The Fetal Central Nervous System



The Fetal Central Nervous System





INTRODUCTION

The evaluation of the fetal central nervous system (CNS) is an important aspect of ultrasound in the first trimester given that several major malformations, such as anencephaly/exencephaly, holoprosencephaly (HPE), among others can be easily identified. However, detection of subtle malformations in the first trimester, such as small encephaloceles, neural tube defects, or posterior fossa abnormalities, requires a detailed ultrasound evaluation of the CNS. In this chapter we present a systematic detailed approach to the first trimester ultrasound examination of the normal CNS, followed by a comprehensive presentation of common CNS malformations that can be diagnosed in early gestation.






Figure 8.1: Three-dimensional ultrasound in multiplanar display of an embryo at 7 weeks 5 days gestation showing early development of the brain. Note the size of the rhombencephalic vesicle (Rb) in the posterior aspect of the brain as the largest brain vesicle at this stage of development. The lateral ventricles (Lat. V) and the third ventricle (3rd V) are also seen. F, falx cerebri.


EMBRYOLOGY

Formation of the fetal brain is seen as early as the fifth week of embryogenesis by outgrowth of the neural tube in its cephalic region to form three brain vesicles: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). By the sixth week of embryogenesis, the prosencephalon differentiates into the telencephalon and diencephalon, the mesencephalon remains unchanged, and the rhombencephalon divides into the metencephalon and myelencephalon. Ultrasound images of the fetal brain at 7 to 8 weeks of gestation (menstrual age) demonstrate these brain vesicles (Figs. 8.1 and 8.2). The falx cerebri, an echogenic structure that divides the brain
into two equal halves, and the choroid plexuses, which fill the lateral ventricles, are seen on ultrasound by the end of the eighth week and beginning of the ninth week of gestation (Fig. 8.3). The cerebellar hemispheres develop in the rhombencephalon and are completely formed by the 10th week of gestation, thus allowing for evaluation of the posterior fossa with optimal ultrasound imaging (Fig. 8.4). Prior to the 9th week of gestation, the cranium is not typically ossified (Fig. 8.5). Cranial ossification begins around the late 9th, early 10th week and is completed by the 12th week of gestation. Figure 8.5 shows progression in fetal cranial ossification from 9 to 13 weeks of gestation.






Figure 8.2: (A, B and C) Three-dimensional ultrasound volume of a fetus at 8 weeks of gestation showing early development of the brain. A: A sagittal view of the fetus in surface mode. B: A coronal plane of the fetal head retrieved from the multiplanar display. C: A rendered image of the fetus in Silhouette® mode. Note the anatomic relationships of the lateral (Lat. V), the third (3rd V), and the rhombencephalic (Rb) ventricles.






Figure 8.3: Axial planes of the fetal head in a fetus at 9 (A) and 10 (B) weeks of gestation. Note in A and B, the appearance of the choroid plexuses (CP) of the lateral ventricles and the falx cerebri (Falx), which are visible from 9 weeks onward. The third ventricle (3rd V) and the aqueduct of Sylvius (AS) are also recognized in these axial planes.







Figure 8.4: The posterior fossa, demonstrating the development of the rhombencephalic vesicle (Rb) into the fourth ventricle (4th V). The sagittal (A) and axial (B) planes of the fetal head at 8 weeks of gestation. Note the appearance of the Rb in the posterior aspect of the brain. The sagittal (C) and axial (D) planes of the fetal head at 10 weeks of gestation. Note the development of the 4th V at this gestational age. The choroid plexus (CP) of the 4th V is also visible in C.






Figure 8.5: Axial planes of the fetal head at 9 (A), 10 (B), and 13 (C) weeks of gestation demonstrating the progression of skull ossification. Note at 9 weeks of gestation (A), the presence of small islands of ossification (arrows). At 10 weeks of gestation (B), partial ossification of the frontal (F), parietal (P) and occipital (O) bones is seen. At 13 weeks of gestation (C), the frontal (F), parietal (P) and occipital (O) bones are clearly seen. The occipital bone (O) is better imaged in a more posterior plane at the level of basal ganglia (see Figs. 8.6 and 8.10).



NORMAL SONOGRAPHIC ANATOMY

Ultrasound evaluation of the fetal intracranial anatomy is commonly performed in the axial (transverse) and midsagittal planes of the fetal head (Figs. 8.6, 8.7, 8.8, 8.9, 8.10, 8.11 and 8.12). Axial (Figs. 8.6, 8.7, 8.8, 8.9, 8.10 and 8.11) and midsagittal (Fig. 8.12) planes of the fetal head are commonly obtained in the first trimester for the measurement of the biparietal diameter (BPD) and nuchal translucency (NT), respectively. Furthermore, the midsagittal plane is also obtained for the evaluation of the fetal facial profile and nasal bones. The axial and midsagittal planes of the fetal head are also part of the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) practice guidelines for the performance of the first trimester ultrasound examination.1 The coronal planes of the fetal head are also occasionally helpful in the visualization of midline structures when certain malformations are suspected. The authors recommend routine evaluation of the axial and midsagittal planes of the fetal head when ultrasound examinations are performed beyond the 12 weeks of gestation. Please refer to Chapter 5 for the systematic approach to the detailed ultrasound examination of the fetus in the first trimester, with comprehensive presentation of standardized planes for the evaluation of the CNS in early gestation.






Figure 8.6: Three-dimensional (3D) volume of a fetal head at 12 weeks of gestation. The 3D volume was obtained from an axial plane and is displayed in tomographic view. From superiorly (top) to inferiorly (bottom) you can see the choroid plexuses (CP), the falx cerebri (Falx), the third ventricle (3rd V), the aqueduct of Sylvius (AS), the cerebral peduncles (Cer. Ped.), and the fourth ventricle (4th V). The lateral ventricles (Lat. V) are seen in the mid-axial plane. Note the appearance of the frontal (F), parietal (P), and occipital (O) bones.


Axial Planes

The systematic detailed examination of the fetal brain in the first trimester includes the acquisition of three axial planes, similar to the approach performed in the second trimester ultrasound examination (see Figs. 5.7 and 5.8). In these planes, the fetal head shape is oval and the skull can be identified from 10 weeks onward (Fig. 8.5). At this early gestation, bone ossification primarily involves the frontal, parietal, and occipital parts of the cranial bones (Figs. 8.5 and 8.6). The use of high frequency linear or transvaginal transducers improves imaging of the fetal CNS in early gestation (Fig. 8.7). The intracranial anatomy is divided by the falx cerebri into a right and left side of equal size (Figs. 8.6, 8.7 and 8.8). The choroid plexus on each side is hyperechoic, typically fills the lateral ventricles, and is surrounded by cerebral spinal fluid (Figs. 8.7, 8.8 and 8.9). In the first trimester, the shape of
the choroid plexus is described to be similar to a butterfly (Fig. 8.8).2 The left and right choroid plexuses are rarely of similar size and shape, and this difference is considered part of the normal variation (Fig. 8.9).3 A small rim of the developing cortex can be seen laterally surrounding the choroid plexuses (Fig. 8.7). In an axial superior plane of the fetal head, a fluid rim can be seen surrounding the choroid plexus on each side, corresponding to the lateral ventricle (Fig. 8.10A). A more inferior axial plane toward the base of the skull shows the two thalami and the third ventricle, forming the diencephalon

(Fig. 8.10B). Posterior to the thalami, the two small cerebral peduncles are identified surrounding the aqueduct of Sylvius and forming the mesencephalon (midbrain) (Fig. 8.10B). The developing cerebellum is identified in the posterior fossa primarily by transvaginal ultrasound and in an axial plane that is tilted toward the upper spine (Fig. 8.11A). A slightly more inferior plane will show the fourth ventricle, the future cisterna magna, and the hyperechogenic choroid plexus of the fourth ventricle (Fig. 8.11B). Table 8.1 gives an overview of the anatomic landmarks and corresponding malformations that can be visualized in the axial planes of the fetal head in the first trimester.






Figure 8.7: Axial view of the fetal head at 12 weeks of gestation from a lateral approach, obtained with three different high-resolution transducers: transabdominal curved array (A), transabdominal linear (B), and transvaginal (C). Note the improved resolution in the transabdominal linear (B) and transvaginal (C) transducers. The falx cerebri (Falx) is seen along with the choroid plexuses (CP) and cerebrospinal fluid (asterisks) in the lateral ventricles (Lat. V). The rim of the developing cortex is also seen (arrows). Note that at this stage of development, the ventricular system occupies the majority of the brain.






Figure 8.8: Axial view of the fetal head at the level of the choroid plexuses (CP) in a fetus at 11 weeks (A) and at 13 weeks (B) of gestation. Note that the right and left CP resemble the shape of a butterfly and are referred to as the “butterfly sign.” The falx cerebri (Falx) is seen in the midline. Compare with Figures 8.24 and 8.25 obtained from fetuses with alobar holoprosencephaly.






Figure 8.9: Axial views of the fetal head obtained in early gestation at the level of the choroid plexuses in three normal fetuses (A-C) and in a fetus with trisomy 13 (D). Note in A to C the presence of choroid plexus asymmetry (double arrows), considered as a normal variant. Fetuses A to C had a normal subsequent second trimester ultrasound. Note the presence of choroid plexus cysts (CPC) and nuchal edema (asterisk) in the fetus with trisomy 13 (D).






Figure 8.10: Axial views of the fetal head at 13 weeks of gestation obtained superiorly at the level of the lateral ventricles (A) and inferiorly at the level of the thalami (B). In A, the two lateral ventricles (Lat. V), the choroid plexuses (CP), and the falx cerebri (Falx) are seen. In B, the thalami (Thal.) with the third ventricle (3rd V) are recognized and constitute the diencephalon. Posterior to the thalami, the cerebral peduncles (Cer. Ped.) with the aqueduct of Sylvius (AS) can be visualized and form the mesencephalon (see Fig. 8.11).






Figure 8.11: Axial view of the fetal head at 13 weeks of gestation obtained at the level of the posterior fossa (inferior to planes A and B in Fig. 8.10). A: At the level of the developing cerebellum (Cer.) and cerebral peduncles (Cer. Ped.). Note the thalami (Thal.) in a more anterior location. B: An oblique, slightly more inferior plane demonstrating the open fourth ventricle (4th V) connecting to the future cisterna magna (CM). The echogenic choroid plexus (CP) of the fourth ventricle can also be seen.






Figure 8.12: Schematic drawing (A) and corresponding ultrasound image (B) of the midsagittal plane of the fetal head in the first trimester (same as nuchal translucency [NT] plane). This plane enables a good assessment of the posterior fossa. The following structures can be seen: thalamus (T), midbrain (M), brainstem (BS), the fourth ventricle presenting as an intracranial translucency between the BS and the choroid plexus (CP), and the cisterna magna (CM). 1, 2, and 3 point to the nasal bone, maxilla, and mandible, respectively.









Table 8.1 • Axial Planes of the Fetal Head and Associated Abnormalities in the First Trimester




































Normal


Suspected Abnormalities


Head shape


Oval shape




  • • Anencephaly/exencephaly: Irregular shape, no skull identified



  • • Holoprosencephaly: circular head


Bony borders


Ossified bones Clear head borders




  • • Osteogenesis imperfecta: no ossification (except for occipital bone). Additional findings such as short broken and bent femur and humerus



  • • Thanatophoric dysplasia: Increased ossification of head, additionally short or abnormally shaped long bones



  • • Encephalocele: Interrupted head contour, commonly in the occipital region


Falx cerebri


Hyperechogenic line from anterior to posterior dividing the brain in two halves




  • • Holoprosencephaly: absent falx cerebri



  • • Encephalocele: Often deviated falx cerebri


Choroid plexuses of lateral ventricles


Large hyperechogenic plexuses on both sides, could be slightly asymmetrical




  • • Holoprosencephaly: fused choroid plexuses



  • • Choroid plexus cysts: typically found in trisomy 13 or other aneuploidies (in general additional markers or abnormalities are present)


Biparietal diameter (BPD)


Within the reference range




  • • Anencephaly: No BPD measurable



  • • Holoprosencephaly and spina bifida: BPD often in the lower range


Thalami, cerebral peduncles, aqueduct of Sylvius


V-Shaped transition, visualized aqueduct. Some distance between occipital bone and cerebral peduncles




  • • Spina bifida: Parallel shaped transition between thalami and cerebral peduncles. Compressed aqueduct. Cerebral peduncles posteriorly shifted and touch or are close to the occipital bones


Fourth ventricle with its choroid plexus


Fourth ventricle well seen with hyperechogenic choroid plexus




  • • Spina bifida: Reduced fluid in fourth ventricle, choroid plexus not well identifiable



  • • Dandy-Walker and Blake’s pouch cyst: Increased fluid in fourth ventricle, anteriorly shifted brainstem



Sagittal Plane

The midsagittal plane is commonly visualized in the first trimester, primarily due to NT measurement (Fig. 8.12). The midsagittal plane reveals more anatomic intracranial information when examined from the ventral approach (fetus back down) (Fig. 8.12). In this midsagittal plane (Fig. 5.6), the following anatomic landmarks can be evaluated: head shape, facial profile, nose with nasal bone, maxilla, mandible, thalamus, midbrain, brainstem (BS), fourth ventricle (called intracranial translucency [IT]) and its choroid plexus, the developing cisterna magna, the occipital bone, and the NT (Fig. 8.12).4 Table 8.2 gives an overview of the anatomic landmarks and corresponding malformations that can be visualized in the midsagittal plane of the fetal head in the first trimester. In the context of spina bifida later in this chapter, the anatomy of the posterior fossa under normal and abnormal conditions is discussed.


Coronal Planes

The coronal planes of the fetal head are obtained along the laterolateral axis of the fetus. In the second and third trimesters, coronal planes of the fetal head are primarily obtained to assess the frontal midline structures such as the interhemispheric fissure, the cavum septi pellucidi, frontal horns of the lateral ventricles, the Sylvian fissure, the corpus callosum, and the optic chiasm. Given that these midline anatomic structures are not fully developed in the first trimester, coronal planes of the fetal head are rarely obtained. Figure 8.13 shows coronal planes of the fetal head in a normal fetus at 12 weeks of gestation.









Table 8.2 • Midsagittal Plane of the Fetal Head and Associated Abnormalities in the First Trimester
























Normal


Abnormalities Suspected


Head shape


Large head in comparison with body, physiologic slight frontal bossing




  • • Anencephaly/exencephaly: Irregular shape, no skull identified



  • • Holoprosencephaly: often abnormal shape


Nuchal translucency (NT)


NT within the normal range


Thickened NT in aneuploidies, complex cardiac malformations, and in many syndromic conditions


Posterior fossa: brainstem, fourth ventricle, cisterna magna


Brainstem diameter within the normal range, slightly s-shaped brainstem, fourth ventricle separated from cisterna magna with choroid plexus




  • • Open spina bifida: brainstem thickened and posteriorly shifted. Compressed or absent fourth ventricle. No cisterna magna seen



  • • Dandy-Walker: thin brainstem, large fourth ventricle



  • • Aneuploidies: often dilated fourth ventricle



  • • Walker-Warburg syndrome: Z-shaped brainstem (kinking)


Facial profile


Normal forehead, nasal bone, maxilla, and mandible


See Chapter 9 for details







Figure 8.13: Coronal planes of the anterior fetal head obtained from a three-dimensional (3D) volume at 12 weeks of gestation and displayed in tomographic view. The posterior fossa is outside the range of the tomographic display and thus is not seen. Note the thalami (Thal.), the choroid plexuses (CP), and the presence of cerebrospinal fluid (CSF) in the anterior segments of the lateral ventricles (asterisks).



CENTRAL NERVOUS SYSTEM ABNORMALITIES


Acrania, Exencephaly, and Anencephaly



Ultrasound Findings

The diagnosis of exencephaly/anencephaly in the first trimester is based upon the demonstration of an absent cranium along with the presence of “abnormal mass of tissue” arising from the base of the remaining skull (Figs. 8.14 and 8.15). On coronal views, the abnormal mass of tissue, representing the amorphous brain matter, has been described as the “Mickey Mouse” sign as it typically bulges to either side of the fetal head (Fig. 8.15A).5 In anencephaly, the absence of the cranial vault is seen along with little to no brain tissue above the level of
the orbits (Fig. 8.16) and on coronal view of the fetal face, the characteristic “frog eyes” appearance is noted (Fig. 8.16B). By the 12th week of gestation, following complete ossification of the fetal skull, ultrasound diagnosis of exencephaly/anencephaly can be performed by the axial, sagittal, or coronal views of the fetal head.6 In these views, the absent calvarium, the abnormal fetal profile, and the disorganized brain tissue protruding from the fetal head can be demonstrated (Figs. 8.14, 8.15, 8.16, 8.17 and 8.18). On transvaginal ultrasound, the amniotic fluid appears echogenic (Fig. 8.16A). The crown-rump length measurements are often
smaller than expected due to loss of brain tissue. The diagnosis of exencephaly/anencephaly can be occasionally suspected at 9 weeks of gestation.6 A follow-up ultrasound after 10 weeks is recommended in order to confirm the diagnosis, especially if pregnancy termination is being contemplated. On occasions, acrania can be diagnosed in the first trimester by the demonstration of absence of cranium and in the presence of membrane (pia mater) covering the brain tissue (Fig. 8.17). In most cases of acrania, follow-up ultrasound examination into the second trimester demonstrates amorphous brain tissue as shown in anencephaly-exencephaly cases. 3D ultrasound can help in providing a complete picture of face and head in anencephalic fetuses (Fig. 8.18).






Figure 8.14: Sagittal view of a fetus with anencephaly-exencephaly at 12 weeks of gestation. Note the absence of normal brain tissue and the overlying calvarium. Amorphous brain tissue is seen protruding from the base of the head region (arrow).






Figure 8.15: Coronal (A) and sagittal (B) views of a fetus with anencephaly-exencephaly at 11 weeks of gestation. The brain is replaced by amorphous tissue (arrows) with absence of the overlying calvarium. The shape of the amorphous brain tissue in the coronal view in A resembles the ears of Mickey Mouse and has been referred to as the “Mickey Mouse” sign. Note in B the absence of the forehead and the protruding brain tissue (arrow).






Figure 8.16: Coronal (A) and direct frontal (B) views of the face in a fetus with anencephaly-exencephaly at 12 weeks of gestation. A: The brain is replaced by amorphous tissue (asterisk) with absence of the overlying calvarium. The direct frontal view of the face in B is lacking the forehead and the eyes (arrows) appear prominent, a view called the “frog eyes.”






Figure 8.17: Sagittal views in two fetuses (A and B) with absence of the calvarium (acrania) in early gestation. Note in A and B the presence of a membrane (pia mater) covering the brain tissue (arrows). The shape of the brain and head is similar in both fetuses. In most cases of acrania, follow-up ultrasound examinations demonstrate amorphous brain tissue as shown in anencephaly-exencephaly cases.

The prenatal diagnosis of exencephaly/anencephaly in the first trimester requires a detailed transvaginal ultrasound looking for the presence of amniotic bands given the association of amniotic band sequence with exencephaly.


Associated Malformations

Anencephaly is commonly associated with other fetal abnormalities to include neural tube malformations such as craniorachischisis, spina bifida, and iniencephaly.6 Other fetal malformations such as cardiac, renal, gastrointestinal, and facial occur in association with anencephaly. Aneuploidy rates are also increased in anencephaly, especially when associated with other malformations. Amniotic band sequence, on the other hand, presents a sporadic association with no increased future risk for recurrence. The in utero mortality rate is high and the malformation is universally lethal.






Figure 8.18: Three-dimensional ultrasound in surface mode in three fetuses (A-C) with anencephaly-exencephaly in the first trimester showing different appearances of the same malformation. Note the fetus in A has part of the calvarium formed (arrow), whereas fetuses in B and C do not.


Cephalocele (Encephalocele)



Ultrasound Findings

The detection of an encephalocele on ultrasound examination is often suspected in the axial view by the presence of a protrusion in the occipital or frontal region of the calvarium (Figs. 8.19, 8.20A, 8.21A, and 8.22A). A sagittal view can reveal the extent of the defect and the size of the encephalocele (Figs. 8.20B, 8.22B, and 9.23A). Transvaginal ultrasound along with image magnification can often reveal the bony defect in the skull (Figs. 8.19B and 8.22). Encephaloceles
are often associated with abnormal brain anatomy that can be detected in the axial or sagittal views of the fetal head (Figs. 8.19, 8.20, 8.21 and 8.22). The larger the encephalocele, the more brain abnormality is seen on ultrasound. As encephaloceles are often part of genetic abnormalities and syndromes, detailed review of fetal anatomy is recommended.7 Special attention should be given to the presence of polydactyly and polycystic kidneys given the association with Meckel-Gruber syndrome (Figs. 8.21 and 13.31).7 Other autosomal recessive ciliopathies can present with posterior cephalocele, such as Walker-Warburg syndrome or the large group of Joubert syndrome-related disorders (Fig. 8.22). Three-dimensional (3D) ultrasound in surface mode can be of help in showing the extent of the encephalocele. Not all cases of cephaloceles are detectable in the first trimester. Smaller defects and internal lesions are difficult to diagnose. The diagnosis of an encephalocele in the first trimester is commonly performed around 13 to 14 weeks unless a meningocele with a dilated posterior fossa is present, mimicking a Dandy-Walker malformation (DWM) enabling an earlier detection (Figs. 8.22 and 13.31). In isolated cases, an attempt should be made to differentiate between an encephalocele and a meningocele given a much improved prognosis of the latter. The absence of brain tissue in the herniated sac on transvaginal ultrasound along

with normal intracranial anatomy make the diagnosis of a meningocele more likely.

Only gold members can continue reading. Log In or Register to continue

Oct 14, 2019 | Posted by in ULTRASONOGRAPHY | Comments Off on The Fetal Central Nervous System
Premium Wordpress Themes by UFO Themes