Overview of Congenital Brain Defects
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:
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.
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.
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.
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
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?
The following factors are associated with the development of
congenital brain defects. They can put unborn babies at a higher risk for brain
of certain prescription drugs (anticonvulsant drugs, warfarin, retinoic acid,
of recreational drugs
(rubella, cytomegalovirus, herpes simplex, and varicella zoster)
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
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.
Treating Congenital Brain Defects
Available treatment of congenital brain defects varies
according to the type and severity of defect. Anticonvulsant medications can
help with seizures. Many defects only can be treated with symptom relief.
Surgical techniques can provide improvement for some
conditions. Decompression surgery can create more space for brain and
cerebrospinal fluid in conditions where there is not enough space for brain
growth. Surgery to correct defective skulls can give the brain space to grow
normally. Shunts can be inserted to drain the cerebrospinal fluid that builds
up with hydrocephalus.
Prognosis for Congenital Brain Defects
The degree to which an individual fetus is affected by a
congenital brain defect varies greatly. The type and severity of defect, the
presence of other physical and/or mental defects, and environmental factors can
contribute to a patient’s prognosis.
Many congenital brain defects cause minor neurological
impairment. These individuals can grow to function independently. Other defects
are so severe that they are fatal before or soon after birth. Some inflict
significant disabilities. Others partially disable individuals so that they are
limited to functioning at a level that is below normal capacity.
Prevention of Congenital Brain Defects
Research and tracking of the incidence of birth defects has
helped medical experts identify specific ways to reduce congenital brain
The Centers for Disease Control and Prevention (CDC) recommends
that women who are pregnant or considering pregnancy do the following:
- Take supplements containing 400
micrograms of folic acid daily. This should begin at least one month before
getting pregnant as well as during pregnancy. Taking these supplements lowers
the risk of having a baby with neural tube defects (CDC).
- Avoid drinking alcohol at any time.
- Quit smoking before getting pregnant
or as early as possible into the pregnancy.
- If you are obese, reach a healthy
weight before getting pregnant.
- Keep blood sugar under control
before and during pregnancy, especially if you have diabetes.
- Do not take any medications or
herbal products before checking with your physician during pregnancy.