Brain and Spinal Cord Defects
Of the many possible defects in the brain and spinal cord, those known as neural tube defects develop within the first weeks of pregnancy. Others, such as porencephaly and hydranencephaly, develop later in pregnancy. Many brain and spinal cord defects result in visible abnormalities in the head or back.
Symptoms of brain or spinal cord damage may develop if the defect affects brain or spinal cord tissue. Brain damage can be fatal or result in mild or severe disability, which may include mental retardation, seizures, and paralysis. Spinal cord damage can result in paralysis, incontinence, and loss of sensation to areas of the body reached by nerves below the level of the defect (see Section 6, Chapter 93). Computed tomography (CT) and magnetic resonance imaging (MRI) can reveal brain and spinal cord defects by showing pictures of the internal structures of those organs.
Some defects, such as those that cause visible openings or swellings, can be repaired surgically. However, brain or spinal cord damage from the defect is usually permanent.
Neural Tube Defects
The brain and spinal cord develop as a groove that folds over to become a tube (the neural tube). Layers of tissue that come from this tube normally cover the brain and spinal cord (meninges). Sometimes the neural tube does not develop normally, which may affect the brain, spinal cord, and meninges. In the most severe form of neural tube defect, the brain tissue may fail to develop (anencephaly); this defect is fatal. Another type of defect results when the neural tube fails to close completely and remains an open channel. In its mildest form, an open channel defect may affect only bone; for example, in spina bifida occulta (which means hidden spine split in two), the bony spine fails to close, but the spinal cord and meninges are unaffected. This common abnormality usually causes no symptoms. Sometimes, a meningocele develops in which the meninges and other tissue, such as brain tissue (meningoencephalocele) or spinal cord tissue (meningomyelocele), can protrude out of the opening. Sometimes the meninges are not involved when tissue protrudes from the brain (encephalocele) or spinal cord (myelocele). Damage to brain or spinal cord tissue is much more likely when tissue protrudes than when it does not.
 See the figure Spina Bifida: A Defect of the Spine.
In occult spinal dysraphism, newborns are born with visible abnormalities on their lower backs. These include birthmarks, overly pigmented areas (hemangioma and flame nevus), tufts of hair, openings in the skin (dermal sinus), or small lumps (masses). The underlying spinal cord may be connected to the surface, which exposes it to bacteria, greatly increasing the chance for development of meningitis. The nerves of the spinal cord may become damaged as the child grows. Or, the spinal cord may have a fatty tumor (lipoma) on it, which also can lead to nerve damage. Therefore, newborns who have these abnormalities should have the underlying soft tissue and spinal cord evaluated using ultrasound or magnetic resonance imaging (MRI).
Genetic factors can make neural tube defects more likely. The defect often develops before the mother knows she is pregnant. Most symptoms from neural tube defects result from brain or spinal cord damage. A defect may be so inconsequential that it is never recognized or so severe that it is fatal. Meningoencephaloceles and meningomyeloceles cause severe disability. These include hydrocephalus, learning disabilities, paralysis with bone and joint abnormalities, decreased sensation of the skin, and bowel and urinary problems.
Many neural tube defects can be detected before birth. A high level of alpha-fetoprotein in the woman's blood or amniotic fluid may indicate a neural tube defect in the fetus (see Section 22, Chapter 256). Ultrasound performed late in pregnancy may show the defect or characteristic abnormalities. Folic acid taken just before a woman gets pregnant through the first three months of pregnancy can decrease the risk of neural tube defects by as much as 70%. Neural tube defects are usually closed surgically.
Hydrocephalus
The fluid surrounding the brain (cerebro-spinal fluid) is produced in spaces within the brain called ventricles. The fluid must drain to a different area, where it is absorbed into the blood. When the fluid cannot drain, hydrocephalus (water on the brain) develops. Hydrocephalus often increases the pressure in the ventricles, which compresses the brain. Many conditions, such as a birth defect, bleeding within the brain, or brain tumors can block drainage and cause hydrocephalus.
An abnormally large head may be a symptom of hydrocephalus. The infant usually fails to develop normally. Computed tomography (CT), ultrasound, or magnetic resonance imaging (MRI) of the head reveals the diagnosis as well as the degree of brain compression.
The goal of treatment is to keep pressure normal within the brain. A permanent alternate drainage path (shunt) for cerebrospinal fluid decreases the pressure and volume of the fluid inside the brain. A doctor places the shunt in the ventricles in the brain and runs it under the skin from the head to another site, usually the abdomen (ventriculoperitoneal shunt). The shunt contains a valve that allows fluid to leave the brain if the pressure becomes too high. Although a few children can eventually do without the shunt as they get older, shunts are rarely removed.
If needed, pressure within the brain can often be temporarily reduced with drugs (such as acetazolamide or furosemide) or repeated spinal taps (lumbar punctures) until a shunt is placed.
Some children with hydrocephalus develop normal intelligence. Others are mentally retarded or have learning disabilities.
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