Errors of Refraction
Nearsightedness (Myopia)

Nearsightedness or shortsightedness is the most common eye problem worldwide.

Myopia is a refractive condition of the eye in which the focusing mechanism when naturally relaxed results in an image that is focused in front of the retina instead of onto it. This could be caused by the cornea protruding too far away from the eye or from an excessive elongation of the eye structure itself. The myopic condition allows nearby objects to be seen clearly by the observer but distant objects appear blurry.

Myopia may be described by clinical appearance. Such a classification would include simple myopia, nocturnal myopia, pseudomyopia, degenerative myopia and acquired myopia. Myopia may also be classified by severity guided by the optical power of a corrective lens used to aid the eye in focusing distant objects onto the retina. This optical power is measured in diopters and a power of 6 diopters or more is considered high with the individual having severe myopia. With severe myopia, there is a greater risk of the individual developing eye problems like floaters, retinal detachment and glaucoma.

The development of myopia is commonly believed to involve both an individual's genes coupled with a reaction to the environment. Susceptible individuals upon extensive exposure to television and computers or near work like reading may encounter the onset and subsequent progression of myopia. Genetically, changes to the PAX6 gene have been shown to contribute to the development of myopia. Environmentally, it has been put forth that excessive near work results in an inflexibility of the ciliary muscles to control the lens for focusing distant objects. Other theories implicate diet and nutrition as well as abnormal breathing patterns in the development of myopia.

Once diagnosed, treatment is often in the form of eyeglasses, contact lenses or refractive surgery. Alternative therapies may involve eye exercises and relaxation techniques depending on the nature of the myopia. The optical power of corrective lenses is measured in negative diopters to counteract the positive diopters of the myopic eye. Special rigid contact lenses may flatten the cornea and hence reduce myopia. LASIK surgery is the other primary option that can correct most forms of myopia.

Farsightedness (Hyperopia)

Far or long-sightedness is a vision problem that arises from a short eyeball or when the lens is inflexible to the point of maintaining a stretched shape under natural conditions of focusing. The refracting mechanism results in a focused image that lies beyond the retina instead of on it. Thus, the eye is unable to focus on near images. As an object comes nearer the power of the cornea and lens are insufficient to keep the image on the retina and image appears blurred.

Hyperopia can be classified by clinical appearance, severity or in relation to the focusing mechanism of the eye. By clinical appearance, hyperopia can be simple, pathological or functional. By severity, hyperopia is defined according to the amount of refractive error. Low hyperopia involves a refractive error of +2 diopters or less. Moderate hyperopia involves a refractive error of between +2.25 and +5 diopters while high hyperopia involves a refractive error of +5.25 diopters or more. Hyperopia can also be categorized by its relationship to the focusing mechanism, leading to hyperopia that is facultative, absolute, manifest or latent.

Minor hyperopia can often be left uncorrected without much impairment to the individual. Higher hyperopia may be corrected with convex lenses in eyeglasses or contact lenses Refractive surgery like LASIK can correct specific types of hyperopia.

Astigmatism

Astigmatism is a refractory condition in which refraction occurs to different degrees in different planes. The cornea is often not spherical, the slope of which creates greater refractive power in one plane compared with that in a perpendicular plane. The image produced may hence be focused on the retina in one plane but in front of or behind it in another. The end result is the observer having difficulty in making out the fine details of a viewed image. Blurring may occur differently at varying distances with horizontal and vertical lines appearing wavy if not blurred. The prevalence of astigmatism often increases with age.

The condition can be classified based on structure, axis or focus. By asymmetry of structure, an irregularly shaped cornea can give rise to corneal astigmatism while an irregularly shaped lens leads to lenticular astigmatism. If defined by axis, astigmatism can be split into regular and irregular. Regular astigmatism would include against-the-rule, with-the-rule and oblique astigmatism. With-the-rule astigmatism has been shown to be a causative risk factor for the development of myopia. If classified by focus, astigmatism would be categorized by simple, compound or mixed groups and additionally stratified by myopic or hyperopic astigmatism.

Mild astigmatism may not significantly affect vision or health. More severe forms however can cause blurry vision, squinting, fatigue and the development of headaches.

An eye examination can detect the presence of astigmatism and help to define the nature and severity of the condition depending on the tests employed. The curvature of the front surface of the cornea can be measured along and the shape of the cornea accurately determined. The refractive error of the eye can be determined along with the planes and powers of astigmatism.

Eyeglasses, contact lenses and refractive surgery are means of correcting astigmatism. In some, the use of rigid gas permeable contact lenses may enable better vision than using eyeglasses. Refractive surgery like LASIK though can enable freedom from vision aids provided one is a suitable candidate for the procedure.

Presbyopia

Presbyopia occurs as the eye ages and becomes less able to focus objects through the natural focusing mechanism. This is most widely believed to be due to the loss of elasticity of the lens. The curvature of the lens may change as well along with the wearing out of the ciliary muscles. These may also lead to presbyopia.

Presbyopia affects just about everyone at a certain age. Symptoms are commonly noticed between the ages of 40 to 50. These may manifest as difficulty in reading fine print under conditions of poor lighting or experiencing eyestrain after prolonged periods of reading. Symptoms of presbyopia can often be nullified by conditions of bright sunlight as the iris closes to a very small size, achieving a greater depth of focus without utilizing the natural focusing mechanism of the eye.

Nevertheless the condition is not usually curable. It can however be improved by using eyeglasses or contact lenses. Depending on the existence of other refractory conditions, multifocal lenses may be of value. With worsening focus, the prescription lenses will need to be adapted accordingly. Bifocals and multifocals may not be the right solution for everyone. They can cause straight lines to appear bent and result in a feeling of dizziness depending on use. Multiple prescription glasses may be a useful alternative. Monovision is yet another solution employed by some to get around wearing bifocals and reading glasses. This involves using an oppositely correcting contact lens for each eye although this can interfere with the perception of depth. The implantation of corrective lenses is an option that has recently become available for those averse to using corrective eyeglasses and contact lenses.

In considering refractive surgery for the correction of myopia, it should be noted that the later development of presbyopia may confer an advantage to myopes in that reading can be comfortably engaged without assistance.

Vision System - How and Why we see

For us to form an image of an object, light has to first enter the eye through the cornea and then the pupil via the aqueous humour. From here it passes through the iris and then through the crystalline lens. The light then traverses the vitreous humour before reaching the retina. It is here that the light transfers it's energy into an electrical impulse that is carried by the optic nerve through the visual pathway and then through the occipital cortex to be finally interpreted by the brain.

The human eye dedicates to focusing light onto the retina. The media for transmission within the eye is significantly transparent to minimize any loss of light while the cornea and lens act to converge the light onto the retina. Most of the light from the external environment is refracted at the cornea. Of the light that reaches the internal regions of the eye, the intensity is maintained at relatively constant levels by the changing size of the pupil. This is adjusted by the relative contractions of the circular and radial muscles of the iris so that the retina is not damaged by excessive light nor is sight hampered by insufficient light entry.

The lens behind the iris functions to focus the light and this is done via contractions of the ciliary muscles. When these muscles contract, they pull the lens to flatten it resulting in a sharp image of a distant object. When released, the lens thickens allowing a nearby object to be focused.

The wall of the eyeball is made up of three layers. The outermost is the sclera, giving the eyeball the characteristic white colour. The middle layer is the choroid, containing blood vessels that nourish and dispose of waste from the retina cells. It is dark to minimize reflections within the eye. The innermost layer is the retina where the light sensitive cells and neurons are located.

Two unique areas exist on the retina, the fovea and the optic disk. The human fovea is an indentation located directly opposite the lens. It allows for increased colour vision and detail in observation. The optic disk, also known as the "blind spot" has a notable absence of light sensitive cells where the optic nerve fuses with the retina. The rest of the retina contains rods and cones. These cells differ in their response to light. Rods function better in darker conditions through a greater sensitivity to light. Cones conversely respond better to higher intensities and different wavelengths of light, providing colour vision and better perception of detail. 

Rods and cones contain variants of the opsin family of light sensitive proteins. Primary opsins break down into secondary opsins upon contact with light of specific intensity and wavelength. These open ion channels and generate electrical impulses in the cell membranes of the rods and cones. Neurons then convey these impulses to the visual cortex in the brain via the optic nerve. Here the projected image begins to be interpreted.

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