Refractive error

# Overview
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.

# Classification

## 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.
- 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
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.
It is worth noting, however, that in the case of myopia, 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.
And although the claims of efficacy are disputed by mainstream optometry and ophthalmology, some claim that various relaxation techniques, such as vision therapy or the Bates method, may ameliorate or eliminate refractive errors .