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Meningocele, Spinal

Summary   Ultrasound Features   Differential Diagnoses   Syndromes   Images 1   Show All

Spina bifida is defined as a congenital bony defect affecting the vertebral arches arising from incomplete caudal neurulation. There is typically a saccular protrusion of neural elements through the spinal defect which may or may not be skin covered. As a consequence of this there is herniation of the posterior fossa contents through the foramen magnum (Arnold-Chiari malformation). If the sac contains meninges and cerebrospinal fluid it is known as a meningocele. When elements of spinal cord and/or nerves are present, it is termed a meningomyelocele (or myelomeningocele). This is the most common lesion, accounting for more than 90% of cases. Myeloschisis occurs when the open neural tube is not covered and is open to the exterior. Craniorachischisis refers to exposure of both cranial and spinal neural elements. Spina bifida can occur at any location along the spine with lumbar and lumbosacral lesions being most common. As part of the Arnold-Chiari malformation, hydrocephalus develops in the majority of cases. Even in experienced hands small closed defects can be missed, especially in the cervical or sacral regions. These lesions may be difficult to diagnose particularly if there is no protruding sac or if the fetus is in an awkward position. In the latter situation or at any time where there is diagnostic doubt the ultrasound examination must be repeated on another occasion. Experience is needed to avoid false positive diagnoses due to incomplete or absent ossification in the second trimester. Ossification of the laminae of the lower lumbar region is not usually seen before nineteen weeks gestation. The arch of the upper sacral region is not always seen until after twenty-five weeks. Transverse views are generally superior as all three ossification centres may be seen in the same plane. Coronal and sagittal views, however, will show many levels at the same time and abnormalities of structure, shape and mineralization of the spine are better appreciated. A protruding sac will be best seen in a sagittal plane as will the integrity of the skin of the fetal back. Coronal views enable comparison of any suspected dysraphism with normal adjacent levels. Cranial findings:- The intracranial findings associated with spina bifida are generally related to the herniation of posterior fossa contents through the foramen magnum. Ventriculomegaly is characteristic, arising from obstruction of the flow of cerebrospinal fluid into the basal cisterns. The prevalence in fetuses with myelomeningocele increases with gestational age so that after twenty-four weeks gestation, 94% of affected fetuses will have a lateral atrial diameter greater than 10 mm. Ventriculomegaly worsens with increasing gestation in open spina bifida while the head circumference remains relatively small. There is a characteristic shape of the head related to the Arnold-Chiari malformation known as the lemon sign, where there is scalloping of the frontal bones within the parietal bones to compensate for the caudad shift in anterior cranial contents. This frontal bone deformity is best demonstrated on transverse views of the fetal head in the transthalamic plane, but may be absent after twenty four weeks gestation as the bones become less malleable. There is also a characteristic shape of the cerebellum in spina bifida known as the banana sign. Displacement of the pons and medulla through the foramen magnum compresses the cerebellum against the occipital bone. This process deforms the cerebellum and gives it the appearance of a banana in the transcerebellar plane. It also effaces the cisterna magna. Therefore obliteration of the cisterna magna or failure to see the cerebellum are excellent pointers to the presence of spina bifida, and if a normal cisterna magna is seen it is unlikely that an open spina bifida is present. Associated ultrasound findings may include other central nervous system malformations such as holoprosencephaly, agenesis of the corpus callosum, and the Dandy-Walker malformation. Diastematomyelia, intraspinal lipoma and dermoid cysts may also be associated with myelomeningocele. Disorders of other systems are common, and include renal, thoracic, gastrointestinal and facial malformations. Aneuploidy has been reported in up to 10% of cases. Title: Prenatally detected fetal myelomeningocele: is karyotype analysis warranted? Author: Babcook CJ, Goldstein RB, Filly RA Journal: Radiology 194: 491-494 Year: 1995 Title: Disorders of the central nervous system Author: Paidas MJ, Cohen A Journal: Semin Perinatol 18: 266-282 Year: 1994 Title: Prevalence of ventriculomegaly in association with myelomeningocele: correlation with gestational age and severity of posterior fossa deformity Author: Babcook CJ, Goldstein RB, Barth RA, Damato NM, Callen PW, Filly RA Journal: Radiology 190: 703-7 Year: 1994 Title: Small fetal transverse cerebellar diameter: a screening test for spina bifida Author: De Courcy-Wheeler RHB, Pomeranz MM, Wald NJ, Nicolaides KH Journal: Br J Obstet Gynaecol 101: 904-905 Year: 1994 Title: The lemon sign: not a specific indicator of meningomyelocele Author: Ball RH, Filly RA, Goldstein RB, Callen PW Journal: J Ultrasound Med 12: 131-134 Year: 1993 Title: Prenatally detected myelomeningoceles: sonographic accuracy in estimation of the spinal level Author: Kollias SS, Goldstein RB, Cogen PH, Filly RA Journal: Radiology 185: 109-112 Year: 1992 Title: Diagnosis and management of neural-tube defects today Author: Hobbins JC Journal: N Engl J Med 324: 690-691 Year: 1991 Title: Early diagnosis of spina bifida: the value of cranial utlrasound markers Author: Thiagarajah S, Henke J, Hogge WA, Abbitt PL, Breeden N, Ferguson JE Journal: Obstet Gynecol 76: 54-57 Year: 1990 Title: Evaluation of the lemon and banana signs in one hundred thirty fetuses with open spina bifida Author: Van den Hof MC, Nicolaides KH, Campbell J, Campbell S Journal: Am J Obstet Gynecol 162: 322-327 Year: 1990 Title: Abnormal US appearance of the cerebellum (banana sign): indirect sign of spina bifida Author: Benacerraf BR, Stryker J, Frigoletto FD Journal: Radiology 171: 151-153 Year: 1989 Title: Effacement of the fetal cisterna magna in association with myelomeningocele Author: Goldstein RB, Podrasky AE, Filly RA, Callen PW Journal: Radiology 172: 409-413 Year: 1989 Title: Abnormalities of fetal cranial contour in sonographic detection of spina bifida: evaluation of the "lemon" sign Author: Nyberg DA, Mack LA, Hirsch J, Mahony BS Journal: Radiology 167: 387-392 Year: 1988 Title: Fetal head shape in spina bifida in the second trimester Author: Furness ME, Barbary JE, Verco PW Journal: J Clin Ultrasound 15: 451-453 Year: 1987 Title: Ultrasound screening for spina bifida: cranial and cerebellar signs in a high-risk population Author: Campbell J, Gilbert WM, Nicolaides KH, Campbell S Journal: Obstet Gynecol 70: 247-250 Year: 1987 Title: Ultrasound screening for spina bifida: cranial and cerebellar signs Author: Nicolaides KH, Gabbe SG, Campbell S, Guidetti R Journal: Lancet ii: 72-74 Year: 1986 Title: Hemangioma presenting as a cystic mass in the fetal neck Author: Grundy H, Glasmann A, Burlbaw J, Walton S, Dannar C, Doan L Journal: J Ultrasound Med 4; 147-150 Year: 1985 Title: Normal fetal sacrum simulating spina bifida: ópseudodysraphismò Author: Dennis MA, Drose JA, Pretorius DH, Manco-Johnson ML Journal: Radiology 155: 751-754 Year: 1985 Title: Ultrasound and amniotic fluid alpha-fetoprotein in the prenatal diagnosis of spina bifida Author: Allen LC, Doran TA, Miskin M, Rudd NL, Benzie RJ, Sheffield LJ Journal: Obstet Gynecol 60: 169-173 Year: 1982

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