Neural tube defects
Neural tube defects: Excerpt from Professional Guide to Diseases (Eighth Edition)
Neural tube defects (NTDs) are serious birth defects that involve the spine or brain; they result from failure of the neural tube to close at approximately 28 days after conception. The most common forms of NTDs are spina bifida (50% of cases), anencephaly (40%), and encephalocele (10%).
Spina bifida occulta is the most common and least severe spinal cord defect. It's characterized by incomplete closure of one or more vertebrae without protrusion of the spinal cord or meninges.
However, in more severe forms of spina bifida, incomplete closure of one or more vertebrae causes protrusion of the spinal contents in an external sac or cystic lesion (spina bifida cystica). Spina bifida cystica has two classifications: myelomeningocele (meningomyelocele) and meningocele. In myelomeningocele, the external sac contains meninges, cerebrospinal fluid (CSF), and a portion of the spinal cord or nerve roots distal to the conus medullaris. When the spinal nerve roots end at the sac, motor and sensory functions below the sac are terminated. In meningocele, less severe than myelomeningocele, the sac contains only meninges and CSF. Meningocele may produce no neurologic symptoms.
In encephalocele, a saclike portion of the meninges and brain protrudes through a defective opening in the skull. Usually, it's in the occipital area, but it may occur in the parietal, nasopharyngeal, or frontal area.
In anencephaly, the most severe form of NTD, the closure defect occurs at the cranial end of the neuroaxis and, as a result, part or the entire top of the skull is missing, severely damaging the brain. Portions of the brain stem and spinal cord may be missing. No diagnostic or therapeutic efforts are helpful; this condition is invariably fatal.
Causes and incidence
NTDs may be isolated birth defects, may result from exposure to a teratogen, or may be part of a multiple malformation syndrome (for example, chromosomal abnormalities such as trisomy 18 or 13 syndrome). Isolated NTDs (those not due to a specific teratogen or associated with other malformations) are believed to be caused by a combination of genetic and environmental factors. Although most of the specific environmental triggers are unknown, recent research has identified a lack of folic acid in the mother's diet as one of the risk factors.
The incidence of NTDs varies greatly among countries and by region in the United States. For example, the incidence is significantly higher in the British Isles and low in southern China and Japan. In the United States, North and South Carolina have at least twice the incidence of NTDs as most other parts of the country. These birth defects are also less common in blacks than in whites.
Signs and symptoms
Spina bifida occulta is usually accompanied by a depression or dimple, tuft of hair, soft fatty deposits, port wine nevi, or a combination of these abnormalities on the skin over the spinal defect; however, such signs may be absent. Spina bifida occulta doesn't usually cause neurologic dysfunction but occasionally is associated with foot weakness or bowel and bladder disturbances. Such disturbances are especially likely during rapid growth phases, when the spinal cord's ascent within the vertebral column may be impaired by its abnormal adherence to other tissues.
In both myelomeningocele and meningocele, a saclike structure protrudes over the spine. Like spina bifida occulta, meningocele seldom causes neurologic deficit. But myelomeningocele, depending on the level of the defect, causes permanent neurologic dysfunction, such as flaccid or spastic paralysis and bowel and bladder incontinence. Associated disorders include trophic skin disturbances (ulcerations, cyanosis), clubfoot, knee contractures, hydrocephalus (in about 90% of patients), and possibly mental retardation, Arnold-Chiari syndrome (in which part of the brain protrudes into the spinal canal), and curvature of the spine.
Clinical effects of encephalocele vary with the degree of tissue involvement and location of the defect. Paralysis and hydrocephalus are common. Infants with this defect have a better chance of survival than anencephalic infants and usually suffer less paralysis; however, surviving infants are usually severely mentally retarded.
Diagnosis
Confirming diagnosis
Amniocentesis can detect elevated alpha fetoprotein (AFP) levels in amniotic fluid, which indicates the presence of an open NTD. Measuring acetylcholinesterase levels can confirm the diagnosis. (Biochemical testing will usually miss closed NTDs.) Because 5% to 7% of NTDs are associated with chromosomal abnormalities, a fetal karyotype should be done in addition to the biochemical tests.
Maternal serum AFP screening in combination with other serum markers, such as human chorionic gonadotropin (HCG), free beta-HCG, or unconjugated estriol, may be offered to some patients who aren't scheduled for amniocentesis, such as those with a lower risk of NTDs and those who will be younger than age 34½ at the time of delivery. Although this screening test can't diagnose either an open NTD or a chromosomal abnormality, it can estimate a fetus's risk of such a defect. Most patients with abnormal maternal serum AFP levels won't have an affected child; however, they may be at increased risk for perinatal complications, such as premature rupture of the membranes, abruptio placentae, or fetal death.
Ultrasound may be used when the fetus has an increased risk of an open NTD based on either the family history or abnormal serum screening results; however, this test alone can't identify all open NTDs or ventral wall defects.
If the NTD isn't diagnosed before birth, other tests are used to make the diagnosis. For example, spina bifida occulta is commonly overlooked, although it's occasionally palpable and spinal X-ray can show the bone defect. Myelography can differentiate it from other spinal abnormalities, especially spinal cord tumors.
Myelomeningocele and meningocele are obvious on examination; transillumination of the protruding sac can sometimes distinguish between them. (In meningocele, it typically transilluminates; in myelomeningocele, it doesn't.) In myelomeningocele, a pinprick examination of the legs and trunk shows the level of sensory and motor involvement; skull X-rays, cephalic measurements, and computed tomography (CT) scan demonstrate associated hydrocephalus. Other appropriate laboratory tests in patients with myelomeningocele include urinalysis, urine cultures, and tests for renal function starting in the neonatal period and continuing at regular intervals.
In encephalocele, X-rays show a basilar bony skull defect. CT scan and ultrasonography further define the defect.
Treatment
Prompt neurosurgical repair and aggressive management may improve the condition of children with some NTDs, but serious and permanent disability is likely.
Spina bifida occulta usually requires no treatment. Treatment of meningocele consists of surgical closure of the protruding sac and continual assessment of growth and development. Treatment of myelomeningocele requires repair of the sac and supportive measures to promote independence and prevent further complications. Surgery doesn't reverse neurologic deficits. A shunt may be needed to relieve associated hydrocephalus.
Treatment of encephalocele includes surgery during infancy to place protruding tissues back in the skull, excise the sac, and correct associated craniofacial abnormalities.
Special considerations
❑ When an NTD has been diagnosed prenatally, refer the prospective parents to a genetic counselor, who can provide information and support the couple's decisions on how to manage the pregnancy.
❑ Recent research sponsored by the March of Dimes and others has indicated that the risk of an open NTD may be reduced 50% to 70% in pregnant women who take a daily multivitamin with folic acid. Urge all women of childbearing age to take such a vitamin supplement until menopause or the end of childbearing potential. (See Folic acid supplement recommendations.)
Pediatric tip
The parents of a child with an NTD will need assistance from physicians, nurses, surgeons, rehabilitation providers, and social workers. Help to coordinate such assistance as needed. Obviously, care is most complex when the neurologic deficit is severe. Immediate goals include psychological support to help parents accept the diagnosis and preoperative and postoperative care. Long-term goals include patient and family teaching, and measures to prevent contractures, pressure ulcers, urinary tract infections (UTIs), and other complications.
Before surgery:
❑ Prevent local infection by cleaning the defect gently with sterile saline solution or other solutions, as ordered. Inspect the defect often for signs of infection, and cover it with sterile dressings moistened with sterile saline solution. Prevent skin breakdown by placing sheepskin or a foam pad under the infant. Keep skin clean, and apply lotion to knees, elbows, chin, and other pressure areas. Give antibiotics as ordered.
❑ Handle the infant carefully, and don't apply pressure to the defect. Usually, the infant can't wear a diaper or a shirt until after surgical correction because it will irritate the sac, so keep him warm in an infant Isolette. Hold and cuddle the infant; on your lap, position him on his abdomen; teach parents to do the same.
❑ Provide adequate time for parent-child bonding if possible.
❑ Measure head circumference daily, and watch for signs of hydrocephalus and meningeal irritation, such as fever or nuchal rigidity. Be sure to mark the spot so you get accurate readings.
❑ Contractures can be minimized by passive range-of-motion exercises and casting. To prevent hip dislocation, moderately abduct hips with a pad between the knees or with sandbags and ankle rolls.
❑ Monitor intake and output. Watch for decreased skin turgor, dryness, or other signs of dehydration. Provide meticulous skin care to genitals and buttocks to prevent infection.
❑ Ensure adequate nutrition.
After surgery:
❑ Watch for hydrocephalus, which commonly follows surgery. Measure the infant's head circumference as ordered.
❑ Monitor vital signs often. Watch for signs of shock, infection, and increased intracranial pressure (ICP) such as projectile vomiting. Frequently assess the infant's fontanels. Remember that, before age 2, infants don't show typical signs of increased ICP because suture lines aren't fully closed. In infants, the most telling sign is bulging fontanels.
❑ Change the dressing regularly as ordered, and check and report any signs of drainage, wound rupture, and infection.
❑ Place the infant in the prone position to protect and assess the site.
❑ If leg casts have been applied to treat deformities, watch for signs that the child is outgrowing the cast. Regularly check distal pulses to ensure adequate circulation.
To help parents cope with their infant's physical problems and successfully meet long-term treatment goals:
❑ Teach them to recognize early signs of complications, such as hydrocephalus, pressure ulcers, and UTIs.
❑ Provide psychological support and encourage a positive attitude. Help parents work through their feelings of guilt, anger, and helplessness.
❑ Encourage parents to begin training their child in a bladder routine by age 3. Emphasize the need for increased fluid intake to prevent UTIs. Teach intermittent catheterization and conduit hygiene, as ordered.
❑ To prevent constipation and bowel obstruction, stress the need for increased fluid intake, a high-bulk diet, exercise, and a stool softener, as ordered. If possible, teach parents to help empty their child's bowel by telling him to bear down, and giving a glycerin suppository as needed.
❑ Urge early recognition of developmental lags (a possible result of hydrocephalus). The child may need to attend a school with special facilities. Also, stress the need for stimulation to ensure maximum mental development. Help parents plan activities appropriate to their child's age and abilities. Refer to early intervention.
❑ Refer parents for genetic counseling, and suggest that parents consider an amniocentesis in future pregnancies. Also refer parents to the Spina Bifida Association of America.
Pictures
Book Source Details
- Book Title: Professional Guide to Diseases (Eighth Edition)
- Author(s): Springhouse
- Year of Publication: 2005
- Copyright Details: Professional Guide to Diseases (Eighth Edition), Copyright © 2005 Lippincott Williams & Wilkins.
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