When rays are focused correctly on the retina of a relaxed eye, the eye is said to be emmetropic. Emmetropia is the medical term for 20/20 vision, vision that needs no corrective lenses, contact lenses, or reading glasses. It occurs because the optical power of the eye can perfectly focus an image to the retina, giving it “perfect” vision.
The opposite of emmetropia is ametropia. With ametropia, the focal point of the eye is some distance in front of or behind the retina. The following vision conditions are types of ametropia.
Hyperopia and Myopia
A family history of hyperopia is a risk factor for developing hyperopia. Babies are often born with hyperopia but they can usually outgrow the condition as their eyes develop into the correct shape.
Hyperopia can be corrected with eyeglasses or contact lenses. There are also new surgical procedures that can correct hyperopia.
Myopia is the condition of being nearsighted. When it is an inherited condition, myopia begins early in life. People with this condition can usually see near objects, but they struggle to see distant objects. Myopia is the opposite of hyperopia, or farsightedness. In myopia, the anatomy of the eyeball, or globe, is longer than normal. This causes the light to focus in front of the retina, blurring the distance vision. Myopia is corrected with glasses and contact lenses, or with laser vision correction. Laser vision correction is only recommended for people over 18 years old, when the eye has finished growing to adult size.
To correct the symptoms of myopia with glasses, lenses are used that are thicker on the edges and thinner in the middle. This is known as a concave lens, which can be cosmetically improved in higher prescriptions with a high index lens.
Myopes are also at increased risk for a retinal detachment. The signs and symptoms of a retinal detachment are flashing lights, black floaters, or a curtain over the vision. The risk of detachment is typically less than 3 percent.
Amblyopia is also known as lazy eye. It is a condition, usually found in children, in which one or both eyes do not develop properly. An easy way to explain this is that the "eye-brain" connection does not communicate properly; therefore, the child does not know what clear vision is...or what 20/20 vision is. The eye anatomy itself is normal, but the neural pathway to the brain is not normal, causing decreased vision.
Prevalence of amblyopia
Amblyopia is one of the most common treatable forms of vision impairment in children. Its prevalence is as high as 3-5% in some studies. It is most common in infants and young children and it is imperative that this condition is caught early. The chance of successful vision restoration goes down dramatically after age 8, therefore the earlier this condition is caught the better chance of successful treatment.
Causes of amblyopia
The causes of amblyopia are varied. A very common condition that can cause amblyopia is strabismus, a misalignment of the eyes. This occurs when one eye has an abnormal turning in or out, causing the brain to stop using the misaligned eye. Other causes may come from a high prescription such as nearsightedness, farsightedness, or astigmatism. In the case of these conditions, the eye’s vision is out of focus and so the brain turns off that image. Eye disease processes can also cause amblyopia. One of these conditions is known as a cataract. A cataract is a condition of the lens of the eye developing an opacity so that light cannot pass through. Abnormal retinal conditions and hereditary factors can also cause amblyopia.
Treatment of amblyopia
In order to increase the chances for success, this condition must be detected early. The recommended ages for early eye examination are 6 months old, then 2-3 years of age, and then school age.
Patching, or occlusion therapy
One of the most common treatments for amblyopia is patching, also known as occluding, the better or stronger eye. This forces the brain to use the weaker eye. An adhesive eyepatch on the skin or a slip-on patch over glasses can be incorporated for occlusion therapy. A blurring contact lens or dilating eye drop can also be used to occlude the good eye.
Cataract, eye muscle, or retinal surgery can be incorporated to help treat the underlying cause of amblyopia in some cases.
Vision therapy has been proven to be successful in the treatment of amblyopia. Vision therapy, or VT, incorporates a series of vision training procedures that helps improve eye movement control, visual acuity, depth perception, and eye coordination. Vision therapy can be done in an office or home setting.
Signs and symptoms of Amblyopia
The most common way amblyopia is diagnosed is a detection of a decreased red reflex in the child's eye. A diagnostic instrument used by the optometrist, ophthalmologist, or pediatrician, can pick up a bright reflection in the normal eye and a dim reflection in the amblyopic eye. Upon further examination, the eye is dilated to see if a refractive error of myopia, astigmatism, or hyperopia is the cause.
Another sign of amblyopia is an eye that turns in or out. A symptom that may be indicative for amblyopia is if the child prefers the vision out of one eye. This can sometimes be detected when occluding the better eye--the child may become fussy and upset because she cannot see out of the lazy eye.
1. Amblyopia informational patient brochure. APOS.org. January 2013
As people get older, usually when they hit their 40s, a condition called presbyopia can set in. Presbyopia is the inability to focus on objects near the eye. One usually notices that it is harder to read or use the computer. Bifocals or reading glasses are a way to remedy this condition.
Presbyopia is a natural consequence of the aging process. There is no known cure, though researchers are constantly looking for one. Even if someone has never had vision problems before, he can still develop presbyopia. It may seem to occur suddenly, but it actually occurs over a long period of time. Symptoms include having to hold things at arm’s length to see them clearly, eye strain, fatigue, and headaches from near work.
Sometimes the cornea is irregularly shaped, causing the eye to focus an object on two different areas of the retina. This is known as astigmatism. For the cornea to bend light correctly, it should be dome-shaped, like a basketball. Astigmatic corneas are shaped more like a football. This causes a distorted view when looking at objects which are close-up and far away.
The cause of astigmatism is unknown. Astigmatism is often associated with myopia or hyperopia, and it usually is present from birth. It may be hereditary, or it may be caused by factors such as pressure on the cornea, incorrect posture, or increased use of the eyes for “near work.”
Mild astigmatism usually does not need to be corrected. Eyeglasses, contact lenses, or refractive surgery can correct moderate to high degrees of astigmatism.
Computer Vision Syndrome
Computer vision syndrome (CVS) affects three out of four computer users. It is a series of symptoms related to extended periods of computer usage. Though it is no cause for panic, measures can be taken to relieve symptoms of CVS.
CVS can appear as a variety of symptoms. Headaches, eye strain, neck and back aches, sensitivity to light, blurred vision, double vision, and dry or irritated eyes are all possible problems related to CVS.
Any computer user can develop CVS. Your vision, your computer, and the environment where you use your computer are all factors which can lead to CVS.