Parkinson's patients come to me all the time to evaluate vision symptoms. There are certain complaints that come up frequently.
Parkinson's disease is all about slowness of movement. It's not just your walking that slows down, but all kinds of movements, and that does affect the eyes.
2.) Eye movements slow down. It is particularly true that vertical eye movements slow down in Parkinson's disease. Many patients don't appreciate this, but it does cause one big problem. If you use eyeglasses, then you need to be thoughtful about what kind of glasses you get. Bifocals are great. (Thanks Benjamin Franklin). They allow us to see both close or distance. Trifocals are even better. We can see near, intermediate or far, but the space for each lens gets smaller. Multifocal lenses are also known as progressive lenses or no-line bifocals. These glasses have gradual variation from near to distance, so theoretically you can focus on any distance. But there is only one small spot in the glasses that focuses each distance. This means the wearer must be very accurate with their eye movements to get the right focus. Most people know when they first got these type of glasses it took some time to get used to it. It is not always easy. So, for patients with Parkinson's disease who have slower than normal eye movements, it might not be practical to use this type of glasses. The easiest glasses to focus with are single vision glasses. Get a pair for reading, a pair for distance and maybe a pair for computer work. Just change the glasses depending on what visual activity you are doing. If having three pairs of glasses is too much to keep up with, then the best compromise is to stick with the old fashioned bifocals.
The other type of double vision occurs when both eyes are not working together. In that case you'll see fine with either eye covered, but when you put the two together it creates double vision. Most often this is horizontal or side-by-side double images. This problem occurs due to weakness of the horizontal eye movements. Depending on what we look at, the eyes move closer together or further apart to find the proper depth of view. Moving together is called convergence and looks like crossed eyes. 
1.) The commonest complaint is dry eyes. Your eyelids are there to protect the ocular surface, and every time you blink the eyelid cleans the eye like a little squeegee. Many Parkinson's patients notice that they have reduced facial expressions. Doctors call this "masked facies" (facies means facial expression- doctors do like their special words.) And reduced facial expressions means reduced eyelid blinking. As you read this you will blink plenty- because you are thinking about it. For the remaining 99.9% of the day, blinking is automatic behavior. Have someone look at your blinking when you are not paying any attention and they can give you an idea of how much you blink. Normal blink rate varies a lot, from 8-20 times per minute and it varies depending on what you are doing, but overall Parkinson's patients tend to blink less and the blinks are slower. If you blink less, the surface of the eye gets dry.
Dry symptoms include blurry vision, burning, itching and redness. It sounds counter-intuitive, but watering is also a symptom of dryness. Once your eyes get dry they oversecrete, but then they just water and run. The ocular surface is important for sharp vision. The reason the surface of the eye, the cornea, is clear is because it is very highly organized on a microscopic level. The cornea is unique tissue. It is one of the only parts of your body that has no blood supply. It can't have any blood because then it wouldn't be clear. So, how does the cornea survive without any blood? It gets all the nutrients it needs from the tear film. If the tear film isn't healthy, the cornea won't be healthy and if it isn't healthy it won't be crystal clear.
Dry symptoms include blurry vision, burning, itching and redness. It sounds counter-intuitive, but watering is also a symptom of dryness. Once your eyes get dry they oversecrete, but then they just water and run. The ocular surface is important for sharp vision. The reason the surface of the eye, the cornea, is clear is because it is very highly organized on a microscopic level. The cornea is unique tissue. It is one of the only parts of your body that has no blood supply. It can't have any blood because then it wouldn't be clear. So, how does the cornea survive without any blood? It gets all the nutrients it needs from the tear film. If the tear film isn't healthy, the cornea won't be healthy and if it isn't healthy it won't be crystal clear.
Treatment of dry eyes is easy in theory but in reality it takes some dedication. Most cases can be treated with lid hygiene and artificial tears. Lid hygiene includes lid scrubs and warm compresses twice per day. Artificial tears ought to be done 4 times per day. If you get preservative free tears you can use them even more than 4 times per day. If you have dry symptoms first thing in the morning, then look into thicker tears for the nighttime, like gel drops or ocular lubricating ointment.
2.) Eye movements slow down. It is particularly true that vertical eye movements slow down in Parkinson's disease. Many patients don't appreciate this, but it does cause one big problem. If you use eyeglasses, then you need to be thoughtful about what kind of glasses you get. Bifocals are great. (Thanks Benjamin Franklin). They allow us to see both close or distance. Trifocals are even better. We can see near, intermediate or far, but the space for each lens gets smaller. Multifocal lenses are also known as progressive lenses or no-line bifocals. These glasses have gradual variation from near to distance, so theoretically you can focus on any distance. But there is only one small spot in the glasses that focuses each distance. This means the wearer must be very accurate with their eye movements to get the right focus. Most people know when they first got these type of glasses it took some time to get used to it. It is not always easy. So, for patients with Parkinson's disease who have slower than normal eye movements, it might not be practical to use this type of glasses. The easiest glasses to focus with are single vision glasses. Get a pair for reading, a pair for distance and maybe a pair for computer work. Just change the glasses depending on what visual activity you are doing. If having three pairs of glasses is too much to keep up with, then the best compromise is to stick with the old fashioned bifocals.3.) Double vision. There are two forms of double vision.
If the eye is very blurry, it can actually go double from the blurriness. You can check for this by covering over each eye. If either eye sees double all by itself then you are dealing with that type of double vision. In that situation see the discussion above about dry eyes.
Moving apart is called divergence. Patients with Parkinson's disease can have deficiency with convergence (double vision at near) or deficiency with divergence (double vision at distance) or both (double vision both at near and distance but a sweet spot in between where the vision is single.)
This symptom is difficult to correct. If the double vision is mild, patients could benefit from prism glasses (especially for near). Prisms are special lenses that bend the light and can help straighten out double vision. However, the challenge is that the strength of the prisms varies depending on the distance you are looking at. So, if you have tried prisms and they don't work well, I would not advise trying any more prisms. Sometimes it is more of a hassle and expense than it is worth.
The simplest solution is a bit crude, but totally effective. You can always cover one eye. If you only see with one eye, you should only have single vision. One could choose the pirate style eye patch.
If you want a less conspicuous option, you could try a bit of scotch tape on one lens of your glasses, or you can also get specialty tape that won't leave a residue to keep your glasses nice.
4) Excessive eye blinking. This is essentially the opposite of #1 above, but everyone living with Parkinson's knows that it can cause some surprising symptoms. Parkinson's patients sometimes get additional movement disorder called dystonia. Dystonia means unusual or unexpected muscle tone. It will often affect just one part of the body. For example, I have seen patients whose big toe sticks up in the air. If dystonia affects the face it will often cause the eyes to blink or scrunch shut. This eyelid dystonia is called blepharospasm. Actually the dryness that Parkinson's patients experience probably contributes to blepharospasm when it occurs. Those patients start out with just dry eyes, but over time, those dry eye symptoms flip something in the brain that causes the eyes to blink too much. So again, treat the dryness.
The blepharospasm itself is definitely treatable. Many Parkinson's disease specialist can inject botulinum toxin to the eyelid. A careful dose of botulinum will weaken the eye squeezing muscles and help the eyelids relax.
In summary, there are four common eye problems among Parkinson's patients.
1. if the eyelids slow down, the eyes get dry
2. if the vertical eye muscles slow down it can make it difficult to use progressive lens glasses
3. if the horizontal eye muscles slow down it can cause double vision
4. if there is too much tone in the eyelid muscles it can cause excessive eye closure
Most of these symptoms can't be "cured" but there are different strategies to manage each.