INTRODUCTION — Malformations of the central nervous system (CNS) are among the most common types of major congenital anomalies. Ultrasound examination is an effective modality for prenatal diagnosis of these anomalies.
This topic will review the evaluation of the fetal CNS and diagnosis of midline CNS anomalies. Prenatal diagnosis of neural tube defects and ventriculomegaly are discussed separately. (See "Neural tube defects: Prenatal sonographic diagnosis" and "Fetal cerebral ventriculomegaly".)
EVALUATION OF THE FETAL CENTRAL NERVOUS SYSTEM
●Basic examination – CNS structures evaluated during a basic fetal ultrasound examination include lateral ventricles, choroid plexuses, cavum septi pellucidi, falx, thalami, cerebellum, cisterna magna, and spine . These structures are seen when scanning transabdominally using the three axial planes, namely the transthalamic, transventricular, and transcerebellar planes (image 1 and image 2 and image 3). In addition to these views, the head shape, face, and neck should be included as part of the routine CNS ultrasound examination.
●Targeted or extended examination – If a suspicion of pathology arises at the basic scan, then targeted or extended examination of fetal neuroanatomy should be performed, adding the coronal and sagittal planes . Ideally, pregnancies at increased risk of fetal CNS anomalies and those with suspicious findings on a basic examination should undergo fetal neurosonography performed by clinicians with expertise in this area. This type of scan utilizes all the available scanning routes, such as transvaginal (if the fetus is in vertex presentation or converts to vertex with gentle transducer pressure or spontaneously), color Doppler, and three-dimensional ultrasound. The most important feature of the targeted or extended scan is the median (midsagittal) plane, which allows better evaluation of the corpus callosum, midbrain and posterior fossa .
●Timing – A thorough understanding of the normal sonographic appearance of the CNS across gestation is crucial for accurate diagnosis because the presence or absence of a structure may be normal or abnormal depending upon the age of the fetus. Poor timing of the examination, rather than poor sensitivity, can be a crucial factor in failing to detect a CNS abnormality . As an example, a sonogram of the fetal brain at 14 weeks of gestation cannot detect agenesis of the corpus callosum since this structure does not become sonographically apparent until 18 to 20 weeks of gestation and does not acquire its final form until 28 to 30 weeks. (See 'Disorders of the corpus callosum' below.)
Traditionally, the routine evaluation of the fetal CNS is performed at 18 to 22 weeks. However, technical advances and improved understanding of early fetal brain development may allow first-trimester neuroanatomy screening to become routine. Imaging of the fetal brain from 16 weeks is now commonly performed in many centers.
●Role of MRI – Magnetic resonance imaging (MRI) shows cortical development much better than ultrasound and may detect additional CNS anomalies. It is appropriate for further evaluation in cases of diagnostic uncertainty when additional information to that obtained by targeted examination by an expert may influence subsequent management of the pregnancy [1,5].
In a multi-center prospective cohort study (MERIDIAN) in which over 800 pregnancies with a fetal brain abnormality on ultrasound underwent MRI within 14 days, overall diagnostic accuracy for ultrasound was 68 percent versus 93 percent for MRI (difference 25 percent, 95% CI 21-29 percent) .
Obstetric providers reported that the additional information obtained on MRI had a "significant or major" influence on management in approximately 35 percent of cases and "no or minor" influence in the remaining 65 percent of cases. However, many details were missing in the report, including the type of abnormalities that changed management and what those management changes were. Other studies have also shown a benefit of fetal MRI [7-9]. However, the incremental value of MRI depends on high-quality sonographic imaging, and the better that imaging is, the lower the perceived value of fetal MRI will be.
●Embryology – The holoprosencephaly sequence develops from failure of the prosencephalon (forebrain) to differentiate into two cerebral hemispheres and lateral ventricles between the fourth and eighth weeks of gestation. This failure results in partial to complete fusion of the cerebral hemispheres and lateral ventricles, which partially or fully communicate across the midline. The third ventricle is usually absent.
●Prevalence – The prevalence is higher in abortuses than in live births (1:250 versus 1:10,000), making holoprosencephaly the most common abnormality of the forebrain [10-12].
●Types – The three most common types of holoprosencephaly are, in decreasing order of severity (image 4) [13-21]:
There are other rare types of holoprosencephaly (eg, middle interhemispheric variant [MIHV] is a milder type).
●Associated anomalies – Variable degrees of facial dysmorphism may be present [13-16,22]. The most severe brain malformation (alobar holoprosencephaly) is associated with the most severe facial dysmorphism (cyclopia [single eye, proboscis located above the eye, nose absent], or ethmocephaly [hypotelorism (image 5), micro-ophthalmia, interorbital proboscis, nose absent], or cebocephaly [hypotelorism, micro-ophthalmia, small flat single-nostril nose]). Other facial dysmorphisms, such as hypertelorism and cleft lip and/or palate, may be seen as well [23,24].
Microcephaly (head size at least two standard deviations below the mean) can be present in all three types of holoprosencephaly .
Extracranial malformations may also be present and include renal cysts and dysplasia, omphalocele, cardiovascular malformations, clubfoot, myelomeningocele, and intestinal abnormalities.
Postnatal clinical features, diagnosis, and management are reviewed separately. (See "Overview of craniofacial clefts and holoprosencephaly", section on 'Holoprosencephaly'.)
●Anatomy – Alobar holoprosencephaly is a lethal anomaly characterized by complete failure of cleavage of the prosencephalon. As a result, the cerebral hemispheres are fused and there is a single large midline fluid collection (ventricle) and absence of the corpus callosum and falx cerebri. The cerebrum is smaller than normal, and the thalami are usually completely fused.
●Etiology – The etiology of holoprosencephaly is varied and most cases are sporadic. However, holoprosencephaly can be the result of a teratogenic exposure (eg, poorly controlled blood glucose levels in maternal diabetes, use of retinoic acid, alcohol use) or inherited. Approximately 75 percent of cases have trisomy 13, making this the most common chromosomal abnormality. Less frequently encountered chromosomal abnormalities include triploidy and trisomy 18, as well as other trisomies [10,25]. Autosomal dominant, autosomal recessive, and X-linked inheritance have been described. Genes associated with holoprosencephaly include Sonic Hedgehog (SHH), ZIC2, SIX3, and TG-interacting factor (TGIF) .
●Prenatal diagnosis – The prenatal diagnosis is based on imaging a single ventricle. In normal brains, the falx in the axial plane is seen between the two brightly echogenic choroid plexus. This appearance has been likened to a butterfly; failure to identify the "butterfly sign" is a reliable first-trimester clue to the presence of alobar holoprosencephaly .
The falx cerebri, which divides the brain into the right and left hemispheres, is sonographically imaged from the 10th week of gestation.
The diagnosis of alobar holoprosencephaly can be made as early as the 10th week of gestation [18,19,21], but not before because the falx cerebri normally becomes sonographically apparent at around 9 weeks of gestation; prior to this time, the normal fetal brain on ultrasound has a single ventricular cavity comparable to what is observed with alobar holoprosencephaly.
●Differential diagnosis – Both alobar holoprosencephaly and hydranencephaly are characterized by characterized by the appearance of a single ventricle; however, in hydranencephaly, some midline structures (falx cerebri, interhemispheric fissure, third ventricle) are present, whereas they are absent in holoprosencephaly. (See 'Hydranencephaly' below.)
Semilobar holoprosencephaly — Semilobar holoprosencephaly is characterized by partial cleavage of the prosencephalon posteriorly. Although anteriorly the cerebral hemispheres are fused and contain a single moderately-sized ventricular cavity, posteriorly the cerebral hemispheres, ventricles, and thalami are partially separated.
We have been able to make this diagnosis at 11+2 weeks of gestation because of the typical appearance of the choroid plexus in these cases: It looks similar to a mustache.
Lobar holoprosencephaly — The sonographic findings associated with lobar holoprosencephaly are subtle and are noted in the midcoronal plane:
●The interhemispheric fissure is present
●The corpus callosum may be absent, hypoplastic, or normal
●The leaflets of the cavum septum pellucidum are absent
●There may be midline fusion of the cingulate gyri
●The frontal horns are fused and have a flat roof that freely communicates with the third ventricle
Lobar holoprosencephaly probably cannot be diagnosed in the first or early second trimester since its most consistent feature, absence of the cavum septi pellucidi, cannot be imaged reliably until at least 18 weeks of gestation.
The differential diagnosis of the lobar holoprosencephaly is septo-optic dysplasia (de Morsier syndrome: hypoplasia of the optic nerve, agenesis or dysgenesis of the septi pellucidi and corpus callosum, pituitary hypoplasia).
Middle interhemispheric variant — The sonographic features of middle interhemispheric variant (MIHV) include abnormalities of the corpus callosum (body may be absent or hypoplastic), absent septi pellucidi, and abnormal midline fusion of the posterior part of the frontal and parietal lobes with variable fusion of thalami [28,29].
DISORDERS OF THE CORPUS CALLOSUM
Overview — The corpus callosum is the largest of the white matter interhemispheric tracts connecting the cerebral hemispheres . These connections are important for the functional integration of sensory, motor, visuomotor, and cognitive processes (language, abstract reasoning, integration of complex sensory information) [30,31].
●Developmental abnormalities – Developmental abnormalities or disorders of the corpus callosum include:
•Complete agenesis (absence)
•Partial agenesis (hypogenesis) – The corpus callosum is shorter in its anterior-posterior length as a result of missing segment(s) such as the splenium and/or the rostrum
•Thinning (hypoplasia) – The corpus callosum is normal in its anterior-posterior length, but there is thinning
•Thickening (hyperplasia) – The corpus callosum is thicker than expected.
●Etiology of abnormal development – Agenesis of the corpus callosum (ACC) is a heterogeneous condition resulting from disruption of multiple developmental steps, ranging from midline telencephalic patterning to neuronal specification and regulation of commissural axons . The cause of the disruption may be genetic, infectious (TORCH infections, Zika virus), vascular, or toxic (fetal alcohol syndrome).
Genetic factors are most common. Among the genetic causes, "syndromic" diagnosis is made in 30 to 45 percent of cases and a monogenic cause can be identified in 20 to 35 percent. Over 200 genetic syndromes, many of which have variable phenotype, include disorder of the corpus callosum as a feature. Selecting the appropriate prenatal diagnostic test requires a multidisciplinary approach including genetics, genetic counselors, maternal fetal medicine, imagers, pathologist and pediatric geneticists.
●Prevalence of ACC – The reported prevalence of ACC is 1:4000 to 1:5000 live births; however, rates of 2 to 3 percent have been reported among patients with neurodevelopmental disabilities [32,33].
●Associated abnormalities – Corpus callosum malformations are often associated with other cerebral or extra-cerebral abnormalities . Among the cerebral anomalies, ventriculomegaly, Dandy Walker spectrum, heterotopias, and cortical dysplasias may be present. Chromosomal abnormalities are seen in approximately 18 percent of ACC, and include trisomy 18, trisomy 13, and mosaic 8. Microarray in postnatal cases has revealed that 9 percent of cases of ACC have at least one de novo pathogenic copy number variant, and 7 percent have at least one de novo large copy number variant greater than 500 kb [30-32].
●Normal appearance – The normal appearance of the corpus callosum is a hypoechoic structure located between the cavum septi pellucidi inferiorly and the cingulate gyrus superiorly. The pericallosal artery is imaged superior to the corpus callosum when using color Doppler. The corpus callosum is composed of three main parts: the rostrum, body, and splenium .
●Timing – A normal corpus callosum (image 6 and image 7) can be seen sonographically by 18 to 20 weeks of gestation on a median section of the brain and should extend to the region of the quadrigeminal cistern [35-40]. The final shape of the corpus callosum is complete by 20 weeks, although it continues to grow during fetal life and the first two months after birth .
Prior to 18 weeks and during the latter part of the first trimester, the pericallosal arteries can be identified using color/power Doppler, which enables a more accurate evaluation of the arteries by eliminating most artifacts and selectively distinguishing between noise and valid flow signal (image 8). Color Doppler is used when there is a question of correctly identifying a normal pericallosal artery or its absence, and can be effectively used from 16 weeks onward. Visualizing the arteries is an indirect sign that a normal corpus callosum will develop [41-43]. In one study, the vascular pattern of the pericallosal artery was seen in 97 percent (144 out of 150) of fetuses at 11 to 14 weeks and two of the fetuses with abnormal arteries were confirmed later as having ACC .
●When to suspect a corpus callosum abnormality – During routine screening for fetal anomalies at 20 to 22 weeks of gestation, the two most important clues that the corpus callosum needs further assessment to exclude a callosal abnormality are:
•Nonvisualization of the cavum septi pellucidi and
•Ventriculomegaly (lateral ventricles measuring >10 mm).
In one study in which both ultrasound and MRI were used, abnormalities of the corpus callosum were detected in 13 percent of fetuses referred for evaluation of ventriculomegaly; callosal dysgenesis was isolated in only 24 percent of the fetuses, the rest had CNS, karyotypic, or another major abnormality .
●Sonographic criteria for diagnosis – Once the suspicion for a callosal abnormality is raised, the sonographic diagnosis can be made using direct and/or indirect sonographic findings.
-Direct sonographic features of complete ACC (image 9) are:
In a median section, complete absence of the corpus callosum and cavum septi pellucidi; and after 25 weeks, additional findings include absence of the cingulate gyrus and radial array of the sulci, which appear to radiate in a perpendicular fashion from the dilated third ventricle in a "sunburst" pattern (image 10 and image 11) [35,45,46].
On color/power Doppler, the normal pericallosal arteries cannot be identified.
-Additional indirect sonographic features can be seen on axial and coronal sections.
In the axial section, the frontal horns appear narrow and laterally displaced, and the atria and occipital horns are slightly dilated (colpocephaly); the shape is similar to a teardrop .
On coronal section, the falx cerebri can be seen in a broad interhemispheric fissure which meets the third ventricle; the lateral ventricles are widely separated and vertically oriented ("Viking's helmet" sign). The thalami may be widely separated due to a dilated third ventricle [46-48].
•Partial agenesis (hypogenesis) – The sonographic features of partial agenesis (hypogenesis) are more subtle, but the key feature is the shorter anterior-posterior length of the corpus callosum seen in the median section of the fetal brain. The splenium and rostrum as well as multiple segments of the corpus callosum can be absent or dysmorphic. Tables of biometry of the normal corpus callosum have been published [49-51].
Diagnosis of a partial agenesis is challenging since the cavum septi pellucidi is almost always present. The cavum septi pellucidi can be present and normal in appearance, dysmorphic, or unusually short and wide . In one study, abnormally shaped cavum septi pellucidi was seen in 27 percent of cases of partial agenesis (hypogenesis) of the corpus callosum .
•Thin corpus callosum (hypoplastic) – Sonographic diagnosis of a thin corpus callosum (hypoplastic) is made when the anterior-posterior length of the corpus callosum is normal, but the body appears thin. As noted above, biometric tables of the corpus callosum are available [49-51].
•Thick corpus callosum – The prenatal diagnosis of a thick corpus callosum is a rare finding, and its significance prenatally is uncertain. Among 59 fetuses with suspected callosal anomalies, 9 were diagnosed with isolated thick corpus callosum at 21 to 29 weeks . All 9 had a normal karyotype. Two patients opted for pregnancy termination, declining autopsy. In five of the seven remaining pregnancies, the corpus callosum normalized over time. In two fetuses the reason for the persistent thick corpus callosum was a variant of the cingulate gyrus. Even though isolated thick corpus callosum was not associated with poor prognosis in this small series, repeat follow-up is indicated.
●CNS anomalies – Associated brain anomalies are present on ultrasound and MRI in 21 to 93 percent of cases of ACC . Among the most commonly associated CNS anomalies are abnormal gyration, interhemispheric cysts, and posterior fossa abnormalities [55,56]. Other CNS anomalies include neural tube defects and lipoma.
●Non-CNS anomalies – Non-CNS anomalies have been reported in as many as 65 percent of the cases, of which craniofacial abnormalities (macrocephaly, hypertelorism, broad and depressed nasal bridge, cleft lip±palate) are the most common.
Other anomalies such as congenital heart defects, limb abnormalities, and growth restriction may be seen in syndromic cases .
●Chromosomal abnormality – Up to 17 percent of ageneses of the corpus callosum has been associated with a chromosomal abnormality, including copy number variants and pathogenic variants of single genes. Exome sequencing or specific gene panels for CNS anomalies should be offered when microarray is nondiagnostic.
Role of magnetic resonance imaging — Magnetic resonance imaging (MRI) is most helpful after the 20th week of gestation, since approximately 20 percent of apparently isolated cases diagnosed by ultrasound have associated CNS anomalies on MRI (image 12) . In addition to morphologic abnormalities, diffusion MRI provides an opportunity to noninvasively study white matter pathways using tractography by following the direction of preferred water diffusion. (See 'Evaluation of the fetal central nervous system' above.)
Outcome — Outcome of individuals with disorders of the corpus callosum is dependent on the presence or absence of associated anomalies and genetic syndromes. Identification of a cause and/or associated abnormalities can result in more appropriate counseling to the pregnant patient regarding the long-term outcome of the child. However, even with isolated agenesis of the corpus callosum, a systematic review found that the frequency, type, and degree of impairment (if any) were difficult to predict due to the large heterogeneity in outcomes measures, time at follow-up, and neurodevelopmental tools used in available studies .
Development of the corpus callosum postnatally appears to be affected by prematurity . Three-dimensional ultrasound derived biometric data relating to the corpus callosum in preterm infants has been correlated with neurodevelopmental outcome .
Counseling — Counseling patients with ACC is extremely difficult due to limitations of existing studies: study designs, selection bias, lack of control groups, and varying definitions and imaging protocols. Counseling should cover possible abnormalities of the corpus callosum, reliability of ultrasound and MRI imaging, associated anomalies and syndromes, prenatal testing, neurological outcome of children, and the limitations of the available studies .
CAVUM SEPTI PELLUCIDI
●Prenatal diagnosis – During fetal development, the space between the two laminae of the septi pellucidi is called the cavum septi pellucidi. It is a fluid-filled space less than 10 mm wide between the frontal horns of the lateral ventricles, and below the anterior portion of the corpus callosum (image 2); it is not part of the ventricular system and it does not communicate with it and it does not contain choroid plexus. Communication with the ventricular system can occur in cases of extreme ventriculomegaly or hydrocephaly and fenestration due to stretching and increased pressure gradient across its walls .
The cavum septi pellucidi should be seen from 18 to 37 weeks; however, before 18 weeks and after 37 weeks of gestation, nonvisualization can be a normal finding. Using transabdominal sonography, nonvisualization has been reported in 60 percent of fetuses at 15 weeks, 18 percent at 16 to 17 weeks, 0 percent at 18 to 37 weeks, and 21 percent at 38 to 41 weeks .
In the midtrimester, nonvisualization of the anechoic cavum septi pellucidi in an axial section of the brain does not always predict absence of the corpus callosum; in these situations, direct visualization of the corpus callosum and pericallosal arteries using the median plane of the brain should be the next step. On rare occasions, the cavum septi pellucidi is obliterated or echogenic, but the corpus callosum is normal [63,64].
●Associated abnormalities and outcome – Nonvisualization of the cavum septi pellucidi between 18 and 37 weeks may be an isolated abnormality (image 13); however, a thorough assessment of the fetal CNS is indicated, as it may be associated with midline malformations of the brain, such as septo-optic dysplasia (de Morsier syndrome), alobar or semilobar holoprosencephaly, schizencephaly, hydranencephaly, Apert syndrome, Chiari type II malformation, rhombencephalosynapsis, and agenesis of the corpus callosum [62,65-68].
In a systematic review of 78 cases of prenatally diagnosed isolated agenesis of the septum pellucidum, the diagnosis was confirmed postnatally in 86 percent of cases while 14 percent had additional or discordant findings . Septo-optic dysplasia occurred in 19 percent of these cases and was the most common new postnatal diagnosis. Prenatal visualization of apparently normal optic pathways halved the chances of a postnatal diagnosis of septo-optic dysplasia. A review of 111 prenatal MRI and 90 prenatal ultrasound reports of fetal absent cavum septi pellucidi noted both modalities had a high degree of accuracy (concordance with postnatal imaging) and that a range of clinical outcomes (including normal development) was possible, so long-term patient follow-up is necessary .
Enlarged (dilated) cavum septi pellucidi
●Prenatal diagnosis – Measurement of the width of cavum septi pellucidi can be performed using the transventricular plane after 18 weeks of gestation. The calipers are placed on the inner portion of its lateral borders (inner-to-inner measurement).
●Outcome – In a retrospective study of 48 euploid fetuses with dilated cavum septi pellucidi (measurements beyond two standard deviations of established ranges) as an isolated prenatal finding on ultrasound or magnetic resonance imaging, six were diagnosed with neurodevelopmental delay in childhood .
In a study of 406 singleton pregnancies, 267 with euploid fetuses, 81 with trisomy 21, 50 with trisomy 18, and 8 with trisomy 13, the mean cavum septi pellucidi width was 4.5 mm (range 1.8-7.4) in the euploid group . The mean width increased from 3.2 to 7.1 mm for biparietal diameter values of 40 to 100 mm, respectively. In fetuses with trisomy 21, 18, and 13, the mean cavum septi pellucidi width was 5.7 mm (range 2.8-10.5), 7.9 mm (range 3.5-12.8) and 5.8 mm (range 4.0-9.0), respectively, and the cavum septi pellucidi was dilated in 92, 40, and 41 percent of cases of trisomy 18, 13, and 21, respectively, suggesting that a large CSP should prompt evaluation of aneuploidy risk. Dilated cavum septi pellucidi has also been noted in children with a microdeletion 22q11 (del. 22q11) [72,73], central nervous system malformations, and normal variant . Thus, dilated cavum septi pellucidi may be an important sonographic marker for del. 22q11, along with conotruncal malformations and thymic hypoplasia. In addition, the length of the cavum septi pellucidi appears to increase in hypoplastic left heart syndrome and both the length and the width increase in dextro-transposition of the great arteries .
ENLARGED CAVUM VERGAE
●Prenatal diagnosis – The cavum vergae (or ventricle of Verga) is the fluid-filled space that continues posteriorly from the cavum septi pellucidi (image 7).
Neither of these structures are part of or connect with the ventricular system as they have a different embryological origin and lack a lining with ependymal or choroid plexus cells . The observed fluid is CSF that was filtered from the ventricles through the septal laminae and reabsorbed by capillaries and veins of the septa .
When the volume and thickness of the cavum vergae was measured in 336 pregnancies between 25 and 41 weeks of gestation using transabdominal ultrasound, 55 had closed cavum vergae and the rate of closure increased with advancing gestational age in the remainder . A borderline positive correlation was noted between biparietal diameter and cavum vergae volume, but no correlation was noted between biparietal diameter and cavum vergae thickness.
●Associated abnormalities – A dilated cavum vergae, when isolated, is considered a benign variant. Case reports have described prenatal diagnosis of dilated cavum vergae associated with other congenital abnormalities, aneuploidy, as well as normal outcome [78-80]. One group concluded that pregnancies with enlarged cava should have ultrasound follow-up, genetic counseling, noninvasive testing and/or an amniocentesis with microarray .
ENLARGED CAVUM VELI INTERPOSITI
●Prenatal diagnosis – The cavum veli interpositi (CVI) is a virtual space inferior to the hippocampus, anteroinferior to the splenium of the corpus callosum, and above the tela choroidea of the third ventricle. Anteriorly it extends to the interventricular foramina (of Monroe) and laterally it is bordered above by the columns of the fornix and below by the thalamus between the choroid membranes of the third ventricle below the splenium of the corpus callosum . This space (cavum) can be seen by ultrasound on a median plane [81,82]. These cysts are also referred to as extra axial cysts .
The most frequent pathology is an anechoic thin-walled cyst that is usually located below and anterior to the tail of the corpus callosum above the third ventricle. Since such cysts are usually small, they do not damage the important white matter, ganglia, or cortex of the brain and have a good outcome (image 14 and image 15) [84,85]. Because the location of the CVI is close to the interventricular foramina (Monroe), enlargement due to cysts may lead to ventriculomegaly [86,87].
CVI cysts may grow during the pregnancy; therefore, we suggest serial ultrasound examination. The aim of ultrasound scanning is to rule out associated anomalies and monitor cyst size as a possible compression by the cysts. However, growth to a large size resulting in ventriculomegaly is rare.
●Differential diagnosis – The main differential diagnosis of CVI cyst is an interhemispheric cyst, usually associated with agenesis or dysgenesis of the corpus callosum [81,86,88,89]. Other structures included in the differential diagnosis are dilated cavum septi pellucidi and cavum vergae . Aneurysm of the vein of Galen is easily differentiated from a CVI cyst by use of color or power Doppler to demonstrate blood flow. Use of axial and serial coronal planes and three-dimensional ultrasound (image 15) may help in the correct localization of pathology and differential diagnosis; magnetic resonance imaging can also be helpful [81,88].
In a meta-analysis of 47 fetuses with extra-axial intracranial cysts, the 23 fetuses with cavum veli interpositi cysts had associated CNS and extra-CNS anomalies in 31 and 6 percent of cases, respectively . No callosal, chromosomal, or associated anomalies were found in these cases. Regression occurred in 23 percent of cases and none had abnormal motor outcome or intelligence.
POSTERIOR FOSSA ABNORMALITIES — The most common pathologies involving the posterior fossa are sonographically suspected or detected by the presence of increased fluid in the cisterna magna (cerebello-peduncular cistern). The size of the cisterna magna during the second trimester is stable with a mean size of 5±3 mm and upper limit of normal of 10 mm .
The four frequent posterior fossa pathologies from the most common to the least common are:
●Mega cisterna magna
●Blake's pouch cyst
Dandy-Walker malformation — Dandy-Walker malformation (DWM) refers to a complex developmental anomaly of the cerebellar vermis in which there is failure of the normal closure of the fourth ventricle with persistence of Blake's pouch that occurs in the 13th to 18th week of gestation, thus explaining some of the concurrent anomalies . The incidence is 1 in 30,000 births .
●Prenatal diagnosis – Prenatal diagnosis and prediction of prognosis can be challenging in mild cases . Sonographic findings include all of the following:
•Cystic dilatation of the fourth ventricle
•Persistent Blake's pouch cyst
•Enlarged posterior fossa with the upward displacement of the tentorium, transverse sinus, and torcular
•Complete or partial agenesis of the vermis
The diagnosis is more easily established when there is complete agenesis of the cerebellar vermis with upward rotation (an elevated tentorium and torcular) and dilation of the third and lateral ventricles (image 16) [94-99].
Diagnosis of cerebellar vermian rotation and elevation of the tentorium and torcular can be facilitated by obtaining a median plane of the brain. Measurement of two angles in the median plane (the brainstem-vermis [BV] and the brainstem-tentorium [BT]) has been used in differential diagnosis of conditions with increased cisterna magna and upward rotation of the vermis (image 17). The BV angle is obtained by drawing a line tangentially to the dorsal aspect of the brain stem and a second line tangentially to the ventral contour of the vermis. The BT angle is obtained by adding a third line tangentially to the tentorium. The BV angle appears to be a useful tool to differentiate between the different pathologies affecting the posterior fossa. It increases with increasing severity of the condition: angles less than 18 degrees are normal, angles between 18 and 30 degrees are suggestive of persistent Blake's pouch, and angles greater than 45 degrees strongly suggest DWM .
The cerebellar hemispheres are often abnormal (eg, compressed) as a result of being separated by the Dandy-Walker cyst. The corpus callosum may be absent.
●Differentiation between DWM and an arachnoid cyst of the posterior fossa depends upon demonstration of a hypoplastic vermis and connection of the cyst with the fourth ventricle in DWM. (See 'Arachnoid cysts' below.)
Knowledge of the developmental changes in the cerebellum and cerebellar vermis are important to avoid a premature, erroneous diagnosis of DWM. The vermis completes its formation from cranial to caudal; the fourth ventricle communicates through a wide opening with the cerebello-medullary cistern (ie, cisterna magna) typically at 13 to 14 weeks of gestation but sometimes as late as the 16th week of gestation. Therefore, the final anatomy of the vermis may not be completed until the 16th to 20th week of gestation. Sonographically, the vermis almost completely closes the gap in the cerebello-medullary cistern (cisterna magna) as it forms, leaving only a narrow passage open (ie, the foramen Magendie or median aperture) . In the first half of pregnancy, this relatively wide opening should not be mistaken for vermian dysgenesis or a variant of DWM . Reevaluation at 20 to 22 weeks of gestation is crucial to confirm presence or absence of a normal vermis.
●Associated abnormalities – In a systematic review of isolated prenatal posterior fossa malformations, associated CNS and extra-CNS structural anomalies were later found to be present in 60.9 and 42.6 percent of cases, respectively . Ventriculomegaly was a common finding later in pregnancy, prenatally diagnosed in 31.3 percent of cases. The prevalence of chromosomal abnormalities in isolated DWM was 16.3 percent, with chromosomal deletions representing the most common anomaly (7.6 percent). Some cases of DWM have been attributed to heterozygous loss of ZIC1 and ZIC4 genes in individuals with a 3q2 deletion .
In the syndromic form of DWM, malformations of the heart, face, limbs, or gastrointestinal or genitourinary system may be present. DWM may occur as part of a Mendelian disorder (eg, Meckel syndrome), a chromosomal aneuploidy (eg, 45X, triploidy), environmental exposures (eg, rubella, alcohol), a multifactorial etiology (eg, congenital heart defect, neural tube defects), or as a sporadic defect (eg, holoprosencephaly) . In a review of 187 individuals of various ages with Dandy-Walker syndrome from 168 case reports, one-third had a chromosomal abnormality or syndrome (n = 8 PHACE), 27 percent had a cardiovascular condition (n = 7 patent ductus arteriosus), 24 percent had a disease of eye and ear (n = 9 cataract), the most common malignancy was nephroblastoma (n = 8, all in patients of Asian descent), almost one-fifth had some type of mental illness, and 6.4 percent had mild or severe intellectual disability . (See "Hydrocephalus in children: Physiology, pathogenesis, and etiology", section on 'CNS malformations'.)
Mega cisterna magna
●Prenatal diagnosis – Mega cisterna magna (MCM) refers to a measurement >10 mm from the posterior portion of the vermis to the inside edge of the skull (on an oblique transverse plane) in the presence of normal-appearing cerebellar hemispheres and vermis. The cerebellum must be of normal size (compare with nomograms) since a small cerebellum with a normal for age cisterna magna can mimic mega cisterna magna.
●Associated abnormalities and outcome – MCM has been associated with chromosomal anomalies, as well as CNS and non-CNS malformations . In the systematic review of isolated prenatal posterior fossa malformations, the rates of additional CNS and extra-CNS anomalies were 12.6 and 16.6 percent, respectively . Ventriculomegaly was the most common associated anomaly, seen in 11.7 percent. No fetus tested prenatally had a chromosomal abnormality.
When isolated, MCM has a favorable outcome almost all cases [107,108].
Persistent Blake's pouch cyst
●Prenatal diagnosis – Blake's pouch is a normal embryological cystic structure of the fourth ventricle that bulges into the cisterna magna; its neck or metapore usually fenestrates in the first trimester and gives rise to the Foramen Magendie. Failure of this normal fenestration to occur results in a persistent Blake's pouch. However, in more than 50 percent of cases, fenestration subsequently occurs by 24 to 26 weeks, with resolution of the findings.
There are three proposed criteria for the diagnosis of persistent Blake's pouch :
•Normal anatomy and size of the vermis
•Mild/moderate rotation of the vermis in the median plane, and
•Normal size of the cisterna magna
When scanning pregnancies <20 weeks of gestation in a semi-coronal plane, a gap between the cerebellar hemispheres may be evident; this Blake's pouch metapore of the posterior fossa is obtained at the connection between the fourth ventricle and cisterna magna.
●Associated abnormalities – As reported in a systematic review of isolated prenatal posterior fossa malformations, the rates of associated CNS and extra-CNS structural anomalies were 11.5 and 25.3 percent, respectively . The likelihood of aneuploidy has been reported to be 5 percent in the absence of associated anomalies. However, trisomy 21 has been reported in cases with and without other anomalies .
Vermian hypoplasia — This term is slowly replacing the term "Dandy-Walker variant."
●Prenatal diagnosis – In vermian hypoplasia, the vermis is normally formed but of smaller size, and the posterior fossa is otherwise of normal size and anatomy.
It is important to concentrate on the normal, pointed shape of the fastigium when assessing the fourth ventricle in the median plane, although sonographic evaluation of the 4th ventricle is not a mandatory part of the basic fetal scan. Prenatal diagnosis of vermian hypoplasia is difficult, with a false-positive rate of 32.4 percent.
●Associated abnormalities – In a systematic review of isolated prenatal posterior fossa malformations, the rates of associated CNS and extra-CNS anomalies in fetuses with vermian hypoplasia were 56.1 and 49.2 percent, respectively . A chromosomal abnormality (deletion) was seen in one fetus among the 30 tested.
CHOROID PLEXUS CYSTS
●Prenatal diagnosis – The choroid plexuses start developing at approximately six to seven weeks of gestation, grow rapidly, and fill approximately 75 percent of the cavity of the lateral ventricles by 9 weeks of gestation. The adult appearance is reached by 20 weeks of gestation [111,112].
Choroid plexus cysts appear to result from filling of the neuroepithelial folds with cerebrospinal fluid . The typical sonographic appearance is a small (usually less than 1 cm), anechoic structure(s) with well delineated borders located within the choroid plexus (image 18). A wide range of appearances are possible, from unilateral single cysts to bilateral septated and multiple cysts (image 19) [111,114-119].
The choroid plexus can be heterogenous with a few small cystic areas. These will not have well-delineated borders and will be best seen on a single plane. Therefore, since the finding of choroid plexus cyst greatly increases patient anxiety, in addition to the routine anatomic survey, we assess the hands, feet, heart, and brain for additional findings. If none are present, we might describe choroid plexus heterogeneity without a true choroid plexus cyst. Alternatively, if the cyst is at least 2 cm and is seen in two orthogonal planes, it is a real finding and indicates that a formal fetal high-risk anatomic survey be performed.
●Clinical significance – Choroid plexus cysts are common sonographic findings during the second trimester. In the absence of other CNS or extra-CNS anomalies and risk factors for chromosomal aneuploidy, isolated choroid plexus cysts are considered to be a variant of normal regardless of shape, size, or laterality. They usually disappear by the third trimester; those that persist are usually asymptomatic, and thus benign.
Isolated choroid plexus cysts are present in 1 to 2 percent of the normal population and not associated with an increased risk of aneuploidy. Fetuses with additional anomalies (non-isolated choroid plexus cysts) are at increased risk of chromosomal abnormalities, especially trisomy 18. Thus, a thorough fetal anatomic survey is important when these cysts are detected to counsel the patient about the need for further evaluation, either noninvasively with cell-free DNA or diagnostically with an amniocentesis. (See "Sonographic findings associated with fetal aneuploidy", section on 'Choroid plexus cysts'.)
●Prenatal diagnosis – Arachnoid cysts are collections of cerebrospinal fluid within the layers of the arachnoid membrane; the cyst may or may not communicate with the subarachnoid space. Primary cysts arise from an abnormal developmental process of the leptomeningeal formation, while secondary or acquired arachnoid cysts result from cerebral spinal fluid entrapment within arachnoid adhesions [120-123]. The cysts are typically located on the surface of the brain, usually close to the cerebral fissures in the region of the sella turcica within the anterior, middle, and posterior fossa. Cysts located in proximity of the splenium of the corpus callosum are commonly referred to as cavum veli interpositi cysts .
The sonographic appearance is an anechoic cystic mass with thin smooth walls lying adjacent to the cerebral hemispheres, cerebellum, or brainstem, which may be compressed or displaced by the cyst (image 20) . Arachnoid cysts do not communicate with the lateral ventricles [120,121]. The earliest diagnosis of arachnoid cysts is in the early second trimester [125,126], but most are diagnosed relatively late in pregnancy.
Magnetic resonance imaging (MRI) is most helpful after the 20th week of gestation. Fetal MRI can be helpful to demonstrate the extra-axial nature of an arachnoid cyst and to determine whether the underlying cortex appears normal.
●Associated abnormalities – The role of ultrasound is to rule out associated anomalies and to monitor cyst size and for signs of compression, such as ventriculomegaly and macrocrania. The mass effect of an arachnoid cyst can decrease over time if the cyst remains stable in size and the remainder of the brain grows; however, cysts frequently increase in size during gestation.
Arachnoid cysts may occur as an isolated lesion or associated with other brain malformations, such as agenesis of the corpus callosum, absent cavum septi pellucidi, deficient cerebellar lobulation, and Arnold-Chiari type I malformation [127-129]. Many of these associated findings do not become sonographically apparent until the late second trimester. A large cyst can cause hydrocephaly from a mass effect, which obstructs flow of the cerebrospinal fluid.
A meta-analysis of 47 fetuses with extra-axial intracranial cysts included 24 fetuses with arachnoid cysts; CNS associated anomalies were noted in 73 percent and extra-CNS anomalies in 14 percent . The most common abnormalities were ventriculomegaly and callosal abnormalities. Chromosomal abnormalities were present in 6 percent, but fetuses with isolated cysts were always euploid.
ANEURYSM OF THE VEIN OF GALEN
●Prenatal diagnosis – The vein of Galen is a midline structure that, together with the inferior sagittal sinus, drains blood to form the straight sinus . The vein of Galen "aneurysm" is not a true aneurysm; it is a rare vascular malformation consisting of one or more arteriovenous shunts from arterial feeders from the carotid and vertebrobasilar systems in the midbrain to the vein of Galen. High flow of blood into the vein causes it to dilate. The high rate of blood flow is also transmitted to the heart, which may lead to high output cardiac failure. The estimated incidence is 1:10,000 to 1:25,000 births and it accounts for approximately 1 percent of all congenital anomalies .
The sonographic appearance of the dilated vein of Galen in the median plane is a large, well-defined, irregular, supratentorial, nonpulsatile structure running from the splenium of the corpus callosum above the cerebellum to the bony cranium. In the coronal plane, the dilated vein appears as a round, cystic, centrally located structure. The structure has better echogenicity on color Doppler imaging, with a wavy venous waveform due to turbulent venous flow (image 21) [132-134].
●Associated abnormalities and outcome – Associated abnormalities are due to the high flow state. These include signs of high output heart failure, such as cardiac dilatation, nonimmune hydrops fetalis, and polyhydramnios. The enlarged vein may cause obstruction of the ventricular system (aqueduct of Silvius) leading to hydrocephaly. Intracranial hemorrhage has also been seen. This venous pathology can also present mimicking intracranial hemorrhage, however, applying color Doppler can help to clarify the diagnosis . Three-dimensional color Doppler techniques can offer additional diagnostic information .
The prognosis for the fetus/neonate depends upon the severity and time of presentation of the cardiovascular symptomatology. If identified before the fetus becomes symptomatic, and if treated rapidly in a setting with experience with newborn brain lesions, the outlook can be quite good. If in-utero signs of progressive cardiac dysfunction are present, the prognosis is poor [137-140] and may indicate a high flow lesion that may not respond to therapy .
●Prenatal diagnosis – Schizencephaly is a rare disorder of neuronal migration in which one or more fluid-filled clefts in the cerebral hemisphere communicate with the lateral ventricle (image 22). It can be unilateral or bilateral and frequently is seen in association with microcephaly and other brain anomalies (image 23) [15,16,142-146]. Two types of schizencephaly have been described :
•Type 1 has small symmetrical clefts and the edges of the clefts are fused within a pia-ependymal seam that is continuous with the ependyma of the lateral ventricle.
•Type 2 has extensive clefts that extend from the ventricle to the surface of the brain and subarachnoid space and the edges are not fused [15,16,142-145].
In an analysis of 32 fetuses with a broad spectrum of anomalies using the transventricular plane to evaluate the "anterior complex" (interhemispheric fissure, the callosal sulcus, the genu of the corpus callosum, the cavum septi pellucidi and the anterior horns of the lateral ventricles) and "posterior complex" (splenium of the corpus callosum, the medial wall of the lateral ventricles, the CS and the parieto-occipital fissure), a normal appearance of the anterior and posterior complexes appeared to be a strong indicator of a normal fetal CNS, whereas morphological abnormalities in both complexes were robust markers of midline defects . Two cases of schizencephaly in this series had an atypical shape of the cavum septi pellucidi and dysmorphic/fused anterior horns. Since schizencephaly is frequently associated with other anomalies such as heterotopic gray matter, evaluating the brain using the transventricular plane may lead to its diagnosis.
●Associated abnormalities and outcome – The degree of impairment depends on the location of the cleft, whether it is unilateral or bilateral, whether it is type 1 or 2, and whether there are associated malformations. The causes of schizencephaly are heterogeneous; both acquired (eg, infection such as CMV, teratogens) and genetic etiologies have been implicated, but it is rare for more than one family member to be affected. Genetic etiologies require consideration of the specific testing platform to utilize; consultation with a genetics expert is recommended.
●Prenatal diagnosis – Congenital porencephalic cysts appear as a fluid-filled cavity in the cerebral hemisphere. They can involve the infratentorial or supratentorial space or both  and more than one cyst may be present. In the third trimester, the cysts are thought to result from ischemic parenchymal vascular damage followed by necrosis and subsequent liquefaction. In the second trimester, vascular injury leads mainly to polymicrogyria with or without a parenchymal cleft from the subarachnoid space to the lateral ventricle (schizencephaly). Some cases of porencephaly and schizencephaly have been associated with COL4A mutation  and some believe that porencephaly cysts and schizencephaly are on the same spectrum.
Porencephalic cysts are lined with white matter, in contrast to schizencephaly, where the cyst is lined with heterotopic gray matter. They are intra-axial, in contrast to arachnoid cysts, which are extra-axial (ie, not arising from the brain).
●Outcome – Prognosis depends on the size and location of the cyst, and the presence of other abnormalities including genetic syndromes.
●Prenatal diagnosis – Hydranencephaly is a congenital brain defect in which fluid-filled cavities replace the cerebral hemispheres. It appears to be the result of either massive brain infarction from bilateral carotid artery occlusion or from primary agenesis of the neural wall [124,150]. The cerebellum, midbrain, thalami, and basal ganglia are usually preserved.
The cerebral hemispheres are initially echogenic in hemorrhagic areas, becoming cystic as the brain becomes necrotic. The amount of cerebral cortex decreases over time and is replaced with fluid and debris. Eventually no cerebral cortex is visible. Sonographic findings may include macrocephaly and a large fluid filled intracranial cavity with variable amount of echogenicity, representing liquified brain and blood. One group described the appearance of the intracranial contents as uniform low-level echogenicity similar to endometrioma fluid .
As discussed above, both alobar holoprosencephaly and hydranencephaly are characterized by absence of the midline echo; however, in hydranencephaly, some midline structures (falx cerebri, interhemispheric fissure, third ventricle) are present (image 24), whereas they are absent in holoprosencephaly (image 4). The cerebral cortex is absent in hydranencephaly but displaced in holoprosencephaly. The face is usually normal in hydranencephaly, while midline facial dysmorphism is common in holoprosencephaly. (See 'Holoprosencephaly' above.)
●Associated abnormalities – If hydranencephaly occurs early, it may result in microcephaly. No cerebral cortex is present anteriorly, but there may be partial preservation of portions of the occipital lobe, and the midbrain and the basal ganglia are variably preserved. Polyhydramnios is usually present by the end of the second trimester since the brain abnormality leads to poor fetal swallowing. However, fetal movement, including breathing and sucking motions, is preserved.
MALFORMATIONS OF CORTICAL DEVELOPMENT
Overview — The four main malformations of cortical development are:
●Periventricular nodular heterotopia
These disorders result from disturbances in migration of the neuroblast during formation of the cerebral cortex. Depending upon the timing of the insult, the cortex will completely lack gyri and sulci (lissencephaly), may have a few coarse gyri (pachygyria), may have multiple small gyri (microgyria or polymicrogyria), or have fragments of gray matter present in an abnormal location of the brain (heterotopias) [13,152]. Abnormal cell proliferation results in microcephaly or megalencephaly, incomplete neuronal migration results in heterotopia and lissencephaly, neuronal over-migration of neural cells results in cobblestone malformations, and anomalous postmigrational cortical organization is responsible for polymicrogyria and focal cortical dysplasias.
These malformations may be caused by genetic, infectious, or vascular etiologies.
Although rarely diagnosed prenatally, dedicated multiplanar neurosonography and magnetic resonance imaging (MRI) can demonstrate signs of malformed fetal cortical development such as delayed, premature or absent cerebral sulcation, thin and irregular hemispheric parenchyma; wide overdeveloped gyri; wide sulci; nodular bulging into the lateral ventricles; cortical clefts; intraparenchymal echogenic nodules; and cortical thickening.
The ultrasound approach to evaluating the fetal brain is based on the classic transabdominal visualization of three different axial planes: the transventricular, the transthalamic, and the transcerebellar; however, they are not sufficient for optimal depiction of malformations of the cortex and midline brain structures. They must be supplemented by a more comprehensive, multiplanar approach in which coronal and sagittal planes are added to the axial planes by a transvaginal or transabdominal scans.
MRI provides excellent contrast resolution between white matter and gray matter and is sometimes needed to characterize the location and development of gyri and sulci and evaluate the formation of white matter, including myelination. In late gestation, the diagnosis of malformations of cortical development by MRI is more accurate than by ultrasound, due to shadowing by the skull bones and lack of an acoustic window. The MRI signs suggestive of MCD are similar to ultrasound signs and include delayed cortical development; dysgenesis of the Sylvian fissure; delayed sulcal appearance; cortical thickening; irregularity of the ventricular wall; absence or abnormal appearance of fissures; abnormal, asymmetric gyri; and discontinuous cortex .
Delayed cortical development was noted in 18 fetuses with different, complex congenital heart diseases . Therefore, thorough evaluation of the fetal brain should be considered in cases of antepartum detection of fetal congenital heart disease.
●Prenatal diagnosis – Lissencephaly or agyria (smooth brain) is a rare anomaly characterized by incomplete or failure of neuronal migration during the 12th to 24th weeks of gestation resulting in a lack of development of gyri and sulci. Microcephaly, ventriculomegaly, wide Sylvian fissures, complete or partial agenesis of the corpus callosum, and a minimal operculum of the insula are associated defects [13,155,156].
There are two types of lissencephaly: type 1 is associated with facial dysmorphism, sometimes with deletion of chromosome 17p (Miller-Dieker syndrome), and type 2 is associated with hydrocephaly and dysgenesis of the cerebellum (eg, Walker-Warburg syndrome) [157,158].
Transvaginal ultrasound allows imaging of the surface of the cerebral hemispheres and should facilitate in-utero diagnosis. Prenatal diagnosis of lissencephaly probably cannot be made reliably until 26 to 28 weeks of gestation, when the normal gyri and sulci become well defined. Up to this time, the normal fetal brain has a smooth appearance [159,160].
At term the lissencephalic brain has a smooth surface on coronal and oblique sections. The interhemispheric fissure is a relatively straight midline-echo and the subarachnoid space and the lateral sulcus (Sylvian grooves) are wide, similar to the appearance of the fetal brain before 20 weeks of gestation. Ventricular dilatation is apparent in both coronal and oblique sections . Microcephaly is often present.
TUMORS — Teratomas are the most common fetal intracranial tumor, followed by astrocytoma, craniopharyngioma, primitive neuroectodermal tumor, choroid plexus papilloma, meningeal tumors, and ependymoma [161-163]. The most common presentation is macrocephaly and an intracranial mass detected on fetal ultrasound examination; hydrocephalus is less common. Most tumors are detected during the third trimester.
SUMMARY AND RECOMMENDATIONS — A wide variety of midline fetal central nervous system (CNS) anomalies can be detected by prenatal ultrasound examination.
●Role of ultrasound – Ultrasound should always be the first imaging study to evaluate the fetal brain. (See 'Evaluation of the fetal central nervous system' above.)
•Structures usually evaluated on a basic ultrasound examination of the fetal CNS include: head size and shape, lateral ventricles, choroid plexus, cavum septi pellucidi, thalami, cerebellum, cisternal magna, and spine. Ideally, patients at increased risk of CNS anomalies, including those in whom the basic examination identified suspicious findings, should undergo targeted /extended fetal neurosonography performed by clinicians with specific expertise in this area. (See 'Evaluation of the fetal central nervous system' above.)
●Role of MRI – Fetal magnetic resonance imaging (MRI) can be helpful when a CNS malformation is incompletely described on ultrasound or to look for associated abnormalities, which affect prognosis. MRI is most helpful after the 20th week of gestation. Similarly to a targeted/extended fetal neurosonogram, MRI should also be performed by specialists in fetal MRI or at least interpreted by those who are experts in Neuro MRI. (See 'Evaluation of the fetal central nervous system' above.)
•When a posterior fossa cyst is detected, absence or hypoplasia of the cerebellar vermis is a key finding for distinguishing between a Dandy-Walker malformation and an arachnoid cyst. (See 'Dandy-Walker malformation' above and 'Arachnoid cysts' above.)
•During routine screening for fetal anomalies at 20 to 22 weeks of gestation, the two most important clues that the corpus callosum needs further assessment to exclude a callosal abnormality are (1) nonvisualization of the cavum septi pellucidi and (2) ventriculomegaly (lateral ventricles measuring >10 mm). (See 'Disorders of the corpus callosum' above.)
•A vein of Galen aneurysm should be suspected if ultrasound examination reveals a cystic midline structure, which fills with turbulent venous flow on color Doppler imaging. (See 'Aneurysm of the vein of Galen' above.)
•Both alobar holoprosencephaly and hydranencephaly are characterized by large intracranial fluid collections; however, in hydranencephaly, some midline structures (falx cerebri, interhemispheric fissure) are present, whereas they are absent in holoprosencephaly. The face is usually normal in hydranencephaly, while midline facial dysmorphism is common in holoprosencephaly. (See 'Alobar holoprosencephaly' above and 'Hydranencephaly' above.)
•Absence of the cavum septi pellucidi between 18 and 37 weeks should be noted, and a detailed fetal brain assessment performed, along with genetic counseling. (See 'Nonvisualization' above.)
•Genetic counseling is advised for most patients with fetal CNS abnormalities, especially when associated with other CNS or non-CNS anomalies. If microarray is performed and is normal, exome sequencing or a targeted panel can be informative. Isolated choroid plexus cysts is an exception as they are present in 1 to 2 percent of the normal population and not associated with an increased risk of aneuploidy. (See 'Choroid plexus cysts' above.)
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