Routine prenatal care includes surveillance for fetal anatomic malformations using ultrasound. Development of ultrasound as an imaging modality began in the 1940s and 1950s, and the first fetal central nervous system (CNS) anomalies were reported in the 1970s. Over the next 2 decades, ultrasound studies described fetal CNS anomalies including Chiari II malformation, agenesis of corpus callosum, and hydrocephalus. In the early 2000s, magnetic resonance imaging (MRI) became an important adjunct to ultrasound both for evaluation of maternal complications and detection of fetal anomalies.1-3
There are several advantages to MRI in fetal diagnostics.4 MRI is not limited by fetal lie, calvarial ossification, maternal obesity, or oligohydramnios. It offers excellent soft tissue contrast and multiplanar visualization of all organs. Limitations include motion artifact from movement of either the fetus or the mother. Typically, MRI scans are not performed in the first trimester due to small fetal size and increased fetal motion. A large study comparing 242 fetal ultrasound and fetal MRI studies showed that MRI changed the diagnosis in about 32% of patients.5 Large multicenter trials have shown that fetal MRI can have a diagnostic accuracy of 93%, whereas ultrasound was found to have an accuracy of 68%.6 A study comparing pre- and postnatal findings found that the primary fetal diagnostic classification remained unchanged in 98% of cases.7 Postnatal MRI identified additional foci of cortical malformations or progression of vascular injury, but these findings did not change the clinical implications that had already been discussed during fetal counseling.
The use of fetal MRI for evaluation of CNS malformations and pediatric neurology consultative services has increased since 2010. Continued efforts at longitudinal studies of outcome of prenatally diagnosed CNS malformations will aid with pregnancy planning, neonatal care, accurate prognostication, and provision of anticipatory guidance. Discussion of the numerous intracranial anomalies is beyond the scope of this chapter. The focus, therefore, will be to provide a brief overview of the more common abnormalities, which include isolated ventriculomegaly, posterior fossa malformations (specifically Dandy-Walker malformation, vermian hypoplasia, and Blake pouch cyst), agenesis/dysgenesis of corpus callosum, congenital aqueductal stenosis, and holoprosencephaly.
The lateral ventricles begin as primitive cerebral cavities that communicate with each other and the third ventricle and are visible at 13 to 14 weeks of gestation. Ventriculomegaly is the most common CNS anomaly identified by prenatal ultrasound, with an incidence of 1 to 2 cases per 1000 births.8,9 Ventriculomegaly can be due to hydrocephalus (either obstructive or communicating) or decreased cerebral volume (either congenital or acquired).10 Fetal ventriculomegaly is defined by a measurement taken at the level of the atrium that is equal to or larger than 10 mm. Mild ventriculomegaly is defined as a measurement of 10 to 12 mm; moderate ventriculomegaly is 12.1 to 15 mm; and severe ventriculomegaly is greater than 15 mm (Figure 30–1...