Abnormal eye movement behavior during reading in Parkinson's disease

Abstract

Introduction: Reading difficulties are common in Parkinson's disease (PD) but not well studied. We report a case of reading difficulties in a 40-year-old man with 6-year history of PD on dopamine replacement therapy.

Methods: We performed detailed neuro-ophthalmic examination and assessment of reading with and without infrared oculography.

Results: Clinical examination revealed visual acuity of 20/20, no evidence of vision loss, and normal eye movement and ocular alignment with normal saccades, pursuit, and normal convergence. During King-Devick test, a rapid number reading task performed on a book, patient had normal number reading speed. More detailed study of number and word reading using infrared oculography revealed that while this patient had normal speed and eye movement behavior during number reading, he had dramatic slowing and eye movement abnormality during word reading. The slower reading speed during word reading was due to increased number of progressive saccades, smaller saccade amplitudes, increased number of regressive saccades, and longer fixation durations.

Conclusions: This case nicely illustrated the importance of comprehensive neuro-ophthalmic evaluations in Parkinson's disease and shows that reading difficulties can arise even when there is good visual acuity, ocular motor abilities necessary to read, and accommodation. In this case, reading difficulty was due to higher order ocular motor planning or cognitive abilities involved in word reading since the patient had no difficulty with ocular motor planning while reading numbers. These findings may have important implications towards our understanding of PD and can serve to spark further research in this important area.

Keywords: Eye movement; Neuro-ophthalmology; Parkinson's disease; Reading; Vision.

Fig. 1.. Slow reading due to abnormal eye movement behavior in Parkinson’s disease.

(A) Schematic diagram of infrared oculography during reading. (B) Representative scan paths during reading in control (ctrl) and PD. Circles denote fixations and lines are saccades; larger circles represent longer fixations. (C) Word reading speed. (D) Line graphs showing horizontal gaze positions during word reading (control on left, PD on right) in (B). (E) Number reading speed. F) Line graphs of horizontal gaze positions during number reading in control (left) and PD (right). (G–J) Bar graphs showing ocular motor behavior during word and number reading. (G) Number of saccades per line. (H) Mean saccade amplitude per line. (I) Mean fixation duration per line. (J) Number of regressive saccades per line. Abbreviations: Amp: amplitude, ctrl: control, Fix: fixation, PD: Parkinson’s disease, Sac: saccades.