Penny CH 24- Fetal Head and Brain

Lissencephaly

"Smooth brain". Condition in which there are no gyri within the cerebral cortex. Agyria and the absence of sulci in the brain is not typically diagnosed until the third trimester or postnatally and almost always carries a poor prognosis

Acrania (Anencephaly and Exencephaly)

Acrania is one of the most common neural tube defects; it is defined as the absence of the cranial vault above the bony orbits. Acrania can be further divided into two main subtypes that are related to the amount of cerebral tissue present: anencephaly and exencephaly. Anencephaly is considered when there are no cerebral hemispheres present; exencephaly denotes a normal amount to of cerebral tissue. Acrania is a fatal condition no matter the type. Sonographic appearance is a "froglike" face or bulging eyes and absence of the cranial vault

Cisterna Magna

Located in the posterior fossa of the cranium. Largest cistern in the head. Appears as an anechoic fluid-filled space posterior to the cerebellum between the cerebellar vermis and the interior surface of the occipital one. It is considered common to find some small septations within the cisterna magna

Choroid Plexus Cysts

Located within the choroid plexus of the lateral ventricles, measures more than 2 mm, round and anechoic with smooth walls; frequently encountered during a routine sonographic examination and typically regress by the third trimester. Associated with an increased risk of trisomy 18.

Effects of Fetal Infections on the Brain

Maternal serum screening for intrauterine infections resulting from toxoplasmosis, other agents, rubella, cytomegalovirus, and Herpes simplex virus (TORCH) can be performed. sonographic intracranial findings consistent with TORCH are calcifications around the ventricles and ventriculomegaly

Lateral Ventricle

Measured in the transaxial plane at the level of the atrium (the atrium is the first region enlargement would occur); the calipers are placed at the level of the glomus of the choroid plexus (largest part of the choroid plexus). The normal lateral ventricle does not exceed 10mm

Doppler Interrogation of the Fetal Brain

Middle Cerebral Artery (MCA) has been shown effective at evaluating for hypoxia in fetuses that are small for dates; the pulsitality index (PI) of the MCA varies with GA, but normally decreases as the pregnancy progresses toward term. The resistane pattern of the MCA should be greater than that of the umbilical artery and thus should be compared when fetal shunting is suspected. Fetal Intracranial vascular anomalies include a vein of galen aneurysm: an arteriovenous malformation that occurs within the fetal brain; the sonographic findings of a vein of galen aneurysm is that of a large anechoic mass within the midline of the cranium that when interrogated with color and pulsed doppler fills with turbulent venous and arterial flow; the fetus will also have signs of hydrops and cardiomegaly; newborns with this condition are prone to suffer from congestive heart failure in the post natal period

Cavum Septum Pellucidum (CSP)

Midline brain structure that can be identified in the anterior portion of the brain between the anterior horns of the lateral ventricles. It appeas as an anechoic "box-shaped" structure in the axial scan plane. The CSP should always be seen between 18-37 weeks, but the closure of this structure is normal in later gestation and often occurs before birth or shortly thereafter. The CSP does not communicate with the ventricular system and its absence is associated with multiple cerebral malformations

Holoprosencephaly

Midlline brain anomaly that is associated with brain abberrations and atypical facial structures. May be detected with TV as early as the first trimester. Three main types include alobar, semilobar, and lobar. Alobar is the most severe form (often results in neonatal death); absence of corpus collosum, CSP, third ventricle, interhemispheric fissure, and falx cerebri; horseshoe shaped monoventricle and fused echogenic thalamus; cerebellum and brain stem remain in tact. Lobar can be consistent with life (may experience severe mental retardation, Trisomy 13 in 50-70% of fetuses). Cyclopia (fused orbits with a single eye) and proboscis (false nose situated above the orbits) are the most disturbing external findings associated with holoprosencephaly; other facial anomalies inlude anophthalmia (absence of eyes), hypotelorism (close set eyes), median cleft lip, cebocephaly (close-set eyes (hypotelorism) and nose with a single nostril).

Neural Tube Defects

Occur when the embryonic neural tube fails to close. Types include cephaloceles, various spinal dysraphisms, anencephaly, spina bifida. Anencephaly and spinal bifida are the most common neural defects (1/1000 pregnancies). Associations: maternal diabetes, the use of valporic acid (seizure medication), edwards syndrome, Patau syndrome, triplidy; supplement of 4 mg of folate (folic acid) in a woman's diet significantly reduces the likelihood of her fetus developing a neural tube defect. Diagnosed with sonography, amniocensesis, and/or maternal serum screening (triple screen of hCG, MSAFP, and estriol). Alpha fetoprotein is initially produced by the yolk sac, gastrointestinal tract, and fetal liver; AFP exits through an opening in the neural tube if one is present, allowing for a greater amount to pass into the maternal circulation (increased levels of AP does not always mean a neural tube defect is present; also found with omphalocele, gastroschisis, multiple gestations, fetal demise, and incorrect gestational dating)

Cephaloceles

Protrusion of intracranial contents through a defect in the skull. Can be distinguished by contents and location . Sonographic findings: open cranial defect (typically posterior in location), small obliterated cisterna magna, complex mass protruding from the cranium. Meningocele is meninges only, cephalocele is brian tissue only, encephalomeningocele is menenges and brain tissue, encephalomeningocystocele is brain tissue and lateral ventricles

Porencephaly

Rare condition in which a cyst communicates with the ventricular system. Can occur after the fetus has experienced hemorrhage within one or both of the cerebral hemispheres; as the hemorrhage changes states, it will form into a cystic cavity and eventually will communicate with the lateral ventricle of the affected size; this condition may caused by ischemic events or vascular occlusion within the brain; most often unilateral. Arachnoid cysts can be confused with porencephaly, but do not communicate with the ventricular system

Cerebrum

The brain can be divided into two main parts, the cerebrum and the cerebellum. The cerebrum is the largest part of the brain. The normal cerebrum contains multiple sulci and gyri. There are six cerebral lobes: the frontal lobe, two temporal lobes, two parietal lobes, and the occipital lobe. The cerebrum is further divided into a right and left hemisphere by the interhemispheric fissure; the falx cerebri, a double fold of dura mater, is located within the interhemispheric fissure and can be readily noted on a fetal sonogram as an echogenic linear formation coursing through the midline of the fetal brain. The cerebral hemisphrers are linked in the midline by the corpus collosum, a thick band of tissue that provides communication between right and left halves of the brain. The meninges are three protective tissue layers that cover the brain and spinal cord. The innermost layer of the meninges is the pia mater, the middle layer is the arachnoid membrane, and the dense outermost layer is the dura mater

Transcerebellar Measurement

The cerebellum grows at a rate of 1 mm per week between 14 and 21 weeks and thus correlates with the gestational age of the fetus. The cerebellum is measured in the transverse plane at the same level of the cisterna magna and thalamus

Fetal Intracranial Hemorrhage (Intraventricular Hemorrhage)

An intracranial hemorrhage is a common finding in premature infants weighing less than 1500 g and those born before 32 weeks gestation. It occurs less often in utero. Risk factors: maternal use of cocaine, trauma, history of amniocentesis, maternal platelet disorders (most common). The most common origin of intracrnial/intraventricular hemorrhage is within the germinal matrix (group of thin walled pressure sensitive vessels located in the subependymal layer of the ventricles; the vessels are prone to rupture secondary to their thin walls). The hemorrhage can spread to the lateral ventricle, often leading to noncommunicating hydrocephalus as the clot obstructs the flow of CSF within the narrowed regions of the ventricular system. Hemorrhage can also occur within the parenchyma of the brain; localized areas of hemorrhage within the cerebral hemispheres will eventually lead to the formation of cystic cavities that will communicate with the ventricular system (porencephaly)

Fontanelles

Anterior or frontal fontanelle is bordered by the frontal and parietal bones and closes by 18 months. Posterior or occipital is bordered by occipital and parietal bones and closes by 6 months. Anterolateral or sphenoidal are paired and bordered byt he frontal, parietal, and sphenoid bones and close by 2 years. Posterolateral or mastoid are paired and borded by the mastoid and occipital bones and close by two years.

Arnold Chiari II Malformation and Spina Bifida

Arnold Chiari II or Chari II malformation is a group of cranial abnormalities associated with the neural tube defect spina bifida. Spina bifida may result in a mass that protrudes from the spine; this mass can be referred to as a meningoecele or myelomeningocele (contains meninges). The most common location of spina bifida is within the distal lumbosacral region. Several notable changes occur within the brain and skull of spina bifida; enlarged massa intermedia, hydrocephalus, the frontal bones become flatten and will yield a "lemon" sign of scalloping of the frontal bones, the cerebellum will become displaced posteiorly and inferiorly and appear curved ("banana sign"), the cisterna magna is completely obliterated, lateral venricles distorted in shape (frontal horns will be small and slitlike whereas the occipital horns will be enlarged (colopcephaly)).

Agenesis of the Corpus Callosum and Cavum Septum Pellucidum

The corpus collosum is a bridge of tissue within the midline of the brain that connects the two lateral cerebral hemispheres. It functionally provides a pathway for communication between the hemispheres and is completely formed by 18 weeks. The CSP, located inferior to the corpus collosum develops at the same time. There can be partial or complete absence of the corpus collosum. Usually, if the corpus callosum is absent, the CSP will be absent as well; their nonexistence has been linked to as many as 50 to 200 different syndromes and anomalies such as holoprosencephaly, Dandy-Walker malformation, aqueductal stenosis, trisomy 18, trisomy 8 (Warkany Syndrome 2), trisomy 18. Sonographic findings consistent with agenesis of corpus callosum include partial or complete absence of the corpus callosum and absence of the CSP after 18 weeks, "sunburst" manifestation of sulci (sulci within the cerebrum typically travel parallel to the corpus collosum, but with agenesis they typically have a more perpendicular or radial arrangement and often appear to have a "spoke wheel" pattern), colpocephaly (small frontal horns and enlarged occipital horns of the lateral ventricles; tear-drop shaped lateral ventricles), superiorly elevated and dilated third ventricle

Cisterna Magna Measurement

The depth of the cisterna magna should not measure more than 10 mm; measurement of more than 10 mm is consistent with mega cisterna magna and Dandy-Walker complex (a spectrum of posterior fossa abnormalities that involve the cystic dilation of the cisterna magna and fourth ventricle) whereas a measurement of less than 2 mm is worrisome for Arnold-Chiari II malformation (group of cranial abnormalities associated with spina bifida)

Ventriculomegaly and Hydrocephalus

Ventriculomegaly is the abnormal enlargement of the ventricles within the brain. It is the most common cranial abnormality . The sonographic finding of the "dangling choroid sign" describes the echogenic choroid plexus hanging limp and surrounded by CSF within the dilated lateral ventricle; it is a highly specific finding for ventriculomegaly. Hydrocephalus is the abnormal enlargement of ventricles due to a build up of pressure/obstruction of CSF flow. Communicating hydrocephalus is apparent when the obstruction lies outside of the ventricular system. Noncommunicating hydrocephalus is when the obstruction level is located within the ventricular system. Aqueductal stenosis is the most common cause of hydrocephalus in utero; a stenosis of the cerebral aqueduct/aqueduct of sylvius between the third and fourht ventricle will cause the third and both lateral ventricles to expand whereas the fourth ventricle remains normal; hemorrhagic obstruction is another possible cause.

Thalamus

Vital brain structure that has numerous functions. On ultrasound, the two lobes of the thalamus are located on both sides of the third ventricle. The massa intermedia or interthalamic adhesion passes through the third ventricle to connect the two lobes of the thalamus. The thalalmus should not be confused with the cerebral peduncles, which are more inferiorly positioned in the brain (paired structures located anterior to the cerebral aqueduct)

Fetal Head Measurements

Biparietal diameter (BPD) can be taken after the first trimester has ended (starting between 13 and 14 weeks); it is obtained in the axial plane at the level of the CSP, thalamus, and falx cerebri (same level as third ventricle which may be seen between the two lobes of the thalamus); the cranial bones must be symmetric on both sides of the head and the measurement is obtained from the outer table of the proximal parietal bone to the inner table of the distal parietal bone (leading edge to leading edge). Head circumference (HC) is also taken starting at 13-14 weeks at the same level of the cranium as the BPD; a measurement around the entire cranium is obtained. The HC can be obtained by taking the occipital frontal diameter (OFD) and taking an outer to outer measurement of the BPD. HC is typically more accurate than the BPD because it is independent of fetal head shape. OFD/fronto-occipital diameter is obtained at the same level of the BPD and HC by measuring from the outside of the occipital bone to the outside of the frontal bone, along the midline of the fetal cranium. The OFD can be added to the BPD and multiplied by 1.57 to obtain the HC. There can be much variability in the shape of the fetal head; the cephalic index is a useful tool for indicating the shape of the fetal head. Brachycephalic head shape (CI>85) is one that is considered round or short and wide while doliocephaly/scaphocephaly (CI <75) denotes an elongated narrow skull shape; the normal to medium sized skull is termed mesocephalic. The formula used to calculate the cephalic index is CI = BPD/OFD X 100

Embryologic Development of the Fetal Brain

By 4.5 weeks, the neural plate (structure that will form the central nervous system) has formed. It gies rise to the neural tube which will become the spine and the brain. Initially, the brain is separated into three primary vescicles termed the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain). Thse vesicles will continue to develop and form cirical brain structures. Sonographically, the rhomboncephalon may be noted within the fetal cranium during the first rimester

Ventricular System

Composed of four ventricles whose primary function is to provide cushioning for the brain. Each ventricle is lined by a membrane called the ependyma. The paired lateral ventricles are located on both sides of the falx cerebri within the cerebral hemispheres (they are frequently referred to as right and left ventricles but may also be called the first and second ventricles). The divisions of the lateral ventricles (horns of the lateral ventricles) correlate with their adjacent cranial bones; each lateral ventricle consists of a frontal, temporal, and occipital horn; the point at which the body, the temporal horn, and the occipital horn meet is called the trigone or atrium of the atrial ventricle. Within the atria of both lateral ventricles lies the echogenic configuration of the choroid plexus, the mass of cells responsible for the generation of cerebrospinal fluid (CSF); the choroid plexus may a lobe found in the roof of the third and fourth ventricles. Each lateral ventricle communicates with the third ventricle in the midline of the brain at the foramen of Monro, or the paired interventricular foramina. The third ventricle is located between the two lobes of the thalamus; the third ventricle connects to the fourth ventricle (found inferiorly) by means of a long tubelike structure called the aqueduct of sylvius, or cerebral aqueduct. The fourth ventricle is located anteriorly to the cerebellum within the midline of the brain; it has three apertures or openings through which CSF travels (two lateral apertures referred to as the foramina of Luschka which allow CSF to travel from the fourth ventricle to the subarachnoid space around the brain) (the median aperature located in the midline also called the foramin of Magendie allows CSF to pass from the foruth ventricle to the cisterna magna and subarachnoid space)

Fetal Sutures

Coronal suture is between the frontal and two parietal bones. Sagittal suture is between the two parietal bones. Lambdoidal suture is between the parietal bones and occipital bone. Squamosal sutures are paired and between the pareital bones annd temporal bones. Metopic suture is located within the frontal bone along the midline of the forehead

Dandy Walker Malformation and Mega Cisterna Magna

Dandy Walker Malformation is a classification within a larger group of anomalies referred to as the Dandy Walker Complex. The Dandy Walker Complex is a spectrum of posterior fossa abnormalities that involve the cystic dilation of the cisterna magna and fourth ventricle. Dandy Walker Malformation is thought to be caused by a developmental abnormality in the roof of the fourth ventricle. The sonographic findings of Dandy Walker malformation include an enlarged cisterna magna that communicates with a distended fourth ventricle through a defect in the cerebellum; the cerebellar vermis is either completely absent or hypoplastic and the tentorium (the structure that separates the cerebrum from the cerebellum) is elevated. Other midline brain anomalies may be present as well; agenesis of the corpus collosum, ventriculomegaly, holoprosencephaly and cephaloceles are all associated with anomalies that can coexist with DWM. Mega cisterna magna is the enlargement of the cisterna magna without the involvement of the fourth ventricle; can be confused with Dandy-Walker malformation but fourth ventricle is normal

Schizencephaly

Development of fluid-filled (anechoic CSF) clefts within the cerebrum. Unknown etiology. Several associated anomalies such as agenesis of the corpus collosum, agenesis of the CSP (50% of the time) and ventriculomegaly.

Cerebellum

Located in the posterior fossa of the cranium. Consists of a right and left hemisphere coupled at the midline by the cerebellar vermis (more echogenic). The cerebellar tonsils (named for their shape) are located on the undersurface of the cerebellum and becoem distorted with spina bifida and Arnold Chiari malformations. The normal cerebellum is dumbbell-shaped or figure eight-shaped. The two hemispheres of the cerebellum should be symmetric, although hypoplasia of one cerebellar hemisphere can occur, resulting in the hypoplastic hemisphere appearing smaller than normal

Hydranencephaly

Fatal condition in which the entire cerebrum is replaced by a large sac containing CSF. The falx cerebri may be partially or completely absent, whereas the brain stem and basal ganglia are maintained and surrounded by CSF; no cerebral cortex identified. Possible causes: bilateral occlusion of the internal carotid arteries with subsequent destruction of the cerebral hemispheres; intrauterine infections such as cytomegalovirus and toxoplasmosis leading to the destruction of the cerebral hemispheres. In both hydrocephalus and holoprosencephaly, there will be a rim of cerebral tissue maintained, but in hydranencephaly there is no cerebral mantle present.

Corpus Collosum

Forms late in gestation but should be completely intact by 18-20 weeks. Corpus collosum connects the two lobes of the cerebrum and consists of four parts. Fetal development of the corpus callosum is anterior to posterior: the rostrum, genu body, and splenium develop in the listed order (anterior to posterior). The sonographic appearance of the corpus collosum is that of an echogenic band of tissue within the midline of the brain connecting the two cerebral hemispheres

Cranial Bones

Frontal bone is anterior. Parietal bones are paired and found superior and lateral. Temoral bones are paired and found inferior and lateral. Occipital bone is found posterior. Sphenoid bone is found laterally. Ethmoid bone is found anteriorly (between orbits)

Abnormal Head Shapes

Lemon Shape: chiari ii malformation. Strawberry Shape: Trisomy 18. Cloverleaf Shape: thanotophoric dysplasia (most common lethal skeletal dysplasia characterized by a cloverleaf skull with frontal bossing and hydrocephalus). Microcephaly: TORCH infections, Trisomy 13, Trisomy 18, Mickel-Gruber Syndrome (fetal syndrome associated with microcephaly, occipital encephalocele, polydactyly, and polycystic kidneys), Fetal Alcohol Syndrome. Macrocephaly: hydrocephalus, hydranencephaly (fatal condition in which the entire cerebrum is replaced by a large sac containing CSF), intracranial tumors, familial inheritence, Beckwith Wiedemann Syndrome. Brachycephaly: craniosynostosis , trisomy 21, trisomy 18. Doliochocephaly: craniocynostosis (premature closure of the cranial sutures with subsequent fusion of the cranial bones)

CSF

The greater part of CSF is produced by the cells of the choroid plexus that are located within the trigone of the lateral ventricles. CSF moves from the lateral ventricles into the third ventricle via the foramina of monro. From the third ventricle, CSF travels to the fourth ventricle through the cerebral aqueduct. Once in the fourth ventricle, the fluid can either exit through the median aperature or the lateral aperatures. CSF also flows inferiorly and around hte spinal chord. Arachnoid granulations (arachnoid villi) are responsible for the resorption of CSF into the venous system; this process occurs at the superior sagittal sinus, located along the superior surface of the cerebrum within its midline

Fetal Intracranial Tumors

The most common intracranial tumor found in utero is teratoma (contain tissue such as hair, sebum, and fat, and most often appear as complex masses that distort the normal architecture of the brain. Chorid plexus papillomas are found within the choroid plexus and produce an increase in the production of CSF, which in turn leads to ventriculomegaly. Other sonographic findings associated with brain tumors include macrocephaly and intracranial calcifications. Corpus callosum lipomas may also be present with agenesis of the corpus callosum. A lipoma will appear as a solid echogenic mass

Normal Fetal Skull and Brain Anatomy

The skull consists of eight cranial bones connected by structures called sutures (may be noted during a routeine sonographic examination as hypoechoic spaces between the bones. The sutures enable the fetal cranial bones remain slightly mobile until delivery to faciliate the passage of the skull through the birth canal. Premature fusion of the sutures is called craniosynostosis (leads to an irregular shaped skull). Spaces that exist between the forming fetal bones are referred to as fontanelles or "soft spots"; several fontanelles persist in the postnatal perior and until infancy; fontanelles are often used as sonograpic windows during neurosonographic examinations to evaluate newborns for intracranial hemorrhage or suspected brain anomalies; the anterior fontanlle, when completely filled with bone is referred to as bregma while the posterior fontanelle is referred to as lambda. The foramen magnum is the opening in the base of the skull through which the spinal cord travels