spina bifida cystica, a developmental defect of the central nervous system in which a hernial cyst containing meninges (meningocele), spinal cord (myelocele), or both (myelomeningocele) protrudes through a congenital cleft in the vertebral column. The protruding sac is encased in a layer of skin or a fine membrane that readily ruptures, causing the leakage of cerebrospinal fluid and an increased risk of meningeal infection. The severity of neurological dysfunction and associated defects depends directly on the degree of nerve involvement. The most severe type is lumbosacral myelomeningocele, which is frequently associated with hydrocephalus and the Arnold-Chiari malformation. Compare spina bifida occulta. See also myelomeningocele, neural tube defect.
