in the sac (spina bifida with meningomyelocele) (Figs. 19.15C and 19.16 A). Occasionally the neural folds do not elevate but remain as a flattened mass of neural tissue (spina bifida with myeloschisis or rachischisis) (Figs. 19.15, D and E, and 19.16B). Hydrocephaly develops in virtually every case of spina bifida cystica because the spinal cord is tethered to the vertebral column. As the vertebral column lengthens, tethering pulls the cerebellum into the foramen magnum, cutting off the flow of cerebrospinal fluid.
Spina bifida cystica can be diagnosed prenatally by ultrasound and by determination of «-fetoprotein (AFP) levels in maternal serum and amniotic fluid. The vertebra can be visualized by 12 weeks of gestation, and defects in closure of the vertebral arches can be detected. A new treatment for the defect is to perform surgery in utero at approximately 28 weeks of gestation. The baby is exposed by cesarean section, the defect is repaired, and the infant is placed back in the uterus. Preliminary results indicate that this approach reduces the incidence of hydrocephalus, improves bladder and bowel control, and increases motor development to the lower limbs.
Hyperthermia, valproic acid, and hypervitaminosis A produce NTDs, as do a large number of other teratogens. The origin of most NTDs is multi-factorial, and the likelihood of having a child with such a defect increases significantly once one affected offspring is born. Recent evidence proves that folic acid (folate) reduces the incidence of NTDs by as much as 70 % if 400 xg is taken daily beginning 2 months prior to conception and continuing throughout gestation.
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