Prenatal ultrasound evaluation of fetal diastematomyelia: two cases of type I split cord malformation

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1 Ultrasound Obstet Gynecol 2000; 15: Prenatal ultrasound evaluation of fetal diastematomyelia: two cases of type I split cord malformation L.M. ALLEN and R.K. SILVERMAN Perinatal Center, SUNY Health Science Center at Syracuse, Department of Obstetrics and Gynecology, Syracuse, New York, USA KEYWORDS: Diastematomyelia, Split cord malformation, Ultrasound, Tethered cord, Magnetic resonance imaging ABSTRACT Isolated diastematomyelia is a rare form of spinal dysraphism characterized by a sagittal cleft in the spinal cord, conus medullaris and/or filum terminale with splaying of the posterior vertebral elements. This condition is the result of the presence of an osseous or fibrocartilaginous septum producing a complete or incomplete sagittal division of the spinal cord into two hemicords. It may be isolated or associated with other segmental anomalies of the vertebral bodies. Prenatal diagnosis of this anomaly is possible in the early midtrimester by sonography, thus allowing for early surgical intervention and a favorable prognosis. Two cases of fetal diastematomyelia diagnosed by prenatal sonography are presented, each demonstrating the typical sonographic features diagnostic of this condition. The first case, detected at 28 weeks gestation, presented with disorganization of the bony processes of the vertebral column with a midline echogenic focus. The second fetus, diagnosed at 17 weeks gestation, had a similar appearance with widening of the posterior elements and the presence of a midline echogenic bony spur. Postnatally, both infants underwent magnetic resonance imaging for a definitive diagnosis. Surgical repair of the defect was performed in the neonatal period in both cases. INTRODUCTION Structural anomalies of the fetal neurovertebral axis (Figure 1) can be diagnosed by prenatal ultrasound screening. A total of 14 cases of prenatally diagnosed diastematomyelia have been reported in the literature, all with a specific sonographic appearance 1 7. A review of these cases indicates that all but one was diagnosed between 15 and 21 weeks gestation. Where maternal serum a-fetoprotein (MSAFP) screening results were available, all cases of isolated diastematomyelia revealed normal levels of MSAFP. No abnormal karyotypes were Figure 1 Schematic sagittal and transverse appearances of diastematomyelia. The intervening bony spur is demonstrated in the widened portion of the vertebral column, with splitting of the spinal cord into two hemicords. reported. Ten out of 14 cases resulted in an elective termination of pregnancy. The unique pathognomonic sonographic criterion for the diagnosis of this condition is the demonstration of an extra echogenic focus located in the midline between localized widenings of the posterior ossification centres of the vertebral column, most commonly identified in the Correspondence: Ms L.M. Allen, The Perinatal Center, SUNY Health Science Center at Syracuse, Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, 90 Presidential Plaza, Suite 3118, Syracuse, New York 13202, USA Received , Revised , Accepted CASE REPORT

2 lower thoracic and upper lumbar regions with coronal scanning 2. Associated anomalies of the vertebral column should he specifically sought. However, isolated diastematomyelia will demonstrate intact posterior soft tissues in the transverse scanning plane and MSAFP screening and the karyotype are expected to be normal. CASE 1 A 21-year-old, married white woman, gravida 1 para 0, was referred for the suspicion of an irregular fetal spine on sonography. The MSAFP level had been normal at 16 weeks gestation (open NTD risk 0.74 MoM). She was evaluated at 28 weeks gestation, at which time a thorough sonographic investigation of the fetal anatomy was performed. There was evidence of disorganization of the bony processes of the vertebral column (Figure 2). In addition, there was protrusion but not disruption of the overlying subcutaneous tissues, splaying of the posterior ossification centers of the vertebral bodies and a midline echogenic focus within the spinal column (Figure 3). There was no evidence of hydrocephalus or intracranial malformation. Amniocentesis was performed for the purpose of establishing lung maturity and assaying amniotic fluid acetylcholinesterase. In addition, chromosome analysis was performed as a precaution. Chromosome analysis showed a normal female karyotype and the amniotic fluid was negative for acetylcholinesterase, ruling out an open spinal defect. Owing to the rarity of this malformation, and the fact that the pediatric neurosurgeons had no experience with prenatally detected cases of diastematomyelia, the patient underwent elective cesarean section at term. A 2910-g female infant was delivered. Evaluation by the neonatal staff confirmed the presence of a large palpable spike just superior to a sacral dimple. Magnetic resonance imaging (MRI) was performed on day 2 of life to evaluate the protuberance in the infant s lower back. Findings were consistent with the prenatal diagnosis of diastematomyelia in the upper lumbar region with splitting of the spinal cord, with two dural sacs that united into one above and below the osseous spike (Figure 4). Additional findings were bony defects of the posterior elements of the upper lumbar spine, a low-lying spinal cord and thickening of the filum terminalis. At 1 month of age, the infant was developing the neurological symptoms associated with a tethered spinal cord and the option of early surgical intervention was elected. Surgery was performed involving lumbar exploration, intradural exploration, resection of the spike, dissection of the fat of the filum terminalis and untethering of the spinal cord. The infant is now 3 years of age and has no medical complications or neurological implications. CASE 2 A 31-year-old, married white woman, gravida 2 para 1, was referred at 16 weeks gestation for evaluation of the fetal spine. There had been suspicion, on sonogram at another unit, of a fetal neural tube defect. MSAFP Figure 2 Case 1. Parasagittal sonogram of the fetal spine, showing disorganization of the vertebral bodies associated with a bony spur (arrow). Figure 3 Case 1. Coronal sonogram of the fetal spine demonstrating the echogenic bony spur (arrow) in the widened portion of the vertebral column. screening at 16 weeks was normal (open NTD risk 0.70 MoM). A complete detailed evaluation of the fetal anatomy was performed at 17 weeks gestation. Images of the fetal vertebral column demonstrated irregularity in the thoracolumbar region, with widening of the posterior elements and the presence of a midline echogenic bony spur (Figure 5). There was no evidence of any intracranial manifestation of this condition. A genetic amniocentesis was performed at the patient s request, and showed a normal male karyotype. The amniotic fluid was negative for acetylcholinesterase. At 38 weeks of gestation, spontaneous vaginal delivery resulted Ultrasound in Obstetrics and Gynecology 79

3 in the birth of a 3090-g male infant. On physical examination, the newborn was noted to have a subcutaneous bump in the thoracolumbar area and a sacral dimple, and was discharged home on day 2 of life. At 3 months of age, MRI was performed to evaluate the split cord malformation. Findings included a bony spur across the spinal canal at the level of L1 2 associated with a split cord (Figure 6). Above the spur, a single spinal cord was visible in a single dural tube. At the level of the spur, the single spinal cord divided into two symmetric hemicords, each within its separate dural tube (Figure 7). Below the spur, the cord reunited to form a single cord, which extended inferiorly to the level of L4. Fatty tissue was associated with the osseous spur and a fibrolipoma of the filum terminalis was identified at the level of S1, extending down to almost the level of the coccyx. A dimple or small soft tissue mass was identified in the subcutaneous tissues posteriorly at the level of L1 with communication to the spinal cord, and may have been either fibrous or a dermal sinus. Because the infant did not demonstrate any adverse effects of the abnormality, surgical intervention was delayed until 5 months of age. This involved resection of the large bony spur to release the spinal cord. Two areas of splitting were noted, as well as an interconnecting bridge of tissue, which was left intact. Partial L4 L5 laminectomies and release of the tethered spinal cord secondary to a fatty filum terminalis were performed. The infant is now 10 months old and has no residual complications, medical or neurological. Figure 4 Case 1. Sagittal magnetic resonance image of the bony spur (curved arrow), correlating well with the prenatal appearance. Figure 6 Case 2. Coronal magnetic resonance image showing the area of the bony septum and splitting of the spinal cord. Figure 5 Case 2. Coronal sonogram representative of the prenatal sonographic appearance of this abnormality. The presence of an extra echogenic focus in the widened segment of the vertebral column (arrow) is diagnostic of the diastematomyelia. Figure 7 Case 2. Magnetic resonance cross-section showing the division of the spinal cord into two hemicords (arrow). 80 Ultrasound in Obstetrics and Gynecology

4 DISCUSSION Diastematomyelia is a rare, complex malformation characterized by sagittal clefting of the spinal cord by a bony, fibrous or cartilaginous spur. This results in segmental duplication of the spinal cord with the formation of two hemicords. Each hemicord contains a central canal and one set of ventral and dorsal nerve roots. In 50% of cases, the hemicords share a single dural tube. In the remaining 50%, the hemicords are enveloped by separate dual sacs. The two hemicords typically reunite below the cleft with the conus lying in an abnormally low position 8. Eighty-two per cent of the septa are located between T1 and L5 9. In addition, diastematomyelia occurs more frequently in females (85%) 10. The unified theory of embryogenesis proposes that all split cord malformations originate from one basic ontogenetic error occurring around the time when the primitive neurenteric canal closes. This basic error is the formation of an accessory neurenteric canal through the midline embryonic disc that maintains communication between the yolk sac and amnion, enabling continued contact between ectoderm and endoderm within the canal. This abnormal fistula causes regional splitting of the notochord and the overlying neural plate 11. The altered state of the emerging split neural tube and the subsequent ontogenetic fates of the constituent components of the endomesenchymal tract ultimately determine the configuration and orientation of the hemicords, the nature of the median septum, the coexistence of various vascular, lipomatous, neural and fibrous oddities within the median cleft, the high association with open myelodysplastic and cutaneous lesions and the seemingly unlikely relationship with fore- and midgut anomalies 11. The proposed classification recommends the term split cord malformation for all double spinal cords. Type I consists of two hemicords, each contained within its own dural tube and separated by a dura-sheathed, rigid, osseocartilaginous median septum. Type II is characterized by two hemicords housed in a single dural tube separated by a non-rigid, fibrous median septum 11. Eighty-five per cent of patients with diastematomyelia have vertebral anomalies, including hemivertebrae, sagittally clefted or butterfly vertebrae, missing vertebrae and midline osseous or fibrocartilaginous spurs 11. Cutaneous signs of occult spinal dysraphism are frequently associated with split cord malformation. Hypertrichosis (hairy patch) is the most common manifestation (56%). Capillary hemangioma (26%), dermal sinuses (22%) and subcutaneous lipomas (11%) are also identified with increasing frequency 2. There is a high association with low-lying or tethered conus with thickened filum terminalis (40%), hydromyelia of one or both hemicords, lipoma of the filum terminalis, intradural lipoma and fibrous bands. Secondary tethering lesions in split cord malformations include thickened filum, terminal lipoma, dorsal lipomas, dermal sinus tract with or without a dermoid cyst, hemimyeloceles, complete myeloceles and limited dorsal myeloschisis 9. It is important that this condition be recognized early for, as the child grows older, the transfixed spinal cord cannot make its normal shift cephalad as the vertebral column lengthens with growth, resulting in tethering of the spinal cord. Patients may be asymptomatic (50%) 10 or present with symptoms and progressive neurological deficits suggestive of tethered cord syndrome. Individual symptoms and neurological deficits include back and extremity pain, gait disturbances, bladder and bowel dysfunction, motor weakness and sensory deficits in the lower extremities, progressive spinal deformity/scoliosis, club foot deformity and sympathetic dystrophy 9. The postnatal radiological evaluation of diastematomyelia begins with plain film findings, including segmental vertebral anomalies, a widened spinal canal and a central osseous spur. Plain computerized tomography (CT) will demonstrate similar findings, whereas CT myelography enables assessment of the spur and the nature of the cord splitting and surrounding dural sac(s). MRI is the modality of choice for definitive demonstration of this abnormality. It offers the most complete evaluation of diastematomyelia because of its superior soft-tissue contrast and its ability to image in any desired plane. MRI can assess the nature and length of the diastematomyelia and is better suited to detect associated soft-tissue abnormalities. Axial and coronal T1W images are best for showing the degree of cord duplication and the associated fibrosis and osteocartilaginous spur, if present 12. The prenatal, second-trimester ultrasound diagnosis of fetal diastematomyelia was first reported in Since that time, over 70% of reported cases have resulted in termination of pregnancy, limiting the information available on postnatal management of infants diagnosed prenatally. In the past, diastematomyelia was not diagnosed until the neonatal period secondary to the presence of cutaneous signs. In the majority of cases, the diagnosis was made later in childhood or adulthood with the onset of neurological symptoms. It appears that, until a larger population of cases becomes available for review, the mode of delivery and age of surgical intervention have been determined on an individual basis. Recommendations for the postnatal management of prenatally diagnosed diastematomyelia have not been clearly established in the literature. ACKNOWLEDGEMENTS The author would like to thank Jeffrey J. Allen for Figure 1, the medical illustration included in this article. 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