Slowed saccades and increased square wave jerks in essential tremor

Abstract

Background: Eye movements in essential tremor (ET) are poorly described and may present useful information on the underlying pathophysiology of the disorder.

Methods: Sixty patients with ET, including 15 de novo untreated patients, and 60 age-matched controls constitute the study population. A video-based eye tracker was used to assess binocular eye position. Oculomotor function was assessed while subjects followed random horizontally and vertically step-displaced targets.

Results: For all reflexive saccades, latencies were increased in ET subjects by a mean of 16.3% (p<0.01). Saccades showed reduced peak velocities with a lengthy, wavering velocity plateau, followed by slowed decelerations. For larger 30°+ saccades, peak velocities were decreased by a mean of 25.2% (p<0.01) and durations increased by 31.8% (p<0.01). The frequency of square wave jerks (SWJs) in patients was more than triple that of controls (p<0.0001). Despite frequent interruptions by SWJs, fixations were otherwise stable and indistinguishable from controls (root mean square [RMS] velocity, p = 0.324). The abnormal eye movement parameters were independent of disease duration, tremor severity, and medication therapy.

Discussion: In contrast to normally swift onset and efficient acceleration/deceleration movements, saccades in ET are characterized by abnormally prolonged latencies and slowed velocity profiles. Although ET subjects maintain highly stable fixations, they are interrupted by increased numbers of SWJs. This study reveals novel oculomotor deficits in ET, which are distinct from the eye movement dysfunction of other movement disorders, supporting a role for eye tracking to assist in the differential diagnoses of not only atypical, but also more common movement disorders.

Keywords: Essential tremor; cerebellum; main sequence; ocular motility; saccades; square wave jerks.

Conflict of interest statement

Conflict of Interests: The authors report no conflicts of interest.

Financial disclosures: None.

Figures

Figure 1. Frequency of Square Wave Jerks while Fixating.

Essential tremor patients exhibit appreciably more square wave jerks than age matched controls (p

Figure 2. Latency of Reflexive Saccades to Randomly Displaced Targets

Patients with essential tremor have increased saccadic latencies compared with age-matched controls (p

Figure 3. Velocity profiles of 10° saccades. Single velocity traces are shown for 18 representative essential tremor (ET) subjects and 18 control subjects.

The velocity profiles were aligned at t = 10 ms at the defined saccadic onset threshold (20°/second). Note the lower peak velocity, velocity plateau, and prolonged duration of saccades in ET subjects. The integral was taken of each individual velocity curve, and no difference was found between each group (p = 0.59). This further supports that despite abnormal velocity behavior during saccadic flight, that subjects with ET maintain accurate saccadic amplitudes, and reach the intended target position.

Figure 4. Saccadic Dynamics along the Main Sequence.

(A) Peak velocity vs. amplitude of saccades. For all movement amplitudes, subjects with essential tremor generally exhibit lower peak velocities than controls (Vmax = 400°/second for essential tremor [ET] subjects vs. 500°/second for controls, C  =  19.9 for ET subjects vs. 9.1 for controls). (B) Saccadic duration vs. amplitude. Saccades are generally slower for subjects with ET compared to controls, with the group differences becoming progressively greater with increasing saccadic amplitude (average duration of a one degree saccade (D1)  =  28.8 ms for ET subjects vs. 20.6 ms for controls, exponential value (n) for ET subjects  =  0.41 vs. 0.39 for controls).