Overview of Congenital Neurologic Anomalies
Congenital brain anomalies usually cause severe neurologic deficits; some may be fatal.
Some of the most serious neurologic anomalies (eg, anencephaly, encephalocele, spina bifida) develop in the first 2 mo of gestation and represent defects in neural tube formation (dysraphism). Others, such as lissencephaly, result from problems with neuronal migration (see Malformed Cerebral Hemispheres), which occurs between 9 wk and 24 wk of gestation. Hydranencephaly and porencephaly are secondary to destructive processes that occur after the basic architecture of the brain has formed. Some anomalies (eg, meningocele) may be relatively benign.
There are many causes of congenital brain anomalies, including many previously unknown genetic factors.
Amniocentesis and ultrasonography permit accurate in utero detection of many malformations, and newer fetal MRI techniques are increasingly useful. Parents need psychologic support when a malformation is detected and also genetic counseling, because the risk of having a subsequent child with such a malformation is high.
Women who have had a fetus or infant with a neural tube defect are at high risk and should take folate (folic acid) supplementation 4 mg (4000 mcg) po once/day beginning 3 mo before conception and continuing through the 1st trimester. Folate supplementation reduces the risk of neural tube defects in future pregnancies by 75%.
All women of childbearing age who have not had a fetus or infant with a neural tube defect should consume at least 400 mcg/day of folate through diet or by taking a supplement (some experts recommend 800 mcg/day to further reduce risk) and continue doing so through the 1st trimester. Although folate supplementation reduces the risk of having a child with a neural tube defect, risk reduction is less than in women who previously had a fetus or infant with a neural tube defect (ie, risk reduction is < 75%).