Spina bifida is a developmental congenital disorder caused by the incomplete closing of the embryonic neural tube. Some vertebrae overlying the spinal cord are not fully formed and remain unfused and open. If the opening is large enough, this allows a portion of the spinal cord often surrounded by a fluid filled sac, to protrude through the opening in the bones and sometimes out between the muscles and the layers of skin to appear like a bubble on the back. The most common locations of such malformations are the lumbar and sacral areas.
Myelomeningocele is the most significant and common form, with a worldwide incidence of about 1 in every 1000 births. This form leads to disability in most affected individuals. This form can be surgically closed after birth, but that does not restore normal function to the affected part of the spinal cord. The defect can also undergo surgical repair before birth by operating through the wall of the uterus. This has been shown to be relatively safe and potentially beneficial to reduce later complications of the malformations, though long-term outcomes are still under study.
Some forms of spina bifida are less apparent because they do not protrude through the skin of the back. These are often referred to as spina bifida occulta, which means "hidden." This is the mildest form of spina bifida. In occulta, the outer part of some of the vertebrae is not completely closed. The splits in the vertebrae are so small that the spinal cord does not protrude. The skin at the site of the lesion may be normal, or it may have some hair growing from it; there may be a dimple in the skin, or a birthmark. Many people with this type of spina bifida do not even know they have it, as the condition is asymptomatic in most cases. The incidence of spina bifida occulta is approximately 10-20 percent of the population, and most people are diagnosed incidentally from spinal X-rays.