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Sci Tech
Screening early for Down’s Syndrome
R. PRASAD
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Nuchal translucency screening with biochemical testing is highly sensitive
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False positive rate is 4.3 per cent with nuchal translucency More the fluid accumulation in the neck the more the chances of the condition showing up
Screening for Down’s Syndrome has got a fillip. Screening for the syndrome using ultrasound, which was traditionally done during 15-21 weeks (second trimester) of gestation can now be done as early as 11-14 weeks.
Down’s Syndrome is a genetic disorder caused by the presence of an extra chromosome or part of it in the 21st chromosome. It is also commonly called trisomy 21.
Unlike the second trimester screening where nuchal fold thickness is measured, what is looked for in the 11-14 week screening is the nuchal translucency (NT). Nuchal translucency is caused by fluid accumulation behind the neck
The advantage
The biggest advantage of screening for nuchal translucency is that its ability to pick up abnormality is about 77 per cent compared with 40 per cent when the screening is done for nuchal fold thickness during 15-21 weeks.
“That is because nuchal translucency is a powerful marker [for detecting Down’s Syndrome] and nuchal fold is not sensitive,” said Dr. S. Suresh, Managing Director of MediScan, based in Chennai. Scning for nuchal translucency requires expertise.
The sensitivity increases to 85 per cent when nuchal translucency is combined with biochemical testing of the maternal blood for two specific parameters. Unlike nuchal fold measurement, biochemical testing of maternal blood done during the second trimester has a sensitivity of 66-67 per cent.
According to Dr. Suresh, while the sensitivity of screening can be increased by combining the results of nuchal translucency with biochemical tests, screening during second trimester cannot combine fold thickness and biochemical test results.
False positive rate
According to a 2004 Editorial published in the British Medical Journal, the false positive rate is 4.3 per cent with NT compared with 6.2 per cent in the case of biochemical test carried out in the second trimester.
According to a Science Commentary published in 1999 in the British Medical Journal, fluid accumulation behind the neck occurs “… partly because of the foetus’ tendency to lie on its back and partly because of the laxity of the skin of the neck.” Fluid accumulation cannot be seen once the skin becomes thick.
And more the fluid accumulation the more the chances of the foetus having Down’s Syndrome. While thickness of more than 6mm is considered abnormal when nuchal fold is measured, the amount of fluid that accumulates behind the neck depends on the gestation period.
One of the reasons for nuchal translucency showing up early in gestation is because any malfunction arises due to a defect in embryogenesis and that happens before 10 weeks of gestation.
A negative result for trisomy 21 would, however, mean that the chances of having a baby with Down’s Syndrome are less but does not totally exclude the possibility.
Dr. Suresh has been screening for Down’s Syndrome using NT combined with biochemical testing for the last two-and-a-half years. “We have screened nearly 4,000 women so far,” he said. Last year alone 1,770 pregnant women were screened at MediScan and Down’s Syndrome was seen in 56 cases.
Despite better sensitivity, the number of women who turn up for NT screening is small compared with second trimester screening. Positive results warrant further testing using invasive techniques such as amniocentesis.
Amniocentesis is an antenatal test where a sample of amniotic fluid is removed and checked for chromosomal abnormalities. Amniocentesis has a less than one per cent risk of causing a miscarriage.
So is it worth the risk to do amniocentesis since early screening has a false positive rate of 4.3 per cent?
“The risk of having a baby with Down’s Syndrome when the nuchal translucency result is positive is higher than the risk of miscarriage through amniocentesis,” Dr. Suresh explained.
The ability to detect trisomy 21 can be increased to 91 per cent when NT is combined with biochemical testing carried out in the first and second trimester. Integrated screening has a false positive rate of just 2.6 per cent.
Dr. Suresh however noted that implementing such a screening, in practice, would be difficult as the patient would have to wait for 4-5 weeks for the result. The first trimester screening result would not be revealed to the patient in integrated screening.
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