Amblyopia

Amblyopia, also called lazy eye, is a condition of reduced vision caused by the inability to process images in the brain. Amblyopia usually affects one eye, but it can affect both. The brain learns to process visual information rapidly in the first few years of life, so anything that interrupts this learning can result in amblyopia. Amblyopia usually arises in the first five years of age. It is the most common visual impairment in childhood.

Types
Amblyopia generally falls into one of three categories, based on the underlying cause.

Strabismic amblyopia
This is amblyopia caused by strabismus, or a deviated eye. If a child is born with a deviated eye, or the eye begins to deviate early in childhood, then the brain will eventually choose to shut down the information coming from the deviated eye rather than dealing with the confusion of having double vision. As a result, the part of the brain responsible for vision in the deviated eye will not develop properly, and amblyopia will occur.

Anisometropic amblyopia
Anisometropia is the condition of having a significant difference in the degree of nearsightedness or farsightedness between the two eyes. If a significant difference exists, the child may see the world as slightly blurry, even if one eye is normal and the other eye is nearsighted, as both the visual images come to the brain together to form a single image. The brain may choose to shut off the information coming from the bad eye (or the worse eye) so that the image will only be coming from the good eye and will be more clear. Once this happens, the brain can no longer processes information from the worse eye and therefore cannot learn how to see using that eye. This will cause amblyopia.

Amblyopia can occur in both eyes if there is a significant myopia (shortsightedness) in both of them. This is called ametropic amblyopia.

Deprivation amblyopia
Deprivation amblyopia, also called occlusive amblyopia, is caused by the images of one eye not reaching the brain correctly because something is preventing clear vision. If a child is born with something blocking the vision, the brain will not receive the correct visual input from that eye and will not develop normal vision. Even if the blockage is eventually removed, the brain will still have a diminished ability to see using that eye. The physical obstruction must be present during the time of visual development, generally before five years of age, and must be present for a prolonged period of time (months or years) in order for it to cause deprivation amblyopia. Common conditions which block vision in one eye include:


 * Congenital cataracts
 * Corneal scarring from forceps injury during birth
 * Drooping of the eyelid (ptosis), which can be caused by damage to the nerve or muscle that raises the eyelid during a forceps delivery, or can be congenital
 * A large hemangioma (birthmark consisting of a large mass of blood vessels and skin) that is located near the eye and is obstructing the vision of that eye.The hemangioma may be partially obstructing the vision, or the hemangioma may be near enough to the eye that it may cause refraction of light as the light travels near the hemangioma and enters the eye. Having a hemangioma close to the eye is an indication to remove the hemangioma at birth rather than waiting for it to grow and then resolve on its own, as it may cause deprivation amblyopia.
 * Any physical obstruction, including the habit of always wearing an eyepatch on one eye. (Wearing an eyepatch for one day, such as on Halloween, will not cause amblyopia, as it requires a period of months or years for it to develop.)
 * A major eye infection, such as orbital cellulitis, which may cause temporary loss of vision, may cause amblyopia if not treated quickly.
 * Severe swelling of the eyelids, as can happen with repeated allergy attacks or with nephrotic syndrome, may also cause enough of an obstruction of vision as to cause amblyopia.
 * In some cases, when amblyopia is being treated by placing a patch on the good eye, amblyopia may develop in the good eye if the patch is left on for too long or there is not enough careful monitoring of the vision of both eyes during treatment.

Signs and Symptoms
In many cases, the child does not notice that one eye is not as effective as the other. This is why screening exams are important; they can detect children with amblyopia while it can still be treated.


 * Increased deviation of the eye: In cases of strabismus (deviated eye) where the strabismus only occurs intermittently, as the visual input from that eye gets ignored, the control of movement and coordination of that eye worsens, and so the eye gets more and more deviated, until the eye is permanently turned outward or inward.
 * Poor depth perception: In more severe cases where the amblyopia causes near-blindness of one eye, the child may notice poor depth perception. He or she may not be as good in sports that rely heavily on depth perception, such as soccer, tennis, and ping-pong. The effect of this poor depth perception is not as dramatic as one would imagine since the child with amblyopia eventually learns to have some depth perception using other cues in the environment, such as size, overlapping of images, and speed of objects in relation to more distant objects.
 * Inability to use one eye in certain situations or occupations: As the child gets older, he or she may eventually notice the poor vision in one eye, and this may be especially disturbing to the individual in certain professions, such as astronomers, microbiologists, and snipers, who rely on using both eyes or use instruments where one or the other eye is needed. Amblyopia may disqualify the individual from military service or from becoming a pilot.

Causes
The brain and the eye work together to produce vision. Light enters the eye and is changed into nerve signals in the [[retina]. These nerve signals travel along the optic nerve to the brain. The brain normally processes images from both eyes. If the signal is impaired, the brain will shut down the messages coming from that eye, and will not be able to get the visual information it needs to "learn" how to see. In time, the brain will not be able to see from the affected eye.

As discussed above in ]]Types, there are several ways in which this impaired signal can arise.

Diagnosis
Amblyopia is a blindness that occurs without there being anything wrong with the eye itself. However, it may be caused by various problems of the eye in early childhood, and as such, the amblyopia may not be discovered and diagnosed until the initial problem with the eye is corrected. Amblyopia may also be diagnosed if the visual impairment is worse than what the actual problem of the eye can account for.

If a child is born with an eye problem such as strabismus (deviation of one eye), cataracts, or severe myopia, then after the underlying problem is treated, it is almost expected that the child will have some degree of amblyopia. Amblyopia is diagnosed in these cases via careful examination of each eye by an ophthalmologist using a visual chart of letters or pictures.

In many cases, amblyopia remains undetected during the early years of childhood and may not be diagnosed until the child has his first vision screening test, where the child's vision is tested using a Snellen chart one eye at a time. The child may not have noticed any problem. This highlights the importance of school vision screening exams, which take place in kindergarten or first grade.

Treatment
The first step in treating amblyopia involves treating the underlying cause. Strabismic amblyopia may require surgery or corrective lenses. Removal of a visual obstacle such as cataract is the first step in treating deprivation amblyopia. Eyeglasses or contact lenses can correct a problem with visual acuity (anisometropic amblyopia). Recent research shows that LASIK appears to be a safe and effective procedure to treat anisometropic amblyopia in children.[1]

Treating amblyopia involves forcing the child to use the eye with the reduced vision (the "bad eye") instead of the normal eye. This trains the brain to use the weaker eye's visual information. The terms "good eye" and "bad eye" are used out of convenience to describe the treatment and management of amblyopia. However, it is important to note that the eye itself is not good or bad or strong or weak, but rather the section of the brain responsible for vision using that eye is undertrained.

It is very difficult to treat amblyopia of both eyes, although the condition may improve with time once the underlying cause of the visual problem is corrected.

Currently, there are two methods used to help the child use the bad eye instead of using the good eye:


 * Use of an eye patch
 * Use of eye drops

Patching
Patching is the standard treatment for amblyopia. An opaque, adhesive patch is worn over the stronger eye for weeks to months. This therapy forces the child to use the eye with amblyopia. Patching stimulates vision in the weaker eye and helps the part of the brain that manages vision develop more completely.

Generally, the patch is prescribed for the entire day while awake except for an hour to shower or bathe. The patch is prescribed for a total period equal to about one week for every year of the child's age. For example, a five-year-old child with amblyopia will have to wear the patch for about five weeks. During this time, the vision of both eyes are frequently tested, and so the actual period may be shorter or longer by one or two weeks.

Compliance with wearing the eye patch is a significant problem in the treatment of amblyopia. This may be a result of the inherent discomfort of having to see with an eye with poor vision or from the discomfort of the eye patch itself. Poor compliance may also be due to social reasons, as some children may be embarrassed about having to wear the eye patch to school.

The image of the one child in a kindergarten or grade-school classroom wearing a black patch on one eye is ingrained in the culture, but nowadays flesh-colored patches are now used, as they are less conspicuous. The patches are more comfortable these days as well. However, there is an easier and less embarrassing alternative for the child with amblyopia: eye drops.[2] The drops are applied to the "good eye" and cause a temporary blurring of vision in that eye, which means the brain must get its visual input from the "bad eye" until the drug wears off. Treatment choice between the patch and eye drops are generally handled on a case-by-case basis, with both being offered. Most parents and children prefer eye drops.

Eye drops
The medicine used in the treatment of amblyopia is atropine, the same eye drops used to dilate the eye before an eye exam at the optometry clinic. A drop of the drug is placed in the good eye once a day to temporarily blur the vision so that the child will prefer to use the eye with amblyopia. The eye drops are in effect being used as a "medical eye patch." Treatment with atropine drops may have the added advantage of stimulating vision in the weaker eye, helping the brain learn to see from that eye more quickly.

Eye drops have been shown to be as effective as eye patches in the treatment of amblyopia.[3]

Treatment of older children
Previously, eye care professionals believed that treating amblyopia in older children would be of little benefit. Many children with amblyopia over the age of 9 years were not treated. However, some pediatric ophthalmologists began to treat older children and saw improvement, and their findings stimulated a nationwide clinical trial to test the treatment on older children. [4] This trial showed that many children age seven through 17 with amblyopia may benefit from treatments that are more commonly used on younger children. This study shows that age alone should not be used as a factor to decide whether or not to treat a child for amblyopia.

Treatment of adults
Studies are very limited at this time and scientists do not know what the success rate might be for treating amblyopia in adults. During the first six to nine years of life, the visual system develops very rapidly. Complicated connections between the eye and the brain are created during that period of growth and development. Scientists are exploring whether treatment for amblyopia in adults can improve vision.

Clinical Trials

 * The Clinical Studies Database of the National Eye Institute: Strabismus, Amblyopia, and Visual Processing: lists of clinical studies related to amblyopia.
 * ClinicalTrials.gov: Amblyopia

History
The cause of amblyopia as an abnormal development of the visual center of the brain was confirmed by direct examination of the brain by David H. Hubel and Torsten Wiesel. They won the Nobel Prize in Physiology or Medicine in 1981 for their work, which involved visual deprivation of kittens during the critical period of visual development.

Etymology
Amblyopia comes from the Greek word ambly, meaning dim, and opia, meaning vision.

Prevalence
Amblyopia is the most common cause of visual impairment in childhood. The condition affects approximately 2 to 3 out of every 100 children. It affects approximately 1-4% of the population.[5] However, many cases of mild amblyopia may remain undiagnosed.

Unless it is successfully treated in early childhood, amblyopia usually persists into adulthood and is a common cause of visual impairment of one eye.[6]

Interesting Facts

 * The term "lazy eye" traditionally refers to amblyopia. However, the term has been frequently and incorrectly applied to many other conditions, such as strabismus (deviated eye) and ptosis (drooping eyelid).
 * Amblyopia is a type of cortical blindness, since it is the visual cortex of the brain that is the cause of the blindness and not the eye itself. However, the term cortical blindness is usually reserved for blindness that begins in adulthood or old age due to strokes or trauma, and amblyopia is used to refer to cortical blindness caused by the brain never having learned to see using the eye in the first place.