refractive errors Flashcards
myopia or nearsightedness.
When the eyeball is too long, as shown in the drawing below, the light entering the eye ends up focusing before it gets to the retina.
Farsightedness, or hyperopia
occurs when the eyeball is too short. As a result, light ends up focusing behind the retina.
Astigmatism
Astigmatism is different from nearsightedness and farsightedness because it makes vision blurry at all distances. To understand why, picture a corneal surface that’s not round like a marble. Instead, the surface is curved like a grape or an olive—that is, more curved in one direction than in the direction perpendicular to it.
presbyopia,
a problem that affects people’s ability to focus up close. This problem usually sets in between the ages of 35 and 45, when the lens becomes less elastic and can no longer focus near objects onto the retina. As you saw in your reading, some researchers think presbyopia may also stem from a weakening of the smooth muscles that help the lens change shape.
Emmetropia
This is the term we use for a person who has no refractive error. This word, too, comes from the Greek—this time from a root meaning well-proportioned.
When a person is emmetropic, the eye and cornea are perfectly spherical, and the rays of light that enter the eye focus directly on the retina without any eyeglass lenses or contact lenses in front of the eye. There are some very fortunate people who stay emmetropic their whole lives until they develop presbyopia and need reading glasses.
Anisometropia
Anisometropia is the term we use to describe people whose eyes have different refractive powers. The first part of this word, aniso-, means unequal. As you learned in today’s reading, anisometropia creates problems with vision and can also be a challenge for the optician since the glasses lenses of a person with this problem will be of different thicknesses.
amblyopia.
If the difference in refractive power between the two eyes is large, the eyes can’t work together and a lazy eye occurs. The medical term for this is amblyopia. This condition affects two or three in every hundred children, so you’ll see it especially often if you work in a pediatric vision clinic. Kids with this problem may have trouble reading and doing schoolwork, and it may be hard for them to play sports.
Amblyopia occurs when the brain can’t take the two differently sized images from each eye and fuse them into one image. It solves this problem by occasionally or continuously shutting off the image from one eye. As a result, one eye will be able to see clearly with prescription glasses, and the other eye, even though it has prescription lenses in front of it, will be unable to see clearly. The eye isn’t really lazy; it’s just that the brain can’t make sense of the images the eye is sending.
