Common Visual Disorders
Visual Disorders – What is myopia, hypermetropia and presbyopia
For clear vision, the refractive components of our eyes must have the power to focus the rays of light that come into our eyes perfectly on the retina. Myopia, hypermetropia and presbyopia are visual disorders caused by the inability of these components to change their refractive power and focus the rays of light on the retina. Read this article to understand more about these refractive disorders and how they can be corrected.
The refractive components of a normal eye are the cornea, the crystalline lens and the vitreous gel which is behind the crystalline lens. The combined refraction of these components focuses the rays of light that is reflected off objects perfectly on the retina. A normal 6/6 (or 20/20) vision exists when this occurs without aid.
Myopia, nearsightedness, or short-sightedness is a refractive disorder where a person can see and perform work near, but can not see clearly objects at a distant. The disorder occurs when the rays of light coming into the eye from a distance focus at the front of the retina and form a blurry image. The disorder is of two types, the type in which there is a genetic myopic state in the eye (axial myopia) and the pathological type in which the retina of the eye is damaged. The most common causes of myopia are increased length of the eyeball and a strong refractive power of the cornea, or lens of the eye. The most common symptoms of myopia are squinting of the eyes and the gluing of the eyes too close to objects. Myopia occurs in early childhood (the development of myopia after 40 may be the first sign of cataract) and increases over time. Some studies show that increased eye strain and too much near work (like sitting in front of a computer for hours) may contribute to the development of myopia. The disorder can be corrected using eyeglasses with concave lenses (their thickness depends on how strong myopia is) and with contact lenses, which enlarge the field of vision and focus the image onto the retina of the eye; in developed stages the disorder can only be corrected with refractive surgery.
Hypermetropia, which is also know as hyperopia, farsightedness and long-sightedness, is a refractive defect where the rays of the light coming into the eye focus behind the retina. People with hyperopia are not able to see clearly objects near to them, or from a far distance. In contrast to primitive humans’ and carnivore animals’ eyes which are continuously hepermetropic, the refraction power of the lens and cornea of the human eye, which is 2.5 to 3.0 dioptres hypermetropic at birth, grows to a normal size as the body grows. Hypermetropia is caused by a shorter eyeball (axial hypermetropia), or a flat cornea, and it is inherited. It can also occur when there is a change in the refraction of the crystalline lens of the eye, for example when a person develops cataracts, or when there is a dislocation of the crystalline lens due to eye injury. The early symptoms of hypermetropia are eye strain, eye watering, redness of the eyes, headaches, blurry vision and squinting. The condition can be treated with corrective eyeglasses, contact lenses and laser eye surgery.
Presbyopia or “aging vision”
Presbyopia is a visual disorder that results in the difficulty of reading/writing and doing near work, and it is another form of hypermetropia. It is also known as aging vision because it starts to affect people between 40 and 50 years of age. Presbyopia is caused by the loss of the elasticity of the crystalline lens, and it progresses with time. The first symptoms of presbyopia are a need for light and difficulty doing near work such as threading a needle, or reading a newspaper. There are several types of corrective lenses for presbyopia, including single vision lenses for reading, which people often have to remove, or look above them when they want to see in the distance, bifocal lenses which are characterized by a glaring boundary line that is an obstacle to the eyes, progressive contact lenses which restore the entire field of vision and allow the person to see clearly. Eyeglasses with progressive lenses have now proven to be very successful in dealing with both distance and near vision corrections.
Poor vision can affect the quality of your life. If you have difficulty seeing objects at a distance or you are squinting to read small signs, you have red or watery eyes and headaches you may suffer from one of these disorders. Visit your Ophthalmologist or Optometrist and have your eyes examined today.
The above figure shows the various visual disorders including astigmatism which we haven’t touched on in this article and will do so soon.