Overview of Refractive Error
In the emmetropic (normally refracted) eye, entering light rays are focused on the retina by the cornea and the lens, creating a sharp image that is transmitted to the brain. The lens is elastic, more so in younger people. During accommodation, the ciliary muscles adjust lens shape to properly focus images. Refractive errors are failure of the eye to focus images sharply on the retina, causing blurred vision (see Figure: Errors of refraction.).
In myopia (nearsightedness), the point of focus is in front of the retina because the cornea is too steeply curved, the axial length of the eye is too long, or both. Distant objects are blurred, but near objects can be seen clearly. To correct myopia, a concave (minus) lens is used. Myopic refractive errors in children frequently increase until the child stops growing.
In hyperopia (farsightedness), the point of focus is behind the retina because the cornea is too flatly curved, the axial length is too short, or both. In adults, both near and distant objects are blurred. Children and young adults with mild hyperopia may be able to see clearly because of their ability to accommodate. To correct hyperopia, a convex (plus) lens is used.
In astigmatism, nonspherical (variable) curvature of the cornea or lens causes light rays of different orientations (eg, vertical, oblique, horizontal) to focus at different points. To correct astigmatism, a cylindrical lens (a segment cut from a cylinder) is used. Cylindric lenses have no refractive power along one axis and are concave or convex along the other axis.
Presbyopia is loss of the lens’ ability to change shape to focus on near objects due to aging. Typically, presbyopia becomes noticeable by the time a person reaches the early or mid 40s. A convex (plus) lens is used for correction when viewing near objects. These lenses may be supplied as separate glasses or built into a lens as bifocals or variable focus lenses.
Anisometropia is a significant difference between the refractive errors of the 2 eyes (usually > 3 diopters). When corrected with eyeglasses, a difference in image size (aniseikonia) is produced; it can lead to difficulties with fusion of the 2 differently sized images and even to suppression of one of the images.
The primary symptom of refractive errors is blurred vision for distant objects, near objects, or both. Sometimes the excessive ciliary muscle tone can cause headaches. Prolonged squinting and frowning with ocular use can also lead to headaches. Occasionally, excessive staring can lead to ocular surface desiccation, causing eye irritation, itching, visual fatigue, foreign body sensation, and redness. Frowning when reading and excessive blinking or rubbing of the eyes are symptoms of refractive error in children.
Visual acuity testing and refraction (determination of refractive error) as needed should be done every 1 or 2 yr. Screening children's visual acuity helps detect refractive errors before they interfere with learning. A comprehensive eye examination, done by an ophthalmologist or an optometrist, should accompany refraction.
Myopia and hyperopia are corrected with spherical lenses. Concave lenses are used to treat myopia; they are minus or divergent. Convex lenses are used to treat hyperopia; they are plus or convergent. Astigmatism is treated with cylindrical lenses. Corrective lens prescriptions have 3 numbers. The first number is the power (magnitude) of spherical correction required (minus for myopia; plus for hyperopia). The second number is the power of cylindrical correction required (plus or minus). The third number is the axis of the cylinder. As an example, a prescription for a person with myopic astigmatism may read -4.50 + 2.50 × 90, and a prescription for a person with hyperopic astigmatism may read +3.00 + 1.50 × 180.