Nearsightedness

Alternative Names
myopia

Definition
An error of visual focusing that causes difficulty seeing distant objects.

Causes, incidence, and risk factors
With this disorder, near objects are seen clearly, while objects in the distance are blurred. This is the result of the visual image being focused in front of the retina rather than directly on it. It may be caused from the eyeball being too long from front to back, or from the lens power being too strong. Nearsightedness often develops in the school-aged child and through the early 20s after which it usually stabilizes. Until that time, it may develop rapidly and require frequent changes in glasses or contact lenses. It affects males and females equally, and family history of nearsightedness is a risk factor.

Farsightedness

Alternative Names
hyperopia

Definition
An error of visual focusing that causes difficulty in seeing near objects.

Causes, incidence, and risk factors
Farsightedness is the result of the visual image being focused behind the retina rather than directly on it. It may be caused from the eyeball being too short from front to back or the lens being too weak. Farsightedness is often present from birth, and children can often overcome a mild condition through adjustment of the eye (accommodation). As aging occurs, glasses or contact lenses are required to correct vision. A family history of farsightedness is a risk factor.

Amblyopia

Alternative Names
cycloplegic; lazy eye

Definition
A condition in which the vision in the non-dominant eye is poor as a result of a visual abnormality early in life.

Causes, incidence, and risk factors
Amblyopia is usually first observed in childhood. Amblyopia may be caused by strabismus (crossed eyes), farsightedness, nearsightedness, astigmatism in one eye, or a childhood cataract. The stronger eye becomes dominant and retains good vision, but in the affected eye, the nerve connections between the brain and that eye fail to develop normally, and the brain learns to ignore visual information from that eye. The resulting vision lacks depth perception.  Strabismus, the most common cause of amblyopia, appears to run in families.

Symptoms
Vision abnormalities where vision in 1 eye is much worse than the other eyes that may not work together eyes that may appear cross-eyed inability to gauge depth when playing with objects (loss of depth perception)

Astigmatism

Definition
A condition in which the cornea of the eye is asymmetrically curved, causing distorted vision.

Causes, incidence, and risk factors
The cause of astigmatism is unknown. It is usually present from birth, may occur along with nearsightedness or farsightedness, and tends to remain fairly constant. A minor degree of astigmatism is considered normal and does not require correction. Astigmatism is very common.

Symptom
inconsistent difficulty focusing on various objects such as printed words and lines

Presbyopia

Alternative Names
old eyes

Definition
The age-associated progressive loss of the focusing power of the lens, resulting in farsightedness.

Causes, incidence, and risk factors
The focusing power of the eye, which depends upon the inherent elasticity of the lens, is gradually lost with the aging process. This results in a gradual decrease in the ability of the eye to focus on objects that are close up. It is usually noticed around the age of 45, when you realize that you need to hold reading materials further away in order to focus on them. Presbyopia occurs in everyone to some degree during the aging process.

Symptoms
decreased focusing ability for near objects
eyestrain
tired eyes
headache

Visual Acuity

Definition
The visual acuity test measures the distance you stand from the eye chart (usually 20 feet) and the smallest line you are able to read at that distance.

How the test is performed
This test may be done in a health care provider's office, a school, a work place, or elsewhere. Stand behind a line 20 feet from the eye chart. Remove glasses or contacts. Keep both eyes open and gently cover one eye with the palm of your hand, a piece of paper, or a paper cup while you read out loud the smallest line of letters that you can read on the chart. If you are not sure of the letter, you may guess. This is repeated with the other eye. Repeat the procedure while wearing glasses or contacts.

Normal values
Visual acuity is expressed as a fraction. The top number refers to the distance you stand from the chart. This is usually 20 feet. The bottom number indicates the distance at which a person with normal eyesight could read the line with the smallest letters that you could correctly read. 20/20 is considered normal.  20/40 indicates that the line you correctly read at 20 feet could be read by a person with normal vision at 40 feet.