Evaluation of Olfactory Impairment in Parkinson’s Disease Using Near-Infrared Spectroscopy

Hyposmia, psychiatric disorders, and cognitive problems are common non-motor manifestations of Parkinson’s disease (PD), but how they are related to the progression of PD remains unclear. Olfactory dysfunction, which is a common non-motor symptom of Parkinson’s disease, is considered to be an early manifestation of this disease. The aim of this study is to investigate the usefulness of near-infrared spectroscopy (NIRS) in measuring olfaction in PD patients and to establish the cortical basis of olfactory function in PD patients. This study was conducted on 9 healthy normosmic volunteers and 24 patients with PD. The authors employed a 22-channel NIRS device with eight light-incident fibers and seven light-detection fibers and placed fibers every 2.5 cm on the forehead of volunteers. Isovaleric acid was used as the odor stimulant. The authors measured the changes in total hemoglobin (tHb) concentration from baseline values and compared the results obtained from healthy normosmic volunteers with those from PD patients. PD patients had higher scores in the subjective olfactory test and smaller changes in tHb concentrations compared with normal volunteers. In particular, no changes in tHb concentration were detected in patients with the awareness of the sense of smell disorder.