Risk of action tremor in relatives of tremor-dominant and postural instability gait disorder PD

Background: Action tremor may be more prevalent in relatives of patients with Parkinson's disease (PD) than in relatives of control subjects. This tremor could represent mild PD or essential tremor. An estimate of the risk of this condition in families of patients with PD is important when studying the genetics of PD. Objectives: To determine the risk of action tremor in first-degree relatives of probands with tremor-dominant PD (TD- PD) and postural instability gait disorder PD (PIGD-PD) compared with first-degree relatives of control probands. Methods: PD and control probands participated in a familial aggregation study of PD. The presence of action tremor in their relatives was ascertained from reports of one or more informants. Relatives who met diagnostic criteria for PD were excluded. Cox proportional hazards models adjusting for gender, education, race, and vital status (dead vs alive) of the relatives were used to assess the relative risk (RR) of action tremor in first-degree relatives of PD probands vs first-degree relatives of control probands. Results: There were 487 PD probands, 409 control probands, and 5,563 relatives. The risk of action tremor was higher in the relatives of TD- PD probands than in the relatives of control probands (RR = 2.14; 95% CI = 1.53 to 2.98) but not in the relatives of PIGD-PD probands compared with the relatives of control probands (RR = 1.81; 95% CI = 0.66 to 5.02). Conclusion: The risk of action tremor was increased in the relatives of PD probands, particularly when they had TD- PD. Whether the tremor in these relatives represents essential tremor or an isolated manifestation of PD requires further investigation.