**7. Posterior fossa abnormalities**

**6. Midline abnormalities**

and the dysmorphic facial features of the neonate.

56 Congenital Anomalies - From the Embryo to the Neonate

parietal lobes.

(**Figures 24** and **25**).

*Holoprosencephaly* is a congenital induction disorder, occurring extremely early in pregnancy (3–6 WA), with failing the segmentation of the neural tube [53–57]. This leads to incomplete separation of the prosencephalon. It has been classified into four subtypes: alobar, semilobar,

**Figure 23.** Complex cortical anomaly. Nodular periventricular heterotopia seen. Cortical hypoplasia. Microcephaly. Periventricular leukomalacia. The postpartum image highlights the abnormal excessive ossification of the coronal suture

The alobar holoprosencephaly is the most severe type, having a complete lack of separation of the cerebral hemispheres; this lead to a single ventricle, absence of the CC and IEF, and fused thalami. In the semilobar type, the cerebral hemispheres are fused anteriorly. In lobar holoprosencephaly, the fusion of the cerebral hemispheres is present at the frontal lobes. The middle interhemispheric fusion variant results from nonseparation of posterior frontal and

The *corpus callosum* is the largest commissure of the brain, and its development is accelerated between 8 and 20 WA. Any disturbance of this process may lead to CC agenesis or partial agenesis (hypogenesis or dysgenesis). Many CC abnormalities (in terms of dimensions and shape) are frequently diagnosed during pregnancy although their significance is still debated [58–67]. Abnormal CC has been described among patients assessed for mental retardation.

In a similar way, the absence of fluid in the CSP (with or without intact *septum pellucidum* and corpus callosum) may indicate subtle or severe midline brain abnormalities. All these conditions may have significant implications for postnatal neurological development [58, 61, 63–68]

**Figure 24.** Different cases of holoprosencephaly in the first trimester (a,b). c. a case which associates proboscis.

lobar, and a middle interhemispheric fusion variant (syntelencephaly).

Posterior fossa abnormalities include the Dandy-Walker malformation (complete or partial agenesis of the cerebellar vermis, cystic dilatation of the fourth ventricle, and enlarged posterior fossa, with upward displacement of the tentorium, torcula, and transverse sinuses); the mega cisterna magna (a CM measuring more than 10 mm and a normal vermis); the Blake's pouch cyst (the presence of an upwardly displaced normal cerebellar vermis, normal appearance of the fastigium, tentorium, and size of the cisterna magna); and isolated vermian hypoplasia (a normally formed vermis but of smaller size, with an otherwise normal size and anatomy of the posterior fossa) [69, 70]. It seems that the Dandy-Walker malformation, even if apparently isolated on ultrasound imaging, carries a high risk for chromosomal and associated structural anomalies. Isolated mega CM and Blake's pouch cyst have a low risk for aneuploidy and associated structural anomalies. The isolated vermian hypoplasia is extremely rare; thus, the literature does not offer definite conclusions about its significance. This needs to be further assessed (**Figure 26**).

**Figure 26.** Isolated mega cisterna magna. US and MRI images of the same third-trimester case.
