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.

Classification

An eye that has no refractive error when viewing distant objects is said to have emmetropia or be emmetropic meaning the eye is in a state of in which 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. This proves to be an evolutionary advantage by automatically focusing the eye on objects in the distance because it allows an individual to be alert in say, a prey-predator situation.

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" or "image formation defects." 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). Myopia can easily be corrected with a concave lens which causes the divergence of light rays before they reach the retina.
  • 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).This can be corrected with convex lenses which cause light rays to converge prior to hitting the retina.

• 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. This is caused by a deviation in the shape of the cornea, a shape other than spherical. This defect can be corrected with refracting light more in one area of the eye than the other. Cylindrical lenses serve this purpose.
  • Presbyopia: When the flexibity of the lens declines typically due to age. Individual would experience difficulty in reading etc. This causes the individual to need visual assistance such as bifocal lenses.

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

An automated refractor is an instrument that is sometimes used in place of retinoscopy to objectively estimate a person's refractive error.. Shack–Hartmann wavefront sensor and its inverse can also be used to characterize eye aberrations in a higher level of resolution and accuracy.

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.

  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.

See also

• Eyeglass prescription

References

• Saladin, Kenneth S. Anatomy & Physiology: the Unity of Form and Function. Dubuque: McGraw-Hill, 2010. Print