SCREENING FOR TUBE NEURAL DEFECTS

In the human embryo the neural groove closes between the third and fifth week of gestation.2 The resulting neural tube separates from the overlying ectoderm to become surrounded by mesodermal elements that later form the vertebral column and supporting soft tissue structures. Interference with this sequence of developmental events at this specific gestational age can result in incomplete closure of the dorsal midline. Depending on the location and the extent of the lesion and whether there is herniation of neural tissue into the sac, the clinical consequences may range from minimal (as in closed spina bifida) to disastrous (as in anencephaly). These types of congenital anomalies are grouped as spinal dysraphism or neural tube defects (NTD). Neural tube defects can be separated into two main categories: (1) abnormalities of the skull and brain (anencephaly, acrania, and encephalocele); and (2) malformations of the spine (meningomyelocele or spina bifida). The latter is the most common defect of the central nervous system. Meningocele denotes a defect in the vertebrae through which the meningeal sac protrudes. When nervous tissue is also involved in the defect, as occurs in some 90% of cases, this is called meningomyelocele. The more common type (85% of cases) is an open defect, with neural tissue covered by a thin membrane or exposed to the amniotic fluid. Closed spina bifida is less common and is characterized by a small, sometimes hidden defect that is covered by fetal skin. This type of defect easily may be overlooked even in experienced hands and is sometimes detected incidentally many years later.