FRONTAL-LOBE SHORTENING IN 2ND-TRIMESTER FETUSES WITH TRISOMY-21 - USEFULNESS AS A US MARKER

PURPOSE: To determine whether the frontal lobe is disproportionately s maller than normal in second-trimester fetuses with Down syndrome by u sing 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, wa s measured in 43 fetuses (mean gestational age, 17.2 weeks +/- 1.3 [st andard deviation], range, 15.0-20.4 weeks) with chromosomally proved t risomy 21 and in: 160-chromosomally normal fetuses (mean gestational a ge, 17.1 weeks +/- 1.5; range, 14.5-22.5 weeks. Other cranial biometri c ratios also were calculated. RESULTS: The FTD was best predicted fro m the estimated gestational ag(ECA) in the euploid population with the quadratic equation FTD = .0.0120 x EGA(2) + 0.6917 x EGA - 5.2349 (R- 2 = .731) or from the biparietal diameter (BPD) with the linear equati on FTD = 0.6837 x BPD + 0.5525 (R-2 = .731). If an observed-to-expecte d ratio of 0.84 is used as a cutoff sign to screen for trisomy 21, a s ensitivity of 16%, specificity of 97%, odd ratio of 6.03 (95% confiden ce interval, 1.81, 20.1), and relative risk of 5.98 are achieved; CONC LUSION: The frontal lobe is statistically significantly smaller in fet uses with trisomy 21. US measurement of the FTD may, prove to-be a use ful adjunctive screening tool if used with other markers for Down synd rome.