17 One study in which first-degree relatives of PD patients and controls were interviewed and screened for tremor showed a trend toward increased risk of ET among family members of patients with PD, with 4.8% of relatives of PD patients having ET versus 3.5% of controls, though this did not reach statistical significance (p = 0.08). 60 The risk increased with younger onset of PD (5.8% of relatives had ET when PD onset was ≤66 years p = 0.006) and with tremor-dominant or mixed forms of PD.
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