Usefulness of near-infrared spectroscopy as olfactmetry for Parkinson's disease

Olfactory dysfunction is a frequent non-motor symptom in Parkinson's disease (PD). This symptom is considered to be an early manifestation of the disease. The aim of this study is to investigate to the usefulness of near-infrared spectroscopy (NIRS) as olfactmetry for PD. We employed a 22-channel NIRS device with eight light incident fibers and seven light detector fibers, each with an interoptode distance of 2.5 centimeters on the frontal head. Isovaleric acid was used as the odor stimulant. We measured the change in total hemoglobin concentrations [totalHb] from pre-baseline values. Then, the changes in [totalHb] of normosmia healthy subjects and patients with PD were compared. In all normosmia healthy subjects and eight patients with PD, isovaleric acid caused remarkable changes of [totalHb], especially in the lower areas of the frontal cortex. These activated areas may be related to the orbitofrontal cortex, corresponding to olfactory cortices. However, in fourteen patients with PD, isovaleric acid caused no changes. In these fourteen patients, eight patients have no subjective olfactory symptom. As the results, in patients with PD, subjective symptoms are different from the results of objective test. This study shows the usefulness of NIRS as olfactory test for PD.