Vision Disorders
How the Eye Works?
Light that strikes the eyeball first passes through the cornea. The cornea is the tough, transparent covering at the front of the eye. It is shaped like a dome.facing outward. Light then passes through the lens. The lens is located just inside the eyeball, behind the cornea. The lens has a double-convex shape. The cornea and lens bend light as light waves pass through them. This process is known as Refraction. In a normal eye, the light waves are brought to focus on the retina. The retina is a thin membrane at the back of the eye. Light waves cause chemical changes in the retina. These chemical changes set off a signal that passes through the optic nerve to the brain. The brain reads that signal as a 'visual image'.Muscles in the front of the eye can change the shape of the lens. These muscles adjust the lens shape to see objects close at hand or far away. This process is known as 'accommodation'.
How Visual Disorders occur?
Visual disorders develop when the cornea and/or the lens do not properly focus light waves. The light waves do not come to a focus (that is, they do not all reach the same point) on the retina. They may come to a focus in front of the retina or behind the retina.The primary symptom of any vision disorder is the inability to see objects clearly. Objects near by or far away will seem blurry to a person with farsightedness or nearsightedness. Objects at almost any distance will seem blurry to a person with astigmatism. Most vision disorders are thought to be inherited. That is, the disorders are passed down from one generation to the next. Most babies are born slightly hyperopic. However, this pattern changes as they grow older. In most cases, children develop normal vision. They may, however, become more hyperopic, or they may develop myopia. Some eyecare specialists argue that vision disorders may be caused by a number of factors in the environment, such as work conditions, stress, and eye strain, can contribute to the development of vision disorders. The development of astigmatism has been attributed to a number of factors. For example, people with allergies often rub their eyes repeatedly. This constant rubbing may cause damage to the cornea. Diabetes (see diabetes mellitus entry) is also known to change the shape of the cornea over time.
Symptoms
Blurry vision may lead to other problems. People who have trouble seeing clearly may begin to squint in order to see better. Constant squinting can lead to discomfort, headaches, and eyestrain.
DIAGNOSIS
There are three major types of visual disorders:Myopia or near-sightedness,Hyperopia or far-sightedness and Astigmatism ,a situation which causes the eye to form a blurred vision of an object.When light waves come to a focus in front of the retina, the person has myopia, or nearsightedness. People who are nearsighted can see objects close at hand, but they cannot see objects far away clearly. When light waves come to a focus behind the retina, the person has hyperopia, or farsightedness. People who are farsighted (hyperopic) can see objects far away but cannot clearly see objects close at hand.In healthy eyes the cornea has a smooth, regular shape. Some people have corneas with an ellipsoidal shape, like a football or a squashed baseball. Such corneas scatter light waves across the retina, causing objects to appear as blurred images. This condition is called astigmatism.
Vision disorders are sometimes difficult to diagnose. They have often grown up seeing the world slightly out of focus and their view of objects may seem perfectly normal.As a result, many vision problems are detected by others. For example, school teachers may realize that a pupil is unable to read the blackboard. Or a parent may notice that a child is constantly squinting while trying to read. In such cases, the individual is referred to an eye specialist for examination. Astigmatism is a condition in which light from a single point fails to focus on a single point of the retina. The condition causes the patient to see a blurred image. The most common tests involve the use of the familiar eye chart. The eye chart contains rows of letters of decreasing size going from top to bottom. The patient is asked to read each row on the chart. The last row the patient is able to read tells how accurate is his or her vision. The results of an eye chart test are usually indicated by a pair of numbers, such as 20/50. These numbers show how well a person can see an object compared to a person with normal eyesight. A person with 20/50 vision can see at 20 feet from an eye chart what a person with normal vision can see at 50 feet from the chart.There are a number of lenses through which the patient is supposed to look at the eye chart.The lens are changed until the patient is able to see the letters on the chart clearly. The degree of error in the patient's eye is determined by the lens needed to produce perfect vision. This degree of error is measured in units called diopters (abbreviated: D). The prescription a doctor writes for glasses will indicate the shape of the lens (the diopter measurements) needed to provide perfect vision.
A simple test for astigmatism consists of a dial containing lines that radiate outward from the center. A person with astigmatism will be able to see some lines more clearly than other lines.
Doctors may also use direct tests on the eye. For example, a device known as a corneal topographer can be used to measure the shape of the cornea and determine whether the shape is correct or not. Doctors can also shine a light directly at the retina in a person's eye to look for any damage. Photorefractive Keratectomy, or PRK, is a type of laser eye surgery for people who suffer from nearsightedness myopia, farsightedness(hyperopia) or astigmatism. It is similar to LASIKin that it involves using an Excimer laser to reshape the cornea to minimize or eliminate your dependence on eyeglasses or contact lenses.It is different in that unlike LASIK the eye surgeon doesn't need to use a microkeratome to make a flap, but instead is able to apply the laser directly to the surface of the cornea to achieve the desired vision correction effect. PRK is better suited for people with thin corneas or certain other corneal abnormalities where using a microkeratome might not be the best choice. Also it may be preferred for those in the United States military where PRK is the approved procedure for those in active combat and certain other positions. PRK has been approved by the FDA for myopia since 1995 and for hyperopia since 1998 and enjoys an excellent safety profile. Since it does not use a microkeratome, there is no risk of flap complications. In PRK to correct myopia, the steep cornea is made flatter by removing tissue from the center of the cornea. To correct hyperopia, the flat cornea is made steeper by removing tissue from the outer edges of the cornea. To correct astigmatism, the cornea is transformed into a more spherical shape.
