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Congenital brain defects are abnormalities in the brain that are present at birth. The defects typically affect the bone and soft tissue in the head and spine. There are many different types of these malformations. They can vary greatly from mild to severe conditions.

A congenital brain defect usually occurs due to an interruption in the normal growth of the nervous system. The brain begins to form in the first month after conception. Development of the brain begins from a small, special plate of cells on the surface of the embryo. These cells grow and form the different regions of the brain.

When this process is disturbed, it can cause structural defects in the brain and skull. Normal brain function can be impaired even if only the skull’s growth is upset.

Types of Congenital Brain Defects

Early in fetal development, a flat strip of tissue along the back of the fetus rolls up to form the neural tube. This tube runs along most of the length of the embryo.

The neural tube typically closes between the third and fourth weeks after conception. It develops into the spinal cord with the brain at the top. If the tube does not close properly, the tissue within the tube can’t develop properly. Several types of congenital brain defects are caused by neural tube defects. They are:

Anencephaly: This occurs when the head end of the neural tube fails to close. A major portion of the skull and brain is missing. Existing brain tissue is exposed.

Encephalocele: This occurs when a portion of the brain bulges through an opening in the skull. It typically is located along the front-to-back midline at the back of the skull.

Spina bifida: This occurs when the neural tube doesn’t close below the level of the brain. This prevents the surrounding vertebrae from fully developing. It leaves the spinal column divided.

Myelomeningocele is the most common type of spina bifida. It occurs when the spinal cord and backbone do not close properly. A portion of the vertebrae may protrude through the gap. Arnold-Chiari, or Chiari II malformation occurs when part of the cerebellum is shifted downward into the upper spinal column. The result is compression on the brain or spinal cord. Other types of congenital brain defects develop within the structure of the brain.

Hydrocephalus, or fluid on the brain, is an excessive buildup of cerebrospinal fluid (CSF) due to impaired circulation of the CSF. When there is excess fluid, it can put too much pressure on the brain.

Dandy-Walker syndrome is a defect that involves the absence or defective growth of the central section of the cerebellum. Holoprosencephaly is a condition in which the brain doesn’t divide into two halves, or hemispheres.

Megalencephaly, also called macrencephaly, is a brain that is abnormally large or heavy. Microcephaly, or a small brain, occurs when the brain doesn’t develop to full size.

What Causes Congenital Brain Defects?

Most congenital brain defects can’t be attributed to a specific single cause. A variety of genetic and environmental factors have been linked to the development of congenital brain defects. These factors may be related to gene defects, infection, drug use, or other trauma to the unborn fetus.

Some brain defects are symptoms of trisomy. Trisomy occurs when an extra, or third, chromosome is present where there typically are only two chromosomes. Dandy—Walker and Chiari II defects are associated with trisomy of chromosome 9. Trisomy of chromosome 13 can cause holoposencephaly and microcephaly. Symptoms of trisomy of chromosomes 13 and 18 can include neural tube defects.

Who Is at Risk for Congenital Brain Defects?

he following factors are associated with the development of congenital brain defects. They can put unborn babies at a higher risk for brain defects:

    alcohol use
    use of certain prescription drugs (anticonvulsant drugs, warfarin, retinoic acid, and tretinoin)
    use of recreational drugs
    infection (rubella, cytomegalovirus, herpes simplex, and varicella zoster)
    nutritional deficiencies
    exposure to X-rays or radiation therapy
    Mothers who have diabetes mellitus or phenylketonuria also are at a higher risk for having babies with congenital brain defects. Any type of trauma to the unborn child also can affect brain development.

Symptoms of Congenital Brain Defects

Symptoms of congenital brain defects vary. Each defect has a distinct set of symptoms and impairments. Some of these symptoms may not be apparent until after birth when the child exhibits developmental or growth delays.

Children born with congenital brain defects also may have cardiovascular disorders, gastrointestinal defects, cleft lip and palate, and seizures. Head pain, muscle weakness, reduced vision, and bladder and bowel problems are also common.

Diagnosing Congenital Brain Defects

In some cases, a congenital brain defect may be identified by detailed ultrasound. If further investigation is needed, magnetic resonance imaging (MRI) can be used to see details of the brain and spine of the fetus.

If a congenital brain defect has been diagnosed during a pregnancy, it may be possible to provide prenatal diagnosis during a subsequent pregnancy. This can be done by using chorion villus sampling (CVS) at the 11-week stage.

In some cases, accurate diagnosis may not be possible until after birth when signs such as mental retardation, delayed behavior, and seizures may be more visible.