Neural tube defects

Definition

Neural tube defects are a group of severe birth defects in which the brain and spinal cord are malformed and lack the protective skeletal and soft tissue encasement.

Description

Incomplete formation and protection of the brain or spinal cord with bony and soft tissue coverings during the fourth week of embryo formation are known collectively as neural tube defects. Lesions may occur anywhere in the midline of the head or spine. These defects are among the most common serious birth defects, but they vary considerably in their severity. In some cases, the brain or spinal cord is completely exposed, in some cases protected by a tough membrane (meninges), and in other cases covered by skin.

Spina bifida accounts for about two-thirds of all neural tube defects. The spine defect may appear anywhere from the neck to the buttocks. In its most severe form, termed "spinal rachischisis," the entire spinal canal is open exposing the spinal cord and nerves. More commonly, the defect appears as a localized mass on the back that is covered by skin or by the meninges.

Anencephaly, the second most common neural tube defect, accounts for about one-third of cases. Two major subtypes occur. In the most severe form, all of the skull bones are missing and the brain is exposed in its entirety. The second form, in which only a part of the skull is missing and a portion of the brain exposed, is termed "meroacrania."

Encephaloceles are the least common form of neural tube defects, comprising less than ten percent of birth defects. With encephaloceles, a portion of the skull bones are missing leaving a bony hole through which the brain and its coverings herniate (protrude). Encephaloceles occur in the midline from the base of the nose, to the junction of the skull and neck. As with spina bifida, the severity varies greatly. In its mildest form, encephaloceles may appear as only a small area of faulty skin development with or without any underlying skull defect. At the severe end of the spectrum, most of the brain may be herniated outside of the skull into a skin-covered sac.

Genetic profile

Most neural tube defects (80-90%) occur as isolated defects. Neural tube defects of this variety are believed to arise through the combined influence of genetic and environmental forces. This multifactorial causation presumes that one or more predisposing genes collaborate with one or more environmental influences to lead to the birth defect. Poor nutrition is believed to be an environmental risk factor and hereditary defects in the absorption and utilization of folic acid are presumptive genetic predisposing factors. After a couple has one infant with a neural tube defect, the recurrence risk is 3-5%. After the birth of two NTD-affected infants, the risk increases to 8-10%.

When neural tube defects occur concurrently with other malformations there is a greater likelihood of an underlying specific genetic or environmental cause. Genetic causes include chromosome aberrations and single gene mutations. Environmental causes include maternal diabetes mellitus, exposure to prolonged hyperthermia, and seizure medications during the early months of pregnancy.

Demographics

Neural tube defects occur worldwide. It appears that the highest prevalence (about one in 100 pregnancies) exists in certain northern provinces in China; an intermediate prevalence (about one in 300-500 pregnancies) has been found in Ireland and in Central and South America; and the lowest prevalence (less than one in 2,000 pregnancies) has been found in the Scandinavian countries. In the United States, the highest prevalence has occurred in the Southeast. Worldwide there has been a steady downward trend in prevalence rates over the past 50-70 years.

Signs and symptoms

Because of the faulty development of the spinal cord and nerves, a number of consequences are commonly seen in spina bifida. As a rule, the nerves below the level

of the defect develop in a faulty manner and fail to function, resulting in paralysis and loss of sensation below the level of the spinal lesion. Since most defects occur in the lumbar region, the lower limbs are usually paralyzed and lack normal sensation. Furthermore, the bowel and bladder have inadequate nerve connections, causing inability to control bladder and bowel function. Sexual function is likewise impaired. Hydrocephaly develops in most infants either before or after surgical repair of the spine defect.

In anencephaly, the brain is destroyed by exposure during intrauterine life. Most infants with anencephaly are stillborn, or die within the initial days or weeks after birth.

Infants with encephaloceles have variable neurologic impairments depending on the extent of brain involvement. When only the brain covering is involved, the individual may escape any adverse effect. However, when the brain is involved in the defect, impairments of the special senses such as sight, hearing, and cognitive thinking commonly result.

Diagnosis

At birth, the diagnosis is usually obvious based on external findings. Prenatal diagnosis may be made with ultrasound examination after 12-14 weeks of pregnancy. Screening of pregnancies can be carried out at 16 weeks by testing the mother's blood for the level of alpha-feto-protein. Open neural tube defects leak this fetal chemical into the surrounding amniotic fluid, a small portion of which is absorbed into the mother's blood.

Treatment and management

No treatment is available for anencephaly. Aggressive surgical and medical management has improved survival and function of infants with spina bifida. Surgery closes the defect, providing protection against injury and infection. A common complication that may occur before or after surgical correction is the accumulation of excessive cerebral spinal fluid (hydrocephaly) in the major cavities (ventricles) within the brain. Hydrocephaly is usually treated with the placement of a mechanical shunt, which allows the cerebral spinal fluid from the ventricles to drain into the circulation or another body cavity. A number of medical and surgical procedures have been used to protect the urinary system as well. Walking may be achieved with orthopedic devices. Encephaloceles are usually repaired by surgery soon after birth. The success of surgery often depends on the amount of brain tissue involved in the encephalocele.

It has been found that 400 micrograms of folic acid taken during the periconceptional period (two to three months prior to conception, and two to three months following conceptions) protects against most neural tube defects. While there are a number of foods (green leafy vegetables, legumes, liver, and orange juice) that are good sources of natural folic acid, synthetic folic acid is available in over-the-counter multivitamins and a number of fully fortified breakfast cereals.

Additionally, a population-wide increase in folic acid intake has been achieved through the fortification of enriched cereal grain flours since January 1998, a measure authorized by the United States Food and Drug Administration. The increased blood folic acid levels achieved in recent years has likely resulted from the synergy of dietary, supplementation, and fortification sources of folic acid.

Prognosis

Infants with anencephaly are usually stillborn or die within the initial days of life. Eighty to ninety percent of infants with spina bifida survive with surgery. Paralysis below the level of the defect, including an inability to control bowel and bladder function, and hydrocephaly are complications experienced by most survivors. Intellectual function is normal in most cases.

The prognosis for infants with encephaloceles varies considerably. Small encephaloceles may cause no disability whether surgical correction is performed or not. Infants with larger encephaloceles may have residual impairment of vision, hearing, nerve function, and intellectual capacity.