Amblyopia (Lazy Eye)

Definition:
The brain and the eye work together to produce vision. Light enters the eye and is changed into nerve signals that travel along the optic nerve to the brain. Amblyopia is the medical term used when the vision in one of the eyes is reduced because the eye and the brain are not working together properly. The eye itself looks normal, but it is not being used normally because the brain is favoring the other eye. This condition is also sometimes called lazy eye.

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amblyopia
amblyopia (Photo credit: ebmorse)

It is a disorder of the visual system that is characterized by poor or indistinct vision in an eye that is otherwise physically normal, or out of proportion to associated structural abnormalities. It has been estimated to affect 1–5% of the population

Amblyopia is the most common cause of visual impairment in childhood. The condition affects approximately 2 to 3 out of every 100 children. Unless it is successfully treated in early childhood, amblyopia usually persists into adulthood, and is the most common cause of monocular (one eye) visual impairment among children and young and middle-aged adults.

Symptoms:
Many people with amblyopia, especially those who are only mildly so, are not even aware they have the condition until tested at older ages, since the vision in their stronger eye is normal. However, people who have severe amblyopia may experience associated visual disorders, most notably poor depth perception. Amblyopes suffer from poor spatial acuity, low sensitivity to contrast and some “higher-level” deficits to vision such as reduced sensitivity to motion.These deficits are usually specific to the amblyopic eye, not the unaffected “fellow” eye. Amblyopes also suffer from problems of binocular vision such as limited stereoscopic depth perception and usually have difficulty seeing the three-dimensional images in hidden stereoscopic displays such as autostereograms. However perception of depth from monocular cues such as size, perspective, and motion parallax is normal.
Causes:
The problem is caused by either no transmission or poor transmission of the visual image to the brain for a sustained period of dysfunction or during early childhood. Amblyopia normally only affects one eye, but it is possible to be amblyopic in both eyes if both are similarly deprived of a good, clear visual image. Detecting the condition in early childhood increases the chance of successful treatment.

Amblyopia may be caused by any condition that affects normal visual development or use of the eyes. Amblyopia can be caused by strabismus, an imbalance in the positioning of the two eyes. Strabismus can cause the eyes to cross in (esotropia) or turn out (exotropia). Sometimes amblyopia is caused when one eye is more nearsighted, farsighted, or astigmatic than the other eye. Occasionally, amblyopia is caused by other eye conditions such as cataract.

While the colloquialism “lazy eye” is frequently used to refer to amblyopia, the term is inaccurate because there is no “laziness” of either the eye or the amblyope involved in the condition. Therefore, some consider the term “lazy eye” to be pejorative.

Types:
Amblyopia can be caused by deprivation of vision early in life, by strabismus (misaligned eyes), by vision-obstructing disorders, or by anisometropia (different degrees of myopia or hyperopia in each eye).

Strabismus:
Strabismus, sometimes erroneously also called lazy eye, is a condition in which the eyes are misaligned in a variety of different ways. Strabismus usually results in normal vision in the preferred sighting eye, but may cause abnormal vision in the deviating or strabismic eye due to the discrepancy between the images projecting to the brain from the two eyes. Adult-onset strabismus usually causes double vision (diplopia), since the two eyes are not fixated on the same object. Children’s brains, however, are more neuroplastic, and therefore can more easily adapt by suppressing images from one of the eyes, eliminating the double vision. This plastic response of the brain, however, interrupts the brain’s normal development, resulting in the amblyopia.

Strabismic amblyopia is treated by clarifying the visual image with glasses, and/or encouraging use of the amblyopic eye with a eyepatch over the dominant eye or pharmacologic penalization of it (usually by applying atropine drops to temporarily paralyze the muscles and weaken vision in the good eye—this helps to prevent the bullying and teasing associated with wearing a patch). The ocular alignment itself may be treated with surgical or non-surgical methods, depending on the type and severity of the strabismus.

Refractive or anisometropic amblyopia
Refractive amblyopia may result from anisometropia (unequal refractive errors between the two eyes). Anisometropia exists when there is a difference in the refraction between the two eyes. The eye with less far-sighted (hyperopic) refractive error provides the brain with a clearer image, and is favored by the brain. Refractive amblyopia is usually less severe than strabismic amblyopia and is commonly missed by primary care physicians because of its less dramatic appearance and lack of obvious physical manifestation, such as with strabismus.[9] Frequently, amblyopia is associated with a combination of anisometropia and strabismus.

Pure refractive amblyopia is treated by correcting the refractive error early with prescription lenses. Vision therapy and/or eye patching can also be used to develop and/or improve visual abilities, binocular vision, depth perception, etc.

Meridional amblyopia is a mild condition in which lines are seen less clearly at some orientations than others after full refractive correction. An individual who had an astigmatism at a young age that was not corrected by glasses will later have astigmatism that cannot be optically corrected.

Form-deprivation and occlusion amblyopia
Form-deprivation amblyopia (Amblyopia ex anopsia) results when the ocular media become opaque, such as is the case with cataracts or corneal scarring from forceps injuries during birth.These opacities prevent adequate sensory input from reaching the eye, and therefore disrupt visual development. If not treated in a timely fashion, amblyopia may persist even after the cause of the opacity is removed. Sometimes, drooping of the eyelid (ptosis) or some other problem causes the upper eyelid to physically occlude a child’s vision, which may cause amblyopia quickly. Occlusion amblyopia may be a complication of a hemangioma that blocks some or all of the eye.

Diagnosis:
An eye exam by a pediatrician or the 20/20 eye chart screening is not adequate for the detection of amblyopia (and other visual conditions). The most important diagnostic tools are the special visual acuity tests other than the 20/20 letter charts currently used by schools, pediatricians and eye doctors. Examination with cycloplegic drops can be necessary to detect this condition in the young.

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Since amblyopia usually occurs in one eye only, many parents and children are unaware of the condition. Many children go undiagnosed until they have their eyes examined at the eye doctor’s office at a later age. Comprehensive vision evaluations are highly recommended for infants and pre-school children.

A March 2002 policy statement by the American Academy of Pediatrics (AAP) recommends that all babies have vision screening (including screening for strabismus and amblyopia) at the earliest possible age, and at regular intervals throughout childhood. Rather than relying on informal or subjective assessments of vision, the policy discusses photoscreening, a new test that can make checking vision easier and more accurate for even very young babies. A specially equipped camera takes their picture and a computer or a person analyzes the eye data obtained. Ask your pediatrician about having your baby screened.

Treatment :
Treatment of strabismic or anisometropic amblyopia consists of correcting the optical deficit and forcing use of the amblyopic eye, either by patching the good eye, or by instilling topical atropine in the eye with better vision. One should also be wary of over-patching or over-penalizing the good eye when treating for amblyopia, as this can create so-called “reverse amblyopia” in the other eye.

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Form deprivation amblyopia is treated by removing the opacity as soon as possible followed by patching or penalizing the good eye to encourage use of the amblyopic eye.

Prognosis:
Although the best outcome is achieved if treatment is started before age 5, research has shown that children older than age 10 and some adults can show improvement in the affected eye. Children from 7 to 12 who wore an eye patch and performed near point activities (vision therapy) were four times as likely to show a two line improvement on a standard 11 line eye chart than amblyopic children who did not receive treatment. Children 13 to 17 showed improvement as well, albeit in smaller amounts than younger children. (NEI-funded Pediatric Eye Disease Investigator Group, 2005)

Some claim the controversial Bates Method can reverse amblyopia. In his book Perfect Sight without Glasses, ophthalmologist William Bates advocated relaxation and thinking relaxing thoughts as a way to correct amblyopia: “When the disturbing thought is replaced by one that relaxes, the squint disappears, the double vision and the errors of refraction are corrected; and this is as true of abnormalities of long standing as of those produced voluntarily.” Bates was of the opinion that the cause of “squint, or of any other functional disturbance of the eye” was simply “wrong thoughts” and these “wrong thoughts” (and therefore the resulting pathology of the visual system) could be cured by relaxing thoughts. “In a fraction of a second the highest degrees of refractive error may be corrected, a squint may disappear, or the blindness of amblyopia may be relieved. If the relaxation is only momentary, the correction is momentary. When it becomes permanent, the correction is permanent.” Some critics contend that this proposed “cure” blames the victim, by implying that amblyopes are in some way responsible for their visual problems.

Can amblyopia be treated in adults?
Studies are very limited at this time and scientists don’t 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.

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Important Points to be noted:
*If not detected and treated early in life, amblyopia can cause loss of vision and depth perception.

*Improvements are possible at any age with proper treatment, but early detection and treatment offer the best outcome.

*Comprehensive vision screenings are needed for infants and pre-school children. An eye exam by a pediatrician or the 20/20 eye chart screening is not adequate for the detection of amblyopia (and other visual conditions).

Disclaimer: This information is not meant to be a substitute for professional medical advise or help. It is always best to consult with a Physician about serious health concerns. This information is in no way intended to diagnose or prescribe remedies.This is purely for educational purpose.

Resources:
http://en.wikipedia.org/wiki/Amblyopia
http://www.medic8.com/eye-disorders/amblyopia.htm
http://www.lazyeye.org/
http://www.drgreene.com/21_1024.html

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