Neural Tube Defects in the Neonatal Period Workup
- Author: Richard G Ellenbogen, MD; Chief Editor: Ted Rosenkrantz, MD more...
Presence of open neural tube defects (NTDs) can be detected with the measurement of AFP in the amniotic fluid or maternal bloodstream. AFP is the major serum protein in early embryonic life and is 90% of the total serum globulin in a fetus. It is believed to be involved in preventing fetal immune rejection and is first made in the yolk sac and then later in the GI system and liver of the fetus. It goes from the fetal blood stream to the fetal urinary tract, where it is excreted into the maternal amniotic fluid. The AFP can also leak into the amniotic fluid from open neural tube defects such as anencephaly and myelomeningocele, in which the fetal blood stream is in direct contact with the amniotic fluid.
The first step in prenatal screening is measuring the maternal serum AFP at 15-20 weeks' gestation. A patient-specific risk is then calculated based on gestational age and AFP level. For example, at 20 weeks' gestation, a maternal serum AFP concentration higher than 1,000 ng/mL would be indicative of an open neural tube defect. Normal AFP concentration in the maternal serum is usually lower than 500 ng/mL.
Determining precise gestational age is essential because fetal AFP levels are age specific and can peak in a normal fetus at 12-15 weeks' gestation. The measurement of maternal serum AFP levels is more than 75% accurate in detecting an open neural tube defect at more than 15 weeks' gestation. In patients in whom a question persists, amniotic AFP can be obtained. It is a significantly more accurate test, especially at 15-20 weeks' gestation, and detects approximately 98% of all open neural tube defects, although this method is not the preferred screening test. Amniotic fluid acetylcholinesterase levels add an increased degree of resolution.
A partial list of the fetal anomalies that are associated with elevated AFP levels is as follows:
Spina bifida cystica
Exstrophy of the cloaca
Urinary tract obstruction
Detection of a neural tube defect with fetal ultrasonography in the hands of a skilled ultrasonographer is usually 98% specific. False-positive findings can result from multiple pregnancies or inaccurate fetal dating. However, closed neural tube defects can sometimes remain undetected, especially in cases of skin-covered lipomyelomeningoceles and meningoceles, in which the AFP levels may also be normal. These closed neural tube defects comprise about 10% or more of total neural tube defects discovered. A skilled ultrasonographer can detect these lesions with almost 95% sensitivity.
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|Days of Gestation||Event||Resultant Malformation|
|0-18||Formation of 3 germ layer and neural plate||Death or unclear effect|
|18||Formation of neural plate and groove form||Anterior midline defects|
|22-23||Appearance of optic vessels||Hydrocephalus (18-60 d)|
|24-26||Close anterior neuropore||Anencephaly|
|26-28||Close posterior neuropore||Cranium bifidum, spina bifida cystica, spina bifida occulta|
|32||Vascular circulation||Microcephaly (30-130 d), migration anomalies|
|33-35||Splitting of prosencephalon to make paired telencephalon||Holoprosencephaly|
|70-100||Formation of corpus callosum||Agenesis of the corpus callosum|
|Anomalies Associated with Myelomeningocele||Approximate Percent of Patients|
|Chiari II malformation||>90%|
|Brainstem malformations (cranial nerve)||75%|
|Cerebral ventricle abnormalities||>90%|
|Agenesis of the corpus callosum||12%|