What Are Neural Tube Defects?
Neural Tube Defects (NTDs) are one of the most common birth defects in the US. Neural tube defects occur in approximately one in every 3,000 pregnancies in the US.
NTDs are caused by the failure of the embryonic neural tube, the primitive spinal cord, to properly close somewhere near the 28th day after conception; typically before most women know they are pregnant. A neural tube defect can occur anywhere along the spinal cord, vertebrae and brain and NTDs range in severity from mild to fatal.
A B-vitamin called Folic Acid can help prevent neural tube defects.
The March of Dimes recommends that all women of childbearing age take a multivitamin with 400 micrograms of folic acid every day before pregnancy and during early pregnancy, as part of a healthy diet.
Open Versus Closed Neural Tube Defects:
Neural tube defects are classified based upon embryological considerations, and the presence (or absence) of exposed neural tissues - open or closed NTDs.
Open neural tube defects are neural tube defects that are not covered by skin. These are also the more common type of neural tube defects. Open NTDs frequently involve the entire central nervous system and are due to a failure of primary neurulation. Neural tissue is exposed in an open NTD, which means there is an associated leakage of cerebrospinal fluid (CSF).
Read more about anencephaly.
Read more about spina bifida.
Closed neural tube defects are neural tube defects that are covered by skin. Closed NTDs are localized and usually confined to the spine (the brain is rarely affected) and result from a defect in secondary neuralization. Neural tissue is not exposed and the defect is fully covered in skin, although the skin covering the defect may be dysplastic.
Closed neural tube defects include tethered cord and lipomylomeningocele.
What Causes Neural Tube Defects?
Scientists believe that a combination of genetics and environmental factors may act together to cause neural tube defects, although in 95% of cases, there is no family history. Certain chromosomal abornalities (trisomy 13, 18, 21) have been associated with neural tube defects.
Development of Neural Tube Defects:
Two distinct processes are involved in the formation of the neural tube - primary and secondary neurulation. The neural plate and the notochord are formed during early embryonic development. The neural groove develops by the third gestational week. Subsequently, the neural folds form bilaterally.
The neural folds elevate and start closing, forming the neural tube. These neural folds initially close at the spinal cord or the head. The cutaneous ectoderm fuses first, followed by the neuroectoderm. The cranial neurospore closes during the fourth week of gestation - the last area that closes is the commissural plate.
At the same time, the cutaneous ectoderm separates from the neuroectoderm to form the overlying skin, while the lateral mesoderm migrates between the two ectodermal layers to form the posterior vertebral arches.
Secondary Neurulation (also called canalization):
This process includes formation of the filum terminale and conus medullaris from a poorly differentiated cell mass of the medial eminence.
Common Types of Neural Tube Defects:
Spina Bifida: The most common neural tube defect, affecting the lower back and occasionally spinal cord. The three forms of spina bifida are:
- Occulta: the mildest form of spina bifida, usually has no symptoms, and does not require treatment.
- Meningocele: the rarest form of spina bifida, a herniated fluid-filled cyst without the spinal nerves pokes through the spinal cord.
- Myelomengingocele: the most severe form of spina bifida, the cyst holds the membranes and nerve roots of the spinal cord and, often, the cord itself. Or, a fully exposed section of the spinal cord without a cyst.
Anencephaly: affected babies with this severe neural tube defect are stillborn or born missing parts of their brain, skull and scalp. They do not survive.
Encephalocele: a sac containing the membranes that surround the brain and often part of the brain itself, bulges through a bony defect in the skull.
Diagnosis of Neural Tube Defects:
Neural tube defects may be diagnosed prenatally or after birth.
Open NTDs may be detected on the Quad Screen, a simple blood test given to expectant mothers. Closed NTDs, however, are not usually detected through the Quad Screen, as they are covered by skin. NTDs are also detected on ultrasound examinations.
Most open neural tube defects are readily visible at birth.
However, closed neural tube defects may present differently. The most common presentation of a closed NTD is an obvious abnormality along the spine - a fluid-filled mass, an area of skin discoloration, a hairy patch of skin at the base of the spinal cord. Also common is a lesion covered with skin without any visible neural tissue.
Neural tube defects may wait until later in life to be diagnosed. Common reasons for seeking a doctor is assymetry of the legs or feet - a thinner calf on one side, a smaller foot on one side. Other children may exhibit progressive spinal deformities like scoliosis.
In newborns with neural tube defects, an MRI is the study of choice for imaging neural tissue and identifying the contents of the defect.
A CT scan allows for direct visualization of the bony defect and anatomy. CT scans also allow for detection of hydrocephalus.
Treatment of Neural Tube Defects:
A newborn with an open neural tube defect should be placed in a warmer in the prone position, and the defect covered with a sterile wet saline dressing.
The bony defect should be closed by surgery as soon as possible.
Hydrocephalus should be managed with the placement of a ventricularperitoneal shunt.
Prognosis for Neural Tube Defects:
Prognosis for neural tube defects depends upon the severity of the defect. Babies with anencephaly die within hours of birth, whereas those with a mild case of spina bifida can live a normal life.
Additional Resources for Neural Tube Defects
The Spina Bifida Association - serves adults and children who live with the challenges of Spina Bifida. Since 1973, SBA has been the only national voluntary health agency solely dedicated to enhancing the lives of those with Spina Bifida and those whose lives are touched by this challenging birth defect. Its tools are education, advocacy, research, and service.
March of Dimes - leading nonprofit organization for pregnancy and baby health. With chapters nationwide, the March of Dimes works to improve the health of babies by preventing birth defects, premature birth and infant mortality. The website contains information on all aspects of pregnancy and baby health, including NTDs.
Preventing Neural Tube Defects - CDC's site regarding neural tube defects and prevention of NTDs.
Clinical Trials of Neural Tube Defects - open trials for those affected by neural tube defects.