The spectrum of square wave jerks in essential tremor: an exploratory study

Square wave jerks (SWJ) are saccadic intrusions interrupting fixation, reflecting an inhibitory deficit linked to impaired cerebellar-brainstem-basal ganglia networks. Cerebellar pathways including fastigial nucleus exert a central role in determining both SWJ subtypes and saccadic adaptation. We aimed to investigate eye movements abnormalities in Essential Tremor (ET) with a particular focus on SWJ subtypes, assessing their possible role in disease characterization. Different SWJ subtypes during primary position fixation task as well as visually-guided saccades (VGS) were evaluated among healthy controls (HCs), pure ET and ET-plus patients through Eyelink 1000-Plus. Forty tremor patients [10 pure ET (25%) and 30 ET-plus (75%)] and 23 HCs were enrolled. Adjusting by age, higher rate of biphasic SWJ (BSWJ) was found in tremor population compared to HCs. The rate of BSWJ was even higher in pure ET as compared to ET-plus. A new pattern of “triphasic” SWJs as well as “staircase” SWJ were found in pure ET and ET-plus but not in HCs. Higher variability on horizontal VGS parameters was found in tremor patients compared to HCs and a trendwise increase was also seen across HCs, ET-plus and pure ET. Fixation and saccadic oculomotor abnormalities were demonstrated in ET patients with clinical-instrumental correlates, showing the possible usefulness of quantitative eye movement assessment in ET characterization.