[Olfactory dysfunction in Parkinson's disease: its role as a new cardinal sign in early and differential diagnosis]

Olfactory dysfunction is a prominent symptom in Parkinson's disease (PD) and found in about 70-100% of patients. In earlier studies significant loss of olfactory function seemed to be unrelated to disease duration, did not correlate with motor function, and was uninfluenced by antiparkinsonian medication. We suggest that the increase of dopaminergic cells in the olfactory bulb is responsible for the hyposmia in PD patients. Interestingly, this olfactory dysfunction is not found in progressive supranuclear palsy or corticobasal degeneration. In multiple system atrophy, the deficit is mild and indistinguishable from cerebellar syndromes of other aetiologies. Intact olfaction has also been reported recently in Parkin disease (PARK 2) and vascular parkinsonism. Olfactory tests may significantly enhance the diagnostic armamentarium in the differential diagnosis of parkinsonian syndromes and indeterminate tremors. Furthermore, olfactory testing may also prove to be a useful aid in the early or "preclinical" detection of PD, once effective disease-modifying therapies are found. Braak and coworkers have confirmed the widespread, extranigral pathology in PD and suggested that pathology in the anterior olfactory region may be one of the earliest appearances of neurodegeneration in PD.