emmetropic |
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| Symptoms: None. |
| Description: Healthy eye or without refractive error. The image passes through the different structures/lenses of the eye and focuses correctly on the concave plane of the retina. The image is captured by the photoreceptors (cones and rods) and is transmitted in the form of an electrical signal that travels to the cerebral cortex that interprets the image. |
| Treatment: Not required. |
Myopia |
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| Symptoms: Blurred vision in the distance, zooming in on texts to read or write, squinting to see distant objects. |
| Description: A myopic eye has a more or less clear vision for near objects while its vision in the distance appears blurry. The most common cause is an eye that is too long for the focusing power of the lens and cornea; myopic people have an excess of power that creates the focus of the image in front of the retina. The correction required in this case is a negative or diverging lens that deflects the light rays so that when they pass through the optical system they converge on the retina (focused). Farsightedness is the same defect in reverse. The rays converge behind the retina. |
| Treatment: Prescription glasses, contact lenses and other techniques. |
Farsightedness |
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| Symptoms: Headache or blurring when working up close. In children it usually goes unnoticed because they can compensate for this defect by "forcing" their eyesight. This causes eyestrain (accommodative asthenopia) and/or learning delays. |
| Description: A farsighted eye focuses the image behind the retina, producing blurred vision of nearby objects. The result is that distant objects are seen clearer than near ones. In this situation this eye has a low power to converge. It is common for hyperopic people not to use correction or are unaware that they are hyperopic due to the ability of the eye to accommodate (correct this defect). The required correction is a positive lens that adds power to the eye for sharp vision of near objects. Myopia is the same defect in reverse. The rays converge in front of the retina. |
| Treatment: Close-up glasses, contact lenses and others. |
| Astigmatism |
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| Symptoms: Headache, tearing, stinging, blurred image. |
| Description: The irregular shape of the cornea causes two foci on the retina, producing a blurred vision or "stretched" image. An astigmatic cornea resembles a rugby ball, while a normal cornea resembles a soccer ball. This problem is usually associated with myopia or hyperopia, is usually congenital and evolves little with age. Symptoms include headache and tearing. Like certain farsightedness, astigmatism may not cause symptoms and may be imperceptible to patients who suffer from it. Toric lenses compensate for this refractive error by creating a single focal point on the retina, thus creating sharper vision. |
| Treatment: Glasses or toric contact lenses. |
| Presbyopia |
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| Symptoms: Difficulty focusing on near objects, moving objects away to be able to read or focus. |
| Description: It is an age-related eye defect that generally appears between 40-45 years of age and causes difficulty in seeing up close. It is due to the reduction in the accommodation power of the eye that causes a decrease in the ability to focus on nearby objects. |
| Treatment: Progressive lenses, bifocal lenses, occupational, multifocal contact lenses. |