Ventriculomegaly, or dilation of the fetal cerebral ventricles, is a relatively common ﬁnding on prenatal ultrasound. This summary of SMFM Consult Series #45 reviews key points for diagnosis, evaluation, and management of mild fetal ventriculomegaly.
Q| How is ventriculomegaly deﬁned?
Fetal cerebral ventriculomegaly is defined as an atrial diameter ≥ 10 mm on prenatal ultrasound. Ventriculomegaly is typically categorized in one of two ways: mild (10–15 mm) or severe (> 15 mm); or as mild (10–12 mm), moderate (13–15 mm), or severe (> 15 mm). Because the chance of an adverse outcome and potential for other abnormalities are higher when the ventricles measure 13–15 mm vs 10–12 mm, the latter classification method should be used for the purposes of patient counseling. The incidence of mild to moderate fetal ventriculomegaly is approximately 1%.
Q| What are the causes of ventriculomegaly?
The differential diagnosis of ventriculomegaly is extensive and includes a normal variant as well as disorders associated with severe impairment. A thorough evaluation is critical to make the correct diagnosis and to provide an accurate prognosis.
Fetuses with a ventricular atrial diameter of 10–12 mm are found to have a normal postnatal evaluation in > 90% of cases. The chance that mild ventriculomegaly represents a normal variant decreases with increasing degrees of dilation. Mild ventriculomegaly is likely to represent a normal variant if no other structural abnormalities are noted and if aneuploidy screening or diagnostic genetic testing results are normal. Because it is not possible to determine with certainty that mild ventriculomegaly is truly isolated during pregnancy, normal variation is a diagnosis of exclusion that cannot be made with certainty until after birth.
Structural abnormalities that can lead to dilation or enlargement of the lateral ventricles include agenesis of the corpus callosum, Dandy-Walker malformation, neural tube defects, cortical defects, and migrational abnormalities or heterotopia. Some structural central nervous system (CNS) anomalies, such as holoprosencephaly, hydranencephaly, porencephaly, or schizencephaly, and cystic lesions, such as arachnoid cysts, result in abnormal fluid collections in the fetal brain that may be misdiagnosed as ventriculomegaly, although these anomalies do not truly represent dilation of the ventricular system. In rare cases, a tumor or choroid plexus papilloma may result in overproduction of cerebrospinal fluid with resultant ventriculomegaly. Large isolated choroid plexus cysts may transiently dilate the fetal cerebral ventricles.
The most common cause of severe ventriculomegaly is aqueductal stenosis, which results from narrowing of the cerebral aqueduct of Sylvius located between the third and fourth ventricle, leading to progressive dilatation of the lateral and third ventricles. Aqueductal stenosis can be genetic or can result from fibrosis secondary to fetal infection (e.g., cytomegalovirus [CMV], toxoplasmosis, or Zika virus) or bleeding (e.g., intraventricular hemorrhage). A mass or congenital tumor can also lead to compression of the aqueduct with resultant ventriculomegaly. In many cases, the cause of aqueductal stenosis is unknown.
Approximately 5% of cases of mild to moderate ventriculomegaly are reported to result from congenital fetal infections, including cytomegalovirus (CMV), toxoplasmosis, and Zika virus. Many cases of ventriculomegaly associated with congenital infection demonstrate other sonographic features, including fetal growth restriction; periventricular, hepatic, and other intra-abdominal calcifications; echogenic fetal bowel; hepatosplenomegaly; ascites; meconium peritonitis; polyhydramnios, and microcephaly. However, these features may not be evident until later in gestation, and not all infected fetuses will have other sonographic signs.
Approximately 5% of fetuses with apparently isolated mild to moderate ventriculomegaly have an abnormal karyotype, most commonly trisomy 21. Another 10% to 15% have abnormal findings on chromosomal microarray. Although hydrocephalus is a component of several congenital syndromes, there are relatively few genetic causes of isolated ventriculomegaly or hydrocephalus. Syndromes typically associated with more severe ventriculomegaly as well as additional abnormalities that may be identified sonographically or by fetal magnetic resonance imaging (MRI) include Walker-Warburg, Bardet-Biedl, Meckel, Joubert, and hydrolethalus syndromes.