Research Insights

Progressive worsening of spatial and chromatic processing deficits in Parkinson’s disease

Diederich NJ, Raman R, Leurgans S, Goetz CG. (2002). Progressive worsening of spatial and chromatic processing deficits in Parkinson’s disease , Aug;59(8):1249-52

This article began by reviewing how the visual system can be affected by Parkinson disease. They cited studies that suggest visual deficits are due to reduced dopamine concentration in the retina, abnormal electrical responses in the brain related to visual processing, and reduced metabolism in the area of the brain that controls visual processing. They also mentioned that intoxication to MPTP (a byproduct of an illicit narcotic drug that when taken persons will have the same signs and symptoms of PD) has been found to cause abnormal electrical functioning in the eye. They suggested that patients have reduced contrast sensitivity (used for spatial processing), which they suggested are related to stage and duration of PD as well as the time of the day (e.g. morning versus evening). They also suggested that poor color discrimination (processing different colors) can be found early in PD. The stated hypothesis was to evaluate patients with PD longitudinally (across time) to evaluate the potential changes in contrast sensitivity as well as color discrimination. Twenty eight patients with PD (mean age 65.2 years, PD duration=13.53 years, duration of treatment with levodopa=8.57 years) were evaluated in their "on" state at two separate times approximately 19.8 months apart. Patients with dementia, other neurological conditions, psychosis, or preexisting low visual acuity or color blindness were excluded. Patients were also given the UPDRS as well as a measure of psychiatric symptoms. The authors found that both contrast sensitivity and color discrimination worsened across time for the patients with PD. The authors specifically made the distinction that both conditions worsened independently versus a decline in one visual ability causing worsening in the other. Deterioration of contrast sensitivity was correlated with age, duration of PD, Hoehn and Yahr stage, and UPDRS motor and ADL assessments. Vision change was not correlated with medications or mental status on the UPDRS. Interestingly, color discrimination was only correlated with age. They also found that those patients that had more severe visual impairments after 19 months showed the highest scores on the measure of psychiatric symptoms. They suggested that aging alone does not explain their findings because PD patients typically have more decline in color discrimination and contrast sensitivity as compared to age matched controls, and aging typically affects different frequencies of contrast sensitivity than seen in this group. However they correctly note that aging combined with PD likely explains some of their findings, as it is difficult to tease out exactly how much each condition explains visual changes in this group of patients. In regards to the psychiatric symptoms, the results of this study do not allow the reader or researcher to determine which difficulty came first, the vision or the psychiatric. The authors noted that previous research has shown that patients with PD and visual deterioration "are at a higher risk of developing visual hallucinations." The authors concluded by discussing how deteriorating vision (both contrast sensitivity and color discrimination) causes difficulties with walking as the patient may over interpret or misinterpret incoming visual information, which may in turn cause falls, tripping, illusions, poor depth perception, poor distance judging, and possible visual hallucinations. They suggested that future research should evaluate more patients in a longitudinal study to evaluate deterioration of vision also using more complex statistical calculations.

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