Refractive imbalance occurs when there is a difference in the quantity and quality of vision between each of the two eyes.
Monovision is an intended refractive imbalance, usually when one eye is corrected for distance vision and the other eye is corrected for close vision. This refractive imbalance is usually no more than 1 or 2 D (diopters, or levels of refractive power).
Those under 40 don’t typically have a problem with close vision, so they usually have both eyes corrected for distance. Those over 40, however, start to lose their ability to focus when looking at things very close to them, such as a book or computer monitor. This is called presbyopia. If someone in this group chooses glasses, they can get one pair of glasses for distance vision and another pair for close vision, or, more commonly, just get a single pair of bifocal glasses. This group can also get contact lenses, correcting one eye for distance vision and the other for close vision (or even some of the newer bifocal contact lenses). Refractive surgery options for monovision almost always correct one eye for distance vision and the other eye for close vision.
Some people tolerate the refractive imbalance from monovision quite well. Others find it extremely frustrating because this imbalance can contribute to mobility issues (the brain uses the information from both eyes to judge distances).
The fixes for refractive imbalance are dependent upon the cause and amount of the imbalance. Annoying refractive imbalance after monovision refractive surgery can sometimes be corrected with glasses. This imbalance can also usually be corrected with contact lenses (in one or both eyes, as necessary), as well as with additional refractive surgery (usually to correct both eyes for vision). Additional surgery usually comes with a risk level similar to the original surgery.
Why don’t glasses always correct refractive imbalance?
When there is a refractive imbalance between the eyes and one wears glasses, one eye has its light refracted on the cornea and the other has its light refracted on the lens of the glasses. This means that the size of the image will be slightly different in each eye. Some people can adapt to this, but others cannot and find it extremely frustrating. The problem is generally worse the greater the amount of refractive imbalance. Someone who started off as very nearsighted (for example) who has refractive surgery on one eye but not the other might only be able to obtain comfortable vision while wearing contact lenses.
Why can’t I just wear glasses again?
After refractive surgery, many people with GASH, multiple images in each eye, and/or loss of contrast sensitivity cannot simply just wear glasses again. This is because glasses cannot mask some of the problems caused by refractive surgery. When you wore glasses and you got a scratch on them, you could go to the eye doctor and get a new pair of eye glasses. But after refractive surgery, the refractive equivalent of this scratch (called an aberration) is now at the surface of your eye, and you can’t just go to the eye doctor and order a new pair of eyes! Glasses and soft contact lenses just can’t compensate for the imperfections on the surface of the eye.
Many people with GASH, multiple images in each eye, and/or loss of contrast sensitivity may find good vision with rigid gas permeable prosthetic contact lenses (RGPs, aka hard lenses). These lenses succeed in masking corneal imperfections because the light entering the eye is refracted by the contact lens, instead of the cornea. Fitting RGPs after refractive surgery can be challenging (due to the changed curvature of the cornea) and requires the expertise of a specially trained optometrist. Not everyone is able to wear these lenses successfully, but generally those with poor vision after refractive surgery are very motivated and dedicated to finding a long-term solution with RGPs, since this is the only way they can see well.