Patients with Parkinson’s disease (PD) experience significantly reduced contrast sensitivity across a range of lighting environments, with deficits particularly pronounced under low-light conditions, according to a new study that could have implications for both clinical assessment and disease monitoring.
Published in Ophthalmic and Physiological Optics, the study is the first to comprehensively evaluate contrast sensitivity function (CSF) in PD under six different illumination conditions while also examining the influence of disease stage. The findings suggest that visual dysfunction in PD extends well beyond standard photopic testing and may be especially evident in situations that mirror everyday visual challenges.
Researchers from the University of Valencia and collaborating institutions assessed 61 patients with Parkinson’s disease and 61 age-matched healthy controls. Participants underwent contrast sensitivity testing under photopic and mesopic conditions, both with and without varying levels of glare. The team also calculated an Index of Contrast Sensitivity (ICS), a summary measure designed to capture overall visual performance across spatial frequencies.
Across virtually all testing conditions, patients with PD demonstrated poorer contrast sensitivity than healthy controls. The deficits were most apparent at intermediate and higher spatial frequencies and became particularly striking under mesopic conditions, which simulate the lower illumination levels encountered during dusk, nighttime driving, or indoor environments with subdued lighting.
“PD participants demonstrated significantly poorer CSF than controls across photopic, mesopic, and glare conditions,” the authors noted, highlighting mesopic testing as especially sensitive to disease-related visual impairment.
The findings help explain many of the visual complaints commonly reported by people with Parkinson’s disease. Reduced contrast sensitivity can make it more difficult to negotiate stairs and curbs, adapt to changing light levels, drive at night, read low-contrast text, or cope with glare from headlights and streetlights. These visual challenges can have a substantial impact on independence and quality of life.
The researchers also explored whether visual performance deteriorates as Parkinson’s disease progresses. Initial analyses suggested that contrast sensitivity worsened with advancing Hoehn and Yahr stage. However, after adjusting for age, most stage-related differences were no longer statistically significant. Age itself emerged as a major factor influencing contrast sensitivity, indicating that some apparent effects of disease progression may actually reflect age-related visual decline.
Similarly, while the ICS showed significant differences between disease stages in unadjusted analyses, these associations disappeared once age was taken into account. According to the study authors, this suggests that contrast sensitivity deficits are a robust feature of Parkinson’s disease but may not serve as a straightforward marker of disease progression without careful consideration of age-related effects.
The authors argue that contrast sensitivity testing under mesopic and glare conditions may offer greater clinical value than conventional photopic assessments alone. They also suggest that testing at mid spatial frequencies – where visual sensitivity normally peaks – could provide the most useful information for identifying functional visual impairment in Parkinson’s disease. They state that future research into the subject should also incorporate a larger, more diverse cohort, particularly in relation to the inclusion of more participants at Hoehn and Yahr stages I and III.
