7-04: Refractive Errors

Refractive error is the most common cause of reduced clarity of vision (visual acuity). In the normal state (emmetropia) objects at infinity are seen clearly. Focusing on objects nearer than infinity requires an increase in the refractive power of the crystalline lens by the process of accommodation. In farsightedness (hyperopia) objects at infinity are not seen clearly unless accommodation is used and near objects may not be seen because accommodative capacity is finite. Farsightedness is corrected with plus (convex) lenses. In nearsightedness (myopia) the unaccommodated eye focuses on objects closer than infinity. The markedly nearsighted eye is able to focus on very near objects without glasses. Objects beyond this distance cannot be seen without the aid of corrective (minus, concave) lenses. Astigmatism, in which the refractive errors in the horizontal and vertical axes differ, is corrected with cylindrical lenses. Presbyopia is the natural loss of accommodative capacity with age. Persons with emmetropia usually notice inability to focus on objects at a normal reading distance at about age 45. Farsighted individuals experience symptoms at an earlier age. Presbyopia is corrected with plus lenses for near work.

Use of a pinhole will overcome most refractive errors and thus allows their identification as a cause of reduced visual acuity. Refractive error can be treated with glasses, contact lenses, or surgery.

TREATMENT

A. Contact Lenses

An estimated 40.9 million US adults wear contact lenses, mostly for correction of refractive errors, though decorative-colored contact lenses are used.

The major types of lenses are rigid (gas-permeable) and soft. Rigid lenses, which need to be removed each day, are more durable and easier to care for than soft lenses but are more difficult to tolerate. Soft lenses are also usually removed every day but are also available for extended wear. Soft lenses are usually renewed every 2 weeks or monthly. Daily disposable soft lenses are available and can be renewed daily.

Contact lens care includes cleaning and sterilization whenever the lenses are to be reused and removal of protein deposits as required. Sterilization is usually by chemical methods. For individuals developing reactions to preservatives in contact lens solutions, preservative-free systems are available.

The major risk from contact lens wear is corneal infection, potentially a blinding condition. Such infections occur more often with soft lenses, particularly extended wear, for which there is at least a fivefold increase in risk of corneal infection compared with daily wear. Decorative contact lenses have a high prevalence of microbial contamination. Contact lens wearers should be made aware of the risks they face and ways to minimize them, such as avoiding overnight wear or use of lenses past ...