Neural Tube Defects

September 17, 2011

Defects of neural tube development are congenital anomalies with an estimated incidence of 1 in 2,000 live births. Formation of the neural tube is completed by 28 days of gestation, before most women are even aware that they are pregnant. Neural tube defects (NTDs) result from the failure of closure of the neural folds at one or more of sites.

The most severe form of Neural tube defects occurs when there is a total failure of neural tube formation. Failure of closure of the anterior portion of the neural tube results in anencephaly, a total or partial absence of the brain and skull. Anencephalocele occurs when there is a limited closure defect of the skull with brain and membrane tissue protruding through the opening. Spina bifida results from failed closure of a portion of the posterior neural tube. Myelomeningocele is a form of spina bifida in which a protruding sac contains nerves, covering membrane tissue, and cerebrospinal fluid. A meningocele is a less frequent type of spina bifida in which the sac protruding through the vertebral defect contains meninges, the membrane tissue enclosing the spinal cord. In another form of spina bifida, intact skin covers the vertebral defect, often concealing the lesion and leading to the term spina bifida occulta. Skin lesions such as hair tufts, sinus tracts, or birthmarks may raise suspicion of an underlying spina bifida occulta, which occurs in approximately 10% of otherwise healthy people. Thus mild forms are considered normal variants. However, some forms of spina bifida occulta are associated with fibrous bands that limit mobility of the lower spinal cord. The tension transmitted to the spinal cord during movement can cause injury and progressive neurological symptoms such as lower back pain, muscle atrophy, and urinary incontinence.

Screening the pregnant population for Neural tube defects is accomplished by measuring maternal serum levels of the major fetal tissue protein, alpha-fetoprotein (AFP). If the fetal skin layer is not intact, greater amounts of AFP leak into the amniotic fluid and then diffuse into the maternal bloodstream. AFP values that are 2-2.5 times above the average identify the pregnancy as high risk and lead to the detection of over 80% of fetuses with Neural tube defects. High-resolution ultrasound is recommended if screening detects an elevated AFP level as it is able to identify nearly 100% of open Neural tube defects. Although AFP screening is noninvasive and does not physically harm the mother or fetus, receiving abnormal results can cause great emotional stress. Awareness of congenital anomalies in the fetus leads to ethically complex decisions about whether to terminate the pregnancy.

Neural tube closure is a complex process, and defects have genetic and environmental etiologies. The incidence of Neural tube defects varies greatly among populations and ethnic groups. In the United States, African and Asian Americans have a lower rate of Neural tube defects than those of Hispanic or European heritage. Risk factors for neural tube defects include diabetes, hyperthermia, and use of the anticonvulsant valproic acid. In addition, alterations in genes involved in the processing of folate, a form of B vitamin, are found in some mothers of affected infants.

Randomized investigations have shown a 60% reduction in the occurrence of Neural tube defects when women received folic acid vitamins. As a result the Public Health Service recommends that women capable of becoming pregnant consume 0.4 mg of folic acid daily and high-risk women take 4 mg of folic acid per day around conception. Estimates of folic acid intake from natural food sources show that only 8% of women consume 0.4 mg daily. Folic acid intake can be increased by consuming foods such as leafy greens, legumes, citrus fruits, liver, and whole wheat. Alternatively, folate fortified processed foods or vitamin supplements can be taken. Since the implementation of grain fortification in 1998, the average woman receives about one quarter of her daily folate requirement from cereal products.

Vitamin supplements were the only form of folate tested in the clinical trials demonstrating decreased incidence of Neural tube defects.

Elective cesarian section and in utero surgery for infants with spina bifida have been advocated to decrease trauma to the spinal cord. Cesarian delivery prior to onset of labor remains controversial because of a lack of planned research studies demonstrating a benefit. Surgery to cover the exposed spinal cord in fetuses with spina bifida during the second trimester of pregnancy is still investigational. Early results show improved neurological outcomes, but the benefit should be weighed against the increased risk of preterm labor and maternal complications.

Anencephalic infants are either stillborn or do not survive beyond a few weeks. In the United States over 90% of infants with spina bifida survive beyond a year, although most experience serious lifelong disability. All have varying degrees of paralysis and decreased sensation, and the majority also have an obstruction to the flow of cerebrospinal fluid. Complications of progressive brainstem dysfunction are the leading early cause of death in infants with spina bifida.

As children with spina bifida mature, the neurological, genitourinary, and musculoskeletal systems require ongoing surveillance. Lack of bladder control may lead to recurrent urinary tract infections and renal damage. Since spina bifida involves the spinal cord, it also inevitably affects sexual functioning. Orthopedic abnormalities include curvature of the spine and imbalanced muscle groups producing joint dislocation. Individuals with spina bifida are also at risk for severe latex allergy from repeated exposures during hospitalizations.

Spinal nerve damage may result in urinary retention, reduced rectal sensation, or an inability to retain stools. Control of urinary and fecal incontinence is essential for achieving personal and social independence. Bladder emptying at routine intervals by insertion of a clean urine catheter is an important management technique, and a range of dietary, pharmacologic, and surgical interventions also exist.

The effects of spina bifida lesions on sexual functioning vary widely between individuals. Puberty may occur earlier in those with spina bifida when compared to their siblings. Altered genital sensation is frequent in both males and females. While women usually have normal fertility, functional problems with erection and ejaculation can lead to decreased fertility in males. The course of pregnancy in women with spina bifida is similar to unaffected women except for an increased risk of urinary tract infections, pressure sores, and lower pelvic pain. Pregnant women with spina bifida are encouraged to deliver vaginally, as cesarian section is associated with higher risk of complication rates. The risk of Neural tube defects in the offspring of individuals with spina bifida is estimated at 1 in 25, which is higher than the risk in the general population. The majority of individuals with spina bifida have never discussed spina bifida-related sexual issues with their physician and providing a supportive environment can greatly improve their quality of life. Assisted reproductive technologies are available to improve male fertility and periconceptual counseling can help in pregnancy planning.

The long-term prognosis for individuals with spina bifida is difficult to estimate as improvements in medical care continue to increase life expectancy. Recent studies show survival to the third decade of life in over 50% of affected persons. Early mortality places Neural tube defects as the fifth leading cause of years of potential life lost. Despite daunting medical problems, most children with spina bifida have normal intelligence and lead a nearly independent life.

SEE ALSO: Activities of daily living, Alpha-fetoprotein screening, Disability, Nutrition, Prenatal care, Quality of life

Suggested Reading

  • Anon. (2002). Spina bifida: Key primary care issues. Australian Family Physician, 31, 66—105.
  • Botto, L. D., Moore, C. A., Khoury, M. J., & Erickson, J. D. (1999). Neural-tube defects. New England Journal of Medicine, 341, 1509-1519.
  • Locksmith, G. J., & Duff, P. (1998). Preventing neural tube defects: The importance of periconceptual folic acid supplements. Obstetrics and Gynecology, 91, 1027-1034.
  • Northup, H., & Volcik, K. A. (2000). Spina bifida and other neural tube defects. Current Problems in Pediatrics, 30, 317-332.
  • Volpe, J. (1995). Neural tube formation and prosencephalic development. In Neurology of the newborn (3rd ed., pp. 3-41). Philadelphia: W. B. Saunders.

Popular:

  • the frequency of neural tube defects such as spina bifida can be greatly decreased by the ingestion of

Tags: ,

Category: N