Common Eye Disorders

Myopia (nearsighted or short sighted)

This is one of the most common vision disorders. It is the inability to focus on objects at a distance. The light rays are focused at a point in front of the retina, causing a blurring of the image. Myopia is typically caused by continued up close activity which causes the muscles to be in a state of constant contraction causing a lengthening of the eyeball. A person with myopia can see things up close, but they will not be as clear as they would be if the eyes were relaxed as in normal eyesight.

Hyperopia or hypermetropia (long sighted or far sighted)

This is the inability to fucus on objects at a close range. This is caused by tension that sortens the length of the eyeball, therefore becoming to short so that light rays are focused on an area behind the retina.

Astigmatism

Astigmatism is an unevenly shaped cornea, almost like a corrugated iron roof. It can be as a result of uneven tension of the extraocular muscles surrounding the eye. This uneven shape causes light rays to focus unevenly, some in front of and some behind the retina.

Presbyopia

This is commonly referred to as “aging vision”. This is the name given to the hardening of the lens and a tightening of the muscles often associated with aging. This condition occurs in nearly everyone at some point in their life, usually around the age of 40. This inflexibility causes ciliary muscle to be unable to change the shape of the lens.

Human Eye Anatomy

The eye is one of the most complex parts of the body. There are more than a billion parts all working in synchronization. The eyesight is one of the most important senses, a fact supported by the amount of brain that is dedicated to process the information received through the eye.

Anatomy of the eye

Anatomy of the eye

How the eye works

The image begins as light waves bouncing or emanating from an object which enter the eye through the cornea. The cornea is a thin transparent protective shield on the front of the eye. The corneal also functions as a lens and begins focusing the rays of light by bending them (refracting) these as they enter.

These rays then enter the pupil. This is the black hole in the center of the eye, and is a door way which, along with the iris, regulates how much light comes through. Hence the term “dilated pupils”, meaning the pupil is very large letting more light rays through. The iris and pupil are constantly regulating how much light enters the eye.

Once the light rays enter through the pupil they then are focused by the lens. The lens is controlled by a band of muscle called the ciliary muscle. The natural relaxed state of these muscles would be focused at a distance of about 7 feet. In order to focus on objects closer than this the ciliary muscle must push on the lens, shaping it in a more convex shape similar to a bowl. To refocus further the muscles shape the lens to a flatter shape. Accommodation is the process by which the eye changes focus between objects that are far and objects that are near. There is much debate whether the eye focuses by the ciliary muscles acting on the lens, or whether it is the varying elongation of the eyeball caused by the extraocular muscles, or a combination of both.

The action of the lens and the shape of the eyeball then permits the light rays to travel through the rest of the eye. The vitreous humour is the transparent gel between the lens and the retina, the final destination of the light rays.

The retina is a complex light sensitive membrane that lines the inner eyeball. It contains hundreds of millions of light sensitive receptors responsible for transmitting the image to the optic nerve. These receptors are made up of cones and rods. The rods monitor how bright the rays of light are. Cones pick up sharp detail (acuity). There are three types of cones, one for each of the primary light colors. Red receptive cones are stimulated by the amount of red light for example. At the center of the retina is the macula. This has a very high concentration of cones. Within the center of the macula is the fovea. The fovea has the highest concentration if cones.

The optic nerve sends all of this information to the brain, where it is translated into what we see.

Each eye has six extraocular muscles which surround the eye and are responsible for controlling the movement of the eye. These muscles are very powerful, many times stronger than would be required to simply move the eyeball. This strength permits rapid acceleration and precise accuracy in eye movement.

What is 20/20 Vision?

In 1864 a test for visual acuity (the clarity or sharpness of vision) was devised by Herman Snellen, a Dutch opthalmologist. Using the eyesight of his assistant Snellen created a chart with letters on it. The “snellen” chart has different sized letters on it and forms the basis of modern eye exams. Snellen decided what a person should be able to see when standing 20 feet away from the eye chart. 20/20 vision is a term which means that you can see clearly at 20 feet what a person with normal vision can see. If you have 20/40 vision this means that you must be as close as 20 feet to see what a person with normal vision can see at 40 feet. 20/20 does not mean “perfect” vision, it only indicates the clarity or sharpness of distance vision. You can also have vision that is better than normal vision, for example a person with 20/10 vision can see things at 20 feet what a normal person can see standing 10 feet from the chart.

Snellen Test Chart

Snellen Test Chart

There are other vision skills including peripheral vision, eye coordination, depth perception, focusing ability and color vision that contribute to your overall vision.