Olfactory deficits in Alzheimer's disease (AD) and idiopathic Parkinson's disease (PD) have been well established.
To clarify and review the literature by evaluating the evidence for olfactory deficits in 3 olfactory domains, including odor identification, recognition, and detection threshold.
A literature search of English-language studies of olfaction in AD, PD, and healthy controls was conducted via online databases (PsycInfo and MEDLINE) and reference lists from review articles.
To meet selection criteria for meta-analysis, each study required a control group and complete and usable data. This review yielded 26 publications of olfactory identification, recognition, and/or detection threshold. Because of the inclusion of more than 1 relevant study of olfaction in several of these publications (eg, both identification and threshold assessed), 43 studies were ultimately appropriate for meta-analysis.
Effect sizes were calculated for each study by expressing differences between patient and control group means in SD units (Cohen's d).
Extremely large effect sizes were shown across all tasks in both AD and PD groups. Both between-group analyses using the Mann-Whitney U test and within-group analyses using Friedman 2-way analysis of variance did not reveal any significant differences (all P>.30).
As expected, severe deficits were found for both patients with AD and PD in each of the 3 olfactory domains relative to controls. However, no discriminating olfactory deficits were seen between patient groups or among the 3 measured olfactory domains, suggesting a similar disturbance in olfactory function between patients with AD and PD.