Alzheimer's Imaging Consortium.

Area of Science:

• Neuroscience
• Neurology
• Gerontology

Background:

• Olfactory deficits are early indicators of dementia, including Alzheimer's Disease (AD).
• Understanding olfactory dysfunction in pre-dementia stages is crucial for early detection.
• This study investigates olfactory identification (OID) in relation to AD biomarkers and brain volumes.

Purpose of the Study:

• To examine associations between cerebrospinal fluid (CSF) biomarkers and central olfactory system volumes with OID.
• To analyze these associations in individuals with subjective cognitive impairment (SCI), mild cognitive impairment (MCI), and AD.
• To identify potential early markers for dementia within the olfactory system.

Main Methods:

• Recruited 154 SCI, 51 MCI, and 31 AD participants.
• Assessed OID using the Sniffin' Sticks test.
• Measured CSF biomarkers (Aβ42, p-tau181, NfL) and central olfactory system volumes (hippocampus, amygdala, entorhinal cortex, orbitofrontal cortex, insula, caudate).
• Analyzed data using partial correlations and multiple linear regressions.

Main Results:

• SCI participants (M=14.1) had better OID than MCI (M=12.7) and AD (M=12.8) groups.
• In SCI, OID correlated with Aβ42 levels and orbitofrontal volume.
• In MCI, OID correlated with hippocampal, parahippocampal, and caudate volumes.
• In AD, OID correlated with hippocampal volume.
• Linear regression showed Aβ42 and orbitofrontal volume predicted OID in SCI; NfL, entorhinal, and insula volumes predicted OID in MCI.

Conclusions:

• Olfactory deficits in SCI are associated with amyloid levels and reduced orbitofrontal volume, indicating early roles for amyloid deposition and frontal lobe degeneration.
• In MCI and AD, olfactory deficits may stem from degeneration in the medial temporal lobe and subcortical structures.
• These findings highlight the potential of olfactory testing and associated neuroimaging/biomarkers for early dementia detection.