Ocular tremor appears to be an early warning sign for Parkinson’s disease (PD), according to a new a study by researchers in Virginia. Measurement of the tremor could even be used to diagnose the disorder—and patients could be provided with therapy—well before more prominent problems manifest.

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

For the study, published online in Archives of Neurology, researchers used a video-based binocular eye tracker to compare the oculomotor function of the study group (112 patients diagnosed with PD) against that of the control group (60 age-matched subjects).

The researchers found that all 112 members of the study group demonstrated “persistent instability” when fixating on a target (averaging a fundamental frequency of 5.7Hz, a horizontal amplitude of 0.27˚ and a vertical amplitude of 0.33˚), while just two of the 60 control group participants demonstrated such tremors (and one of those two eventually displayed symptoms of PD).

Researchers also determined that angle of gaze bore no impact on the instability, that the movements were conjugate (i.e., in both eyes), and that each patient’s tremors were consistent over the course of each recording—saccades, blinking or other eye movements did not disrupt them.

The researchers also found that the amplitude and frequency of PD patients’ visual instability did not correlate with the duration of disease, the United Parkinson’s Disease Rating Scale score or the medications being taken. In fact, no differences were seen between those taking medication and those not taking medication for PD.

“The fact that this behavior was universally observed in every tested patient with PD, including unmedicated patients, suggests that ocular tremor is a function of the disease process and not induced by medication,” the researchers wrote.

Although the tremor requires special equipment to detect it, “The test is actually very simple so [it]…could be used as a screen by all or many physicians,” said study author Mark S. Baron, M.D., of the VA Medical Center in Richmond, Va., in an interview with MedPage Today.

“The study clearly demonstrated that patients previously diagnosed with PD, regardless of treatment regimen, displayed unique ocular motility deficits, similar to nystagmus,” comments Michael N. Block, O.D., who is in private practice and is a consultant in geriatric nursing facilities in New York. “The authors posit that these deficits could serve as a biomarker for PD.”

Then again, Dr. Block says, the study does not establish the predictive value of these oculomotor tests.

Gitchel GT, Wetzel PA, Baron MS. Pervasive ocular tremor in patients with Parkinson disease. Arch Neurol. 2012 Apr 9 [epub ahead of print].