Congenital Brain Defects
Brain development begins shortly after conception and continues throughout the growth of a fetus. A complex genetic program coordinates the formation, growth, and migration of billions of neurons, or nerve cells, and their development into discrete, interacting brain regions. Interruption of this program, especially early in development, can cause structural defects in the brain. In addition, normal brain formation requires proper development of the surrounding skull, and skull defects may lead to brain malformation. Congenital brain defects may be caused by inherited genetic defects, spontaneous mutations within the genes of the embryo, or effects on the embryo due to the mother's infection, trauma, or drug use.
Early in fetal development, a flat strip of tissue along the back of the fetus rolls up to form a tube. This so-called neural tube develops into the spinal cord, and at one end, the brain. Closure of the tube is required for subsequent development of the tissue within. Many different types of brain defects are caused by improper closure of this neural tube. One such congenital brain anomaly, anencephaly (literally "without brain") results when the topmost portion of the tube fails to close and the brain does not develop. Anencephaly is the most common severe malformation seen in stillborn births. It is about four times more common in females than males. Anencephaly is sometimes seen to run in families, and for parents who have conceived one anencephalic fetus, the risk of a second is as high as 5 percent. Fewer than half of babies with anencephaly are born alive, and survival beyond the first month is rare.
Another congenital brain defect, encephalocele, is a protrusion of part of the brain through a defect in the skull. The most common site for encephalocele is along the front-to-back midline of the skull, usually at the rear, although frontal encephaloceles are more common among Asians. Pressure within the skull pushes out cranial tissue. The protective layer over the brain, the meninges, grows to cover the protrusion, as does skin in some cases. Defects in skull closure are thought to cause some cases of encephalocele, while defects in neural tube closure may cause others. Encephaloceles may be small and contain little or no brain tissue or may be quite large and contain a significant portion of the brain.
Failure of neural-tube closure below the level of the brain prevents full development of the surrounding vertebral bones and leads to spina bifida, or a divided spinal column. Incomplete closure causes protrusion of the spinal cord and meninges, called meningomyelocele. Some cases of spina bifida are accompanied by another defect at the base of the brain, known as the Arnold-Chiari malformation or Chiari II malformation. For reasons that are unclear as of 2004, part of the cerebellum is displaced downward into the spinal column. Symptoms may be present at birth or delayed until early childhood.
The Dandy-Walker malformation is a brain defect marked by incomplete formation or absence of the central section of the cerebellum and the growth of cysts within the lowest of the brain's ventricles. The ventricles are fluid-filled cavities within the brain, through which cerebrospinal fluid (CSF) normally circulates. The cysts may block the exit of the fluid, causing hydrocephalus. Symptoms may be present at birth or delayed until early childhood.
Soon after closure of the neural tube, the brain divides into two halves, or hemispheres. Failure of division is termed holoprosencephaly (literally "whole forebrain"). Holoprosencephaly is almost always accompanied by facial and cranial deformities along the midline of the head, including cleft lip, cleft palate, fused eye sockets and a single eye (cyclopia), and deformities of the limbs, heart, gastrointestinal tract, and other internal organs. Most infants are either stillborn or die soon after birth. Survivors suffer from severe neurological impairments.
The normal ridges and valleys of the mature brain are formed after cells from the inside of the developing brain migrate to the outside and multiply. When these
If damaged during growth, especially within the first 20 weeks, brain tissue may stop growing, while tissue around it continues to form. This causes an abnormal cleft or groove to appear on the surface of the brain, called schizencephaly (literally "split brain"). This cleft should not be confused with the normal wrinkled brain surface, nor should the name be mistaken for schizophrenia, a mental disorder. Generalized destruction of tissue or lack of brain development may lead to hydranencephaly, in which cerebrospinal fluid fills much of the space normally occupied by the brain. Hydranencephaly is distinct from hydrocephalus, in which CSF accumulates within a normally-formed brain, putting pressure on it and possibly causing skull expansion.
Excessive brain size is termed megalencephaly (literally "big brain"). Megalencephaly is defined as any brain size above the 98th percentile within the population. Some cases are familial and may be entirely benign. Others are due to metabolic or neurologic disease. The opposite condition, microcephaly, may be caused by failure of the brain to develop or by intrauterine infection, drug toxicity, or brain trauma.
Researchers estimate that central nervous system anomalies, congenital brain defects included, occur in approximately 15 of every 10,000 live births. Some congenital brain defects, such as those associated with spina bifida have a higher prevalence, affecting as many as two to three per 1,000 live births. In general, birth defects of the brain are not more prevalent in one gender; however, specific defects such as anencephaly, affect girls four times more often than boys, and in families who have conceived one child with anencephaly, the risk to another pregnancy increases nearly 5 percent.
Congenital brain defects may have genetic, infectious, toxic, or traumatic causes. In most cases, no certain cause can be identified.
GENETIC CAUSES Some brain defects are caused by trisomy, the inclusion of a third copy of a chromosome normally occurring in pairs. Most trisomies occur because of improper division of the chromosomes during formation of eggs or sperm. Trisomy of chromosome 9 can cause some cases of Dandy-Walker and Chiari II malformation. Some cases of holoprosencephaly are caused by trisomy of chromosome 13, while others are due to abnormalities in chromosomes 7 or 18. Individual gene defects, either inherited or spontaneous, are responsible for other cases of congenital brain malformations.
DRUGS Drugs known to cause congenital brain defects when used by the mother during critical developmental periods include:
OTHER Other causes of congenital brain defects include:
- intrauterine infections, including cytomegalovirus, rubella, herpes simplex, and varicella zoster
- maternal diabetes mellitus
- maternal phenylketonuria
- fetal trauma
Besides the features listed above, symptoms of congenital brain defects may include:
- Chiari II malformation: Impaired swallowing and gag reflex, loss of the breathing reflex, facial paralysis, uncontrolled eye movements (nystagmus), impaired balance and gait.
- Dandy-Walker malformation: Symptoms of hydrocephalus, lack of muscle tone or floppiness, seizures, spasticity, deafness, irritability, visual impairment, deterioration of consciousness, paralysis.
- Lissencephaly: Lack of muscle tone, seizures, developmental delay, spasticity, cerebral palsy.
- Hydranencephaly: Irritability, spasticity, seizures, temperature oscillations.
- Megalencephaly due to neurological or metabolic disease: Mental retardation, seizures.
Congenital brain defects are diagnosed either from direct physical examination or imaging studies
Prenatal diagnosis of neural tube defects causing anencephaly or meningomyelocele is possible through ultrasound examination and maternal blood testing for alpha-fetoprotein, which is almost always elevated. Ultrasound can also be used to diagnose Dandy-Walker and Chiari II malformations. Amniocentesis may reveal trisomies or other chromosomal abnormalities.
Spina bifida may be treated with surgery to close the open portion of the spinal cord. Surgery for encephalocele is possible only if there is a minimal amount of brain tissue protruding. Malformations associated with hydrocephalus (Dandy-Walker, Chiari II, and some cases of hydranencephaly) may be treated by installation of a drainage shunt for cerebrospinal fluid. Drugs may be used to treat some symptoms of brain defects, including seizures and spasticity or muscle rigidity.
Most congenital brain defects carry a very poor prognosis. Surgical treatment of meningomyelocele and encephalocele may be successful, with lasting neurological deficiencies that vary in severity. Early treatment of hydrocephalus may prevent more severe brain damage.
Some cases of congenital brain defects can be prevented with good maternal nutrition, including folic acid supplements. Folic acid is a vitamin that has been shown to reduce the incidence of neural tube defects. The Centers for Disease Control and Prevention recommends that all women of childbearing age consume 0.4 mg of folic acid daily to prevent neural tube defects. All over-the-counter multivitamins contain this amount of folic acid. Pregnant women should avoid exposure to infection, especially during the first trimester. Abstention from drugs and alcohol during pregnancy may reduce risk. Genetic counseling is advisable for parents who have had one child with anencephaly, since the likelihood of having another is increased.
Some congenital brain anomalies, such as anencephaly, are not compatible with life, and fetuses affected by them will die. Many more brain anomalies are not lethal. Most congenital brain anomalies, however, will impact mental functioning, development, and, in some cases, physical mobility. When functions of the brain are severely limited, the child may be placed on a life support system. If the essential tasks of the brain are not impaired, the child will most likely face brain dysfunction, including any or all of the following: memory and language problems, neuromotor functioning problems, and behavioral and social problems. It is important that the child's abilities are accurately assessed and that an appropriate plan of treatment is developed. A multidisciplinary
Many children with congenital brain anomalies, such as Chiari I malformation and nearly 50 percent of those affected by hydrocephalus, have normal intellectual functioning, and some have unimpaired physical mobility. These children, however, still require medical management and benefit from a multidisciplinary healthcare team. Many face multiple surgical procedures and hospital admissions. It is important to prepare children for these medical treatments and surgeries. Children's hospitals, local school systems, and state governments can assist parents in finding the support and resources they need.
Amniocentesis—A procedure performed at 16-18 weeks of pregnancy in which a needle is inserted through a woman's abdomen into her uterus to draw out a small sample of the amniotic fluid from around the baby for analysis. Either the fluid itself or cells from the fluid can be used for a variety of tests to obtain information about genetic disorders and other medical conditions in the fetus.
Cerebrospinal fluid—The clear, normally colorless fluid that fills the brain cavities (ventricles), the subarachnoid space around the brain, and the spinal cord and acts as a shock absorber.
Congenital—Present at birth.
Fetus—In humans, the developing organism from the end of the eighth week to the moment of birth. Until the end of the eighth week the developing organism is called an embryo.
Spasticity—Increased mucle tone, or stiffness, which leads to uncontrolled, awkward movements.
Moore, Keith L., et al. Before We Are Born: Essentials of Embryology and Birth Defects. Kent, UK: Elsevier— Health Sciences Division, 2002.
"Congenital Birth Defects." Dr. Joseph F. Smith Medical Library. Available online at <www.chclibrary.org/micromed/00043570.html> (accessed December 8, 2004).
"Congenital Birth Defects." Principal Health News. Available online at <www.principalhealthnews.com/topic/topic100586649> (accessed December 8, 2004).
Richard Robinson Deborah L. Nurmi, MS