Evaluation of the sense of smell by using near-infrared spectroscopy

Olfactory impairment is a characteristic of early stage of Parkinson's disease (PD). Over 95% of PD patients present with severe olfactory loss. Almost all olfactory tests are subjective. Therefore, olfactometry cannot be performed unless the patient himself/herself notices olfactory impairment. When the condition of the patients was diagnosed as a PD, they had completely lost their sense of olfaction. Thus, an objective olfactory test is required to measure the olfactory function of patients with PD. In this study, we examined the subjective olfactory test (T&T olfactometry) and the objective olfactory test (cerebral blood volume changes under odor stimulation by using near-infrared spectroscopy [NIRS]) in PD patients and age-matched elder normosmic volunteers. The results of the subjective and objective olfactory tests of PD patients were significantly worse than those of the normal volunteers. These results suggested that most of the PD patients had olfactory impairment and half of the PD patients did not notice their olfactory impairment. In case of the latter group of patients, subjective olfactometry is not useful. The objective olfactory test is required to measure the olfactory function of patients with PD. Multi-channel NIRS is expected to be widely used in the future for clinical objective olfactometry. Olfactometry may be an early screening test for the diagnosis of PD.