Neural tube defects (or NTDs) are a congenital defect that occurs in around 3,000 births a year in the US. NTDs are considered the second most common birth anomaly, after congenital heart defects.
NTDs occur at a rather early stage of fetal development (when the spine is supposed to form a tube shape). When this defect occurs, part of the nervous system remains outside the spine.
There are two types of NTDs – cranial NTDs (that involve the skull and brain, these defects prevent the fetus from developing), and caudal NTDs (that occur in the spinal cord).
Symptom severity differs according to the number of affected vertebrae and the location of the defect. However, in most cases these defects cause severe functional impairments that include leg paralysis and bowel control difficulties.
In some cases, the defect occurs as an isolated finding but in other cases it is part of a set of symptoms that appear as a known syndrome. These cases can occur due to Meckel-Gruber syndrome, which is the most common type of syndromic neural tube defect.
How are NTDs inherited?
Only five percent of all NTD cases are part of a family history of this condition. The remaining cases are sporadic. Generally, this is a multifactorial disorder, which means that several factors must exist for the condition to develop in the fetus.
The gene mutations that cause NTDs have not yet been discovered. Women who are homozygous for a mutation called MTHFR are at an increased risk for having a child with neural tube defects, due to low levels of folate in their blood.
The CDC (Centers for Disease Control and Prevention) has recommended women who have had a child with NTD and are planning on becoming pregnant to consume 4,000 mcg of folic acid a day starting one month before becoming pregnant until three months of pregnancy. The recommendation to all women of reproductive age is to consume 400 mcg of folic acid daily.
When NTDs occur as part of a syndrome, the inheritance pattern varies and depends on the syndrome. Meckel-Gruber syndrome has an autosomal recessive inheritance pattern, and so do other syndromes such as Jarcho-Levin syndrome and more.
Prenatal diagnosis
The alpha-fetoprotein test is performed at week 16-18 of gestation, this blood test screens for congenital anomalies. When AFP measurements are higher than normal (more than 2.5 multiples of median), this can indicate a risk for NTDs.
In these cases, the recommendation is to continue testing and undergo an ultrasound scan and amniocentesis, to determine AFP levels in the amniotic fluid.
It is important to note that when defects occur at the top of the spinal column, they can be easily detected during an ultrasound scan during weeks 12-16 of gestation. However, of the defect occurs along the spine, diagnosis during these weeks of pregnancy could be harder to achieve.
In any case, NTDs can easily be diagnosed during week 21 of pregnancy in an ultrasound scan.
