We studied a group of patients with PD for a mean duration of 11 years. Our study confirms the presence of substantial convergence insufficiency in subjects with PD as compared with control subjects. Furthermore, the severity of this deficit fluctuates throughout the day in association with the dosing schedule of anti-Parkinson medications. Whereas our data show that duration and severity of the PD do not affect the size of the convergence insufficiency, severity of PD adversely affects vision-related quality of life. This appeared to be greater for near activities, rather than all vision tasks.
In our subjects with known ‘wearing-off’ symptoms, convergence ability improved substantially after a discrete dose of dopamine, although convergence did not normalize. Anti-Parkinsonian treatment typically involves taking multiple doses of dopaminergic medications throughout the day, with more frequent dosing as the disease progresses (for example, up to six to seven times daily). Therefore, convergence ability is likely to be in nearly constant flux during waking hours as dopamine levels in the central nervous system rise and fall. Consequently, there is nearly constant but varying reading difficulty and double vision at near.
Ophthalmologic management of individuals with PD requires an understanding of typical PD-related ocular motor dysfunction and cognitive dysfunction as well as the impact of PD treatments. Most subjects had no exodeviation at distance, yet a substantial exodeviation at near. Thus, a custom-fabricated split lens with base-in prism only in the lower segment or single vision readers with prism would be necessary. The clinical situation is aggravated, however, by the sizeable fluctuation of convergence amplitudes in response to dopaminergic therapy. Fluctuating convergence insufficiency compounds the difficulty of prescribing a single satisfactory prism correction even for a single fixation distance. In addition, the reduced convergence ability, even when optimally treated with dopamine, increases the difficulty of alleviating the symptoms.
Our findings suggest that optical correction be prescribed after consideration of the timing of the eye exam relative to dopaminergic treatment. Orthoptic measurements during the “on” and “off” states may be useful in selecting a suitable prism correction, but the effectiveness of this approach remains to be studied. In addition, the typical timing for use of the near spectacle correction by the patient relative to dosing seems essential. Patients with PD should also be queried about on-off fluctuations and wearing-off with respect to their visual complaints. Collaboration with the treating neurologist to ensure that the anti-Parkinsonian regimen is optimized to minimize motor fluctuations may be an appropriate step before prescribing the visual correction. Referral to a program specializing in PD may be appropriate for patients with unmanaged motor fluctuations.15
Afferent visual system function including visual acuity, contrast sensitivity, and color vision in PD subjects did not differ from those observed in controls and did not fluctuate with dopaminergic treatment state. CS measured in the PD subjects (1.63) was similar to normal values described among individuals 50 years of age and older for the Pelli-Robson chart.16,17
Vision-related quality of life was much worse in PD subjects compared to controls using the VFQ-25. This finding seems most likely related to ocular motor function since acuity, color vision, and contrast sensitivity did not appear to vary significantly from controls. However, impaired vision-related quality of life was evident for nearly all of the VFQ-25 subscales, including those specific for near visual activities, driving, and peripheral vision. Although the vision-specific questions in the VFQ-25 specifically direct the respondent to answer only with respect to vision, there could in theory be a generalized impact of PD on the vision-related quality of life assessments. To address that issue, we reviewed the questionnaire results for a correlation between the general health question and the subscale for general vision. There was no correlation for either the controls or the PD subjects suggesting that the subjects were able to separate these two issues. In addition, reviewing the data for general health and near activities, there was no correlation for controls (0.12), but a modest correlation for PD subjects (0.40). This suggests that the effect of PD on vision quality of life is mostly for near activities. This is consistent with the clinical ocular motor measurements made in this study. Lastly, our small sample size prevented a comparison of oral medication alone to oral medication plus deep brain stimulation.
Our study has several strengths. It utilized prospective data collection including functional neurological status to examine the impact of dopaminergic treatment on ocular sensory and motor findings. The examiners were masked to the PD severity and the “on/off” state of the subjects they were assessing. We are limited by a relatively small sample size and the potential for unmasking of the examiners by the presence of the tremor, mobility difficulty, and other features of PD. The PD group included fewer females than the controls because of our use of a convenience sample for controls. We are uncertain of any bias in ocular function that may have been introduced. Lastly, all of our patients were white so we cannot generalize to other races or ethnic groups without further study.
Future study among patients with PD should investigate the effectiveness of optical interventions such as base-in prism or single vision reading glasses for improvement in the visual function of PD patients. Single vision reading glasses may allow the necessary refractive correction and prism to allow restoration of more comfortable reading vision. In addition, attention to the timing of the determination of prism power and medication adjustments that limit ocular motor fluctuations, in cooperation with the treating neurologist, may have an important impact on success of such therapy. Lastly, given the difficulty of finding a single prism correction that is suitable for the varying convergence insufficiency experienced by these patients, evaluation of the effectiveness of vergence exercises to improve convergence as well as varied strength prism correction seems warranted.