Myelomeningocele and myeloschisis (myelocele) are the most common open spinal dysraphisms and accounting for 85 % of all cases of spinal dysraphisms [16]. Abnormalities in neurulation can result from defects in disjunction, which involves the process by which there is separation of the neural tube from the overlying ectoderm. Complete failure of a large site of disjunction may result in a myelomeningocele [15].

Differentiation of myeloschisis from MMC can be detected on fetal US and fetal MRI. The distinction is based on the position of the neural placode with respect to the level of the skin surface. When there is elevation of the neural placode due to expansion of the subarachnoid space, the lesion is a MMC. In MMC the intramedullary structures lined with meninges protrude or herniate beyond the level of the skin surface (Fig. 8.1). When there is no elevation or protrusion of the neural placode beyond the plane of the skin but a posterior osseous and cutaneous defect is present, it is referred to as a myeloschisis or myelocele. In myeloschisis the spinal canal is directly exposed to the exterior (Fig. 8.2). Besides the aforementioned difference, the imaging findings are identical in MMC and myeloschisis. MMC and myeloschisis are nearly always found with Chiari type II malformation. This association is thought to be due to the collapse of the primitive fetal ventricular system (brought about by the loss of cerebrospinal fluid through the spinal defect), which prevents the rhombencephalic vesicle (from which the brainstem, fourth ventricle, and cerebellum are derived) from expanding and results in deficient development of the posterior fossa and herniation [17].

The stigmata of Chiari II malformation include a small posterior fossa, beaking of the tectum, hindbrain herniation, and effacement of the supratentorial CSF spaces. Additional findings of dysgenetic corpus callosum, and subependymal gray matter heterotopia may be present. Ventriculomegaly may be seen in many of these cases. Both US and MR can visualize the direct and indirect stigmata of Chiari II malformation such as fetal cranium (the lemon sign: special shape of the cranium due to deformity of the frontal bones) and the banana sign (special shape of the posterior fossa secondary to displacement of the cerebellar hemispheres toward the cervical canal), which results in the obliteration of cerebrospinal fluid at the foramen magnum [17]. The size of the defect and of the sac in MMC is variable and could be small or large (Fig. 8.3). MR can demonstrate the neural placode elements entering into the sac. Detection of presence of hindbrain herniation is ready apparent with fetal MRI but may be sometimes difficult to detect with screening US. Excellent detail and soft tissue resolution can be obtained with MRI, but fetal US may be more accurate in determining the level of the defect and determination of the neural placode, making the two modalities complementary [1, 18].

An extremely rare type of open dysraphism is the hemimyelomeningocele or hemimyelocele: in this anomaly, diastematomyelia is associated with one of the abovementioned anomalies in which one of [15] the hemi cords does not complete neurulation [15]. This entity has been discussed separately in the later part of this chapter.

In addition to the standard options of termination of the pregnancy and continuation of the pregnancy until term with cesarean or vaginal delivery, fetal closure of the dysraphic defect as part of the management of myelomeningocele study (MOMS) is now an option for some families in the United States [9, 10].

The results of the MOMS trial convincingly demonstrated that the cases treated with prenatal MMC closure had a decreased incidence of hindbrain herniation [10, 19]. The ascent of hindbrain structures could be demonstrated within 3 weeks of the fetal closure using serial MRI (Fig. 8.4). The overall fetal head size and posterior fossa volume have been demonstrated to be small in MMC patients and have been shown to increase normally after fetal surgery [10] (Fig. 8.5). The return of the head size to normal is likely attributed to restoration of cerebrospinal fluid volume after reversal of hindbrain herniation [10]. Recently published results from the MOMS study indicate that prenatal fetal MMC repair leads to reversal of hindbrain herniation, improvement in the lower extremity function, and decreased need for CSF diversion compared with postnatal repair controls [10, 19, 20].

In our institute, the fetus was only considered for prenatal myelomeningocele surgery if the gestational age at the time of the proposed surgery was 26 weeks or less, if the transatrial ventricular dilation was less than 16 mm (normal being less than 10 mm), if the estimated level of the lesion was at or above S1, and if there was convincing leg and foot motion on ultrasound in the absence of foot or leg deformity. To provide all this accurate information, fetal MRI was an integral part of the MOMS trial workup prior to prenatal surgery [10]. The discussion of the MOMs trial and fetal myelomeningocele repair in detail is beyond the scope of this chapter. Excellent papers on this topic are available to the interested reader [10, 18–25].