Among the NMS (non-movement symptoms) of Parkinson’s disease, visual changes are often confused with normal aging. However the mechanisms and treatment are very different and some are not even correctable at all. Eye changes of normal aging mainly involve a stiffening of the lens of the eye, making focus on close objects at acute angles difficult to impossible, This is correctable with the aid of reading glasses or a bifocal lens.
The visual changes in Parkinson’s:
1. The eyes show sluggish pupil constriction, required for close vision and bright lights. The ciliary muscle attached to the iris acts like any other muscle affected by PD in this regard. This is correctable with reading glasses/bifocal lenses.
2. Sluggish lens focusing for the same reason, ie., both lens and iris around the pupil need a strong and functional set of ciliary muscles to work.
3. Medications, particularly the anticholinergic drugs given for tremor like benztropine, can impair the ciliary apparatus as well, making close focus even more difficult.
4. The decrease in spontaneous muscle movement of the face or “mask face” of Parkinson’s affects the eyelids as well. Blinking is decreased and often requires the use of artificial tears to keep the cornea and eyeball moist.
5.The retina in the back of the eye shows a slowed electrical response to flashes. This may be responsible for part of the blurring and color loss seen. This would not be correctable with eyeglasses.
6. The eye-movement control region of the brainstem relies on dopamine. In some cases of PD, the eyes when neutral at rest, end up fixated in an upward gaze that can result in an aesthetically disturbing or “weird” look.
7. The eye-movement control region relies on the basal ganglia (refer to previous articles) to help smooth-out and initiate movement so that dynamic eye functions like following a moving object can become impaired in Parkinson’s when the basal ganglia misfire due to lack of dopamine.
8. When looking at close objects the eyes converge, i.e, they assume a slight “cross-eyed” position. With Parkinson’s, this ability to converge can become weak and this can result in actual double vision when doing close work. This can be corrected by blurring out one eye of a pair of reading glasses with translucent nail polish and using only one eye for close work or reading.
9. Patients with advanced Parkinson’s can experience disturbing visual hallucinations.
10. Because the visual system is tied in with balance, the impaired ability to smoothly follow moving objects may contribute to the postural imbalance that occurs in later stages of Parkinson’s.