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- Refraction is the bending of a wave of light as it passes from air through the more dense optical system of the eye. The degree of bending is dependent on the index of refraction through the air, cornea, and lens and is described quantitatively by Snell law. A refractive error refers to the inability of the eye to produce a focused image on the fovea or central part of the retina.
- Emmetropia: When light rays are in perfect focus, the image being viewed is seen clearly.
- Ametropia: any refractive error of the eye that prevents normal focusing of the image
- Hyperopia: When the cornea of the eye is too flat or the eye is too short, light rays fall in focus behind the retina, and the individual is “farsighted.”
- Myopia: When the cornea is too steep or the length of the eyeball is too long, the focal point for light rays lies short of the retina, and the individual is “nearsighted.”
- Presbyopia: the natural tendency of the crystalline lens to harden (sclerosis and enlarge) with age, limiting the focusing of the eye on near objects (accommodation). By the age of 40 years, most people do not have enough room within the eye to allow for normal excursion of the lens and accommodation; viewing of near objects is blurred, and reading glasses are required to provide the additional power to compensate.
- Astigmatism: When the cornea is steeper in one meridian more than the other or the globe is not round (i.e., is oval or almond shaped), visual blurriness occurs due to the absence of a single point of focus.
- Anisometropia: an unequal refractive error between the two eyes
- System(s) affected: central nervous
Presbyopia occurs after the age of 40 years.
Refractive errors should be detected early in life.
- Predominant age: Refractive errors may be present at birth and can increase in magnitude with age until the eye is fully developed.
- Individuals >40 years of age are more likely to experience presbyopia/the normal loss of accommodation that occurs with age, necessitating the use of reading glasses for close work.
70% of U.S. population has some form of ametropia.
Etiology and Pathophysiology
- Developmental (most common)
- Ocular trauma
- Iatrogenic (e.g., postrefractive or cataract surgery)
Commonly Associated Conditions
Patients with diabetes mellitus have fluctuating myopia as a result of poorly controlled blood glucose and concomitant swelling of the crystalline lens.