Refractive Errors: Nearsightedness, Farsightedness, Astigmatism and Presbyopia Understanding Visual Acuity
Understanding 20/20 Visual Acuity (Image Courtesy of AMO)
At 20 feet, the person with 20/20 vision sees the stop sign clearly.
At 20 feet, the person with 20/40 vision sees the stop sign as if they were 40 feet away. | Most people realize that 20/20 vision is considered about "perfect" visual acuity, at least in humans. However, many human eyes can see even better than that, in fact, 20/15 vision is fairly common (this is one line smaller on the Snellen eye chart than 20/20). A very small percentage of the population can read down to 20/10, which is two lines smaller on the Snellen eye chart than 20/20. So what does 20/40 visual acuity mean? If an individual has 20/40 vision, it means that person must have the eye chart at 20 feet to see the same detail an individual with 20/20 visual acuity could see with that same chart at 40 feet. By further example, if an individual had 20/800 vision, that person would have to have an object at 20 feet to see it with the same detail that an individual with 20/20 vision could see with that object at 800 feet. Refractive Error The term refractive error encompasses myopia (nearsightedness), hyperopia (farsightedness), astigmatism and presbyopia. Those without any refractive error are said to be emmetropic. Myopia (Nearsightedness) Myopia, or nearsightedness, is the condition of the eye in which images are formed in front of the retina, resulting in a blurred image. This occurs when the eye is relatively too long or the refractive powers of the cornea and lens of the eye are relatively too strong. Myopia usually begins in childhood, and progressively worsens until adulthood is achieved, usually about 18 to 21 years of age.  | | Myopic (Nearsighted) Eye
The Image is Formed in Front of the Retina |
Hyperopia (Farsightedness) Hyperopia, or farsightedness, occurs when images are formed behind the retina, resulting in a blurred image. This is due to an eye that is relatively too short, or the refractive powers of the cornea and lens of the eye are relatively too weak. Depending on the degree of farsightedness and the age of the individual, some farsighted people may see clearly at both distance and near through the process of accommodation, or focusing of the lens in the eye. This may cause significant eyestrain, however, and accommodative abilities are gradually lost with aging.
 | | Hyperopic (Farsighted) Eye
The Image is Formed at a Theoretical Point Behind the Eye |
Astigmatism
Astigmatism generally indicates that the cornea is not spherical. That is, the cornea in astigmatism is sometimes referred to as having a football shape rather than a basketball shape. More specifically, the cornea is steeper in one meridian, and flatter in the opposite meridian 180 degrees away. In some cases, astigmatism occurs primarily as a consequence of the lens of the eye (lenticular astigmatism). In either case, or even if there is a combination of corneal and lenticular (lens induced) astigmatism, the result is two separate points of focus in the back of the eye. To the affected individual, vision is simply blurred or in some cases even distorted. Astigmatism of some degree is present in approximately 30 to 40% of individuals who wear glasses or contact lenses.  | | Astigmatism: The cornea is steeper in one meridian and flatter in the opposite meridian 180 degrees away, resulting in two planes of focus | Irregular Astigmatism Astigmatism indicates that vision is distorted by a variation in the curvature of the cornea. In other words, the cornea has a different curvature in one meridian as compared to the opposite meridian 180 degrees away. When the two meridians of curvature are exactly 180 degrees apart, the condition is known as regular astigmatism. Significant amounts of regular astigmatism occur in about 30 to 40 percent of patients that wear prescription glasses or contact lenses. Regular astigmatism of significant degree may be corrected with cylinder in eyeglasses, toric soft contacts, or rigid gas-permeable contacts. Irregular astigmatism is present when the different meridians of curvature of the cornea are not 180 degrees apart or the cornea takes on multiple meridians of curvature. This condition may be thought of as bumpy surface topography. The visual result of irregular astigmatism is distortion of vision that may not correct well even with glasses or contacts. Irregular astigmatism frequently occurs with the following conditions: keratoconus, corneal scarring, pressure on the cornea from lid masses such as chalazia, and corneal warpage associated with contact lens wear. Treatment of the condition depends on the underlying cause. Presbyopia In infancy and childhood the natural lens of the eye is soft, pliable, and easily altered in shape by the ciliary muscles of the eye through a process called accommodation. Throughout life the lens of the eye gradually hardens, becomes less pliable, and progressively increases its resistance to change in shape. This leads to greater difficulty in focusing at near. By approximately 45 years of age, most people require reading correction, assuming their vision is corrected for distance. By 51 years of age, virtually all people have some degree of difficulty focusing at near. This condition is known as presbyopia. Symptoms of Presbyopia Most patients with presbyopia first notice difficulties while attempting to perform near distance tasks, such as reading. There may be blurring of near objects, or discomfort and fatigue with attempted near vision tasks. Some presbyopic individuals complain that their vision remains blurred for a few moments (or even minutes) after looking up from sustained near vision tasks. This is due to the lag of relaxation of accommodation after excessive near focusing in the presbyopic age group. Many patients complain that their "arms are too short", a statement that indicates accommodative powers are declining. Refractive Error and the Appropriate Refractive Surgical Procedure To understand which procedures are available for the different types of refractive error, please review Chapter Six, "Contemplating refractive surgery – What Are the Options for Your Eyes?" |
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