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Refractive
error
A refractive
error, or refraction error, is an error in the focusing of light by the
eye and a frequent reason for reduced visual acuity.
Epidemiology
The global
prevalence of refractive errors has been estimated from 800 million to
2.3 billion.
Types
An eye that
has no refractive error when viewing a distant object is said to have
emmetropia or be emmetropic. An eye that has a refractive error when viewing
a distant object is said to have ametropia or be ametropic.
Refractive
errors are frequently categorized as spherical errors and cylindrical
errors. Spherical errors occur when the optical power of the eye is either
too large or too small to focus light on the retina. People with refraction
error frequently have blurry vision. When the optics are too powerful
for the length of the eyeball (this can arise from a cornea with too much
curvature or an eyeball that is too long), one has myopia. When the optics
are too weak for the length of the eyeball (this can arise from a cornea
with not enough curvature or an eyeball that is too short), one has hyperopia.
Cylindrical
errors occur when the optical power of the eye is too powerful or too
weak across one meridian of the optics. It is as if the overall lens tends
towards a cylindrical shape along that meridian. People with this refraction
error see contours of a particular orientation as blurred, but see contours
with orientations at right angles as clear. When one has a cylindrical
error, one has astigmatism.
Causes
Refractive
errors are thought to occur due to a combination of genetic and environmental
factors. Trauma or ocular disorders such as keratoconus may induce refractive
errors.
Diagnosis
Blurry vision
may result from any number of conditions not necessarily related to refractive
errors. The diagnosis of a refractive error is usually confirmed by an
eye care professional during an eye examination using an instrument called
a phoropter which contains a large number of lenses of varying optical
power. In combination with a retinoscope (a procedure entitled retinoscopy),
the doctor instructs the patient to view an eye chart while he or she
changes the lenses within the phoropter to objectively estimate the amount
of refractive error the patient may possess. Once the doctor arrives at
an estimate, he or she typically shows the patient lenses of progressively
higher or weaker powers in a process known as refraction or refractometry.
Cycloplegic agents are frequently used to more accurately determine the
amount of refractive error, particularly in children.
An automated
refractor is an instrument that is sometimes used in place of retinoscopy
to objectively estimate a person's refractive error.
Treatment
and management
How refractive
errors are treated or managed depends upon the amount and severity of
the condition. Those who possess mild amounts of refractive error may
elect to leave the condition uncorrected, particular if the patient is
asymptomatic. For those who are symptomatic, glasses, contact lenses,
refractive surgery, or a combination of the preceding three may be used.
Although the
claims of efficacy are disputed by mainstream ophthalmology, some claim
that various relaxation techniques, such as the Bates method, may correct
or eliminate refractive errors.
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