Refractive errors affect a large population worldwide and account for half the cases of avoidable vision impairment in over 153 million persons globally. If these errors are not detected and corrected early they may adversely affect development of the eye, academic and social performance. Over the age of 50 years, refractive errors are responsible for accidents, social and emotional problems. Around two thirds of all vision impairment in developed nations is due to refractive errors and their under-correction.
The eye receives light from the outside world and as it falls on the retina or the light sensitive layer at the back of the eye, we process the image in our brain. This is the basis of normal vision. There are two parts of this function –
For normal vision and depth of the picture both eyes need to function correctly for perception of a single vivid image.
Refraction is a term used in optical physics to describe the bending of light rays as they pass across a particular medium. In natural eye the light rays pass through the transparent cornea and then through the biconvex lens. The lens has a power that means its capability to focus parallel rays of light. The higher this value measured in dioptres (D), the stronger is its focusing ability. Refraction is also caused by a small extent on the air/tear interface on the surface of the cornea and the cornea itself.
The accommodative response is the response of the eye muscles to alter the shape of the lens for allowing projection of an image onto the retina.
The accuracy of focus depends on some factors that include –
With age these factors change leading to difficulty in vision. An eye with normal refraction is termed emmetropic eye that can achieve a perfect focus. Ametropia is the global term referring to any refractive error.
Refractive errors occur when there is an error or defect in any of the four factors. Common problems include those in –
Myopia – This is commonly termed short sightedness or near sightedness. The myopic eyes have excessive power that means the image is focussed in front of the retina. This occurs if there is a variation in the length of the eye or an excessively curved cornea. In this type of refractive error distant objects appear to be blurred while close-up objects remain in focus. If myopia is severe, even closer objects may appear blurred.
This condition often manifests during teenage years or in early adulthood and myopia affects a quarter of normal populations worldwide. Degree of myopia includes –
Severe myopia is rare but is associated with severe consequences like retinal detachment, cataract and glaucoma. Severe myopia may run in families. Adolescents and young adults who do a lot of close up work are likely to worsen pre-existing myopia. Some conditions are associated with myopia. These include premature birth, Marfan's syndrome, Ehler Danlos syndrome, Homocystinuria etc.
Treatment is by correction of myopia using corrective concave or minus lens.
Hypermetropia – This is also called far sightedness or distant vision. The problem is opposite to myopia. In this case, the eye does not have the optical power to focus the object on the retina and thus the object is focused behind the retina, so giving rise to a blurred image. The patients can see distant objects clearly but have blurred vision on close up. Working with close up objects or reading may give rise to eye strain and headache in these patients.
Hypermetropia to a smaller degree is common in babies and infants as the eye ball develops and grows. This condition usually resolves by three years. If the condition persists in pre-schoolers beyond three years of age, it may give rise to complications such as squint, lazy eye (amblyopia) or glaucoma in the child. Hypermetropia may be caused due to conditions like congenital cataracts, retinitis pigmentosa, microphthalmia and corneal dystrophies.
Treatment is by correction of hypermetropia using corrective convex or plus lens.
Astigmatism – The symmetrical curve of the cornea and the lens helps in focus of the light rays onto the retina in a uniform manner. In astigmatism this uniformity is altered. There are variations in the symmetry of these curvatures (usually corneal) and thus the light rays fail to focus on a single point. Astigmatism is often present in association with some degree of myopia or hypermetropia.
The patient experiences blurring of vision despite correction of near or far sightedness with glasses.
The degree of astigmatism is measured in cylinders (cyl). Some mild amount of astigmatism is common during growing years but it resolves without corrective lenses in most cases. More severe astigmatism may lead to complications like squint and lazy eye.
Astigmatism may be seen spontaneously with or without myopia or hypermetropia and also result with certain conditions. Some of the causes of astigmatism include an eye surgery, cornea injury, corneal dystrophies, cataract, retinitis pigmentosa, nystagmus or albinism.
For correction a cylindrical lens is used to 'neutralise' the variations in symmetry of the cornea. The axis of the cylinder depends on the meridian of asymmetry in the patient's cornea. Cylindrical lenses may be combined with spherical lenses (convex or concave) depending on concurrent presence of hypermetropia or myopia.
Presbyopia – This condition refers to refraction errors that occur due to aging. With age there is loss of the accommodative response. In addition to weakness of the eye muscles there is also decline in the elasticity of the lens. This condition usually takes years to develop.
These symptoms most commonly occur after the age of 40 although this varies depending on presence of pre-existing refraction errors, reading, pupil size and the patient's usual visual tasks. Worldwide there are 517 million cases of uncorrected presbyopia. Patients with this condition often find it difficult to carry out near-tasks. Corrective glasses as per the refractive error may be prescribed for Presbyopia.
Anisometropia – This is a condition where there are unequal refractive errors in the two eyes. This could mild, for example different degrees of myopia in each eye, which is a common situation. In some patients however this condition could be more severe. A difference of more than 2.0 D between two eyes is considered to be a large difference. Problem may also arise with one eye is myopic and the other hypermetropic. The patient usually complains of difficulty in focussing with double vision, headaches, nausea, dizziness, fatigue and sensitivity to light. The condition is usually corrected using various types of spherical and cylindrical lenses depending on the type of anisometropia.
The diagnosis of refractive error can be made using a refraction testing. Both eyes are tested individually for near and distance vision. Refractive correction or refraction testing refers to the process by which the best possible visual acuity can be obtained for a patient.
The distant vision eyes are tested using the Snellen chart with random letters of different sizes placed at 6 metres distance. Depending on the ability to read a particular line, myopia is determined. Then the affected eye is corrected with test lenses to check for the exact optical power. Refraction is generally tested by an optometrist.
Near vision is tested by using reading material of varying sizes held at a hand holding distance.
Refraction errors may be managed with the use of lenses. Spectacles are the simplest, safest and most cost-effective method for correcting refractive errors. Spherical lenses are characterised by a constant curvature over the entire surface and may be concave or convex. Cylindrical lenses have focusing powers in one meridian only and are used for astigmatism.
Another option for correction of refractive errors is with the use of contact lenses. Contact lenses work on the same principle as spectacle lenses but the space between the lens and the surface of the cornea is reduced to the tear film alone.
For prevention of deterioration of refractive errors, even patients with no vision problems and those who have achieved vision correction with lenses need to be checked. For patients between 19 and 40 tests should be every 10 years, for those between 41 and 44, tests every 5 years and for those 56 to 65, tests every 3 years and those over 65 tests every 2 years are recommended. Those with other problems such as diabetes or familial eye problems need more frequent checking.
Laser surgery or surgical correction for refractive errors may be chosen in severe cases. There are a number of surgical procedures available for refractive errors. These include corneal surgery where the shape of the cornea is altered to alter its refractive power. This is the commonest type of refractive surgery at the current time. There are a number of options available, but the commonest procedure is LASIK (Laser-Assisted In-Situ Keratiomileusis). Another type of surgery is by removal of the patient's own lens and replacement with an artificial one of the correct refractive power. This is not a commonly practiced surgery.