Facts About Refractive Errors, Presbyopia & Cateracts
In order for a person to see an image clearly, it must be focused properly on the retina, the posterior layer of the eye that contains photoreceptor cells that transmit the image to the brain.
Only a minority of people are fortunate enough to have perfect vision without correction. The rest of us are nearsighted, farsighted, or have astigmatism:
- Myopia (nearsighted): near vision may be clear, distance blurry
- Hyperopia (farsighted): both near and distance vision are blurry, a person may notice that near vision is even worse than distance vision
- Astigmatism: this type of focusing error arises because the person's eye is not spherical (like a basketball), but is oval shaped (like a football). This distortion causes blurry vision at near and distance.
The term "presbyopia" refers to the inability to focus in and out using the natural lens/muscle system, a condition that we all experience beginning around age 40. When we are young, most of us see well at a distance with the proper corrective lenses. In order to focus up close, we use our ciliary muscle to control the natural lens inside the eye. As we get older, the lens becomes inflexible and the ciliary muscle becomes weaker. By around age 40, we lose the ability to focus at various distances and require bifocals or reading glasses to help us see things up close.
After around age 40:
- Emmetropic (normally sighted) people require reading glasses
- Myopic (nearsighted) people require bifocals, or they may be able to read by taking off their distance glasses
- Hyperopic (farsighted) people require bifocals
Correction of Refractive Errors
Eyeglasses are the most common, and safest, method for correcting refractive errors. They have a number of advantages over other forms of correction, including:
- Protection from foreign material and injury
- Ability to change lenses as needs change
- Opportunity to use special features such as tints, polarized lenses, sunglasses, and antireflective coating
Contact lenses are preferred by some individuals for a variety of reasons, including appearance and full peripheral vision. Contact lenses are relatively safe when good lens hygiene is practiced. This includes daily lens cleaning, disinfection, and periodic enzyme treatment. Contact lenses that are taken out at night (daily wear lenses) are safer than those that are left in overnight (extended wear lenses). Some contact lenses are replaced after a year of use; others (disposable lenses) are replaced every day or after several weeks.
Vision Correction Surgery can also be used to correct focusing errors. In the 1970's and 1980's, the most common corrective surgery was radial keratotomy, the use of a calibrated diamond scalpel to make partial thickness grooves or scores on the surface of the cornea. Radial keratotomy was limited for use in mild degrees of nearsightedness, and sometimes resulted in visual side effects such as glare or starbursts.
Laser Vision Correction was developed in the late 1980's. The first method was photorefractive keratectomy (PRK). In PRK, the excimer laser is used to reshape the surface of the cornea. The cells on the surface usually heal over the treated area in 3 to 5 days. The most advanced method is laser in-situ keratomileusis (LASIK). In LASIK, the surgeon creates a flap on the surface of the cornea followed by the use of the excimer laser to reshape the cornea. Because the corneal flap covers the treated area, LASIK has quicker visual improvement and relatively mild discomfort.
Over time, the natural lens inside the eye hardens and becomes opaque. When the lens becomes cloudy, we refer to it as a "cataract." If the clouding is mild, the vision may be only slightly affected. Once the opacity becomes more severe, vision is impaired and cataract surgery may be beneficial. A cataract is not a growth or film over the eye.
If vision is only a bit blurry, a change in glasses may help for a while. If a person is not able to see well enough to do their daily activities, cataract surgery should be considered. Unlike many other kinds of eye surgery, cataract surgery is not performed with a laser. The most advanced method of cataract surgery is known as phacoemulsification.
Cataract surgery is performed with an injection of anesthetic around the eye or the use of drops for anesthesia. This type of surgery involves the use of a very small incision about 1/8 of an inch in size. A small ultrasound instrument is used to break up and suction the lens out of the eye. The cataract is then replaced with an artificial intraocular lens implant. The incision is constructed in a way that allows the natural pressure in the eye to close the incision at the end of surgery without the need for stitches in most patients.