Is Parkinson's disease a primary olfactory disorder?

It has been known for over 30 years that olfactory function is disordered in idiopathic Parkinson's disease (IPD). The severity and partial specificity of anosmia was not realized until recently, with the advent of more detailed analysis and sophisticated measurement. The olfactory vector hypothesis suggests that the causative agent for IPD enters the brain via the nasal route, but the reason for olfactory dysfunction may be more subtle. Evidence for olfactory disturbance is reviewed from pathological, psychological, neurophysiological and genetic standpoints. It is proposed that the initial causative event in IPD may start in the rhinencephalon (olfactory brain) prior to damage in the basal ganglia.