Assessment of Midfacial Hypoplasia in down Syndrome Fetuses-Validity of a Two-Line Approach and Introduction of a Novel Angle (Maxilla-Mandible-Nasion Angle)

Purpose: To scrutinize the validity of a novel angle (maxilla-mandible-nasion angle, MMN) as objective proof of midfacial hypoplasia in trisomy 21 fetuses. Materials and Methods: Volume data sets of 2^ndtrimester fetuses were reviewed in this retrospective study. After achievement of the correct midsagittal position, the fetal profile line (FP line) and the mandibulo-maxillary line (MML) were applied and the resulting angle was calculated. Additionally, the prefrontal space ratio (PFSR) was assessed. Both measurements were obtained from 401 euploid fetuses and 42 fetuses with trisomy 21. Values for MMN and PFSR<5^thpercentile were considered abnormal. Results: The study included 443 fetuses with a mean gestational age of 21.3 weeks (range: 14.0-26.3). The MMN angle sufficiently identified hypoplasia of the midface in trisomy 21 fetuses (mean: 14.6°; range: 10.1°-22.0°) compared to controls (mean: 20.5°; range: 17.3°-23.7°; p<0.0001). Concomitantly, the PFSR of Down syndrome fetuses was significantly lower (mean: 0.53; range: 0.21-1.22) than in euploid individuals (1.38; range: 0.54-2.23; p<0.0001). Conclusion: Calculation of the novel MMN angle in 2^ndtrimester fetuses reliably allows rapid assessment of craniofacial anatomy in order to rule out the midfacial hypoplasia frequently found in trisomy 21.