DOWN SYNDROME - New test could diagnose earlier in pregnancy

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New Down syndrome test could help diagnose condition earlier in pregnancy

By Emma Ross, Associated Press, 11/16/2001 01:35

LONDON (AP) Using ultrasound to look for the nose bone in early pregnancy could help to identify Down syndrome babies sooner, new research suggests.

Some experts say the test, described for the first time this week in The Lancet medical journal, could be a major step forward, bringing the detection rate of Down syndrome screening up from about 65 percent today to about 98 percent, and allowing diagnosis in the first trimester rather than in the second, when the condition is normally confirmed.

Because the method has a lower false positive rate than current screening tests, it could also reduce the number of women having to undergo unnecessary diagnostic tests, which involve inserting a needle into the womb and trigger miscarriages about 1 percent of the time, experts said.

''Adding the nasal bone test is a big leap forward,'' said Dr. Howard Cuckle, professor of reproductive epidemiology at Leeds University in England, who was not connected with the study.

However, other obstetricians said that while the findings were impressive, detecting nasal bone on an ultrasound requires special skills and it would be difficult to maintain quality measurements everywhere.

''In the U.S., we are still not fully using first trimester screening, partly due to insurance reimbursement issues most health insurers pay for only one routine sonogram,'' said Dr. Joshua Copel, chief of maternal fetal medicine at Yale University School of Medicine.

Doctors are also waiting for the results of a major U.S. study comparing first- and second-trimester screening techniques, he said.

One of every 700 children is born with Down syndrome, a chromosomal abnormality that is a common cause of mental retardation and is marked by a broad, flat face with slanting eyes.

The condition is caused by having three Chromosome 21s, instead of two.

Prenatal diagnosis is made by one of two tests that analyze chromosomes in cells taken from the mother's belly with a needle.

The most popular test is amnioscentisis, which draws out amniotic fluid during the second trimester of pregnancy.

The other, which is most widely used in Europe, is chorionic villus sampling, or CVS. The CVS test extracts a sample of the placenta and is performed during the first trimester.

Both tests trigger miscarriages about 1 percent of the time.

For many years, only women over 35 who have a higher chance of a Down syndrome baby were sent for a diagnostic probe, but there are now three additional screening methods to help determine who should get one.

The most common screening approach, which involves a blood test for certain red flags, is done in the second trimester of pregnancy. The standard practice in the United States, it detects about 65 percent of Down cases before they are confirmed by amnioscentisis.

Another technique, which relies on whether an ultrasound shows a shaded area suggesting a thickening at the back of the fetus' neck, is done in the first three months of pregnancy, between weeks 11 and 14. That test, used by some of the top centers in Europe, spots about 75 percent of cases.

The third approach is to combine both the fetal neck test and the blood test in the first trimester, which gives a detection rate just under 90 percent.

In the study, led by Dr. Kypros Nicolaides, head of fetal medicine at King's College Hospital Medical School in London, doctors examined 701 routine ultrasound scans done at 11-14 weeks for women about to undergo prenatal diagnosis in the first trimester because of positive results on a test of the thickness of the back of the fetus' neck.

The nasal bone was absent in 43 out of the 59, or 73 percent, of the fetuses that were later confirmed to have Down syndrome and in three of the 603 normal ones, or 0.5 percent.

Adding the nasal bone screening to the neck thickness test pushed the detection rate up from 75 percent to 85 percent, while cutting the rate of false positives from 5 percent to 1 percent, the study showed.

Cuckle estimates that combining the nose bone test with the neck thickness scan and the blood test, all in the first trimester, could allow screening to detect almost 98 percent of Down syndrome cases.

It's a compelling argument in favor of shifting Down syndrome diagnosis policies from the second trimester to the first, he said.

''First-trimester screening has obvious benefits over second-trimester screening, other than efficiency,'' Cuckle said. ''These advantages include, for some, an early diagnosis with consequent safer and less traumatic therapeutic abortion, and, for most, an earlier reassurance.''

On the Net:

National Institute of Child Health and Human Development Down syndrome page:

-- Anonymous, November 16, 2001


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