Pace-Induced Saccades in Essential Tremor Differ from Those in Parkinson's Disease and Degenerative Ataxias

Background: Essential tremor (ET) is increasingly recognized as a heterogeneous disorder that may present with additional Parkinsonian or cerebellar features, complicating its differential diagnosis from Parkinson's disease (PD) and degenerative ataxias (DA). Oculomotor assessment has been proposed as a potential tool for identifying disease-specific patterns. Methods: We evaluated eye movement parameters in 50 patients with ET (including subgroups with Parkinsonian and/or cerebellar signs), 50 patients with PD, 42 patients with DA, and 42 healthy controls. Reflexive, pace-induced, and cued saccades were recorded using saccadometry, while smooth pursuit and fixation were assessed with electrooculography. Group comparisons focused on saccadic metrics and the frequency of abnormalities. Results: Hypometria of pace-induced saccades was observed in patients with PD (32.0%) and DA (57.1%) but was not detected in any ET subgroup, including those with Parkinsonian or cerebellar features. Reflexive saccade hypometria was less frequent in ET patients with Parkinsonian features compared to PD (50.0% vs. 66.0%, p = 0.016). Other oculomotor parameters showed substantial overlap between groups. Conclusions: ET patients, including those with additional Parkinsonian or cerebellar signs, showed partially distinct oculomotor features compared with PD and DA, particularly in pace-induced saccades. These findings require confirmation in larger, well-characterized cohorts.