Dandy-Walker syndrome

NEW YORK -- If you can do only one ultrasound for a pregnant woman, do it at 18 weeks, Dr. Beryl Benacerraf said at a meeting on ob.gyn. ultrasound sponsored by the American Institute of Ultrasound in Medicine.

"You can really see a fetus that is completely formed[cdots]and you can really identify a lot more of the anatomy at that time than you can in the first trimester," said Dr. Benacerraf of Harvard Medical School, Boston.

While nuchal translucency seen on a first-trimester sonogram is sensitive in detecting fetuses that have chromosomal abnormalities, it's still an indirect marker for fetal abnormalities.

"I'd like to see the organs rather than do this indirectly," Dr. Benacerraf said at the meeting.

Far more fetal anomalies are detected during the second trimester, compared with the first trimester.

For example, the second-trimester sonogram has a much higher sensitivity for detecting cardiac defects, which are often missed by clinicians. Before 18 weeks, resolution of the fetal heart is generally poor and dependent on the fetal position.

One gets a better look at the cerebellum as well. On a 14-week first-trimester sonogram, the cerebellum is not completely formed and may appear abnormal, which the clinician may incorrectly interpret as a Dandy-Walker variant.

"We have to watch out as we're getting better and better equipment that we're not trying to diagnose things that really aren't ready to be diagnosed until later," Dr. Benacerraf cautioned.

Other anomalies that can be detected at 18 but not 14 weeks include those of the neural tube, gastrointestinal tract, and kidneys, as well as facial cleft, agenesis of the corpus callosum, hydrocephalus, fetal tumors, and limb and skeletal abnormalities. In addition, a fetus with a normal foot in the first trimester can develop a clubbed foot in the second trimester.

Second-trimester ultrasound can be as sensitive and specific as first-trimester nuchal translucency in predicting Down syndrome. In fact, a completely normal 18-week sonogram in a woman over 35 years can reduce the risk of Down syndrome or trisomy 18 such that she could consider deferring having an amniocentesis, pending the outcome of the sonogram.

In one study, Dr. Benacerraf and colleagues devised a scoring system to assess the risk of Down syndrome based on 18-week sonographic markers.

A nuchal fold and major structural abnormalities received a score of two; soft markers such as short femur and pyelectasis received a score of one each. Fetuses without findings have a score of zero. Fetuses that had a score of two or more were candidates for amniocentesis.

A 40-year-old woman with a score of zero on an 18-week scan has the same risk as a 35-year-old woman without a scan--0.5 per 1,000--of having a fetus with Down syndrome. However, a woman over 40 should get an amniocentesis regardless of her score, since a normal scan doesn't confer reduced risk below 1 per 250 after this age.

Although fetal anatomy is more defined at 18 weeks, clinicians in this country are still missing anomalies at a disappointingly high rate, said Dr. Benacerraf. A recent study showed that the identification rate of ultrasound-detectable malformations was 13%-35% in non-teaching hospitals and 36%-77% in teaching hospitals.

This may be due to a lack of standardized training, she noted. The American Institute of Ultrasound in Medicine and the American College of Radiology have just begun a voluntary accreditation process designed to improve clinicians' ultrasonographic skills.

But in an ideal world with limitless resources and patients who come in for early prenatal care, "I would choose to do a scan during each of the three trimesters if I could," Dr. Benacerraf said.

COPYRIGHT 1999 International Medical News Group
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