Spina bifida (Latin: "split spine") is a birth defect where there is incomplete closing of the backbone and membranes around the spinal cord. There are three main types: spina bifida occulta, meningocele, and myelomeningocele. The most common location is the lower back, but in rare cases it may be the middle back or neck. Occulta has no or only mild signs. Signs of occulta may include a hairy patch, dimple, dark spot, or swelling on the back at the site of the gap in the spine. Meningocele typically causes mild problems with a sac of fluid present at the gap in the spine. Myelomeningocele, also known as open spina bifida, is the most severe form. Associated problems include poor ability to walk, problems with bladder or bowel control, hydrocephalus, a tethered spinal cord, and latex allergy. Learning problems are relatively uncommon.