Functional imaging studies of hyposmia in Parkinson's disease

Hyposmia in Parkinson's disease (PD) was evaluated by using neuroimaging techniques. It is well known that olfactory impairments are one of the cardinal non-motor symptoms in PD. However, all smell tests used in previous studies depend on subjective answers by examinees and on sniffing of odorants, the latter of which may be impaired in PD as a consequence of motor impairments. We developed an fMRI system, which can visualize brain activation by olfactory stimuli during natural breathing. Although 7 age-matched controls demonstrated significant activations in various brain areas including precentral gyrus (BA6/6) and middle temporal gyrus (BA19/39) by the odorant stimuli, 9 patients with PD showed little activations by the same stimuli. These data suggest that the olfactory dysfunction in PD is not a simple reflection of impaired sniffing. Recent epidemiological studies demonstrate that the olfactory impairments may precede the onset of motor symptoms. Moreover, several pathological studies suggest that amygdala is one of the most frequently affected regions and is closely related to hyposmia in PD. Further brain imaging studies of hyposmia will shed light on the early pathological changes in PD.