Spina Bifida
(Neural Tube Defect)

The human nervous system develops from a small, specialized plate of cells (the neural plate) along the back of an embryo. Early in fetal development, the edges of this plate begin to curl up toward each other, creating the neural tube -- a narrow sheath that closes to form the brain and spinal cord of the embryo. As development progresses, the top of the tube becomes the brain and the remainder becomes the spinal cord. This process is usually complete by the 28th day of pregnancy. But if problems occur during this process, the result can be brain disorders called neural tube defects, including spina bifida.

Spina bifida, which literally means "cleft spine," is characterized by the incomplete development of the brain, spinal cord, and/or meninges (the protective covering around the brain and spinal cord). It is the most common neural tube defect in the United States -- affecting 1,500 to 2,000 of the more than 4 million babies born in the country each year. There are an estimated 166,000 individuals with spina bifida living in the United States.

There are four types of spina bifida: occulta, closed neural tube defects, meningocele, and myelomeningocele.

Occulta is the mildest and most common form in which one or more vertebrae are malformed. The name "occulta," which means "hidden," indicates that the malformation, or opening in the spine, is covered by a layer of skin. This form of spina bifida, present in 10 to 20 percent of the general population, rarely causes disability or symptoms.

Closed neural tube defects make up the second type of spina bifida. This form consists of a diverse group of spinal defects in which the spinal cord is marked by a malformation of fat, bone, or membranes. In some individuals there are few or no symptoms; in others the malformation causes partial paralysis with urinary and bowel dysfunction.

In the third type, meningocele, spinal fluid and the meninges protrude through an abnormal vertebral opening; the malformation contains no neural elements and may or may not be covered by a layer of skin. Some individuals with meningocele may have few or no symptoms while others may experience symptoms similar to closed neural tube defects.

Myelomeningocele, the fourth form, is the most severe and occurs when the spinal cord/neural elements are exposed through the opening in the spine, resulting in partial or complete motor paralysis and sensory deficits within the parts of the body below the spinal opening. The paralysis may be so severe that the affected individual is unable to walk and may have urinary and bowel dysfunction.

Source: MedicineNet.com
http://www.medicinenet.com/spina_bifida_and_anencephaly/article.htm