What are refractive errors?
Refractive errors are eye disorders where the light is not refracted properly by eye causing reduction in visual acuity (blurred vision).
Normal eye with no refractive error: light rays are focused on retina
In order for eyes to be able to see, the cornea and the lens must refract the light entering the eyes and focus it on the retina (which is the layer of light sensitive cells lining the back of eye). The retina then receives the image formed by the light rays and sends the information to the brain through the optic nerve.
What are the different types of Refractive Errors?
There are 4 types of refractive errors:
1. Myopia (near-sightedness): difficulty in seeing distant objects clearly
In myopic eye, the eye is too long or the cornea is too steep causing the light rays to be focused in front of the retina instead of on it. As a result, close objects appear to be clear but distant objects appear to be blurry. Myopes need glasses or contact lenses to help them to see more clearly at distance.
Myopic eye: light rays are focused in front of retina
Unfortunately, myopia is increasing in both frequency and severity and starting younger in Singaporean children. It is unusual for our young in Singapore to have perfect eye-sight without glasses.
In fact, myopia is not only a cosmetic condition, it is an eye disease. We recommend myopia control to prevent the complications of high myopia later on in life, such as retinal tears, retinal detachments, macular degeneration, premature cataracts, and glaucoma. A secondary aim is keep the degree of myopia as low as possible so as to achieve some degree of spectacle/contact lens independence: eg being able to play soccer or go jogging; enjoy water sports such as swimming, snorkelling, scuba diving; or attend a social function without spectacles.
2. Hyperopia (far-sightedness): difficulty in seeing close objects clearly
In hyperopic eye, the eye is too short or the cornea is too flat causing the light rays to be focused beyond the retina instead on it.
Hyperopic eye: light rays are focused behind retina
Most children are far-sighted, yet may not experience any blurry vision. With focusing (accommodation), children’s eyes are able to bend the light rays and place them directly on the retina. As long as the hyperopia is not too severe, most hyperopic children will not need glasses and they will have clear vision to see objects both at distance and near. As we get older, we slowly lose our ability to focus, and adults with hyperopia may experience increased difficulties with reading or other tasks up close, they may also need glasses to focus for small objects at distance. This is not to be confused with presbyopia where your eyes gradually lose the ability to see near objects due to ageing process of the eye. In presbyopes, the lens in the eye loses its flexibility and elasticity to focus for reading and this usually occurs after age 40.
However, for high or severe hyperopic children, they may have difficulty in seeing close objects as they need to focus extra hard. This may even cause eye strain or headache. High hyperopia may cause crossed eyes known as ‘accommodative esotropia’. Glasses wear will be needed for such cases.
3. Astigmatism (cylinder): distorted vision resulting from an irregularly curved cornea
In astigmatic eye, an irregular shaped cornea or lens prevents light from focusing properly on the retina and resulting in blurred vision at any distance. In normal eye without astigmatism, the cornea is round and smooth, like a basketball; in an eye with astigmatism, the cornea is more curve in one direction than the other, like a rugby ball.
Astigmatic eye: light rays are focused in front of and behind retina
Astigmatism distorts and blurs vision for both distance and near. It is possible to have astigmatism in combination with myopia or hyperopia. Patient with astigmatism will benefit from glasses or contact lenses to see more clearly.
4. Presbyopia: gradually lost the ability to see near objects
This is not an eye disease or disorder, but a natural ageing of the eye. In presbyopes, the lens in the eye loses its flexibility and elasticity to focus for reading and this usually occurs after age 40. You can also have presbyopia in combination with myopia, hyperopia or astigmatism.
Reading glasses, bifocals or progressive glasses, or contact lenses can be prescribed.
Presbyopes reading newspaper w glasses
What is Anisometropia (Imbalance in degree)?
Anisometropia is the condition in which the two eyes have unequal refractive error. Each eye may have different degrees of myopia (nearsightedness), hyperopia (farsightedness), or astigmatism (cylinder).
Anisometropia can adversely affect the development of vision in infants and children if there is a large difference in clarity between the two eyes. If one eye has a much higher refractive error than the other eye, it may be suppressed if the brain cannot fuse the 2 images together. As a result, the blurrier eye will not develop its visual function as well. This is known as amblyopia, or lazy eye. Children with amblyopia due to anisometropia will require glasses and patching of the better or stronger eye to improve and exercise the lazy eye.
In more severe cases of amblyopia, the amblyopic eye may begin to wander creating a condition known as strabismus or squint.
People with severe anisometropia have impaired binocular stereoscopic vision and they may not be as good in judging depths and distances. This is because of the difference in image size between the 2 eyes. This may affect performance in 3-D activities, e.g. racquet sports, parking, threading a needle etc.
The options for correcting anisometropia depend on the severity. Tolerance for anisometropia varies with the individual and tolerance can sometimes be acquired over time. Spectacles and contact lenses are the main options. In occasional cases, refractive surgery may be recommended.