Refractive Error and Presbyopia
Refractive error exists when light does not focus perfectly onto the retina. Most of us have some degree of refractive error, whether we know it or not. If you have one or more refractive errors, your eye care practitioner may prescribe eyeglasses or contact lenses. In contrast, someone with "emmetropia" has perfect vision with no refractive error. Emmetropes see clearly far away with no effort.
There are three types of refractive error: myopia, hyperopia, and astigmatism.
- Myopia or "nearsightedness". If you are myopic, you can see clearly up close, but your distance vision is blurry. (In some areas, myopia is referred to as "hypermetropia."
- Hyperopia or "farsightedness". If you have hyperopia, you may be able to see clearly at all distances, but you have to use extra focusing effort to do so, which can cause eyestrain, headaches, and intermittent blurred vision, especially up close. Hyperopia is often confused with presbyopia (see below), but farsightedness can exist with or without presbyopia.
- Astigmatism. If you have astigmatism, your vision is blurred or distorted at all distances. Many people who are nearsighted or farsighted also have astigmatism.
Presbyopia is the normal, age-related loss of ability to focus on things up close. It affects all of us once we reach our 40s. If you are already in your 40s or 50s, you know what presbyopia does. If you're younger and not yet presbyopic, one of the best ways to understand presbyopia is to consider what your vision is like when your eyes are dilated at the doctor's office. The dilating drops simulate presbyopia by relaxing your eyes' ability to focus (besides making your pupils large). If you wear eyeglasses to see clearly far away, put them on when your eyes are dilated. Then try reading. It'll be hard to focus, and this is what presbyopia is like!
Presbyopia is due to age-related changes inside the eye, either within or around the crystalline lens. If you are presbyopic, your eyes will need at least two different prescriptions: one for far away, and one for up close.
Current conventional refractive surgery procedures cannot correct presbyopia. If you have your refractive error (nearsightedness, farsightedness, and/or astigmatism) corrected with refractive surgery, you will still need reading glasses if you are presbyopic even if you currently are myopic and can see clearly up close without your glasses on! Nearsighted presbyopes can get their distance vision corrected with refractive surgery, but they will give up their good near vision in the process.
An alternative way to deal with presbyopia is called monovision. In monovision, your dominant eye is given a distance prescription, while your other eye is given a near prescription. Contact lens practitioners have used monovision for years. Refractive surgeons use it as well. While monovision can decrease your need for reading glasses, it can take some getting used to. Monovision can hinder depth perception, and you may not feel comfortable driving or reading for extended periods without glasses to bring both your eyes into optimal focus. If you’ve never experienced monovision, try it with contact lenses first to make sure you're comfortable with it. If you are, your eye surgeon can give you permanent monovision LASIK or other refractive surgery.