Ventriculomegaly, a term commonly employed to indicate the enlargement of the lateral cerebral ventricles, is found in ~1% of fetuses at midgestation and is the most common abnormal fetal cerebral diagnosis.

Although the enlargement of the lateral ventricles encompasses a wide spectrum of severity, at present a width of the atrium <10 mm is considered normal, 10 to 15 mm indicates mild ventriculomegaly, and >15 mm represents severe ventriculomegaly. This categorization has prognostic implications. Fetuses with normal ventricles have an exceedingly low risk of cerebral anomalies. Fetuses with mild ventriculomegaly in the majority of cases are normal at birth but have an increased risk of an abnormal outcome. Fetuses with severe ventriculomegaly have a very high probability of an abnormal outcome.

Enlargement of the cerebral lateral ventricles is not an anomaly per se. The clinical significance of this finding is that it signals to the possibility of associated anomalies of the brain or other organs. The final prognosis depends more on such anomalies than on the degree of ventricular dilation.

Fetuses with mild ventriculomegaly in particular have an increased risk of chromosomal aberrations.

Synonyms

Mild hydrocephaly, borderline ventriculomegaly

Definition and Diagnosis

The widely accepted definition of mild cerebral lateral ventriculomegaly is an atrial width of 10 to 15 mm on the transverse plane (Figure 4–1)^1,2,3

Prevalence

Seen in 1% of fetuses⁴

Pathogenesis and Pathology

In many cases, it probably represents a normal variant. In other cases, mild enlargement of the lateral ventricles may be the only obvious epiphenomenon of heterogeneous cerebral anomalies.

Differential Diagnosis

Isolated mild ventriculomegaly should be differentiated from more complex abnormalities of the fetal brain that frequently have a different prognosis (eg, agenesis of the corpus callosum and cortical malformations). Several reports suggest that magnetic resonance imaging (MRI) may be a useful adjunct to sonography, particularly in late gestation.^5,6

Implications for Targeted Examination

The main problem in cases that are referred with mild dilatation of the lateral ventricles is to exclude other neural and extraneural malformations. We recommend careful multiplanar examination of the fetal brain, performed if possible with a high-resolution vaginal probe, and a detailed evaluation of the spine. Both lateral ventricles should be visualized and assessed, as this condition can be unilateral (Figure 4–2). A stepwise ultrasound (US) evaluation of the fetal brain should be performed in order to exclude associated anomalies (Figure 4–3). Although the use of MRI has also been advocated,^5,6 we believe that when following the protocol suggested in Figure 4–3, the role of fetal MRI remains limited to those cases in which technical issues impairs US visualization. A detailed evaluation of the entire fetal anatomy, including fetal echocardiography, should also be performed. These examinations may be incomplete or limited during the third trimester.

Implications for Sonographic Screening

In all standard sonographic examinations, a view of the lateral ventricles should be obtained, and at least one of the atria should be visualized and assessed. A qualitative -evaluation is acceptable, and the presence of the choroid plexus filling the cavity of the atrium, being closely apposed to both the medial and lateral walls of the ventricle, is indicative of normalcy. A quantitative approach, however, is favored, and a measurement <10 mm is considered normal between 15 and 40 weeks.⁷ Congenital ventriculomegaly may develop late in gestation, and a normal midtrimester exam does not exclude this condition. The correct measurement of the lateral ventricles is important to avoid false-positive diagnoses of ventriculomegaly. Common errors in measurement include placing the calipers in the brain parenchyma instead of at the inner border of the ventricular wall, measuring the ventricle not perpendicular to their walls, measuring the occipital horn, or measuring the lateral ventricle not in a truly axial plane. A specific approach has been described that proposes placing the calipers at the level of the parietooccipital fissure, an easily recognizable landmark beginning around 20 weeks’ gestation⁸ (Figure 4–4).