Neural tube defects
Neural tube defects: Excerpt from Handbook of Diseases
Neural tube defects (NTDs) are serious birth defects that involve the spine or brain; they result from failure of the neural tube to close about 28 days after conception. The most common forms of NTD 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 (meningo-myelocele) and meningocele. With 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. With meningocele, which is less severe than myelomeningocele, the sac contains only meninges and CSF. Meningocele may produce no neurologic symptoms.
With 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 also occur in the parietal, nasopharyngeal, or frontal area.
With anencephaly, the most severe form of NTD, the closure defect occurs at the cranial end of the neuroaxis and, as a result, part or all of the top of the skull is missing, severely damaging the brain. Portions of the brain stem and spinal cord may also be missing. No diagnostic or therapeutic efforts are helpful; this condition is invariably fatal.
Causes
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 a risk factor.
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 lower in southern China and Japan. In the United States, Appalachian and East Coast areas have a slightly higher incidence of NTDs than most other parts of the country. These birth defects are also more common in whites than in blacks.
Signs and symptoms
Spina bifida occulta is commonly 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.
With 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.
Signs and symptoms of encephalocele vary with the degree of tissue involvement and location of the defect. However, surviving infants are usually severely mentally retarded, with paralysis and hydrocephalus common effects.
Diagnosis
Amniocentesis may detect elevated alpha-fetoprotein (AFP) levels in amniotic fluid, which indicates the presence of an open neural tube defect. 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 betaHCG, 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 34 ½ 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. The majority of patients with abnormal maternal serum AFP levels won’t have an affected child, but they should be offered diagnostic testing by amniocentesis. If the amniocentesis results are normal, abnormal AFP levels may still indicate an increased risk of 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.
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.
Meningocele and myelomeningocele are obvious on examination; trans-illumination of the protruding sac can sometimes help distinguish between them. (In most patients with meningocele, it can be transilluminated; in those with myelomeningocele, it can’t.) With 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 for patients with myelomeningocele include urinalysis, urine cultures, and tests for renal function starting in the neonatal period and continuing at regular intervals.
With 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 disabilities are 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.
❑ The risk of an open NTD may be reduced by 50% in pregnant women who take a daily multivitamin with folic acid.
CLINICAL TIP: Urge all women of childbearing age to take a vitamin supplement with folic acid until menopause or the end of childbearing potential. (See Folic acid supplement recommendations.)
❑ The parents of a child with an NTD will need assistance from physicians, nurses, surgeons, rehabilitation provi-ders, 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. 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, and teach the 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 can be a complication following 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, or infection.
❑ Place the infant in the prone position to protect and assess the site.
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). If present, stress the importance of follow-up IQ assessment to help plan realistic educational goals. 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 parents for genetic counseling, and suggest that amniocentesis be performed in future pregnancies. Also, refer parents to the Spina Bifida Association of America.
Pictures
Book Source Details
- Book Title: Handbook of Diseases
- Author(s): Springhouse
- Year of Publication: 2003
- Copyright Details: Handbook of Diseases, Copyright © 2003 Lippincott Williams & Wilkins.
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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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More About This Book:
Title: Handbook of Diseases
Authors: Springhouse
Publisher: Lippincott Williams & Wilkins
Copyright: 2003
ISBN: 1-58255-266-5
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