Purpose : Vision difficulties have been well described in patients with Parkinson’s Disease (PD), but the etiologies underlying PD patients’ difficulties in performing functional tasks like reading are not well understood. In this study, we performed clinical and infrared oculography studies of reading in PD in order to better delineate the causes.
Methods : We performed a prospective cross-sectional study of 23 treated patients with PD and 25 controls with no known visual symptoms. We assessed subjective visual disability using the National Eye Institute Visual Function Questionnaire (VFQ-25) (n=16), measured reading speed using a rapid number naming task called the King-Devick test (n=23), and further analyzed ocular motor behavior during reading in PD patients with good visual acuity using 500-Hz infrared oculography (RED500, SensoMotoric Instruments) (n=9). Stimuli included fixation, number-reading, and word-reading tasks. Statistical analysis was performed using the Mann-Whitney test (Prism).
Results : PD patients reported decreased visual function with a mean VFQ-25 score of 81±3.6 (n=16), and 56% had moderate or extreme difficulties with near activities. Using the King-Devick (KD) test, we found that PD patients were significantly slower readers than age-matched controls (PD: 68.3±6.5s, ctrl: 48.8±2.4s, p = 0.009), which correlated with subjective decrease in near activity on VFQ-25 (r2=0.49). On infrared oculography, PD patients had significantly slower number reading speed (PD: 100.2 ±12.1 numbers per minute (npm), ctrl: 145.0±20.6 npm, p=0.05) and word reading speed (PD: 135.3±15.1 words per minute (wpm), ctrl: 230.2±21.0 wpm, p=0.01). The slower reading was attributable to more frequent (PD: 6.6±0.3 saccades/line, ctrl: 4.9±0.1 saccades/line, p=0.08) and smaller saccades, more regressions (PD: 9.1%, ctrl: 5.3%, p=0.05), and longer fixations (PD: 266±13 ms/line, ctrl: 233±14 ms/line, p=0.05).
Conclusions : The majority of treated PD patients reported visual disability, especially with reading and near work which correlated with worse performance on a rapid number reading task. Using infrared oculography to study reading behavior, we found that PD patients exhibited abnormalities in saccades, fixations, and reading behavior, which were attributable to ocular motor abnormality, impaired ocular motor planning, and cognitive dysfunction.