Fetal ventriculomegaly is a relatively common finding on second trimester obstetrical ultrasound examination. It is clinically important because it can be caused by a variety of disorders that result in neurological, motor, and/or cognitive impairment. Many cases are associated with other abnormal findings, but in some fetuses, ventriculomegaly is the only abnormality . This topic will discuss the prenatal diagnosis and clinical significance of ventriculomegaly and options for management of affected pregnancies.
DEFINITION AND NOMENCLATURE
The atrium of the lateral ventricle is the portion where the body, posterior horn, and temporal (inferior) horn converge. Atrial diameter remains stable between 15 and 40 weeks of gestation [2,3]. Ventriculomegaly is defined as an atrial diameter ≥10 mm, which is 2.5 to 4 standard deviations above the mean depending on the study [2-4]. Ventriculomegaly is generally considered mild if the atrial diameter is between 10 and 15 mm and severe if >15 mm, although some authors use the categories of mild (10 to 12 mm), moderate (13 to 15 mm), and severe (≥16 mm) .
Ventriculomegaly is "isolated" when the fetus has no other anomalies, except those that are a direct result of the ventricular enlargement. Many cases that appear isolated prenatally are ultimately found to have other abnormalities, particularly when ventriculomegaly exceeds 15 mm [5-8]. These abnormalities include Chiari malformations, neural tube defects, Dandy Walker malformations, agenesis of the corpus callosum, and genetic syndromes.
Hydrocephalus is the correct term for pathologic dilatation of the brain's ventricular system from increased pressure, usually due to obstruction. Ventriculomegaly is the appropriate term when dilatation is due to other causes, such as brain dysgenesis or atrophy. Because ventricular pressure cannot be measured prenatally, the two terms are sometimes used synonymously when applied to the fetus. Most commonly, the term "ventriculomegaly" is used when the ventricles are mildly enlarged, and "hydrocephalus" is used when they measure >15 mm.
Ventriculomegaly may or may not be accompanied by macrocephaly (ie, head circumference greater than two standard deviations above the mean).