Center for Prenatal Diagnosis
at St. Vincent Women's Hospital

 

 

Central Nervous System Abnormalities


Neural Tube Defects:

Anencephaly | Meningoencephalocele | Myelomeningocele | Spina bifida occulta

Other CNS Defects:

One of the most common central nervous system defects is a neural tube defect. The neural tube is a portion of the developing embryo that becomes the brain and the spinal cord. If the neural tube fails to close properly, the result may be spina bifida. Prenatal screening for neural tube defects can be based on maternal serum screening or on routine ultrasound. Approximately 80% of babies with open spina bifida and 90% of babies with anencephaly have elevated maternal serum AFP (alpha fetoprotein). Sonography may identify up to 90% of babies with a spina defect and nearly 100% of babies with anencephaly. It is important to keep in mind that this detection rate is largely due to the expertise of the sonographer performing the procedure. Extensive counseling and amniocentesis are recommended when a NTD is identified.

Approximately 1/800-1/1,000 individuals in the US are born with neural tube defects. Neural tube defects are multifactorial conditions, which means that both a genetic and environmental components are involved in the etiology of the condition. In 90% of cases, there is no family history of neural tube defects, so most parents are unaware that they may have a predisposition for having a child with a neural tube defect. It is also estimated that approximately 10% of babies with spina bifida have an underlying chromosome problem. If the baby were to have an underlying chromosome abnormality as a cause of the spina bifida, and we would expect the baby to have more significant health concerns than babies with an isolated spina bifida.

Neural tube defects are described using often very complicated terms. These terms help healthcare professionals describe the specific nature of each defect. For example, the meninges is the membranes that surround the brain and spinal cord while the encephalon refers to the brain itself, and the prefix myelo- refers to the spinal cord itself. Therefore a meningocele is the protrusion of the meninges (membranes surround the brain and/or spinal cord) while a meningoencephalocele involves the protrusion of the membranes as well as the brain itself.

 Anencephaly and Exencephaly

Anencephaly and exencephaly are very severe forms of a neural tube defect and can often be diagnosed in the first trimester. These neural tube defects are identified when the cranial vault (portion of the skull which holds the brain) cannot be seen. Often times the face itself, including the orbits (eyes), can be seen. When brain appear flat, the term anencephaly is used. When brain appear as an irregular bulging structure, the term exencephaly is usually preferred. Unfortunately the prognosis for these babies is extremely poor. Polyhydramnios is frequently seen with this condition, as well as spina bifida and cleft lip and palate.

 

fetus with anencephaly    normal fetal skull

 

Meningoencephalocele

This is a neural tube defect where the meninges (membranes that surround the brain and spinal cord) as well as the brain protrude through a hole in the skull. This is sometimes called an encephalomeningocele. In Caucasians, meningoencephalocele is usually located at the back portion of the skull. Large encephaloceles can be diagnosed rather easily in the second or occasionally even in the first trimester. Smaller lesions may remain undiagnosed. Establishing their prognosis prenatally consists in evaluating the impact of the lesion on brain development. In cases of an isolated meningocele or small and isolated encephalocele the prognosis may be virtually normal. Ruling out associated abnormalities is essential, since encephalocele may be part of a multiple malformation such as Meckel-Gruber syndrome. 

Myelomeningocele  

Myelomeningocele is often identified on a second trimester ultrasound with a collection ultrasound findings including a “lemon sign”, Arnold-Chiari malformation, and open spinal defect. A “lemon sign” is when the head shape is slightly pinched in at the sides of the forehead of the baby and an Arnold-Chiari malformation is the displacement of the hindbrain into the spinal column. The problems associated with a myelomeningocele are variable, and it is known that they are associated with the size and position of the opening. In general, the higher the defect on the spine the more severe the handicap. Most myelomeningoceles are located in the lumbarsacral region of the spine. Problems associated with myelomeningoceles usually include paralysis of the lower limbs and incontinence of the bowel and bladder. If the spina bifida is isolated, the majority of individuals have normal intelligence. 
fetus with lemon shaped skull  fetus with lemon shaped skull

One should keep in mind that spina bifida occulta, which has a better neurological outcome than myelomeningoceles can also be detected by ultrasound, and should be identified as such. Spina bifida occulta involves the meninges without involvement of the spinal cord itself.

fetus with a myelomingocele    fetus with a myelomingocele

Other Central Nervous System Abnormalities

Some brain abnormalities are still very difficult to diagnose despite the technological progress in sonographic imaging.

 

Hydrocephalus

Aqueductal stenosis is the most common form of obstructive hydrocephaly and results from narrowing of the aqueduct of Sylvius, which connects the third and fourth ventricles. Aqueductal stenosis may be inherited as an X-linked recessive condition, a component of an autosomal recessive syndrome, or a multifactorial disorder. Also, intraventricular hemorrhage, intrauterine infection, and compression by adjacent masses may be possible etiologies of this condition. Sonographic findings of aqueductal stenosis include third ventricular and lateral ventricular dilatation. This dilation is usually severe and progressive, and it usually can be identified during the second trimester. However, if the etiology is intraventricular hemorrhage or infection, the abnormality may not be present until the third trimester (or even evolve after birth). Fetuses with aqueductal stenosis have a good survival rate, because they usually do not have associated abnormalities. However, neurological outcome varies. Males with X-linked hydrocephalus generally have a less favorable prognosis, and flexion abnormalities of the thumb.

Communicating hydrocephalus, also known as external hydrocephalus, is the result of obstruction of the flow of Cerebral Spinal Fluid (CSF) outside the ventricles or an impaired reabsorption of CSF. Findings of communicating hydrocephalus are dilatation of the subarachnoid space and the lateral, third, and fourth ventricles. The etiology of this condition is unknown, and familial transmission is rare. If the communicating hydrocephalus is isolated, the prognosis is better than with other forms of hydrocephalus. If other anomalies are present, the prognosis may depend on the type of anomaly.

Ventriculomegaly

Ventriculomegaly is among the most common brain abnormalities that can be picked up by routine ultrasound. Fetal ventriculomegaly is the result of the accumulation of cerebrospinal fluid in the ventricles, or chambers, in the brain. Within the brain, there are four ventricles that allow the fluid to circulate from one chamber to another through several channels. If a blockage occurs that prevents this normal flow of fluid, the ventricles or channel above the blockage will become dilated. The result is enlarged ventricles (ventriculomegaly). This blockage can lead to hydrocephaly when the head size becomes increased to accommodate the excess fluid. There are many causes of ventriculomegaly such as an underlying chromosome abnormality, genetic syndrome, developmental defect of the fetal brain, or a fetal infection or hemorrhage. Often it is difficult to determine the cause. Ventriculomegaly is associated with a wide spectrum of abnormalities ranging from isolated "mild" ventriculomegaly with a normal neurological outcome to extremely severe, complex cerebral malformations.

Moderate ventriculomegaly is a nonspecific prenatal sonographic feature that may be "physiological" or may be the hallmark of a severe abnormality. The size of the lateral ventricles is remarkably stable throughout gestation after 22 weeks, the 95th centile corresponding to 10 mm and the 98th to 12 mm. We tend to consider that ventriculomegaly is moderate when it does not exceed 15 mm. However, this definition is somewhat arbitrary, since postnatal outcome is probably related to the presence or absence of structural brain abnormalities rather than to the size of the ventriculomegaly.

Prenatal diagnosis of moderate ventriculomegaly should prompt a chromosomal abnormality or a severe fetal infection such as toxoplasmosis or cytomegalovirus to be ruled out by performing amniocentesis and/or by maternal serology. However, ventriculomegaly due to cytomegalovirus is usually associated with extracerebral sonographic abnormalities. Associated abnormalities and spina bifida should also be ruled out at initial evaluation. A careful follow-up based on sonography and third trimester MRI is required to ascertain the morphological normality of the central nervous system.

Infants with moderate nonprogressive ventriculomegaly, without associated structural abnormalities, and with normal growth of the head and brain ascertained by US and MRI, are expected to do well postnatally. However, one should be aware that occasionally, our diagnostic tools may fail to spot subtle brain abnormalities that have yet to make a significant clinical impact.

 
Dandy-Walker malformation

Dandy-Walker malformation (DWM) is a constellation of abnormalities consisting of complete or partial absence of the cerebellar vermis, cyst in the posterior fossa, and hydrocephalus. However, hydrocephalus is not usually present at birth but does develop within the first months of infancy. DWM accounts for approximately 5 to 12 percent of all cases of hydrocephalus. The etiology of DWM may be a inherited disorder, infection, teratogen exposure, maternal condition, chromosome abnormality, or isolated sporadic event. Both CNS anomalies and extra-CNS anomalies are frequently associated with DWM. Prognosis for fetuses with a DWM is worse than with other types of hydrocephaly. 

Holoprosencephaly

Holoprosencephaly results from failure of the hemispheres of the brain to cleave into the cerebral and lateral hemispheres. This defect occurs at approximately 6 to 10 weeks gestation, so it is conceivable to identify this defect before the fetus is viable. Holoprosencephaly can be divided into three categories, alobar, semilobar, and lobar depending on the degree of cleavage. Alobar holoprosencephaly is the most severe form. Findings of this type include absence of the interhemispheric fissure, a single primitive ventricle, fused thalami, and absent third ventricle, olfactory bulbs, and optic tracts. Semilobar holoprosencephaly is characterized by partially separated cerebral hemispheres and a single ventricular cavity. The interhemispheric fissure is well-developed in lobar holoprosencephaly; however, there is some fusion of structures. Facial anomalies, consisting of aplasia or hypoplasia of midline structures such as the nose are characteristic of holoprosencephaly. Prognosis depends on the type of holoprosencephaly and the presence of associated anomalies. Chromosome abnormalities, especially trisomy 13 and 18, may be associated with holoprosencephaly. For this reason, if holoprosencephaly is identified, amniocentesis is recommended. 

Choroid plexus cysts

Choroid plexus cysts are believed to result from the accumulation of CSF and cellular debris within the neuroepithelial folds. These cysts are found in approximately 50% of individuals of all ages in serial autopsy. These cysts can generally be visualized in the second trimester and usually resolve by 24 weeks gestation. Isolated choroid plexus cysts are usually benign. However, serial scans should be used to monitor the size of the cyst and exclude the development of hydrocephalus. If the cyst does not resolve, is unusually large, or other abnormalities are detected, the risk of the fetus having a chromosome abnormality, particularly trisomy 18, increases.

Cephaloceles are defects in the cranium through which intracranial contents can protrude. When meninges are protruding, the term cranial meningocele is often used, and when a portion of the brain is protruding it is called an encephalocele. These defects are subdivided into frontal, parietal, and occipital, depending on the location of the defect, with occipital cephaloceles being the most common. Hydrocephalus is frequently associated with the presence of a cephalocele, and spina bifida and microcephaly are also associated findings. A paracranial mass on ultrasound may be suggestive of a cephalocele, but one should try to identify the actual cranial defect. This may not be possible, but the defect should be differentiated from a cystic hygroma or scalp edema. The prognosis of fetuses with a cephalocele is variable depending on the presence of brain in the sac, hydrocephalus, and microcephaly. If the brain is involved the prognosis may be quite poor. However, approximately 60% of individuals with a cranial meningocele are of normal intelligence. 

Craniostenosis

Craniostenoses are a group of disorders in which the plates of the cranium (skull) in children fuse prematurely, resulting in deformities of the skull of present at (or even before) birth. There are many different types of craniostenosis determined by which plates are fused. Scaphocephaly (also known as dolichocephaly) is a malformation of the skull which is long and narrow. Plagiocephaly is a malformation of the skull most commonly caused by the plates of one side of the forehead (coronal sutures) closing prematurely resulting in a flattening on one side of the skull and a slight bulging on the other. This is similar to positional deformation of the skull sometimes due to the child sleeping on only one side. Brachycephaly may be caused by the premature closure of the coronal sutures on both sides of the forehead resulting in a head which is disproportionately wide. Brachycephaly is a feature of several genetic syndromes such as Apert, Crouzon, Pfeiffer, and Carpenters syndromes. Trigonocephaly is the result of premature closure the plates at the center of the forehead (metopic suture), resulting in a longer head shape from front to back and possible ridging of the forehead.

Craniostenoses occur in approximately 1/2,000-1/2,500 live births. The prognosis is usually good for these children providing other abnormalities or syndromes are ruled out and surgical treatment is provided when necessary. It is important to determine whether other abnormalities are present and whether or no the craniostenoses is related to a genetic condition. This is crucial to accurately determine the prognosis.

REFERENCES
Nyberg, D.A., Mahony, B.S., and Pretorius, D.H. Cerebral Malformations in Diagnostic Ultrasound of Fetal Anomalies. Mosby-Year Book, Inc: St. Louis, 1990. Pp 83-146.

Nyberg, D.A., Mahony, B.S., and Pretorius, D.H. The Spine and Neural Tube Defects in Diagnostic Ultrasound of Fetal Anomalies. Mosby-Year Book, Inc: St. Louis, 1990. Pp 147-202.

Prenatal Diagnosis of Central Nervous System Abnormalities. Curr Probl Obstet Gynecol Fertil March/April, 1995: 43-68.

Romero, R., Pilu, G., Jeanty, P., Ghidini, A., Hobbins, J.C. The Central Nervous System in Prenatal Diagnosis of Congenital Anomalies. Appleton & Lange: Connecticut, 1988. Pp 1-80.

M. C. Aubry, J. P. Aubry and M. Dommergues. Sonographic prenatal diagnosis of central nervous system abnormalities. Springer-Verlag 2003.

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  Page updated on December 5, 2010
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