Human sense of Vision - Refractive-error

  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.

Classification

An eye that has no refractive error when viewing a distant object is said to have emmetropia . An eye that has a refractive error when viewing a distant object is said to have ametropia Refractive errors are frequently categorized as spherical errors and cylindrical errors:

1. 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.
2. 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. 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.

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. 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 people 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. Lasik Eye Surgery LASIK, Laser-Assisted In Sito Keratomileusis, is a laser vision correction surgery that is meant to reduce a person!!!s need for glasses or contacts. The procedure is highly specialized and can only be performed by a trained LASIK surgeon. The procedure begins with a formal consultation, where you will meet your LASIK expert, answer some questions, and together, determine which LASIK procedure it right for you, and what you can expect. Unfortunately, those with Cataracts are typically not candidates for the LASIK procedure. Once you!!!ve had your consultation, and determine that you are qualified for the surgery, you!!!ll make another appointment to have the procedure done. Prior to your procedure, you!!!ll arrive at the LASIK center, where the staff there will make you feel relaxed and welcome. Most patients are given a mild sedative, and led to a quiet waiting room where the lights are dimmed. When you go in for the actual procedure, you!!!ll come to find the whole process takes less than 15 minutes. You!!!ll be given anesthetic drops for your eyes, and you!!!ll lay back on the table. While everyone!!!s pain threshold is different, few people describe the procedure as !!!painful!!! - most find it to be uncomfortable, but bearable. The exact LASIK process involves an incision in your corneal flap so your inner eye is exposed. A laser will shoot down waves that smooth the abnormalities in the cornea that cause imperfect vision. Once you!!!re done with this part, you!!!ll be sat up, and asked to read an eye chart. If all has gone well, you should start to see a difference immediately! Following the surgery, you!!!ll be given lubricating drops to keep your eyes moist. Most patients are able to see the results in a matter of hours to days. Again, LASIK, CustomLASIK, and PRK are not procedures designed to treat Cataracts. However, if you!!!re unsure about the state of your vision, or have ruled out Cataracts as a definite cause to your vision problem, why not explore LASIK more?