What Are the Many Types of Vision Disorders?

("normal" or "perfect" vision) is the condition in which the image of a faraway object is focused exactly on the retina, resulting in clear distant vision.

Myopia (nearsight) is the condition in which the image of a faraway object is focused in front the retina because the eyeball is too long or the refractive power of the eye is too strong. This results in the far vision being blurry. When a near object is viewed, it is focused on the retina, resulting in the near sight being clear.

Hyperopia (farsight) is the condition in which the image of a faraway object is focused behind the retina, either because the eyeball is too short, or because the refractive power of the eye is too weak. When a near object is viewed, the image becomes focused even further behind the retina, resulting in the far sight being clearer than the near sight. This may cause headaches, eyestrain, and/or fatigue.

Astigmatism is the condition in which the images of both faraway AND near objects become split into two, making the vision blurry at ALL distances. Astigmatism may cause eye strain and may occur with nearsightedness or farsightedness. Symptoms include headache, eye strain, fatigue, and fluctuating vision.

Eyestrain can result from an imbalance in either the extraocular (eye turning) muscles or the internal accommodative (focusing) muscles.

There are 6 muscles attached to the outside of the eyeball that control its movements:

• Superior rectus: main movement is elevation (upward)
• Inferior rectus: main movement is depression (downward)
• Medial rectus: main movement is adduction (inward)
• Lateral rectus: main movement is abduction (outward)
• Superior oblique: actions are depression, abduction, and medial (inward) rotation
• Inferior oblique: actions are elevation, abduction, and lateral (outward) rotation

When one or more of these muscles exert more force on the eyeball than their opposite counterpart, the eye will either want to turn in a particular direction (“phoria”), or, if the imbalance is too great, it may indeed turn in a particular direction (“tropia” or “strabismus”).

There are 4 phorias:

1. Esophoria: eye “wants” to turn in
2. Exophoria: eye “wants” to turn out
3. Hyperphoria: eye “wants” to turn up
4. Hypophoria: eye “wants” to turn down

There are 4 tropias:

1. Esotropia: eye turns in
2. Exotropia: eye turns out
3. Hypertropia: eye turns up
4. Hypotropia: eye turns down

The tropias can be further classified as:

1. Constant: the eye always turns
2. Intermittent: sometimes the eye turns and sometimes it doesn’t
3. Alternating: one eye turns sometimes, and the other eye turns sometimes

Eyestrain occurs as the imbalanced extraocular muscles struggle to try to work together, as in phorias. When the imbalance is too great, the muscles cannot overcome it, and one eye “gives up”, as in tropias. The visual result is either diplopia (double vision), where the brain sees the image of each eye pointing to different targets, or “suppression”, where the brain uses only one eye while ignoring the other. The brain prefers suppression to diplopia, therefore the eyes are not straining to work together anymore. Unfortunately, if this occurs in childhood before each eye has developed a 20/20 pathway with the brain and is not treated before approximately age 5 years, that eye will likely never see 20/20 in the future.

Imbalances occur with the accommodative (internal focusing) muscles as well:

1. Accommodative excess (overaction of the focusing muscles).
2. Accommodative insufficiency (underaction of the focusing muscles).
3. Accommodative infacility (decreased ability of the focusing muscles to change focus from far to near and/or near to far).

Each of these can result in:

• Blurred near and/or far vision
• Unstable (variable) near and/or far vision
• Strained feeling
• Eye aches, pulling sensation
• Headaches
• Fatigue
• Movement of print
• Diplopia
• Avoidance of near work
• Dry eyes
• Light sensitivity

Spending hour after hour on a computer causes all of the eye muscles to work excessively, leading to eyestrain and eye related headaches.

In general, there are 4 muscle tasks that are involved in doing any kind of near work:

• The focusing muscles must move the position of the image onto the retina.
• The turning muscles must aim the eyes together at the object viewed.
• The tracking muscles must move smoothly across rows of words.
• The targeting muscles must jump accurately from one word to the next and from one row to the next.
• This is a lot to ask for 8 hours a day. Constant eye muscle stress causes you to be tired physically and mentally. Your ability to concentrate decreases and you become less productive.

At Eye To Eye, when we recommend that you get your vision examined every year, we do a lot more than get you to read the 20/20 line. We test all of these muscles and make our recommendations to provide you with not just clear, but the most COMFORTABLE vision possible. This often means wearing a special pair of computer glasses or doing specialized eye exercises to keep your eyes relaxed and functioning smoothly and efficiently.

Most eye doctors prescribe progressive no-line bifocals for people over 40. But for computer use, progressives are not a great option. The vision channel is narrowest at the distance that most people work at their computers, so it is very difficult to get and maintain a clear view. People come in all the time describing this problem to us while they move their heads up and down and side to side trying desperately to find a clear area in the lens.

An option that we often recommend is a "computer trifocal". This lens design has 2 lines across the lens, like a regular trifocal, but in this case, the lines are set with a much taller intermediate distance viewing area, allowing much better vision at the computer distance and height.

Amblyopia (“lazy eye”) is a vision developmental disorder in which an eye fails to achieve normal visual acuity, even when corrected with prescription eyeglasses or contact lenses. It typically begins during infancy and early childhood. If detected early and treated promptly, normal visual acuity may be attained, but if left untreated it will result in permanent loss of acuity. It is estimated that 2-3% of the U.S. population has some degree of amblyopia.

Because amblyopia typically is a problem of infant vision development, symptoms of lazy eye can be difficult to spot. A common cause of amblyopia is misalignment of the eyes, a condition known as strabismus, where one of the eyes turns in or out or up or down. If you notice your baby has an eye turn, schedule an appointment with an eye doctor who specializes in children’s vision immediately.

Another clue that your child may have a lazy eye is if he or she cries or fusses when you cover one eye while they are actively engaged in some sort of visual task such as watching TV. If your child is not bothered when one eye is covered but objects or fusses when the other eye is, this may suggest that the eye you covered is the “good” eye and that the uncovered eye is amblyopic, causing blurry vision.

It is recommended to schedule your child’s first eye exam at around 6 months of age to make sure vision is developing normally and the eyes function together as a team.

There are 3 types of lazy eye, based on the underlying cause:

1. Strabismic amblyopia (where the brain ignores the visual input from the misaligned eye to avoid double vision, leading to amblyopia in that eye)
2. Refractive amblyopia (where the eyes have unequal refractive errors, even though they may be perfectly aligned. The brain relies on the eye with less error and ignores the eye with the greater error, causing the unused eye to develop amblyopia)
3. Deprivation amblyopia (where something inside the eye, usually a congenital cataract, obstructs light from entering the eye. Prompt cataract surgery is necessary to allow normal visual development to occur)

Treatment of lazy eye:

• Strabismic amblyopia treatment may involve wearing an eye patch or opaque eyeglass or contact lens or using atropine (blurring) eyedrops on the good eye (to force the brain to start using the lazy eye), vision therapy (specific eye exercises to help both eyes work together as a team), and possibly muscle surgery to align the eyes.
• Refractive amblyopia may be treated simply by fully correcting the refractive errors in both eyes with glasses or contact lenses. Usually, some eye patching is needed to force the brain to start using the lazy eye.