Neural tube defects (NTDs), are the second most common congenital anomaly, and are the cause of chronic disability of between 70,000 and 100,000 individuals in the United States. Myelomeningocele (spina bifida) is the most common neural tube defect.
Myelomeningocele is characterized by a cleft in the vertebral column, with a corresponding defect in the skin so that the meninges and spinal cord are exposed. Because the neural tissue is exposed, it is also known as open spinal dysraphism, or spina bifida aperta. By contrast, occult spinal dysraphism is characterized by a cleft in the vertebral column, without a corresponding epithelial defect, and neural tissue is not exposed. There are many different forms of occult spinal dysraphism, ranging from asymptomatic vertebral anomalies to clinically significant defects in the spinal cord and related structures. Occult spinal dysraphism is discussed separately. (See "Closed spinal dysraphism: Pathogenesis and types".)
The embryology and pathophysiology of myelomeningocele will be reviewed here. The management of infants with myelomeningocele, prenatal aspects, and prevention of neural tube defects are discussed separately. (See "Overview of the management of myelomeningocele (spina bifida)" and "Prenatal screening and diagnosis of neural tube defects" and "Ultrasound diagnosis of neural tube defects" and "Folic acid supplementation in pregnancy".)
EMBRYOLOGY OF THE NEURAL TUBE
Primary neurulation — The central nervous system (CNS) initially appears as a plate of thickened ectoderm, called the neural plate, at the beginning of the third week of embryonic life . The lateral edges of the neural plate become elevated to form the neural folds . These folds subsequently become further elevated, approach each other, and fuse to form the neural tube (figure 1) . The fusion begins in the cervical region and proceeds in both the cephalad and caudal directions (figure 2).
The cranial neuropore closes on the 25th day after conception. Fusion is delayed at the caudal end of the embryo so that the caudal neuropore forms an open communication between the lumen of the neural tube (the neurocele) and the amniotic cavity. Closure of the caudal neuropore normally occurs approximately two days later. This process is called primary neurulation and forms all of the functional central nervous system, which extends to the mid sacral levels of the embryo .