Characterizing olfaction, anxiety, and depression in subjective cognitive decline

BackgroundOlfactory dysfunction and neuropsychiatric symptoms are well-established hallmarks of Alzheimer's disease (AD) and serve as predictors of progression from mild cognitive impairment to AD. However, the literature remains unclear on whether olfactory dysfunction is already evident in the prodromal form of subjective cognitive decline (SCD).ObjectiveThis study aimed to characterize olfactory function and neuropsychiatric symptoms, including anxiety and depression, in individuals with SCD compared to healthy controls (HC).MethodsA total of 110 community-dwelling participants aged 60 years and older were recruited, with 59 classified as SCD (42 women) and 51 as HC (35 women). Participants completed validated anxiety and depression questionnaires (Geriatric Anxiety Inventory and Geriatric Depression Scale) and underwent a comprehensive olfactory assessment using the complete Sniffin' Sticks battery (odor threshold, discrimination, and identification).ResultsThe SCD group exhibited lower global olfactory scores (small to moderate effect size; Cohen's <i>d</i> = 0.41), and higher depression (large effect size; Cohen's <i>d</i> = 1.07) and anxiety (moderate effect size; Cohen's <i>d</i> = 0.64<i>)</i> scores compared to HC. Notably, global olfactory performance enhanced group status prediction and classification (SCD versus HC) when combined with anxiety and depression measures.ConclusionsThese findings highlight subtle but significant lower olfactory performance in individuals with SCD. The neurophysiological underpinnings are unclear and may be unveiled with electrophysiological methods such as olfactory-evoked potentials.