When fetal ventriculomegaly is diagnosed or suspected, detailed ultrasound to look for associated brain abnormalities as well as extracranial abnormalities is warranted. When severe ventriculomegaly is present (atrial diameter ≥15 mm), the rate of associated abnormalities may be 50 % or greater [10]. Fetal MRI may assist in differentiating ventriculomegaly due to obstruction from cases occurring due to abnormal cortical development, hemorrhage, or abnormal vascular supply. Fetal MRI should be considered in all cases of ventriculomegaly, including those with borderline measurements [11].

Ventriculomegaly, including borderline cases, has been associated with genetic abnormalities, and workup should include karyotype as well as microarray to look for microdeletions and duplications not detectable on karyotype. In a male fetus, findings consistent with aqueductal stenosis may represent X-linked hydrocephalus, and testing for mutations of the L1CAM gene should be included. Toxoplasmosis and cytomegalovirus (CMV) infection are also associated with aqueductal stenosis. While classic features include periventricular calcifications, maternal serology and/or PCR of amniotic fluid for DNA of these agents should be considered for any unexplained cases.

If fetal intracranial hemorrhage is suspected to be the cause of ventriculomegaly, workup should include testing for alloimmune thrombocytopenia, as this condition is known to recur in future pregnancies and maternal medical therapy can prevent catastrophic outcomes [12].

In general, the most important factor predicting outcome is whether ventriculomegaly represents obstruction of cerebrospinal fluid, such as aqueductal stenosis, or whether dilation of the ventricles is due to abnormal cortical development or cortical damage from hemorrhage or vascular insufficiency. Lesions occurring due to obstruction are amenable to treatment, and outcomes are variable. If aqueductal stenosis is due to toxoplasmosis or CMV, poor outcomes, including mental retardation and deafness, are more common. X-linked hydrocephalus is also associated with poorer outcomes compared to sporadically occurring aqueductal stenosis.

Ventriculomegaly due to fetal intracranial hemorrhage is associated with high rates of poor prognosis, though this will depend on the size and location of the lesion(s). When ventriculomegaly is due to conditions characterized by abnormal cortical development, the prognosis will be determined by the specific condition, though poor outcomes are very likely.

When borderline ventriculomegaly is isolated and stable, normal outcomes are likely. Studies assessing long term outcome of prenatally diagnosed mild or borderline ventriculomegaly (unilateral or bilateral) suggest low rates of abnormal neurological development [7–9].

If macrocephaly is present, cesarean delivery is usually recommended [13]. Delivery should occur at a facility with an experienced pediatric neurosurgeon, as neonatal transport can delay proper evaluation and management. When ventriculomegaly is severe and/or progressive, delivery as soon as fetal lung maturity can be confirmed may be considered to facilitate earlier postnatal treatment.

Dandy–Walker Malformation is characterized by complete or partial absence of the cerebellar vermis, widely splayed cerebellar hemispheres, and an enlarged cisterna magna that is seen communicating with the fourth ventricle (Fig. 17.2). Ventriculomegaly is typically present.

Inferior vermian agenesis, also referred to as a Dandy–Walker Variant, is less likely to be identified than the Dandy–Walker Malformation. The posterior fossa will appear normal in transverse views through the superior portion of the vermis, and the ventricles typically appear normal. Real-time scanning through the entire cerebellum can identify absence of the inferior portion of the vermis, and coronal views can confirm the diagnosis [17].

In the second-trimester, the transverse cerebellar diameter measures approximately 1.0 mm for each week of gestation as measured from the first-day of the last menstrual period [19]. Though this is actually overstates by 2 weeks the time from conception, this method of describing gestational age is standard in the United States. Cerebellar growth is typically maintained in the growth-restricted fetus, and trans-cerebellar diameter has been proposed as an accurate method of estimating gestational age in the second or third trimesters when dating is unclear and growth restriction may be present [20].

Cerebellar hypoplasia as an isolated finding appears to be uncommon, and the prognosis is unclear. As with microcephaly, larger differences between expected and actual measurements are more concerning [18]. A lack of data correlating prenatal findings with long term outcomes precludes providing precise prognostic information.

Obstetric management is usually routine. When hydrocephalus associated with the Dandy–Walker Malformation is present, cesarean delivery may be advised [13].

Cephaloceles are defects in the skull through which intracranial tissue can herniate. If only meningeal tissue has herniated, it is referred to as a cranial meningocele. If brain tissue is present within the meningeal sac, it is referred to as an “Encephalocele ”.

A midline cephalocele in most cases is likely to represent an open neural tube defect affecting the cranium, though this is considerably less common than meningomyelocele or anencephaly. While maternal serum alpha-fetoprotein (MSAFP) may be elevated, lesions are often skin covered and MSAFP screening is less sensitive in detecting cephaloceles compared to spina bifida or anencephaly. Most midline encephaloceles occur in the occipital region (Fig. 17.3). A midline encephalocele in a fetus with polydactyly and cystic kidneys is likely to represent Meckel-Gruber syndrome , a lethal autosomal recessive condition [21].

Non-midline cephaloceles occur sporadically, and are associated with the amniotic band syndrome. Anterior cephaloceles, in which intracranial structures herniate into facial structures, have also been described, though prenatal diagnosis is uncommon [22].

Cephaloceles can appear similar to other masses, such as lipomas, teratomas, and hemangiomas. To distinguish a cephalocele from a mass overlying an intact skull, a skull defect must be identified. It is important to not confuse sutures and fontanelles with a pathologic skull defect. Large encephaloceles typically are associated with ventriculomegaly or distortion of brain anatomy. Fetal MRI is very helpful in verifying or ruling out a skull defect when cephalocele is suspected, as well as evaluating associated changes in brain anatomy.

The Chiari II Malformation is present in most cases of meningomyelocele. Hindbrain herniation typically causes collapse of the frontal bones and a “lemon-shaped” calvarium in the second-trimester, as well as a “banana-shaped” cerebellum in the second and third trimesters. The cisterna magna will be absent, and ventriculomegaly typically is present [24]. Early in the second-trimester, head measurements will often be small for gestational age, though macrocephaly often occurs later in pregnancy due to progressive ventriculomegaly.

When spina bifida is present, the cranial findings may be more apparent than the spinal defect, and the description of these findings led to a significant increase in the sensitivity of ultrasound in detecting spina bifida. When these findings are present, a thorough evaluation of the fetal spine is obviously important, as spina bifida cannot be diagnosed based on cranial findings alone.