Frontal lobe shortening in second-trimester fetuses with trisomy 21: usefulness as a US marker

Abstract

Purpose: To determine whether the frontal lobe is disproportionately smaller than normal in second-trimester fetuses with Down syndrome by using prenatal ultrasonographic (US) measurements of the frontothalamic distance (FTD).

Materials and methods: The FTD, measured from the inner table of the frontal bone to the posterior margin of the thalamus, was measured in 43 fetuses (mean gestational age, 17.2 weeks +/- 1.3 [standard deviation]; range, 15.0-20.4 weeks) with chromosomally proved trisomy 21 and in 160 chromosomally normal fetuses (mean gestational age, 17.1 weeks +/- 1.5; range, 14.5-22.5 weeks). Other cranial biometric ratios also were calculated.

Results: The FTD was best predicted from the estimated gestational age (EGA) in the euploid population with the quadratic equation FTD = -0.0120 x EGA2 + 0.6917 x EGA - 5.2349 (R2 = .731) or from the biparietal diameter (BPD) with the linear equation FTD = 0.6837 x BPD + 0.5525 (R2 = .731). If an observed-to-expected ratio of 0.84 is used as a cutoff sign to screen for trisomy 21, a sensitivity of 16%, specificity of 97%, odds ratio of 6.03 (95% confidence interval, 1.81, 20.1), and relative risk of 5.98 are achieved.

Conclusion: The frontal lobe is statistically significantly smaller in fetuses with trisomy 21. US measurement of the FTD may prove to be a useful adjunctive screening tool if used with other markers for Down syndrome.