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 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 –
Each of the refractive errors has specific symptoms related to clarity of vision of near or far objects. Some of the symptoms that are common with all kinds of refractive errors include:
- Haziness or blurring of vision
- Double vision
- Glare or halos around bright lights
- Squinting especially seen in children with refractive errors
- Eye strain
- Nausea and sometimes vomiting
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.
Most of the refractive errors are associated with severe difficulty in normal vision that may affect the quality of life and in severe cases make the person dependent on others for performing their daily activities (e.g. crossing the road, driving etc.) Blindness due to refractive errors is a rare occurrence. Some of the complications of refractive errors include
- Severe myopia may be associated with consequences like retinal detachment, cataract glaucoma and macular degeneration
- In children with hypermetropia, there is a risk of development of squint or amblyopia. This leads to progressive decrease of visual acuity due to a lack of interaction between the brain and the eye.
- Astigmatisim usually is a stable eye condition. In rare cases it may progress rapidly. This may be due to development of keratoconus. Keratoconus is a condition characterised by the progressive deformation of the cornea making it conical in shape. This results in reduction of vision
- Presbyopia may lead to loss of or diminished vision resulting in a high risk of falls and injury. Since it affects mainly elderly persons, this is of particular concern.
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.