Background Down syndrome (DS) is caused by trisomy 21, leading to increased risks of Alzheimer's disease (AD) and Obstructive sleep apnea (OSA). Traditional diagnostic methods for AD and OSA, like cerebrospinal fluid analysis and polysomnography, are invasive and challenging for people with DS. Facial morphology has emerged as a non-invasive biomarker for these conditions. Therefore, we conducted a comprehensive 3D analysis of facial shape variation in DS and euploid control (EU) populations, assessing the effect of critical factors such as age, sex, and facial size, to investigate its association with AD and OSA. Methods Facial shape differences among groups were analyzed from the coordinates of 21 landmarks automatically registered on 3D facial models generated from head magnetic resonance images in a cross-sectional sample of 131 individuals with DS and 216 EU controls from 18 to 90 years old, including subjects diagnosed with AD and OSA. We used Procrustes ANOVA and MANOVA tests to quantify the amount of facial shape variation attributable to sex, age, and facial size, and quantified facial shape differences among diagnostic groups using geometric morphometrics. Results Besides facial shape differences between DS and EU individuals, our results detected significant interactions between diagnosis and sex, and between diagnosis and age, indicating sex-dependent differences and an altered pattern of facial shape change over adulthood in DS, with females presenting more severe alterations. Multivariate regression analyses showed that facial shape significantly correlated with the concentration ratio between amyloid beta peptide 1-42 and amyloid beta peptide 1-40 (AB;1-42/AB1-40) in cerebrospinal fluid, a common biomarker for AD diagnosis. In the DS population, facial shape differences between individuals with and without AD diagnosis did not achieve statistical significance after adjusting for age and facial size, but significant shape differences were detected in the EU population. Regarding OSA, facial shape significantly correlated with the apnea-hypopnea index, and individuals with DS and severe OSA presented a significantly different facial morphology in comparison to individuals with DS and no signs of OSA, suggesting that facial morphology could be associated with sleep respiratory disturbances. Conclusions Overall, our study underscores the potential of facial morphology as a diagnostic biomarker for the early detection and clinical management of AD and OSA.

Mateus Rozalem-Aranha Funding: Alzheimer's Association Research Fellowship to promote Diversity (AARFD-21-852492) Mateus Rozalem-Aranha COI: Payed consultancy to VERANEX Board participation in Masima Solucoes em Imagens Medicas LTDA

The research in this paper was supported by the Fondation Jerome Lejeune with grant 2020b cycle-Project No.2001 and a postdoctoral fellowship to SL, as well as Beca predoctoral Fundacion Alvaro Entrecanales-Lejeune to LME-Q and Joan Oro grant (2024 FI-3 00160) from the Recerca i Universitats Departament (DRU) of the Generalitat de Catalunya with grant 2023 FI-2 00160 and the European Social Fund to AH-L. This study was funded by the Fondo de Investigaciones Sanitario, Instituto de Salud Carlos III and co-funded by the European Union (PI14/01126 and PI17/01019 to JF, and PI20/00836 to SG). This work was also supported by the National Institutes of Health (NIH grants 1R01AG056850-01A1; R21AG056974 and R01AG061566 1RF1AG080769-01 to JF) and Jerome Lejeune Foundation (JLF1801), and from the Alzheimer's Association and Global Brain Health Institute) GBHI ALZ UK-23-971107) to SG. The authors would also like to thank the Agencia de Gestio d Ajuts Universitaris i de Recerca (AGAUR) of the Generalitat de Catalunya (2021 SGR00706 and 2021 SGR0139), and the International Collaboration and Exchange Program (ICEP).

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Ethics committee of Hospital de la Santa Creu i Sant Pau gave ethical approval for this work.

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Raw image data cannot be made available due to restrictions imposed by the ethics approval. Landmark data supporting the findings of this study are available from the corresponding authors upon reasonable request.