Can Botox Impair Vision?

When Botox is injected it can spread almost 3 cm (~1 inch from the injection site). When it is injected, it is kind of similar to predicting what an ink spot will look like when you deposit a drop of ink on a tissue. It's not the same everytime. One of the best ways to control this is by using concentrated doses and injecting very small quantities to limit the diffusion.
In the eye area, 3 cm is a large area and it includes the area 3cm around and 3cm deep. Therefore it is possible to affect the vision muscles or the eyelid lifting muscles (levator).
Sagging of the eyelid is called blepharoptosis.
When ptosis occurs, it can be temporarily managed by stimulating another muscle that lifts the eyelid called Muller's muscle. Ptosis can be treated with apraclonidine 0.5% eyedrops. This is one of the preferred agents used to stimulate Muller's muscle. Other second line (alternative) medications include phenylelphrine 2.5% eye drops. Both of these medications require a prescription and should be used under the guidance of a physician due to contra-indications, interactions, side effects and adverse reactions.

For patients that have some degree of upper lid ptosis (an eyelid that is slightly lower than it should be) or for patients with significant brow ptosis your frontalis muscle (your forehead muscle) may have been compensating for the extra skin. Botox may be blocking the action of that muscle causing you to be unable to use your forehead muscle to keep that skin out of the way.
Botox can also diffuse into muscles that are responsible for keeping your lid elevated though this is rare.
Botox should not affect your ability to focus.
You should consult a physician about these problems.
I hope this helps.

Botox Cosmetic migrates a little bit away from the injection site, like a drop of water in the pool. As a result, Botox may affect facial muscles not intended. Injections around the eyes, which are helpful for wrinkle reduction, may spread to a muscle responsible for lifting the upper eyelid, called the levator. This migration occurs with anyone who injects Botox, including dermatologist, facial plastic surgeons, and cosmetic specialists.
Ptosis, otherwise known as a droopy or saggy eyelid, occurs because of a weakened levator muscle of the upper eyelid. Although uncommon, Botox may affect the vision by causing a droopy eyelid obstructing the upper visual field. Ptosis from Botox is temporary and resolves within a few weeks, but may last 3-4 months.
Upper eyelid ptosis from Botox can be temporarily managed by stimulating another muscle that helps lift the upper eyelid, called Muller's muscle. Eye drops, such as apraclonidine 0.5%, are used twice daily in the interval time until the natural muscle movement of the levator returns.
Speak to your plastic surgeon and ask about treatment for eyelid ptosis from Botox Cosmetic.

It is a rare adverse effect for Botox injection. The migration of the product into the orbicularis muscle will lead to the lid ptosis.Application of alpha blocker eye drops will help to a certain extent. Otherwise, if you wait, it will go away.

Drooping of the eyelid is not common. Sometimes when the botox is injected to close to the eyeball, it can affect the muscles that move the eye as well.  The effects of Botox will last 3-4 months.