Neural Tube Defects

Formation of the Neural Tube

The central nervous system starts to develop in week five of pregnancy. At that time, a thickening of cells called the neural plate forms, which folds in on itself to create what we know as the neural tube. One end of the neural tube will form the brain, and the remainder of the neural tube will form the spinal cord. In week six, openings in the neural tube close. If these openings do not close properly, abnormalities called neural tube defects can form. These defects are rare, but they account for most of the abnormalities of the brain and spinal cord.

Abnormalities of the spinal cord

Spina bifida

Spina bifida is the most common permanently disabling birth defect. In spina bifida, there are openings in the spinal cord because the neural tube did not close properly during early pregnancy. The disease affects every person differently, and varies in its severity. People with spina bifida may have some of the following symptoms:

• hydrocephalus: a build-up of cerebrospinal fluid in the brain, which causes the brain to swell
• paralysis
• problems controlling their bowel movements
• learning disabilities

There are different types of spina bifida. Spina bifida occulta, a mild condition, is when the spinal cord is normal with no openings to the back. There may be a small gap in some of the small bones that make up the spine. Although there may be no motor and sensory problems at birth, there may be subtle neurological deterioration later in childhood or as the child grows into adulthood.

Spina bifida cystica is a severe condition associated with myelomeningocele, where the protective coating of the spinal cord comes through the openings in the spine. This causes a sac filled with cerebrospinal fluid to protrude through the baby’s back. In rare cases, spinal nerves may also protrude into the sac outside the back. This can cause nerve damage and severe disabilities, but usually not mental retardation. Myelomeningocele, which causes paralysis of the legs and sometimes the arms, may be corrected with surgery.

Spina bifida cystica is also associated with another condition called meningocele, where the cerebrospinal fluid and membranes covering the spinal cord and brain bulge beneath the skin. Meningocele is treated with surgery. 

Abnormalities of the brain

Encephaloceles

Encephaloceles are when parts of the brain protrude through openings in the skull. The protruding parts of the brain are encased in a sac. Usually, encephaloceles are dramatic deformities associated with neurological problems. They are often associated with abnormalities of the brain, skull, face, and other problems. Some examples of these abnormalities include:

• a build-up of cerebrospinal fluid in the brain, called hydrocephalus
• paralysis of the arms and legs
• an abnormally small head, called microcephaly
• uncoordinated movement
• mental retardation
• developmental delay
• vision problems
• seizures

Encephaloceles are usually treated with surgery to place the tissues back into the skull and correct any facial abnormalities. Hydrocephalus may be treated with a shunt to drain out the extra fluid.

Anencephaly

Anencephaly is the absence of a major part of the brain, skull, and scalp. A baby born with anencephaly is usually blind, deaf, unconscious, and unable to feel pain. She may still be able to breathe. The prognosis for babies born with anencephaly is extremely poor, and there is no corrective surgery for this congenital abnormality. If the infant is not stillborn, then he or she will usually die within a few hours or days after birth.

Hydranencephaly

Hydranencephaly is a condition where the brain's cerebral hemispheres are missing and replaced by sacs of cerebrospinal fluid. A baby with hydranencephaly may appear normal at birth and have spontaneous reflexes such as sucking, crying, and swallowing. She may move her arms and legs normally. Her head size may appear normal. However, after a few weeks, the baby usually becomes irritable and her muscles may start to tense more and more. She may develop seizures and hydrocephalus, where there is a build-up of cerebrospinal fluid in the brain. She may be deaf, blind, paralyzed, and have intellectual problems. Some babies with this condition also have respiratory problems. There is no corrective surgery for hydranencephaly, and many children with this condition die before the age of one year. Some children with hydranencephaly may survive for a few years.

Arnold-Chiari malformation

Arnold-Chiari malformation is a condition where a part of the brain protrudes into the spinal canal. Arnold-Chiari malformation can cause symptoms of headache, vomiting, difficulty swallowing, and hoarseness. Another symptom is hydrocephalus, which is a build-up of fluid in the brain. Hydrocephalus can be corrected with a tube-like device called a shunt, to drain the cerebrospinal fluid from the brain. Some ibabies with a form of spina bifida called myelomeningocele may have the Arnold-Chiari malformation. Myelomeningocele is a defect of the spine where the protective covering of the spine and the spinal nerves protrude through openings in the spine. Myelomeningocele, which causes paralysis of the legs and sometimes the arms, may be corrected with surgery.

Detecting neural tube defects

The baby can be screened for neural tube defects using integrated prenatal screening, which uses a combination of ultrasound and blood tests to determine the risk of having a baby with these conditions. Since this is only a screening test, any unusual results must be followed up by a diagnostic test such as amniocentesis.

Some neural tube defects have a genetic component. If you have a previous history of a baby with a neural tube defect, you may wish to seek advice from a genetic counsellor to determine what your options are for the next pregnancy. You may also be referred to a genetic counsellor if you are pregnant and your baby shows signs of a neural tube defect.

Preventing neural tube defects

Studies show that folic acid is very important to the prevention of neural tube defects, especially in early pregnancy. The recommended folic acid intake for women with no health risks or with health risks such as epilepsy, diabetes, or obesity is 0.4 to 1.0 mg per day with a daily multivitamin. Folic acid should taken from two to three months before conception, throughout pregnancy, and for the first four to six weeks after birth or as long as breastfeeding continues.

A woman who has previously conceived a baby with a birth defect such as anencephaly, myelomeningocele, cleft lip or palate, structural heart disease, limb defect, a defect of the urinary tract, or hydrocephalus should take 5.0 mg of folic acid daily from three months before conception until 10 to 12 weeks after conception. After that time, she needs to continue taking folic acid 0.4 to 1.0 mg per day throughout the rest of pregnancy and for the first four to six weeks after birth or as long as breastfeeding continues.