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Neural Tube Defects and Spina Bifida

By Stephen J. Falchek, MD, Director, Residency Program and formerly Division Chief of Pediatric Neurology;Instructor, Nemours/Alfred I. duPont Hospital for Children;Sidney Kimmel Medical College of Thomas Jefferson University

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Neural tube defects are a certain type of birth defect of the brain, spine, and/or spinal cord.

  • Neural tube defects can result in nerve damage, learning disabilities, paralysis, and death.

  • The diagnosis can be made before birth and is based on a blood test, an amniotic fluid test, and an ultrasound.

  • Folate (folic acid) taken by the mother before conception and during the first trimester can help prevent these defects.

  • Surgery is needed to close neural tube defects.

In the fetus, the brain and spinal cord develop as a groove that folds over to become a tube called the neural tube. Layers of tissue that come from this tube normally become the brain and spinal cord and the tissues that cover them, including part of the spine and the meninges. Sometimes the neural tube does not develop normally, which may affect the brain, spinal cord, and meninges.

Anencephaly is the most severe form of neural tube defect. In anencephaly, the brain tissue fails to develop. This defect is fatal.

Chiari malformation may be present. In this abnormality, the cerebellum (the part of the brain that controls balance) protrudes through the opening in the bottom of the skull. The protruding cerebellum may put pressure on the brain stem or spinal cord. Children may develop hydrocephalus (water on the brain).

Syringomyelia, a dilation of the normally small fluid-filled central canal of the spinal cord, cord may be present.

Spina bifida

Spina bifida results when the neural tube fails to close completely and remains an open channel. In spina bifida, the bones of the spine (vertebrae) do not close over the spinal cord. It most commonly affects the spine in the lower back. One or more of the vertebrae may be involved.

Spina bifida occulta is the mildest form of spina bifida. Only the bone is affected and the spinal cord and meninges are unaffected. This common defect is called occulta because it is hidden (covered) by a layer of skin that typically appears normal except sometimes there may be a small tuft of hair or different color of the skin overlying the defect. It usually causes no symptoms, but children who have a larger defect may have symptoms such as leg weakness or bladder dysfunction.

In occult spinal dysraphism, a more severe form of spina bifida, newborns are born with visible abnormalities on their lower back. These include birthmarks, overly pigmented areas (hemangiomas and flame nevus [stork bite]), tufts of hair, openings in the skin (dermal sinus), or small lumps (masses). The underlying spinal cord may be abnormal such as having a fatty tumor (lipoma) or other problems that can lead to nerve damage.

In spina bifida cystica, the most serious form of spina bifida, tissues of the meninges and/or spinal cord protrude through the opening in the vertebrae, causing the following defects:

  • A meningocele: Only the meninges protrude

  • A meningoencephalocele: The meninges and brain tissue protrude

  • A meningomyelocele: The meninges and spinal cord tissue protrude

  • An encephalocele: Only brain tissue protrudes

  • A myelocele: Only spinal cord tissue protrudes

Damage to brain or spinal cord tissue is much more likely when the tissue protrudes than when it does not. Also, when spinal cord tissue or meninges are exposed, they may be infected by bacteria, causing meningitis.

Spina Bifida: A Defect of the Spine

In spina bifida, the bones of the spine (vertebrae) do not form normally. Spina bifida can vary in severity.

In occult spinal dysraphism, one or more vertebrae do not form normally, and the spinal cord and the layers of tissues (meninges) surrounding it may also be affected. The only symptom may be a tuft of hair, a dimpling, or a pigmented area on the skin over the defect.

In a meningocele, the meninges protrude through the incompletely formed vertebrae, resulting in a fluid-filled bulge under the skin. The spinal cord is in its normal location.

The most severe type is a meningomyelocele, in which the meninges and spinal cord protrude. The affected area appears raw and red, and the infant is likely to be severely impaired.

Causes

There are many causes of neural tube defects. Deficiency of a vitamin, folate, during pregnancy is a significant factor. Genetic factors and use of certain drugs during pregnancy (such as valproate) can make neural tube defects more likely. The defect often develops before the mother knows she is pregnant.

Symptoms

Many children who have minor neural tube defects have no symptoms.

Most symptoms caused by neural tube defects result from brain or spinal cord damage.

Brain damage can cause problems, including water on the brain (hydrocephalus), learning disabilities, and difficulty swallowing.

Spinal cord damage can cause severe problems, typically involving the bowels, bladder, and legs. Problems include

Other problems, such as clubfoot, arthrogryposis (joints, usually ankles, that become frozen and cannot bend), a dislocated hip, or an abnormally curved spine (kyphosis), may also be present at birth.

Diagnosis

  • Before birth, blood tests or amniocentesis to measure alpha-fetoprotein levels and prenatal ultrasonography

  • After birth, physical examination and additional imaging tests

Many neural tube defects can be detected before birth with prenatal screening tests. A high level of alpha-fetoprotein in a pregnant woman's blood or in amniotic fluid may indicate a neural tube defect in the fetus. So during the 2nd trimester, blood tests or amniocentesis (removing a sample of fluid from around the fetus) may be done to measure these levels.

Prenatal ultrasonography may show the defect or characteristic abnormalities.

After birth, some defects are obvious during the physical examination. If newborns have abnormalities that suggest occult spinal dysraphism, ultrasonography or magnetic resonance imaging (MRI) is done to check for defects in the spine. X-rays may also be done.

After spina bifida is diagnosed, tests to evaluate bladder function are done. They include urinalysis, urine culture, blood tests, and ultrasonography.

Prognosis

With appropriate care, most children do well. However, complications, such as loss of kidney function and problems with shunts needed to treat hydrocephalus, may occur and sometimes cause death in older children.

Prevention

  • Folate

All women of childbearing age who have not had an infant with a neural tube defect should consume folate (folic acid) through diet or by taking a supplement and continue doing so through the first 3 months of pregnancy.

Women who have had an infant with a neural tube defect are at high risk of having another affected infant and should take high-dose folate supplements beginning 3 months before getting pregnant and continuing through the first 3 months of pregnancy. Folate can reduce the risk of neural tube defects by as much as 75%.

Did You Know...

  • Taking folate before and during pregnancy can decrease the risk of neural tube defects.

Treatment

  • Surgery

Health care practitioners, usually a team of specialists (including a neurosurgeon, a urologist, a pediatrician, a pediatric rehabilitation medicine specialist, an orthopedic surgeon, a physical therapist, a nurse practitioner, and a social worker), evaluate the type and severity of the defect and talk to the family about how treatment and care can be implemented.

Neural tube defects are usually closed surgically. Certain defects, such as a myelomeningocele, are typically repaired soon after birth.

Bladder, orthopedic, and other problems are treated as needed.

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