Types of Neural Tube Defects
There are several types of neural tube defects (NTDs) that vary in their severity. Depending on the type of defect and the location, the symptoms can range from mild to severe and the complications can vary from minor to serious, including mental and physical disabilities, and/or be life-threatening. The two most common types of neural tube defects are spina bifida and anencephaly.
Spina bifida, which literally means "cleft spine," is the most common type of NTD. According to the National Institute of Neurological Disorders and Stroke, spina bifida affects about 1,500 to 2,000 of the babies born in the United States each year. It occurs when the neural tube does not close completely somewhere along the spine.
Cases of spina bifida are generally classified as either "closed," where the skin covers the defect, or as "open," where the skin is not intact. Different types include:
Closed Neural Tube Defects
- Occulta. Spina bifida occulta is often called hidden spina bifida. A small gap may exist in one or more of the vertebrae, but the spinal cord and tissues are normal. Many people with this condition do not have any symptoms and may never know that it exists unless it is identified during testing that is done for another reason.
- Closed neural tube defects are frequently grouped with spina bifida occulta. They consist of a wide variety of spinal defects with malformed bone, tissue, and/or fat deposits at the location of the defect. Often, there is a visible sign on the outside of the baby's skin. This may be a tuft of hair, a dimple, a birthmark, lump, or skin opening. Many with this condition will have few symptoms, but some may have nerve damage that affects normal bowel or urinary function and/or can cause lower body weakness and pain.
Closed neural tube defects are often not detected by screening tests.
Open Neural Tube Defects
- Meningocele. With this type of spina bifida, cerebrospinal fluid and meninges have ballooned out of the spinal opening to form a bulge or sac, but the spinal cord is usually in its normal location. This bulge may or may not be covered with a layer of skin. This bulge will be noticeable on the back of the baby. Symptoms of those with this type are variable.
- Myelomeningocele. This is the most severe type of spina bifida. It develops when the cerebrospinal fluid, meninges, and the spinal cord protrude through the opening in the spine. It leaves the spinal cord vulnerable to damage and can cause paralysis in those parts of the body below the opening. Those affected with a myelomeningocele frequently have bowel and urinary dysfunction. Some will require assistance to walk and others will require a wheelchair. Newborns with this condition are at an increased risk of developing meningitis.
According to the Spina Bifida Association, approximately 70-90% of those affected by myelomeningocele will develop hydrocephalus. This complication occurs when the flow of cerebrospinal fluid is obstructed and accumulates in the brain. The child's head becomes larger and the fluid applies pressure on the brain. Left untreated, this condition can cause mental retardation and learning disabilities and can, in some cases, be fatal.
Those with myelomeningocele may also have a Chiari malformation. This is a structural defect in which the cerebellum, the portion of the brain that sits just above the spinal cord, protrudes down into part of the space normally occupied by the spinal cord. The cerebellum controls balance. People with a Chiari malformation may experience a range of symptoms, including sensory disturbances, vision problems, headaches, neck pain, and dizziness.
Anencephaly occurs when the neural tube fails to close properly at the head. The result is the lack of development of a large portion of the brain and skull. The Centers for Disease Control and Prevention (CDC) estimates that about 1 in 4,800 babies born every year in the U.S. will be born with anencephaly. Those who are born alive are usually unconscious, unable to feel pain, deaf, and blind. Most of those affected die within a few hours or days.
There are varieties of other, rarer forms of NTDs. All are associated with a lack of proper neural tube formation. Examples of these include:
- Encephaloceles – in those affected, part of the brain and its membranes bulge out through openings in the skull; newborns with this condition may have microcephaly (small head) and develop hydrocephalus. Prognosis depends on severity.
- Iniencephaly – in this condition, the head is bent back at a severe angle and the baby does not usually have a neck; associated with other abnormalities, most affected infants do not survive.