Impaired sense of smell is one of the earliest clinical features in both Alzheimer's disease (AD) and Parkinson's disease (PD). A meta-analysis was performed on articles obtained from the PubMed database in order to determine what aspects of olfaction are affected in these two diseases. By applying strict criteria, we included a total of 81 studies meeting the following criteria: (1) patients had a clinical diagnosis of AD or PD; (2) patients were compared to a healthy control group; (3) patients and controls were age-matched; (4) olfactory function was assessed by means of a psychophysical olfactory test; (5) mean and standard deviation were reported. Results indicate that AD and PD patients are more impaired on odor identification and recognition tasks than on odor detection thresholds task. In addition, PD patients are more impaired on detection thresholds than AD patients. These results suggest that PD patients are more impaired on low-level perceptual olfactory tasks whereas AD patients are more strongly impaired on higher-order olfactory tasks involving specific cognitive processes. The effect appears more pronounced for AD than PD, which seem to be affected more homogeneously. In conclusion, olfactory identification and recognition appear as the most interesting candidates to be included in a battery to detect subclinical cases in AD. In parallel, detection thresholds should be included in such a battery for subclinical PD patients.
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