Refractive error

Refractive error, also known as ametropia, occurs when an eye cannot focus light accurately on the retina. The retina is the light-sensing tissue located at the back of the eye. For clear vision, the eye's refractive power must perfectly match its physical length. When this balance is lost, light focuses away from the central retina instead of directly on it. This results in blurry or distorted vision.

To understand how this happens, we must look at the process of refraction. Refraction is the bending of light as it passes through the eye. This process is performed primarily by the cornea and the lens in the anterior segment. When you look at distant objects, which are more than 6 meters or 20 feet away, light enters the eye as parallel rays. If the eye is emmetropic, it focuses these rays perfectly on the retina without extra effort. However, when objects are close, the eye must perform accommodation. This means the eye thickens the lens to increase its refractive power and bring the rays together on the retina.

There are four primary types of refractive error. Myopia, or nearsightedness, happens when the eyeball is too long or the refractive power is too strong. This causes light to focus in front of the retina, making distant objects blurry. Hyperopia, or farsightedness, occurs when the eyeball is too short or the refractive power is too weak. This makes near objects appear blurry. Astigmatism is caused by an asymmetrical cornea. Because the refractive power is not uniform, the eye focuses light more strongly in one direction than another, which distorts images. Finally, presbyopia is a result of aging. As the lens loses its flexibility, it can no longer change shape sufficiently for near vision.

Doctors use several methods to diagnose these conditions. During an eye examination, a professional might use a trial frame and various lenses. They may also use a retinoscope, which is a tool used to observe the pattern of light reflection inside the eye. This process is called retinoscopy. To get an objective estimate, clinicians may use an automated refractor. For even higher resolution, they can use a Shack–Hartmann wavefront sensor. To confirm a refractive error, doctors sometimes use a pinhole occluder. If vision improves when looking through the pinhole, it suggests a refractive error is present.

Refractive errors are a massive global health issue. It is estimated that between one and two billion people worldwide have them. In 2013, approximately 660 million people had uncorrected refractive errors. This represents about 10 per 100 people. Sadly, 9.5 million of those people were blind due to these errors. Rates vary significantly by region. For example, about 25% of Europeans are affected, while 80% of Asians are affected. Myopia is the most common disorder. Among adults, rates range from 15% to 49%, while children see rates between 1.2% and 42%.

Several factors influence the risk of developing these errors. Genetics play a major role, as people with affected parents are more likely to have similar conditions. Myopia is linked to several heritable connective tissue disorders, such as Marfan syndrome, Knobloch syndrome, and Stickler syndrome. Environmental factors also matter. Myopia has been observed in people with visually intensive occupations. In children, spending significant time reading or focusing on near objects like screens can be a predictor. Interestingly, children who spend more time playing outdoors appear to have a lower risk than those who focus heavily on near tasks.

Management of these errors involves several different tools. Eyeglasses are considered the easiest and safest method of correction. They act as added lenses to bend light toward the retina. People can use single vision lenses for one type of error or multifocal lenses to correct both near and far vision. Contact lenses are another option. Some are soft, made of silicone or hydrogel, while hard lenses can physically change the shape of the cornea. Finally, refractive surgery is available. This can involve permanently changing the cornea's shape or implanting intraocular lenses inside the eye.