Background: Olfactory deficits in Alzheimer's disease (AD) and idiopathic Parkinson's disease (PD) have been well established.
Objective: To clarify and review the literature by evaluating the evidence for olfactory deficits in 3 olfactory domains, including odor identification, recognition, and detection threshold.
Data sources: 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.
Study selection: 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.
Data extraction: Effect sizes were calculated for each study by expressing differences between patient and control group means in SD units (Cohen's d).
Data synthesis: 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).
Conclusions: 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.