Refractive error


Refractive error
Refraction error
Classification and external resources
ICD-10 H52.0-H52.4
ICD-9 367.0-367.2-367.9
DiseasesDB 29645

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.

Contents

Classification

An eye that has no refractive error when viewing distant objects is said to have emmetropia or be emmetropic. The eye can focus parallel rays of light (light from distant objects) on the retina, without using any accommodation. A distant object in this case is defined as an object 6 meters or further away from the eye.

An eye that has refractive error when viewing distant objects is said to have ametropia or be ametropic. This eye, when not using accommodation, cannot focus parallel rays of light (light from distant objects) on the retina.

The word "ametropia" can be used interchangeably with "refractive error" as they refer to the same thing. Types of ametropia include myopia, hyperopia and astigmatism. They 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.
    • Myopia: When the optics are too powerful for the length of the eyeball one has myopia or nearsightedness. This can arise from a cornea with too much curvature (refractive myopia) or an eyeball that is too long (axial myopia).
    • Hyperopia: When the optics are too weak for the length of the eyeball, one has hyperopia or farsightedness. This can arise from a cornea with not enough curvature (refractive hyperopia) or an eyeball that is too short (axial hyperopia).
  • Cylindrical errors occur when the optical power of the eye is too powerful or too weak across one meridian. It is as if the overall lens tends towards a cylindrical shape along that meridian. The angle along which the cylinder is placed is known as the axis of the cylinder, while 90 degrees away from the axis is known as the meridian of the cylinder.
    • Astigmatism: People with a simple astigmatic refractive 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.

Diagnosis

A doctor uses a trial frame and trial lenses to estimate the patient's refractive error.

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 [1]

An automated refractor is an instrument that is sometimes used in place of retinoscopy to objectively estimate a person's refractive error.[2]

Vision defects caused by refractive error can be distinguished from other problems using a pinhole occluder, which will improve vision only in the case of refractive error.

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 three are typically used.

In the case of myopia, however, some believe that such treatments may also have the long-term effect of exacerbating that refractive error — i.e., making the patient even more nearsighted. This would be due to the very same prescription that is tailored for use at a 12-to-20-foot distance also commonly being used for close-up work as well, thus artificially amplifying the focusing stress that would normally be presented to the accommodation mechanisms of the eye at that distance.[citation needed]

However, this exacerbating effect is not generally believed to exist in the general case, although in cases where the myopia is due to accommodative spasm, removing the corrective lenses for a time may lead to improvement.[citation needed]

Epidemiology

Statistical number of people with refractive error in 2004.[3]
  no data
  less than 100
  100-170
  170-240
  240-310
  310-380
  380-450
  450-520
  520-590
  590-660
  660-730
  730-800
  more than 800

The global prevalence of refractive errors has been estimated from 800 million to 2.3 billion.[4]

References

See also


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • refractive error — an abnormality of the eye resulting in a blurred image on the retina as a result of abnormal focusing, which can be corrected by glasses, contact lenses, or refractive surgery. Refractive errors include myopia, hypermetropia, and astigmatism …   Medical dictionary

  • refractive error — an abnormality of the eye resulting in a blurred image on the retina as a result of abnormal focusing, which can be corrected by glasses, contact lenses, or refractive surgery. Refractive errors include myopia, hypermetropia, and astigmatism …   The new mediacal dictionary

  • Error (disambiguation) — Error may refer to: *Error, a mistake. *Error coram nobis, a type of writ. *Postage stamp error *Error coin *Error, a character from cience*Approximation error, error in scientific measurements *Standard error (statistics) *Errors and residuals… …   Wikipedia

  • error of refraction — refractive error deviation from optimal focusing of light (emmetropia) by the lens of the eye onto the retina, such as myopia, hyperopia, astigmatism, or anisometropia …   Medical dictionary

  • Error — The word error has different meanings and usages relative to how it is conceptually applied. The concrete meaning of the Latin word error means wandering or straying . To the contrary of an illusion, an error or a mistake can sometimes be… …   Wikipedia

  • refractive surgery — any surgical procedure that has as its primary objective the correction of any refractive error. It includes such procedures as clear lens extraction, LASIK, LASEK, photorefractive keratectomy, and thermokeratoplasty …   Medical dictionary

  • refractive surgery — any surgical procedure that has as its primary objective the correction of any refractive error. It includes such procedures as clear lens extraction, LASIK, LASEK, and photorefractive keratectomy …   The new mediacal dictionary

  • Refractive index — Index In dex, n.; pl. E. {Indexes}, L. {Indices}(?). [L.: cf. F. index. See {Indicate}, {Diction}.] [1913 Webster] 1. That which points out; that which shows, indicates, manifests, or discloses; as, the increasing unemployment rate is an index of …   The Collaborative International Dictionary of English

  • Index error — Index In dex, n.; pl. E. {Indexes}, L. {Indices}(?). [L.: cf. F. index. See {Indicate}, {Diction}.] [1913 Webster] 1. That which points out; that which shows, indicates, manifests, or discloses; as, the increasing unemployment rate is an index of …   The Collaborative International Dictionary of English

  • Kurzsichtigkeit — Klassifikation nach ICD 10 H44.2 Degenerative Myopie/Maligne Myopie H52.1 Myopie H52.5 A …   Deutsch Wikipedia


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