Neural tube defect (NTD) is one of the most common birth defects. NTD develops when during the early development of the fetus, specialized cells on the back of the fetus fail to roll into a neural tube (precursor of spinal cord) and does not close completely . This condition starts at the very early stage of pregnancy (3rd week) often before a woman even knows that she is pregnant. Since it is a malformation of brain or spinal cord or both, hence the babies born with NTD have serious disabilities or die within few hours to few days after birth.
NTD are of two major types: Open NTD or close NTD.
Here are the further division of these two:-
A. Open NTD : This condition happens when the brain or spinal cord remain exposed due to the incomplete formation of cranium or
vertebrae. It is more more common among the two types. Further division of Open NTD :
1. Anencephaly : ( An = without, encephalon = brain )Anencephaly happens when the head end of the neural tube fails to close properly, resulting in the absence of major portion of brain and cranium. This happens during the 23rd and 26th days of pregnancy. The infants born with this condition generally die within few days after birth.
2.Encephaloceles : It is the protrusions or hernia of the brain due to incomplete formation of cranium or brain case. The protruded brain remains covered with a sac-like membrane.
3.Hydranencephaly : (Hydran = watery, encephalon = brain) In case of hydranencephaly the formation of cerebral hemispheres doesn’t happen. The missing cerebral hemispheresare filled with cerebrospinal fluid.
4.Iniencephaly : (Ini = nape of the neck, encephalon = brain ) In this condition the head bends extremely towards the spine. Generally infants born with iniencephaly die within few hours after birth.
5. Spina Bifida : (spina = spine, bifida = split ) The incomplete development of the spine and spinal cord is called Spina Bifida. The most common location of Spina Bifida
is Lumber or Sacral region of the spine, for this reason generally the babies with Spina Bifida shows weakness in legs or leg paralysis. It is of two following types:
a. Spina Bifida Occulta : (Occulta = hidden ) In this case the gap between the vertebrae are very small and spinal cord doesn’t protrude out. This condition is generally asymptomatic. Approximately 10% of healthy people have Spina Bifida Occulta and may remain unnoticed throughout life.
b. Spina Bifida Cystica : In this condition the hernial cyst containing meninges (meningocele), spinal cord (myelocele), or both (myelomeningocele) protrudes through a gap in the vertebral column.
B. Closed NTD : In the case of closed NTD the spinal defects are covered by skin. This condition is localized and generally confined to the spine without affecting the brain.
Detection of NTD : Most of the NTD are detectable at approximately 16-18 weeks of pregnancy by these prenatal tests:
1. Maternal Serum Alpha Fetoprotein (MSAFP), a measurement of the level of Alpha Fetoprotein in the pregnant woman’s blood ( it is a screening test not a confirmative one ).
2.High Resolution Ultrasound : NTD may be detected visually by high resolution ultrasound.
3. Amniocentesis : The confirmative test of NTD is Amniocentesis. This test is performed taking the amniotic fluid from the pregnant woman’s uterus.
The cause of NTD
The exact cause of NTD is still unknown. But it is seen that several genetic and environmental factors may be responsible for this condition. The common environmental factors are deficiency of folic acid,maternal diabetes and maternal use of certain anticonvulsant. NTD may be congenital also, studies show that identical twins having NTD are more common than fraternal twins. Chance of having NTD increases in second child if the first child has NTD.
The most important factor that increases the chance of NTD in the foetus is the amount of folic acid in maternal blood. Folic acid is a water soluble B vitamin. Human body cannot synthesize folic acid, so it is important to supply it through diet to maintain daily requirement. Leafy vegetables and citrus fruits are good sources of folate.
The definite role of folic acid in reducing the chances of NTD is not yet clear. It is found that folate is required for production and maintenance of new cells, for DNA synthesis and RNA synthesis, which is very important in the organization of the embryo or early stage of foetus.
As previously stated, NTD starts at a very early stage of pregnancy, even before a woman is aware of her conception so it is very important to take adequate amounts of folic acid one month prior to pregnancy.
It is recommended that all women of childbearing age should take 400 micrograms of folic acid each day. Women having previous conception with NTD must increase the dose to 4,000 micrograms (4 mg).
Folic acid is a water soluble vitamin so it doesn’t have any serious toxicity (excess amount of folic acid gets excreted through urine).
Folic acid also helps in synthesis of RBC ( red blood corpuscles) hence preventing anemia, which is also an important aspect in pregnancy.