Contribution of Olfactory Tests to Diagnosis of Neurodegenerative Diseases

In many neurodegenerative diseases, changes in olfactory perception have been identified that can be tested with psychophysical tests. Olfactory testing is of particular significance in Alzheimer's and Parkinson's disease, where olfactory decline precedes other clinical symptoms. Olfactory decline may indicate a neurological disorder, aid in differential diagnosis, or help to estimate disease prognosis. This review presents olfactory dysfunction profiles in selected neurodegenerative diseases with an emphasis on Alzheimer's and Parkinson's disease, assessment of quanti-and qualitative olfactory dysfunction, focusing on widely used psychophysical tests suitable for routine olfactory testing in everyday clinical practice, and contribution of olfactory testing to the diagnosis of the selected neurodegenerative diseases. In Alzheimer's disease, identification is more severely affected than detection thresholds, whereas in Parkinson's disease, decline is more homogeneous across various olfactory measures and increase in detection thresholds is more prominent.