Spina Bifida

Spina bifida (SB) refers to protrusion of the spinal contents through a bony defect in the spine. The abnormality probably results from maldevelopment at the fourth week of gestation: the bony sheath surrounding the spinal cord does not form properly due to failure of the posterior vertebral arches to close. Although the spinal opening can be surgically repaired shortly after birth, the nerve damage is permanent, resulting in varying degrees of paralysis of the lower limbs. Even when there is no lesion present, there may be improperly formed or missing vertebrae and accompanying nerve damage. In addition to physical and mobility difficulties (such as congenital scoliosis, kyphosis, and hip deformities), most individuals have some form of learning disability. SB may also cause bowel and bladder complications, and many children with SB have hydrocephalus (excessive accumulation of cerebrospinal fluid in the brain).

SB is one of the most common birth defects, with an average worldwide incidence of one to two cases per 1,000 births, but certain populations have a significantly greater risk. In the United States, the average incidence of SB is 0.7 per 1,000 live births. The incidence is higher on the East Coast than on the West Coast, and higher in Caucasians (one case per 1,000 live births) than in African Americans (0.1–0.4 case per 1,000 live births). Immigrants from Ireland have a higher incidence of spina bifida than do nonimmigrants. This corresponds to the fact that the highest incidence rates worldwide are found in parts of the British Isles, mainly Ireland and Wales, where three to four cases of myelomeningocele per 1,000 population have been reported, along with more than six cases of anencephaly (both live births and stillbirths) per 1,000 population. The reported overall incidence of myelomeningocele in the British Isles is 2 to 3.5 cases per 1,000 births.

The three most common types of SB are myelomeningocele, the severest form, in which the spinal cord and its protective covering (the meninges) protrude from an opening in the spine; meningocele in which the spinal cord develops normally but the meninges protrude from a spinal opening; and occulta, the mildest form, in which one or more vertebrae are malformed and covered by a layer of skin. Ten to 15 percent of spinal defects are closed by normal skin covering the bony defect. The remainder are open and most are detectable by maternal serum alpha-fetoprotein screening. Approximately 80 percent of lesions occur in the lumbar, thoracolumbar, or lumbosacral areas of the spine, with the remainder in the cervical and sacral areas.