Olfaction testing in patients with tremor‐dominant Parkinson's disease: Is this a distinct condition?

There is considerable controversy regarding the relationship between essential tremor (ET) and Parkinson's disease (PD), especially when tremor is the dominant feature of PD or there is a family history of tremor. Reduced olfaction function is one of the initial signs of PD. In contrast, ET has relatively preserved olfaction. To infer whether the tremor‐dominant subgroup of PD is intrinsically different from mainstream PD, we tested olfaction using the University of Pennsylvania Smell Identification Test‐40 (UPSIT) in this group and compared the findings with those of patients with non–tremor‐dominant “regular” PD. We then evaluated predictors of reduced UPSIT scores within the tremor‐dominant group. Overall, olfaction did not differ between tremor‐dominant PD and regular PD; however, the subgroup of tremor‐dominant PD with a family history of tremor had less olfaction loss than those without a family history ( P = 0.0007) or those with regular PD ( P = 0.0350). Other clinical features of this tremor‐dominant PD with a family history of tremor group mostly resembled those without a family history. This finding suggests that patients with a family history of tremor may represent a different disease process even though, aside from differences in olfaction, they are clinically similar to other patients with tremor‐dominant parkinsonism. It additionally suggests phenotypic overlap between PD and ET. © 2004 Movement Disorder Society