Dr Nigel Simpson, C Desjardins RDMS 01 January 2001
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.
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
Title: Disorders of the central nervous system
Author: Paidas MJ, Cohen A
Journal: Semin Perinatol 18: 266-282
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
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
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
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
Title: Diagnosis and management of neural-tube defects today
Author: Hobbins JC
Journal: N Engl J Med 324: 690-691
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
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
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
Title: Effacement of the fetal cisterna magna in association with
Author: Goldstein RB, Podrasky AE, Filly RA, Callen PW
Journal: Radiology 172: 409-413
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
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
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
Title: Ultrasound screening for spina bifida: cranial and cerebellar signs
Author: Nicolaides KH, Gabbe SG, Campbell S, Guidetti R
Journal: Lancet ii: 72-74
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
Title: Normal fetal sacrum simulating spina bifida: ópseudodysraphismò
Author: Dennis MA, Drose JA, Pretorius DH, Manco-Johnson ML
Journal: Radiology 155: 751-754
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