The birth prevalence of holoprosencephaly is only 1 in 10,000 to 15,000. However, the abnormality has been identiied in nearly 1 in 250 early abortuses, which attests to the extremely high in-utero lethality (Orioli, 2010; Yamada, 2004). The alobar form accounts for 40 to 75 percent of cases, and 30 to 40 percent have a numerical chromosomal abnormality, particularly trisomy 13 (Orioli, 2010; Solomon, 2010). Conversely, two thirds of trisomy 13 cases are found to have holoprosencephaly. Fetal karyotype or chromosomal microarray analysis should be ofered when this anomaly is identiied.
This posterior fossa abnormality is characterized by agenesis of the cerebellar vermis, posterior fossa enlargement, and elevation of the tentorium. Sonographically, luid in the enlarged
FIGURE 10-15 Alobar holoprosencephaly. A. Transverse cranial image of a fetus with alobar holoprosencephaly, depicting fused thalami (Th) encircled by a monoventricle () with a covering mantle (M) of cortex. The midline falx is absent. (Reproduced with permission from Rafael Levy, ROMS.) B. In this profile view of the face, a soft tissue mass-a proboscis (arrow), protrudes from the region of the forehead.
cisterna magna visibly communicates with the fourth ventricle through the cerebellar vermis defect, with visible separation of the cerebellar hemispheres (Fig. 10-16). The birth prevalence approximates 1 in 12,000 (Long, 2006). Associated anomalies and aneuploidy are common. These include ventriculomegaly in 30 to 40 percent, other anomalies in approximately 50 percent, and aneuploidy in 40 percent (Ecker, 2000; Long, 2006). Dandy-Walker malformation is also associated with numerous genetic and sporadic syndromes, congenital viral infections, and teratogen exposure, all of which greatly afect the prognosis. Thus, the initial evaluation mirrors that for ventriculomegaly (p. 193).
Inerior vermian agenesis, also called Dandy-Walker variant, is a term used when only the inferior portion of the vermis is absent. But, even when vermian agenesis appears to be partial and relatively subtle, the prevalence of associated anomalies and aneuploidy is still high, and the prognosis is often poor (Ecker, 2000; Long, 2006).
Schizencephaly is a rare brain abnormality characterized by clefts in one or both cerebral hemispheres, typically involving the perisylvian fissure. The cleft is lined by heterotopic gray matter and communicates with the ventricle, extending through the cortex to the pial surface (Fig. 10-17). Schizencephaly is believed to be an abnormality of neuronal migration, which explains its typically delayed recognition until after midpregnancy (Howe, 2012). It is associated with absence of the cavum septum pellucidum, resulting in the frontal horn communication shown in the image below.
FIGURE 10-16 Dandy-Walker malformation. This transcerebellar image demonstrates agenesis of the cerebellar vermis. The cerebellar hemispheres (+) are widely separated by a fluid collection that connects the 4th ventricle (asterisk) to the enlarged cisterna magna (eM).