Astigmatism, color blindness, hyperopia (farsightedness), myopia (nearsightedness), presbyopia, strabismus (aka heterotropia)….Most people who wear eyeglasses or contact lenses are affected by one of these common visual problems. Here are some definitions to better understand them.
What is astigmatism?
People with astigmatism do not see clearly at any distance. Vertical, horizontal or oblique lines are poorly perceived and are distorted. Certain people thus confuse similar-looking characters such as M, N and H, or 8 and 0.
This visual defect is due to the shape of the cornea, which is oval instead of round. When one’s eyesight is correct, the sun’s rays converge at the retina. With astigmatism, the rays focus on different places, in front of and behind the retina. The image transmitted to the brain is thus blurry.
What is color-blindness?
Color-blindness is an anomaly in color perception: dichromatic color-blindness means one can only perceive two colors and monochromatic color-blindness means one can only perceive black and white. Abnormal trichromatism causes a person to have a poor perception of red, green or blue. The retina is composed of photoreceptors called cones that make day-vision and color perception possible, whereas night-vision is made possible by the presence of photoreceptors called rods. The number and nature of cones a person has determines their capacity for color perception. Color-blindness is an incurable condition occurring predominantly in males. Early diagnosis is important to help affected people adapt the condition.
What is hyperopia?
People with hyperopia generally see poorly at close range and well at long range. In children, hyperopia is sometimes accompanied by esotropia, or squinting, resulting from the extra effort exerted by the eyes in order to adapt to the condition. The hyperopic eye is too short, resulting in image formation "behind" the retina; the image is thus blurred when the unadapted eye is at rest. Before 40 years of age, weak hyperopia may go unnoticed due to the ability of the eyes to adapt to the condition. After 40, it becomes increasingly difficult for the eyes to adapt and hyperopia becomes manifest.
What is myopia?
Myopia means that one’s long-range vision is poor, usually improving as we move closer to an object.
Generally, myopia appears between the ages of 7 and 14 and stabilizes between the ages of 20 and 25. Nevertheless, certain myopias can appear at any age and evolve in diverse ways. This visual problem is characterized by a convergence of light rays before the retina, making eyesight blurry. In effect, this convergence should occur on the retina itself, whose job is to capture the light from the environment and transmit the signal to the brain.
What is presbyopia?
Presbyopia is responsible for poor vision; it obliges us, for example, to hold a label or book at arm’s length in order to read it.
This problem begins to appear around the age of 45 and stabilizes around the age of 60. It occurs as a result of the aging of the crystalline lens, a part of the eye that changes shape in order to focus on objects at different distances.
After 45 it becomes less elastic and no longer functions correctly; the capacity of the eyes to re-focus decreases until it disappears completely at the age of 60.
What is strabismus?
From the Greek work strabismus, strabismus is a squinting problem usually found in young children. Beyond the esthetic considerations involve, this visual anomaly is often a sign of other more or less serious eye problems.
The deviation of the eyes is caused by poorly-functioning eye muscles which do not have the same strength and are thus “out of balance”. A person affected by strabismus does not have correct binocular vision, which means that they cannot use both eyes simultaneously and thus have a flat and reduced field of vision.