Neural tube defects
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.
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.
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.
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.