Congenital condition produced by failure of fusion of the laminae of the lumbar vertebrae or of the sacrum. It occurs as a secondary consequence of failure of the posterior part of the developing neural tube to close correctly. It may be relatively mild (spinal cord remains in the vertebral canal) or severe (spinal cord or cauda equina is extruded into a meningeal cyst). In mild cases there may be no neurologic deficit unless other regions of the developing nervous system have been affected. In the severe cases where a meningomyelocele is present, there often is a type II Chiari malformation resulting in additional neurologic problems such as hydrocephalus and brainstem/cranial nerve abnormalities.