in the lumbar region of a 33-week-old fetus. In addition, the CM terminates at the lumbosacral junction, representing spinal cord tethering (arrowhead). B, A postnatal lateral spine film shows multiple segmentation anomalies of the lumbar spine and thoracic spine anomalies that were not seen with ultrasound imaging. Tethered cord was confirmed using neonatal ultrasound.

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Prenatal Diagnosis of Tethered Spinal Cord

FIGURE 3. VACTERL. A, Coronal oblique image of the sacrum of a 31-week-old fetus shows the termination of the CM in the distal sacrum (arrowhead). In addition, there is a fluid-filled ventriculus terminalis (arrow) and minimal scoliosis. B, Axial view of the renal fossae shows a normal right kidney (arrow) and an absent left kidney. Fetal colon fills the left renal fossa. C, Postnatal magnetic resonance (MR) imaging shows left renal agenesis and segmentation anomaly of the lumbar spine with scoliosis. This neonate also had pulmonary stenosis and esophageal atresia with trachea-esophageal fistula (neither of which were identified in utero).

maternal serum >-fetoprotein (AFP) levels are often normal or only minimally elevated when these anomalies are present.16 Myelocystocele, a skin-covered neural tube defect, appears as a lumbosacral mass containing a combination of cerebrospinal fluid, neural tissue, and fat (Figs. 4 and 5). Associated spinal dysraphism and tethered cord are always present. Large masses can mimic cystic sacrococcygeal teratoma and open neural tube defects.16 Smaller masses are associated with subtle neural tube defects and can easily be missed on prenatal examinations. The much more common distal fatty tethering malformation, lipomyelomeningocele, may be associated with a large subcutaneous lumbar fatty mass or with a relatively normal-appearing back. The lumbar region in children with tethering due to a thickened or fatty

FIGURE 4. Myelocystocele. A, Coronal ultrasound of the distal spine of a 34-week-old fetus shows a skin-covered cystic mass (arrows). The spinal cord extends into the sacrum (arrowhead). B, Fetal MR imaging better reveals associated spinal dysraphism (arrows) than ultrasound. On MR imaging, spinal cord parenchyma is seen extending to the dysraphic defect. Nevertheless, cord tethering was best visualized using ultrasound. * 2009 Lippincott Williams & Wilkins

filum terminale may appear normal or may show only subtle abnormalities such as a sacral dimple or deviated gluteal fold, which are exceedingly difficult to visualize using prenatal imaging.2,3,15 More subtle fetal skin stigmata include back lipomas, fibrolipomas, hair tufts, hemangiomas, and caudal appendages

FIGURE 5. Myelocystocele. A, Sagittal ultrasound of the lower back of a 37-week-old fetus illustrates the location of the CM (arrowhead) and identifies a ventriculus terminalis in the lower lumbar spine. Spinal dysraphism was suggested to underlay normal skin, but was not clearly seen. B, Postnatal ultrasound of a lumbar mass demonstrates a spinal defect at L3/L4 with neural elements extending dorsally (arrow). C, Postnatal MR imaging shows a tethered cord (arrowhead) and skin-covered neural tube defect. www.ultrasound-quarterly.com |


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FIGURE 6. Human tail. A, Axial ultrasound of the lower back of a 27-week-old fetus with a clubfoot shows an echogenic mass (arrow). B, Magnified oblique view of the mass shows that it is narrow, extending a few centimeters from the skin surface (arrows). C, 3D ultrasound of the mass shows its ‘‘tail-like’’ morphology and surface attachment to the skin. This finding on 3D ultrasound suggested a risk for tethered spinal cord. We therefore carefully imaged the entire spinal canal using 2D ultrasound to evaluate this possibility. D, The CM extended into the sacrum (arrowhead) and contained a distended central echo complex (arrows). The normally single echogenic line representing the central canal was replaced by 2 echogenic lines because of the presence of excess fluid. E, Fetal MR imaging confirmed the presence of a distally tethered cord (arrowhead). F, Clinical photograph of the neonate shows the caudal appendage. G, Postnatal MR imaging confirmed the diagnosis of tethered cord (arrowhead) and showed an associated dermal sinus tract with intraspinal extension (arrow).

(Fig. 6). Neuroectodermal caudal appendages, also known as Bhuman tails,[ are associated with spinal dysraphism, and 81% have tethered cord.17

OPEN NEURAL TUBE DEFECTS Tethered cord is always a feature of open spina bifida and can be seen using prenatal ultrasound (Fig. 7). The prenatal diagnosis of spina bifida is well described and will

not be reviewed here. Elevated levels of maternal serum AFP and the presence of associated Chiari 2 malformation and hydrocephalus are highly suggestive of the diagnosis. These clues lead the sonographer to carefully assess the spine for spinal dysraphism, which is almost always detected in utero. The identification of the level and severity of the open neural tube defect is important for prenatal counseling.

FIGURE 7. Open neural tube defect. A, 3D ultrasound images through the spine of a 19-week-old fetus with spina bifida show a lumbar meningomyelocele sac (arrows). B, A coronal-reformatted image from the 3D volume set clearly identifies the CM extending to the proximal sacrum (arrowhead).

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TABLE 1. Anomalies Associated With Tethered Cord 1. Vertebral body segmentation anomaly 2. Scoliosis 3. VACTERL 4. Myelocystocele 5. Lipomyelomingocele 6. Fetal back mass/appendage 7. Open neural tube defect

CONCLUSIONS Ultrasound is routinely used in the pediatric population to diagnose tethered cord in high-risk patients. Table 1 summarizes the list of conditions in which it would be suitable to spend extra effort looking for a tethered cord. Like the neonatal spine, the fetal spinal cord is also seen well with ultrasound, and the CM can be consistently visualized. In our pictorial essay, we have shown examples of tethered cord in a variety of fetuses with spine anomalies. In some cases, the tethered cord is arguably easier to see than the subtle bony abnormality. We think that looking for tethered cord in highrisk fetuses will enable the practitioner to make more accurate diagnoses of spinal anomalies. Because tethered cord almost always requires surgical correction, prenatal diagnosis and counseling allow families to prepare for the postnatal course.

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Prenatal Diagnosis of Tethered Spinal Cord 2. Lowe LH, Johanek AJ, Moore CW. Sonography of the neonatal spine: part 2, spinal disorders. AJR Am J Roentgenol. 2007;188:739Y744. 3. Kose N, Campbell RM. Congenital scoliosis. Med Sci Monit. 2004;10(5):RA104YRA110. 4. Selcuki M, Vatansever S, Inan S, et al. Is a filum terminale with a normal appearance really normal? Childs Nerv Syst. 2003;19: 3Y10. 5. Michelson DJ, Ashwal S. Tethered cord syndrome in childhood: diagnostic features and relationship to congenital anomalies. Neurol Res. 2004;26(7):745Y753. 6. Zalel Y, Lehavi O, Aizenstein O, et al. Development of the fetal spinal cord: time of ascendance of the normal conus medullaris as detected by sonography. J Ultrasound Med. 2006;25:1397Y1401. 7. Robbin ML, Filly RA, Goldstein RB. The normal location of the fetal conus medullaris. J Ultrasound Med. 1994;13:541Y546. 8. Lee W, Chaiworapongsa T, Romero R, et al. A diagnostic approach for the evaluation of spina bifida by three-dimensional ultrasonography. J Ultrasound Med. 2002;21:619Y626. 9. Johnson DD, Pretorius DH, Riccabona M, et al. Three-dimensional ultrasound of the fetal spine. Obstet Gynecol. 1997;89:434Y438. 10. Bradford DS, Heithoff KB, Cohen M. Intraspinal abnormalities and congenital spine deformities: a radiographic and MRI study. J Pediatr Orthop. 1991;11:36Y41. 11. Drvac DM, Ruderman R, Coonrad R, et al. Congenital scoliosis and urinary tract abnormalities. J Pediatr Orthop. 1987;7:441Y443. 12. MacEwen G, Winter R, Hardy J. The evaluation of kidney anomalies in congenital scoliosis. J Bone Joint Surg Am. 1972;54:1341Y1354. 13. Reckles L, Peterson H, Bianco A, et al. The association of scoliosis and congenital heart defects. J Bone Joint Surg Am. 1975;57: 449Y455. 14. Dick EA, Patel K, Owens CM, et al. Spinal ultrasound in infants. Br J Radiol. 2002;75:384Y392. 15. Meyer SH, Morris GF, Pretorius DH, et al. Terminal myelocystocele: important differential diagnosis in the prenatal assessment of spina bifida. J Ultrasound Med. 1998;17:193Y197. 16. Yu JA, Sohaey R, Kennedy AM, et al. Terminal myelocystocele and sacrococcygeal teratoma: a comparison of fetal ultrasound presentation and perinatal risk. AJNR. 2007;28:1058Y1060. 17. Herman TE, Siegel MJ. Human tail-caudal appendage: tethered cord. J Perinatol. 2008;28:518Y519.

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