08/05/2024
Differentiating Fetal Brain/Head Anomalies Part Two:
1.Cystic Hygroma - A thin-walled, uni or multilocular cystic structure near the fetal head and neck, eccentrically situated with respect to the long axis of the fetus. The presence of septations may indicate a poorer outcome. This happens when there’s a malformation or blockage in the lymphatic system during fetal development, which leads to the formation of a cystic hygroma also called nuchal lymphangioma. A cystic hygroma can be found in the nuchal region, which may extend the length of the fetus, take care not to confuse with nuchal translucency. Usually there is evidence of fetal anasarca/hydrops fetalis.
2.Fetal Hydrocephalus - (Fluid with Brain) is typically identified by measuring the size of the ventricles in the fetal brain, when the fetal lateral ventricular diameter is greater than 15 mm. A normal size of the ventricular atrium is less than 1 cm. Measurements between 1-1.2 cm are considered borderline ventriculomegaly, 1.2-1.5 cm is mild ventriculomegaly, and greater than 1.5 cm is deemed Hydrocephalus. The choroid plexus may appear compressed and thinned due to increased CSF pressure. A separation of more than 3 mm between the choroid plexus and the margin of the ventricle is considered abnormal. “Dangling” Choroid Plexus Sign is observed when the dependent choroid plexus hangs down towards the dependent part of the head by gravity.
3.Fetal Hydranencephaly (Hydra=Fluid, An=without, Ceph=Brain) - A type of encephalopathy characterized by the destruction of the cerebral hemispheres, which are transformed into a membranous sac containing cerebrospinal fluid (CSF) and remnants of cortex and white matter. Fetal head appears “empty” because the cerebral hemispheres are missing. Instead, the skull contains a sac-like structure filled with CSF. Essentially, there is no remaining cortical tissue in the brain. Occasionally, small islands of residual tissue may be preserved at the occipital poles and orbitofrontal regions. Unlike hydrocephalus, where the choroid plexus may appear compressed, in hydranencephaly, the choroid plexus is absent or severely reduced.
4.Anencephaly (An= without, Ceph=Brain) - Absence of cortical tissue as well as an absence of the cranial vault. No parenchymal tissue is seen above the orbits. If a small amount of neural tissue is present, it is termed exencephaly. Exencephaly may be seen at an earlier stage. The calvarium (skull) is absent.
5.Acrania (A=without, Crania=Cranium) - Absence of the fetal skull, resulting in brain tissue being exposed to the amniotic fluid. It is considered the precursor to anencephaly in what is called the acrania-exencephaly-anencephaly sequence. The exposed brain tissue may appear as a normal amount of brain tissue initially, creating the “Mickey Mouse sign” (two semicircular structures floating above the fetal face, resembling Mickey Mouse’s rounded ears). As the sequence progresses, a significant amount of brain tissue is lost, leading to the “frog face sign” (absence of recognizable tissue superior to the level of the fetal orbits).
6.Septo-Optic Dysplasia - Presence of at least two of the following three clinical or radiographic findings: optic nerve hypoplasia, pituitary hypofunction, and midline brain abnormalities, namely, malformation of the CSP or corpus callosum. Fused frontal horns are also commonly seen in lobar holoprosencephaly, sometimes making the two entities indistinguishable. Key findings are fused frontal horns, absent CSP, and a normal posterior fossa. The primary differential diagnosis is lobar holoprosencephaly.
