the errors of refraction

8/6/2019 The Errors of Refraction

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The errors of refraction

The Normal Eye

In the normal eye the refractive components of the

eye include the cornea, the crystalline lens (shownin white) and the vitreous gel (behind the crystallinelens). The combined refraction of these elements isthus that rays of light reflected off objects in thefront of the eye are focused on the retina (the foveacentralis). When such a refractive state occurs, astate of normal vision exists as shown in the diagramabove. Unaided vision will then be declared 6/6 (or

20/20).

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Myopia

What is it?

Also called short-sightedness or short-sight, because theperson affected is able to perform near work withoutcorrection, and has difficulty defining distance objects.Rays reflected off an object in front of the eye focus infront of the retina. A "-" or concave lens is needed for

correction of this refractive state.Why does it occur?

In the great majority of cases the cause of myopia is anincreased length of the eyeball. But it may also be due toan increase in the curvature of the cornea or thesurfaces of the crystalline lens. Cataract can alsoproduce myopia due to a change in the refractive index

of the crystalline lens of the eye. Myopia is of two types.A simple myopia in which there is only a myopic state inthe eye, the commonest form of myopia, and no damageto the retina, and pathological myopia in which there isdamage to the retina of the eye.

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Myopia can also be progressive if the negative powercontinues to increase over a time period. Progressivemyopia may require surgical intervention by a retinaspecialist.

Myopia in childhood may be associated with a divergent

squint.

Presbyopes with myopia are able to do their near workwithout glasses as the negative correction in myopiacancels the positive correction of presbyopia (this is ofcourse applicable to lower refractive errors); ascompared to hypermetropes who need presbyopiccorrection at an earlier age.

Symptoms

Typically uncorrected myopes do not have "eye-strain","watering" of the eyes or headaches as often ashypermetropes do. The myopia is usually detected in theyoung when children playing with each other discover

that they can not see distant objects as well as theirfriends do, or the class-teacher complains that the childmakes too many mistakes copying things from the black-board.

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Hypermetropia (or hyperopia) and presbyopia

What is it?

Also called far-sightedness, or far-sighted and hyperopia.Rays of light relected off objects in front of the eye

are focused behind the retina as shown above. Suchpersons can see well in the distance provided they areyoung and the refractive error is not large. This errorof refraction is corrected by a "+" (convex) lens.

Why does it occur?

It was at one time accepted that in the majority of caseshypermetropia was axial, that is the length of theeyeball is shorter than it should be. But this is not alwaystrue, the corneal may be flatter than it should be(curvature hypermetropia). Hypermetropia actuallyforms a stage in normal development of the eyes.

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At birth practically all eyes are hypermetropic to theextent of 2.5 to 3.0 Diopters and as the growth of thebody proceeds, the lenght of the eye increases untiladolescence is passed, the eye should theoretically be"normal" in size and refraction. Hypermetropia thereforerepresents an imperfectly developed eye when it

persists in adult life. Most primitive peoples and many ofthe lower animals are hypermetropic, carnivora forexample, are almost constantly so. Hypermetropia canalso occur due to change in refractive index of thecrystalline lens of the eye, as in cataract, or due toinjury to the eye with dislocation of the crystalline lensof the eye.

Another form of Hyperopia is Presbyopia, which affectsindividuals after or near 40 years of age. This is becauseage related loss of "accomodation" due to decreasedelastictiy of the crystalline lens surface (capsule).Persons with presbyopia have difficulty inreading/writing and doing near work. This refractiveerror is corrected by adding a "+" lens to the currentrefractive correction the person is wearing.

Symptoms

The earliest symptoms of uncorrected hypermetropia are"eye-strain", "watering", "redness" of the eyes, and oftenheadaches in the later part of the day. Presbyopicpersons complain of blurring of text specially after sometime. Young children with significant hypermetropia canalso develop a convergent squint.

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Astigmatism

What is it?

Rays of light reflected off objects in front of the eye arefocused differently on the retina of the eye, becausemore than one focal point exists. Astigmatism is dueto distortion of the corneal surface in one or multipleplanes. A cylindrical lens with either "+" or "-" poweris required to correct the refractive error.

Why does it occur?

Astigmatism may either be an error of curvature, ofcentring, or of refractive index. Curvature astigmatismof any degree occurs in the cornea. The commonesterror is one wherein the vertical curvature is greaterthan the horizontal. Injuries to the eyes can also causeastigmatism, so do surgeries to the eyes.

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Symptoms

Astigmatic persons often complain of headaches, lowvision or reduced vision for distance and near,

"watering", "redness", "foreign body sensation" in theeyes as the day progresses. Such persons may alsocomplain of "running together of text" while reading. Anastigmatic person can see well neither a distant nor anear object, more so the difficulty is with distantobjects. In uncorrected astigmatism with oblique axesthere may be a compensatory tilt in the head posture.

Correcting refractive errors of the eye

Spectacles (Eyeglasses)

By far the commonest used correction for refractiveerrors are spectacles. An optometrist performs either asubjective or an objective assessment of your vision andprescribes the lenses (and even dispenses) you will needfor your refractive error.

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The advantages of using spectacles:

1.Cost effective2.Freely available even in remote places

3.Can be worn and removed easily4.Can be cleaned easily

The disadvantages:

1.Difficult to handle for children and sportspersons

2.Misplaced or lost easily

3.Accidental breakage and "scratching" morecommon

4.Provided limited field of peripheral vision5.Peripheral distortion specially in high powered

lenses6."Heavy" to wear specially in high powered lenses7.Some persons are prone to dermatitis over the

bridge of the nose (contact dermatitis)8.Some persons find it difficult to adjust to

bifocals, trifocals and multifocals9.Social stigma specially in children

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Contact Lenses

Contact lenses rest on a thin layer of tearfilm on thefront surface of the eye (the cornea). Variousderivatives of Acrylic material are used in themanufacture of contact lenses. Such lenses can be of

3 types;1.Hard lenses2.Semisoft or Gas Permeable lenses3.Soft lenses

The most commonly used contact lenses now a days arethe soft lenses, though semisoft (gas permeable lenses)are indicated occasionally. Both soft lenses and semisoft

lenses are gas permeable, that is, they allow the corneato "breathe" through the lenses, making them morecomfortable to wear.

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The Advantages;

1.Better field of vision2.Overcome peripheral distortions in spectacles

specially if the power requirement is high3.More suitable for sports persons and similar

professions4.Cosmetically more acceptable5.Freely available6.The only form of correction for irregular corneal

surfaces (irregular astigmatism) and mild

corneal scarsThe Disadvantages;

1.Require more maintenance (or meticulousmaintenance)

2.Chances of infection in the eye are morecommon if cleanliness is inadequate

3.Require better hand-eye co-ordination whilewearing4.Cannot be used without supervision by children

and mentally handicapped5.More chance of injury to the eye in case of an

accident specially with hard and semisoft lenses6.Cannot be "over-worn" (most lenses have a

wearing time of 12-14 hours/day)7.Uncomfortable to use in dusty, humid, or

polluted environments, or in the presence ofchemical fumes, etc.

8.Costlier than spectacles

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The Surgical Correction of Refractive Errors

There are numerous surgical procedures for thecorrection of all types of refractive errors. Most of theprocedures are aimed at altering the front surface of theeye (the cornea) so that a state of normal refraction can

be acheived. Surgical procedures such as RadialKeratotomy are performed using a special knife, andLASIK (Laser In Situ Keratomileusis), PRK(Photorefractive keratectomy) using a special LASER.The latest in the series of surgeries is in the form ofimplants within the corneal stroma "INTACS (link)" orlens implants in the eye "phakic intraocular lensimplants"

Apart from changing the surface, in case of cornealopacities and irregularities the cornea itself may bechanged (corneal transplant surgeries, which may bepartial thickness, or full thickness).

The Advantages;

1.In case of high refractive errors, the error may

be brought down to a more manageable level sothat it can be corrected by other means

2.No need for glasses, contact lenses, nomaximum wearing time

The Disadvantages;

1.All surgical procedures have a statistical failure

rate which may range from improper correctionto loss of the eye

2.Costly instrumentation and surgery3.Not freely available

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the errors of refraction

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