Lower odor identification in subjective cognitive decline: a meta-analysis.

INTRODUCTION

Odor identification correlates with Alzheimer's disease (AD) biomarkers, and its decline may emerge before measurable cognitive deficits - as early as the subjective cognitive decline (SCD) stage. We aimed to compare odor identification between SCD and cognitively normal (CN) stages.

METHODS

A systematic search of four databases identified studies assessing olfactory identification and cognitive screening in individuals aged 50+. A random-effects meta-analysis was performed on 11 studies (660 SCD, 574 CN).

RESULTS

Individuals with SCD exhibited lower olfactory identification scores compared to CN participants (SMD = -0.67, 95% CI [-1.31, -0.03],  = 0.04). Meta-regression revealed a negative association ( = -1.79,  = 0.02) between cognitive and olfactory differences; lower olfactory identification scores in SCD occurred despite minimal cognitive differences across groups.

DISCUSSION

Odor identification is lower in pre-mild cognitive impairment individuals reporting SCD. Olfactory decline may emerge prior to measurable general cognitive decline, supporting its role as a screen for AD.

HIGHLIGHTS

Individuals with SCD exhibit lower odor identification scores.Olfactory identification may serve as an early marker of neurodegeneration in preclinical AD.Heterogeneity highlights the need for multimodal biomarker approaches.