Cerebellar and frontal lobe hypoplasia in fetuses with trisomy 21: usefulness as combined US markers.

Abstract

To confirm that cerebellar hypoplasia is ultrasonographically recognizable in second-trimester fetuses with Down syndrome and determine whether the combination of frontal lobe shortening and cerebellar hypoplasia is superior to either measurement alone as a marker of this abnormality. The frontothalamic distance (FTD) and transcerebellar diameter (TCD) were measured in 52 middle-trimester fetuses with euploid karyotypes and in 52 fetuses with Down syndrome. Receiver operating characteristic (ROC) curves were constructed with various thresholds for observed-to-expected ratios (O/Es) of the FTD, TCD, and average of these two parameters. The area under the average ROC curve, 0.80, was greater than that for either the FTD alone (0.75) or the TCD alone (0.76). At a 6% false-positive rate, the sensitivity for the detection of Down syndrome obtained with the average parameter was 34% better than that obtained with only the FTD and 12% better than that obtained with only the TCD. With an O/E threshold of 0.92 for the average parameter, an odds ratio of 16.3 and positive predictive value of 12.7% in the high-risk population were achieved. Although both measurements are individually statistically significant, the combination of TCD and FTD measurements may be superior to the use of either parameter alone as a marker of trisomy 21.