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.