Functional Optical Hemodynamic Imaging of the Olfactory Cortex in Patients with 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 was to establish the cortical basis of olfactory function in patients with PD. This study was conducted on ten healthy, normosmic subjects and seven patients with PD (one with subjective olfactory dysfunction and nine without subjective olfactory dysfunction). We employed a 22-channel near-infrared spectroscopy (NIRS) device with eight light-incident fibers and seven light-detector fibers, each with an inter-optode 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 and compared the results obtained for healthy normosmic subjects and patients with PD. In all healthy normosmic subjects and three patients with PD, isovaleric acid caused remarkable changes in (totalHb), especially in the lower areas of the frontal cortex. However, in four patients with PD, isovaleric acid caused no changes. This result indicates that subjective symptoms are different from objective test results in patients with PD. These activated areas may be related to the orbitofrontal cortex corresponding to the olfactory cortices. This study suggests that normosmic subjects with PD already have damage to their olfactory function.