Resting and action tremor in Parkinson's disease: pathophysiological insights from long-term STN-DBS

Tremor is a well-recognized sign of Parkinson's disease (PD), yet its long-term evolution remains unclear, particularly regarding the relationship between resting and action tremor. This retrospective study examined resting and action tremor using specific UPDRS-III subitems in 301 PD patients treated with bilateral subthalamic nucleus deep brain stimulation (STN-DBS), assessed preoperatively and at one-year follow-up, with 108 and 57 patients re-evaluated at ten and 15 years, respectively. Most patients (61.8%) had both tremor types, with smaller subsets showing isolated resting (14.3%), action (10.6%), or no tremor (13.3%). Resting tremor responded better to L-Dopa, STN-DBS, and their combination than action tremor (p < 0.05) and remained stable long-term. In contrast, action tremor worsened over time, particularly in tremor-dominant and mixed phenotypes (p < 0.05). A moderate association between tremor types was observed OFF-medication (ρ = 0.59), weakening under treatment (ρ = 0.23/0.30). Action tremor also correlated weakly with bradykinesia and rigidity (ρ = 0.19/0.21). Overall, these differences suggest distinct pathophysiology and neural circuits for resting and action tremor in PD.