Mild (borderline) ventriculomegaly [Fig5, Fig6] is used when the atrial width is 10-15 mm. Some consider early ventricular dilatation as separation (>3 mm) of the dependent choroid from the medial ventricular wall. This occurs bilaterally in 0.15-0.7% of fetuses and unilaterally in 0.07% of pregnancies. This can be related to an increased risk of CNS and non-CNS anomalies. Isolated borderline ventriculomegaly is normal variant in most cases, however, it may represent the earliest manifestation of cerebral anomalies (4%), chromosomal abnormalities (4%) (mostly trisomy 21) and overall abnormal outcomes (20%).
However, early and unexplained mild ventriculomegaly appears to have a good prognosis but a long postnatal clinical follow-up is required.
Overt (or moderate to severe) ventriculomegaly [Fig7, Fig8, Fig9, Fig10] is usually defined for an atrial width of more than 15 mm in the second and third trimesters(8). This may involve isolated cases (30-60%) or cases associated with other CNS anomalies, most frequently neural tube defects (NTDs) and midline defects. The degree of antenatal ventriculomegaly is related to pediatric neurological morbidity and, when it is >15 mm, it is associated with an increase in abnormal neurological development.
Unilateral ventriculomegaly is a rare finding usually due to focal obstruction of the CSF pathway at the level of Monro’s foramen. Examination of the lateral ventricles using the angled technique helps to distinguish between unilateral and bilateral hydrocephalus. Unilateral ventriculomegaly is usually an isolated finding and when isolated has little measurable effect on developmental outcome.