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.
The most useful findings in suggesting the diagnosis of spina bifida are the features of spinal dysraphism (with disruption of the dorsal skin line by a protruding sac), and the presence of associated cranial defects. Spinal findings:- Transverse views – the normal spinal cord appears as a closed circle with three prominences at each level representing two posterior (laminar) ossification centres and one anterior (vertebral body) ossification centre. In spina bifida the posterior laminae are absent and the lateral processes are splayed and displaced laterally, forming a characteristic V or U shape. Coronal views – normally, three parallel lines are seen, the lateral two representing the posterior ossification centres, the middle representing the anterior ossification centres. Separation or splaying of the two lateral lines implies the presence of spina bifida at that level. Sagittal views – in the normal spine two parallel lines representing the posterior and anterior ossification centres are seen converging towards the sacrum. Absence of the posterior aspect of these lines indicates a spinal defect. Exaggerated curvatures of the spine are often present in spina bifida and sagittal views will demonstrate any coexisting lordosis and/or kyphosis. Some points for consideration:- 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 mineralisation 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.
There are other spinal defects requiring differentiation: Myelocystocele, which is a localised dilatation of the spinal cord, may be difficult to distinguish from a closed simple meningocele with a small spinal defect. In this condition, however, there is no bony defect apparent. Lipomyelomeningocele is a rare lesion involving a fatty tumour of the spinal cord with associated spinal dysraphism which may be difficult to differentiate from meningomyelocele. The lesion is predominantly solid, however, in comparison with the cystic appearance of the defect in spina bifida. Diastematomyelia, which may coexist with spina bifida, will be apparent by the presence of the midline bony spur arising between the posterior ossification centres on coronal views. Sacrococcygeal teratomas not only have a solid and cystic appearance, but they may be associated with extensive local destruction, and are likely to become progressively larger.
Anterior calvarial collapse
Obliteration of cisterna magna by cerebellum
Splaying of posterior ossification centres
Kyphosis and scoliosis
Open neural defect.
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