Objective:
To further assess oculomotor control of patients with Parkinson disease (PD) during fixation and with movement.

Design:
Case-control study.

Setting:
A Parkinson disease research, education, and clinical center.

Patients:
One hundred twelve patients with PD, including 18 de novo untreated patients, and 60 age-matched controls.

Intervention:
Modern, precise eye tracking technology was used to assess oculomotor parameters. Oculomotor function was compared between groups during fixation and while tracking a randomly displaced target on a PC monitor.

Main Outcome Measures:
Fixation stability and saccadic parameters.

Results:
- All patients with PD and 2 of 60 control subjects showed oscillatory fixation instability (ocular tremor), with an average fundamental frequency of 5.7 Hz and average magnitude of 0.27°.
- Saccadic parameters and occurrences of square wave jerks did not differ between subjects with PD and controls.
- The amplitude and frequency of fixation instability did not correlate with disease duration, clinical Unified Parkinson's Disease Rating Scale scores, or dopa-equivalent dosing.
- No differences in oculomotor parameters were found between medicated and unmedicated patients with PD.

Conclusions:
All patients with PD exhibited persistent ocular tremor that prevented stability during fixation. The pervasiveness and specificity of this feature suggest that modern, precise oculomotor testing could provide a valuable early physiological biomarker for diagnosing PD.