Conditions and Treatments
Nearsighted people older than 40 who are accustomed to removing their glasses for close work need to give extra thought to vision correction surgery. Because their eye gradually becomes a single-focus optical system which can view either near objects or distant objects clearly (but not both), they will probably need glasses to read if they have surgery to focus both eyes for distant objects. For some, this may be an advantage, but for others, it may not.
It is possible to correct one eye for distance and leave the other slightly nearsighted for reading. This technique, called monovision, may give presbyopes the best chance of eliminating corrective eyewear entirely. If you are in the presbyopic age range, you must determine if your goal is to achieve best vision correction for distance in both eyes or to leave one eye slightly nearsighted for reading.
If you are older than 40 and a contact lens wearer, you may have already experienced monovision by wearing one contact lens that has slightly less power. With refractive surgery, you may have monovision by leaving one eye slightly nearsighted.
About a third of patients who try monovision adapt to it successfully, and the other two-thirds elect to have both eyes corrected optimally for distance and wear simple "dime-store" reading glasses for near work.
If you try monovision and do not like it, your under-corrected eye can be enhanced to the full correction, allowing you to see clearly at a distance with both eyes.
Please discuss monovision with your doctor so that an appropriate surgical plan can be made.
The Nearsighted Eye
About one in four Americans is nearsighted, totaling 70 million people. The degree of myopia can vary from low (-1 to -3 diopters) to high (greater than 6 diopters).
Myopia is not caused by reading at an early age, prolonged reading, reading in the dark, watching TV too closely, wearing glasses too strong, wearing glasses too weak or other similar folklore. Some controversy exists over whether myopia can be made worse by prolonged near work as a child, but this has never been proven.
In studies, myopia has been correlated with higher IQs, but this is probably related to the fact that myopes tend to read more because that is where they can see the best without glasses.
Myopia tends to run in families, so it is probably inherited. Myopia also tends to start in the early teens (earlier for high myopes), and increases as the eye grows in length during puberty. It tends to stabilize after age 18.
In a nearsighted eye, the cornea is too steeply curved for the length of the eye, causing light rays to focus in front of the retina. Distant objects appear blurred or fuzzy because the light rays are not in focus by the time they reach the retina. The greater the myopia, the more the light rays converge and the more blurred distant objects are.
Near objects, when viewed at the proper distance, can be seen clearly because the focus of their light rays matches the refractive error of the nearsighted eye. For example, a patient with -2 diopter correction is naturally in focus at 0.5 meters (18 inches).
Having the focus point of the eye within arm's length is an intrinsic advantage to the myope, who always has the option of seeing well at near without optical correction. Indeed, many myopes instinctively remove their spectacles for near work, particularly after the onset of presbyopia.
Spectacle Correction for Myopia
The correction of myopia with spectacle lenses accrues advantages and disadvantages for the myope. The obvious advantage is bringing distant objects into focus. The disadvantages increase in rough proportion to the strength of the lens. Most myopes begin wearing spectacles in childhood and therefore incorporate the less desirable cosmetic, convenience and optical disadvantages into their daily routine; however, this adaption varies among individuals.
Among the optical drawbacks of minus spectacles lenses is the minification of the image. There is a 2% minification for every diopter of spectacle power. For example, a -10 diopter spectacle correction results in a 20 percent reduced image size. Thick lens edges and supporting frames also distort and reduce the peripheral vision.
Glasses fog up under certain conditions, slide down the nose, fall off, need cleaning, and have other issues that prevent optimum performance under all conditions.
Contact Lens Correction
Contact lens correction reduces the optical and physical problems that plague spectacle lenses, but they have their own unique problems, including high-maintenance care solutions, corneal warpage, aggravating dry eye, corneal suffocation, corneal infections and eyelid allergies.
Some people lose their ability to wear contact lenses, particularly individuals with dry or sensitive eyes. It appears that 20 to 30 years of contact lens wear causes a chronic dry eye syndrome.
Presbyopia affects 100 percent of the population by age 50. Currently, bifocals or monovision are the only successful ways to treat presbyopia.
The crystalline lens allows the eye to vary its optical power, permitting individuals with normal vision to view distant objects and refocus their eyes to see near objects sharply. This ability begins to decline around age 40 with the onset of presbyopia (literally "old eye") when the lens starts to lose its flexibility.
Presbyopic individuals with normal vision need reading glasses, while nearsighted and farsighted patients need bifocals for clear viewing at both distance and near.
People who have low myopia (3 diopters or less) often notice that they can read fine print comfortably without their glasses well past the age of 40. This is because their nearsightedness allows them to focus at near without the use of any additional optical power from their crystalline lens.
Bifocal lenses allow the user to view distance objects through the top portion of their glasses, and near objects with magnifiers added to the bottom portion of their glasses. These lenses can be blended together to produce a "progressive add" or "no line" bifocal.
Special Refractive Surgery Considerations
It is important that you understand that refractive surgery does not prevent the age-related loss of the eye's ability to vary its focusing power. If you are older than 40 and have both your eyes fully corrected for distance vision, you will need reading glasses for near work. As an alternative to reading glasses for near work, you may elect to leave 1 eye slightly nearsighted; this outcome is called monovision.