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Spina Bifida Ultrasound
Spina bifida is a neural tube defect (NTD) that happens because of incomplete closure of the neural tube along the backbone during very early fetal development. The exact cause of this incomplete closure is unknown, but genetic and environmental factors are suspected to be involved in its causation . Additionally, studies have shown that an adequate intake of folic acid during pregnancy reduces the incidence of neural tube defects.
Prenatal screening for spina bifida
It is possible to diagnose spina bifida during the antenatal period. Prenatal screening detects the (Neural Tube Defects) NTDs and assists the obstetrician in deciding on the proper management. Usually, the following screening tests are carried out in antenatal period to diagnose spina bifida:
One of the important prenatal screening tests is the maternal serum alpha-fetoprotein (MSAFP) test. It is a common test used to check for neural tube defects. A blood sample is sent to the laboratory, where it is tested for alpha-fetoprotein (AFP). AFP is a protein, produced by the baby. A small amount of AFP crosses the placenta and enters the mother’s bloodstream, but very high levels of AFP in mother’s blood suggests that the baby has a neural tube defect, most commonly, spina bifida or anencephaly. Note that some cases of spina bifida do not produce high levels of AFP. Moreover, abnormally high levels of AFP do not always indicate the presence of spina bifida. Other conditions associated with high levels of AFP include:
So, whenever there are high levels of AFP do a repeat blood test for confirmation. If the levels of AFP are still high, then further evaluate the patient by using other tests like ultrasonography. Other blood tests used to diagnose spina bifida prenatally include:
These tests detect other hormones, such as human chorionic gonadotropin (HCG), inhibin-A and estriol. Remember that the objective of these tests is to screen for trisomy 21 (Down syndrome), not neural tube defects .
Using Ultrasound for Diagnosing Spina Bifida
Many obstetricians totally depend on ultrasonography to screen for spina bifida. If blood tests show very high alpha-fetoprotein or AFP levels, an ultrasound exam must be performed to confirm the presence of neural tube defect. Ultrasonography will also help you to rule out the other causes of high levels of AFP such as miscalculation of gestational age, twin pregnancy or the presence of ovarian tumor. An advanced ultrasound can also detect signs of spina bifida such as an open spine or special features in the baby’s skull that show spina bifida. The detection of the type of Neural Tube Defects or NTDs from ultrasound depends upon the experience of the radiologist. Experienced radiologists can detect spina bifida and its severity. Ultrasound is not harmful to mother or baby. The favorable period for detection of NTDs is between 16 and 20 weeks of gestation .
Findings from Spina Bifida Ultrasonography
Ultrasonography is carried out in standard planes, that is, coronal, tranverse and sagittal. The technician usually starts by examining the spine in the coronal plane, confirming that an equal proportion of fetal tissue is present in both parts of the fetus. Then, each vertebra is observed in the transverse plane. If the defect is detected during the examination in the coronal or tranverse plane, then sagittal sections are obtained to determine the extent of the defect .
Ultrasound is generally very effective for detection of spina bifida and it may show the following signs suggestive of spina bifida:
Presence of dorsal ossification centers/lateral pedicles as being splayed apart gives a V shaped appearance to the posterior elements.
Spina bifida can be suspected when the two indirect signs are present on ultrasound. The first is referred to as the “lemon sign”, consisting of a more pronounced depression at the level of the metopic suture, giving to the calvarium the shape of a lemon. The second sign is called as the “banana sign” and is seen at the level of the cerebellum. It is caused by the protrusion of the cerebellar vermis through the foramen magnum, giving the cerebellum the shape of a banana.
Clefts in the spinal canal
Remember a normal cranial ultrasound does not always exclude a neural tube defect .
The amniocentesis test is suggested to a pregnant lady whose blood test shows high levels of alpha-fetoprotein or AFP but the ultrasound is normal. During amniocentesis, a needle is used to remove a sample of fluid from the amniotic sac that covers the baby. The analysis of this sample indicates the level of AFP present in the amniotic fluid. Normally amniotic fluid contains a small amount of AFP. However, when an open neural tube defect such as spina bifida is present, it increases the levels of AFP in the amniotic fluid due to its leakage from the abnormal spine of the fetus. For the confirmation of presence of NTD (Neural Tube Defects), a second test can be done on the same sample .
There are many tests to diagnose spina bifida in the prenatal period. Initially, blood tests are performed. If the blood tests are suspect, other tests such as ultrasound and amniocentesis are performed to confirm the diagnosis. The screening of spina bifida or neural tube defects is an important feature in the antenatal care of the pregnant mother.
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Guibaud S, Capella-Bonnet M, Simplot A. Contribution of amniocentesis to prenatal screening of neural tube closing defects: evaluation of 10 years of a center’s experience. J Genet Hum 1985;33(3-4):339-45.
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