Odor Discrimination as a Marker of Early Alzheimer's Disease

Abstract

Background: Olfactory dysfunction is an early symptom of Alzheimer's disease (AD). However, olfactory tests are rarely performed in clinical practice because their diagnostic efficacy in detecting early AD is unclear.

Objective: To investigate odor discrimination in patients with early AD and the efficacy of olfactory discrimination tests in differentiating these patients from subjects with normal cognition (CN).

Methods: Thirty patients each with mild dementia due to AD (MD-AD) and mild cognitive impairment due to AD (MCI-AD) and 30 older subjects with CN were enrolled. All participants underwent cognitive examinations (CDR, MMSE, ADAS-Cog 13, and verbal fluency) and odor discrimination tests (Sniffin' Sticks test, Burghart®, Germany).

Results: The MD-AD group achieved significantly worse scores on the olfactory discrimination test than the MCI-AD group, and the MCI-AD group achieved significantly worse results than the CN group (p < 0.05). A cut-off score of≤10 had a diagnostic accuracy of 94.44% (95% CI, 87.51-98.17%) in differentiating patients with MCI-AD/MD-AD from subjects with CN and of 91.67% (95% CI, 81.61-97.24%) in differentiating those with MCI-AD from subjects with CN. Our multinomial logistic regression model with demographic data and ADAS-Cog 13 scores as predictor variables correctly classified 82.2% of the cases (CN, 93.3%; MC-AD, 70%; MD-AD, 83.3%); on adding the olfactory discrimination score to the model, the percentage increased to 92.2% (CN, 96.7%; MCI-AD, 86.7%; MD-AD, 93.3%).

Conclusion: Odor discrimination is impaired in cases of early AD and continues to deteriorate as the disease progresses. The olfactory discrimination test showed good diagnostic efficacy in detecting early AD.

Keywords: Alzheimer’s disease; mild cognitive impairment; olfaction; olfactory impairment.

Fig. 1

Olfactory discrimination scores of the three groups. Lines represent medians, error bars represent Interquartile ranges, and dots represent individual data points. MD-AD, mild dementia due to Alzheimer’s disease; MCI-AD, mild cognitive impairment due to Alzheimer’s disease; CN, normal cognition.

Fig. 2

Relationship between olfactory discrimination scores and ADAS-Cog 13 results. ADAS-Cog 13, Alzheimer’s Disease Assessment Scale-Cognitive Subscale, version 13. MD-AD, mild dementia due to Alzheimer’s disease; MCI-AD, mild cognitive impairment due to Alzheimer’s disease; CN, normal cognition.

Fig. 3

Performance of olfactory discrimination score in differentiating between participants with AD and CN. MD-AD, mild dementia due to Alzheimer’s disease; MCI-AD, mild cognitive impairment due to Alzheimer’s disease; CN, normal cognition.