What is refractive error?
The term refractive error encompasses myopia (shortsightedness), hypermetropia (longsightedness), astigmatism and presbyopia. These disorders of the eye are routinely corrected with glasses or contact lenses.
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.
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.
The term astigmatism is used to describe a cornea that isn’t round. Instead, it’s shaped like a football rather than a round, spherical basketball. Like a football, the astigmatic cornea has two curves: a steeper one, and a flatter one 90 degrees away. These two curves bend light entering the eye, causing two images to form on the retina.
Patients with astigmatism may notice blurred or ghost images close up or far away. Astigmatism may be present in various degrees and is found in approximately 30 to 40% of individuals who wear glasses or contact lenses.
In infancy and childhood the natural lens of the eye is soft and pliable and is 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 and, by the age of about 45 years of age, most people require reading correction, assuming their vision is corrected for distance. This condition is known as 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.