FIGURE 10-13 Ventriculomegaly. In this transverse view of the cranium, the white line depicts measurement of the atrium of the lateral ventricle, which measured 12 mm, consistent with mild ventriculomegaly.
Prognosis is generally determined by etiology, severity, and rate of progression. However, even with mild-appearing and isolated ventriculomegaly, prognosis can vary widely. In a systematic review of nearly 1500 mild-to-moderate cases, 1 to 2 percent were associated with congenital infection, 5 percent with aneuploidy, and 12 percent with neurological abnormality (Devaseelan, 2010). A neurological abnormality was significantly more common if ventriculomegly progressed with advancing gestation.
Agenesis of the Corpus Callosum
The corpus callosum is the major iber bundle connecting reciprocal regions of the cerebral hemispheres. With complete agenesis of the corpus callosum, a normal cavum septum pellucidum cannot be visualized sonographically. Also, the frontal horns are displaced laterally, and the atria show mild enlargement posteriorly-such that the ventricle has a characteristic "teardrop" appearance (Fig. 10-14). Callosal dysgenesis involves only the caudal portions-the body and splenium-and consequently may be more diicult to detect prenatally.
In population-based studies, agenesis of the corpus callosum has a prevalence of 1 in 5000 births (Glass, 2008; Szabo, 2011). In a review of apparently isolated cases, fetal MR imaging identiied additional brain abnormalities in more than 20 percent (Sotiriadis, 2012). If the anomaly was still considered isolated following MR imaging, normal developmental outcome was reported in 75 percent of cases, but severe disability occurred in 12 percent. Agenesis of the corpus callosum is associated with other anomalies, aneuploidy, and more than 200 genetic syndromes. Thus, genetic counseling can be challenging.
FIGURE 10-14 Agenesis of the corpus callosum. This image demonstrates a "teardrop" shaped ventricle with mild ventriculomegaly (dotted line) and laterally displaced frontal horns (arrow). A normal cavum septum pellucidum cannot be visualized.
In early normal brain development, the prosencephalon or forebrain divides as it becomes the telencephalon and diencephalon. With holoprosencephaly, the prosencephalon fails to divide completely into two separate cerebral hemispheres and underlying paired diencephalic structures. Main forms of holoprosencephaly are a continuum that contains, with decreasing severity, alobar, semilobar, and lobar types. In the most severe form-alobar holoprosencephay-a single monoventricle, with or without a covering mantle of cortex, surrounds fused central thalami (Fig. 10-15). In semilobar holoprosencephay, partial separation of the hemispheres occurs. Lobar holoprosencephay is characterized by a variable degree of fusion of frontal structures and should be considered when a normal CSP cannot be seen.
Diferentiation into two cerebral hemispheres is induced by prechordal mesenchyme, which is also responsible for diferentiation of the midline face. Thus, holoprosencephaly may be associated with anomalies of the orbits and eyeshypotelorism, cyclopia, or micro-ophthalmia; lips-median cleft; or nose-ethmocephaly, cebocephaly, or arhinia with proboscis (see Fig. 10-15).