The Prenatal Ultrasound Assessment for the Risk of Down's Syndrome

September 14, 2006

OBGYN.net Conference CoverageFrom 45th Annual Conference of the AIUM - Orlando, FL 2001

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Dr. Allen Worrall: "I'm Dr. Allen Worrall from Fairbanks, Alaska and I'm an OB-GYN doctor who does a good deal of prenatal and GYN ultrasound. My job today is to interview Dr. Art Fougner from Forest Hills, New York, and I'm interested in that because I was born and raised on Long Island and used to drive by Forest Hills on the Long Island Railroad when I went into New York City. It used to be a big tennis center; I remember that when I was a kid. Art is going to talk about the prenatal assessment of the risk of Down's syndrome, and my interest in this was started by several things; I'm due to give a talk on April 21st and I chose that as my topic. I had a very recent case that sparked my interest even more; I scanned a thirty-four year old woman who would be thirty-five at the time of her delivery and her biophysical triple screen risk was something like 1 in 7,000 - very low risk. I scanned her at about nineteen weeks and found what I interpreted to be echogenic bowel. I was not sure whether the echogenic bowel overrode the very low triple screen risk or whether an amniocentesis was indicated or not indicated. I decided that I should recommend to my obstetrical colleague that he do an amniocentesis and he did. He is also here in Orlando at Walt Disney World but on vacation and not coming to this meeting. By one chance in five million, I ran into him yesterday and he told me that the amniocentesis revealed a mosaic for 46-XY and 45-X. I don't know what that's got to do with echogenic bowel but, Art, tell us how do you integrate these two findings - the age of the patient and the triple screen and, of course, the triple screen includes the age of the patient and the ultrasound risk? We could talk about lowering the risk and raising the risk."

Dr. Arthur Fougner: "In order to do that, we almost have to go back to the beginning. Amniocentesis was first used to diagnose Down syndrome in the older woman. It was first used prenatally in the 1970's and the situation used to be very easy, the counseling and risk assessment simply consisted of asking a woman - how old are you and that was it. If you were over a certain age you were offered amniocentesis, and if you were under a certain age you weren't. That's the way it stood until Merkatz in the eighties serendipitously discovered four fetuses with Down syndrome whose maternal serum AFP's were much lower than the medium. Then this progressed rapidly into using other analytes to nail down and make the test even more sensitive without sacrificing specificity so now by serum screen we have Down syndrome detection rates of conservatively around 60%-70%. Subsequently several investigators, most notably the group out of Massachusetts headed by Dr. Beryl Benacerraf, reported on several ultrasound findings which might also be used to rule in or rule out or I should say modify risks for Down's such as nuchal folds and then later certain characteristics of the fetal hand and the fetal heart. It's almost gotten to the point now that there are so many markers - I counted over twenty the last time I looked - for Down's syndrome that one could almost justify an amniocentesis on virtually every pregnant woman simply based on the presence or absence of markers. One of the problems with risk assessment is you have to know what the individual relative risk is of the individual finding. You bring up echogenic bowel, for example, what's not well appreciated is that the most common correlate with echogenic bowel and probably the most common cause is bleeding into the amniotic fluid and subsequent fetal swallowing that let's you get some bio breakdown and that shows up as inspissated meconium and echogenic material. The second thing is that the equipment over the years is getting better and so the transducer frequencies are getting higher and higher and produce more and more false-positives. Now we can detect things that we never saw previously so the first thing we need to know is relative risk of the individual findings. The reason that's important is if you want to modify risks you have to know by how much does this finding contribute to a risk. That's not well defined in various populations, as a matter of fact, in any population and the problem with jumping in using markers right now is that all these things were first described in populations that were already considered to be at high risk because the original use of ultrasound was not to rule in an amniocentesis, it was to tell which patient was truly reluctant - actually your risks are probably lower because your fetus really doesn't have any of these findings. Now it's gone completely the opposite where we're taking young women who are very happy with their pregnancy and all of a sudden you see the jaws begin to drop and now they're rapidly becoming scared to death. Yesterday it was announced that there's going to be a consensus panel over the summer to address many of these issues. In answer to your question, I'm not sure that really answers your question because we're all kind of stuck so the best you can do is what you did - you present the findings to your patient, you have to tell her that these things have been associated with things, and ultimately the final choice is up to her. I think if this were a cancer-screening test, we wouldn't be doing any of these things because the yield is so poor. No cancer detections test would pass muster if a positive finding consisted of 1 in 300, it just wouldn't pass muster which leaves me to believe that, perhaps, the only thing our society tolerates less than cancer is Down syndrome. Now whether that's a positive or negative can be debated but that seems to be what the facts are right now."

Dr. Allen Worrall: "Is it true that you can reduce a woman's risk between the ages of 35 and 40 by a negative ultrasound?"

Dr. Arthur Fougner: "Yes, I believe that you can. You can probably reduce it conservatively by a factor of 75%, 3 out of 4 and some centers aggressively claim that their detection rates are around 90%. I'm always leery of that because my own personal bias is that if you want to see the chromosomes you have to do a specific test that's for chromosomes. As of yet, ultrasound hasn't gotten down to the high frequency necessary to visualize chromosomes so, therefore, you almost have to look but I do believe that you could at least modify the risks."

Dr. Allen Worrall: "Now how about the woman under 35, let's say, a low risk woman who has a triple screen that indicates a risk of 1 in 200 but a negative ultrasound. How would you advise her?"

Dr. Arthur Fougner: "Again, the trick is given the practice environment in which we live, you can't advise anybody to do anything. What you can do is you can say - these are your risks and ultimately it's your pregnancy, you need to talk to people. This is a decision that I can't make for you, you need to make that decision that your risks have to be reduced but they're not zero. A common analogy I use is that in our neighborhood we have Queen's Boulevard, it's easily one of the widest thoroughfares in the world, and people get killed periodically crossing the street so the risk of getting killed on Queen's Boulevard given the number of people that cross the street is very low but it's not zero. I think this is an analogous situation."

Dr. Allen Worrall: "We do expect a paper, I believe, this afternoon from Dr. Nyberg. He has some interesting new information I think about the independence of ultrasound and biochemistry. This is a field that is important and we know that we're overusing these minor markers, and Doctors Filly and Benacerraf have written recent papers on this subject that stimulated the AIUM to call a consensus panel so we all will know how to use these things. I think it takes a good deal of courage to find a soft marker in an otherwise low risk patient and just ignore it and not even mention it because any baby can be born with Down syndrome, and if that one is born with Down syndrome and the patient gets it to a lawyer, and the lawyer finds that the baby had an echogenic focus that you didn't mention - why you might be in trouble so that's the problem."