Refractive Errors

Refractive error is a term used to describe an inability of images to focus properly on the retina of the eye. It is the most common cause of blurred vision, and may be a treatable component of poor vision in many patients. These errors are inherited in a complex manner involving many variables. Emmetropia is absence of refractive error and ametropia is the presence of refractive error. Some of the most common [Page 1474]refractive errors include presbyopia, myopia, and hyperopia.

The loss of accommodation that comes with aging is called presbyopia. A person with emmetropic eyes will begin to notice inability to read small print or discriminate fine, close objects at about age 44 to 46. These symptoms increase until about age 55, when they stabilize but persist.

When the image of distant objects focuses in front of the retina in the unaccommodated eye, the eye is myopic, or nearsighted. If the eye is longer than average, the error is called axial myopia. As the object is brought closer, the image moves closer to the retina and comes into sharper focus. The point reached where the image is most sharply focused on the retina is called the far point.

The myopic person has the advantage of being able to read at the far point without glasses even at the age of presbyopia. A high degree of myopia results in greater susceptibility to degenerative retinal changes, including retinal detachment. Concave spherical (minus) lenses are used to correct the image in myopia. These lenses move the image back to the retina.

Hyperopia (farsightedness) is the state in which the unaccommodated eye would focus the image behind the retina. If hyperopia is not too great, a young person may obtain a sharp distant image by accommodating. The young hyperopic person may also make a sharp near image by accommodating more—or much more than one without hyperopia. This extra effort may result in eye fatigue that is more severe for near work. The degree of hyperopia a person may have without symptoms is variable. However, the amount decreases with age as presbyopia increases. If the hyperopia is too high, the eye may be unable to correct the image by accommodation.

The hyperopia that cannot be corrected by accommodation is termed manifest hyperopia. The degree of hyperopia overcome by accommodation is known as latent hyperopia. A moderately farsighted person may see well for near or far when young. However, as presbyopia comes on, the hyperope first has trouble with close work—and at an earlier age than the nonhyperope. Finally, the hyperope has blurred vision for near and far and requires glasses for both.

Spectacles continue to be the safest method of refractive correction. Alternatives include contact lenses, intraocular lenses, and keratorefractive surgery.