Abstract
Changes in olfactory perception, which can be tested with psychophysical tests, have been noted in many neurodegenerative diseases. Olfactory testing is of greatest significance in Alzheimer's and Parkinson's disease, in which olfactory decline precedes other clinical symptoms.
It can indicate the presence of a neurological disorder, aid in differential diagnosis, or help estimate the future development of the disease. This review presents olfactory dysfunction profiles in selected neurodegenerative diseases with an emphasis on Alzheimer's and Parkinson's disease, assessment of quantitative and qualitative olfactory dysfunction, focusing on the widely used psychophysical tests which can be employed in routine olfactory testing in everyday clinical practice, and the 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, the decline is more homogeneous across the various olfactory measures and the increase in detection thresholds is more prominent.