Neural tube defect
|Neural tube defect|
|Classification and external resources|
|ICD-10||Q00, Q01, Q05|
|ICD-9||740, 741, 742|
Neural tube defects (NTDs) are one of the most common birth defects, affecting over 300,000 births globally each year. For example, spina bifida affects approximately 1,500 births annually in the USA, or about 3.5 in every 10,000, which has decreased from around 5 per 10,000 since folate fortification was instituted. A NTD is an opening in the spinal cord or brain that occurs very early in human development. In the 3rd week of pregnancy called gastrulation, specialized cells on the dorsal side of the fetus begin to change shape and form the neural tube. When the neural tube does not close completely, an NTD develops.
There are two types of NTDs: open, which are more common, and closed. Open NTDs occur when the brain and/or spinal cord are exposed at birth through a defect in the skull or vertebrae (back bones). Examples of open NTDs are anencephaly, encephaloceles, hydranencephaly, iniencephaly, schizencephaly, and spina bifida. Rarer types of NTDs are called closed NTDs. Closed NTDs occur when the spinal defect is covered by skin. Common examples of closed NTDs are lipomyelomeningocele, lipomeningocele, and tethered cord.
Anencephaly (without brain) is a neural tube defect that occurs when the head end of the neural tube fails to close, usually during the 23rd and 26th days of pregnancy, resulting in an absence of a major portion of the brain and skull. Infants born with this condition are born without the main part of the forebrain—the largest part of the cerebrum—and are usually blind, deaf and unconscious. The lack of a functioning cerebrum will ensure that the infant will never gain consciousness. Infants are either stillborn or usually die within a few hours or days after birth.
Encephaloceles are characterized by protrusions of the brain through the skull that are sac-like and covered with membrane. They can be a groove down the middle of the upper part of the skull, between the forehead and nose, or the back of the skull. Encephaloceles are often obvious and diagnosed immediately. Sometimes small encephaloceles in the nasal and forehead are undetected. http://www.ninds.nih.gov/disorders/encephaloceles/encephaloceles.htm
Iniencephaly is a rare neural tube defect that results in extreme bending of the head to the spine. The diagnosis can usually be made on antenatal ultrasound scanning, but if not will undoubtedly be made immediately after birth because the head is bent backwards and the face looks upwards. Usually the neck is absent. The skin of the face connects directly to the chest and the scalp connects to the upper back. The infant will usually not survive more than a few hours.
Spina bifida is further divided into two subclasses, spina bifida cystica and spina bifida occulta.
Spina bifida cystica
This includes meningocele and myelomeningocele. Meningocele is less severe and is characterized by herniation of the meninges, but not the spinal cord, through the opening in the spinal canal. Myelomeningocele involves herniation of the meninges as well as the spinal cord through the opening.
Spina bifida occulta
In this type of neural tube defect, the meninges do not herniate through the opening in the spinal canal. It is a common condition, occurring in 10–20% of otherwise healthy people. By definition, spina bifida occulta means hidden split spine. The most frequently seen form of spina bifida occulta is when parts of the bones of the spine, called the spinous process, and the neural arch appear abnormal on a radiogram, and is generally harmless. Usually the spinal cord and spinal nerves are not involved. The risk of recurrence in those who have a first degree relative (e.g. parent, sibling) is 5–10 times greater than that in the general population. The genetic risk of recurrence with symptomatic forms of spina bifida occulta is uncertain.
Folate (vitamin B9) and vitamin B12 are very important in reducing the occurrences of NTDs. Folate is required for the production and maintenance of new cells, for DNA synthesis and RNA synthesis. Folate is needed to carry one carbon groups for methylation and nucleic acid synthesis. It has been hypothesized that the early human embryo may be particularly vulnerable to folate deficiency due to differences of the functional enzymes in this pathway during embryogenesis combined with high demand for post translational methylations of the cytoskeleton in neural cells during neural tube closure. Vitamin B12 is also an important receptor in the folate biopathway such that studies have shown deficiency in vitamin B12 contributes to risk of NTDs as well. Importantly, a deficiency of folate itself does not cause neural tube defects. The association seen between reduced neural tube defects and folic acid supplementation is due to a gene-environment interaction with the 'splotch' gene mutation. Supplementing folic acid during pregnancy reduces the prevalence of NTDs by not exposing this otherwise sub-clinical mutation to aggravating conditions, but not all such mutations are folate-dependent.
Other potential causes can include folate antimetabolites (such as methotrexate), maternal diabetes, maternal obesity, mycotoxins in contaminated corn meal, arsenic, hyperthermia in early development, and radiation. Studies have shown that both maternal cigarette smoking and maternal exposure to secondhand smoke increased the risk for neural tube defects in offspring. A mechanism by which maternal exposure to cigarette smoke could increase NTD risk in offspring is suggested by several studies that show an association between cigarette smoking and homocysteine levels. The study suggests that cigarette smoke, including secondhand exposure, is not only hazardous to the mother, but may also interfere with neural tube closure in the developing embryo.
Tests for neural tube defects include ultrasound examination and measurement of maternal serum alpha-fetoprotein (MSAFP). Amniotic fluid alpha-fetoprotein (AFAFP) and amniotic fluid acetylcholinesterase (AFAChE) tests are also used to confirming if ultrasound screening indicates a positive risk. Often, these defects are apparent at birth, but occult defects may not be diagnosed until much later in life. An elevated MSAFP measured at 16–18 weeks gestation is a good predictor of neural tube defects.
In 1996, the United States Food and Drug Administration published regulations requiring the addition of folic acid to enriched breads, cereals, flour and other grain products. It is important to note that during the first four weeks of pregnancy (when most women do not even realize that they are pregnant), adequate folate intake is essential for proper operation of the neurulation process. Therefore, women who could become pregnant are advised to eat foods fortified with folic acid or take supplements in addition to eating folate-rich foods to reduce the risks of serious birth defects. In Canada, mandatory fortification of selected foods with folic acid has been shown to reduce the incidence of neural tube defects by 46%.
Women who may become pregnant are advised to get 400 micrograms of folic acid daily. Women who are pregnant should receive 1.0 mg (1000 mcg), and women who have previously given birth to a child with a neural tube defect should get 4.0 mg/5.0 mg in the UK mg daily.
Treatments of NTDs depends on the severity of the complication. No treatment is available for anencephaly and infants usually do not survive more than a few hours. Aggressive surgical management has improved survival and the functions of infants with spina bifida, meningoceles and mild myelomeningoceles. The success of surgery often depends on the amount of brain tissue involved in the encephalocele. The goal of treatment for NTDs is to allow the individual to achieve the highest level of function and independence.
Neural tube defects resulted in 71,000 deaths globally in 2010.
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