Background: Down syndrome (DS) shows neuropathology similar to Alzheimer disease, which presents olfactory impairment. Previous work showed olfactory impairment in DS, but a comprehensive evaluation of olfactory function in DS is lacking.
Methods: We investigated a large number (n = 56; M = 31, F = 25) DS participants (age range18-57y) using the "Sniffin' Sticks" Extended test. This comprises three subtests (threshold, discrimination, and identification) yielding a global score (TDI) defining normosmia, hyposmia, and functional anosmia. To the best of our knowledge, this is the second largest group of DS people investigated for olfactory function ever. Age- and sex matched euploid individuals (n = 53) were the control.
Results: In DS, TDI was lower (16.7±5.13 vs. 35.4±3.74; P<0.001), with DS people performing worse in any subtests (P<0.001 for all); 27 DS participants showed functional anosmia (i.e., TDI<16). In DS, age was weakly and negatively correlated with TDI (r = -0.28, P = 0.036) and identification (r = -0.34, P = 0.012). When participants were stratified in young adults (18-29y) and older adults (30-61y), a significant effect of age was found for identification in both DS (young adults, 8.3±2.58; older adults, 6.9±2.99; P = 0.031) and control (young-adult, 14.3±1.18, older adult, 13.0±1.54; P = 0.016).
Conclusion: Olfactory function is overall severely impaired in DS people and may be globally impaired at relatively young age, despite of reportedly normal smell. However, specificity of this olfactory profile to DS should be considered with some caution because cognition was not evaluated in all DS participants and comparison with a control group of non-DS individuals having cognitive disabilities was lacking. Further study is required to longitudinally assess olfactory dysfunction in DS and to correlate it with brain pathology.