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Amblyopia |
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 Amblyopia, or lazy eye, is a disorder of the eye
that is characterized by poor or blurry vision in an eye that is otherwise
physically normal, or out of proportion to associated structural abnormalities.
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 disuse during
early childhood. The condition will only arise at this young age because most of
the visual system's development in humans is complete and "locked in" by 8 to 10
years of age. 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.
Amblyopia has been estimated to affect 1 to 5% of the population. It is a
developmental problem in the brain, not an organic problem in the eye (although
organic problems can induce amblyopic symptoms which persist after the organic
problem has resolved). The part of the brain corresponding to the visual system
from the affected eye is not stimulated properly, and develops abnormally. This
has been confirmed via direct brain examination. David H. Hubel and Torsten
Wiesel won the Nobel Prize in Physiology or Medicine in 1981 for their work
demonstrating the irreversible damage to ocular dominance columns produced in
kittens by sufficient visual deprivation during the so-called "critical period".
Many amblyopics, 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 vision disorder, most notably poor depth perception.
Symptoms
Amblyopes suffer from poor spatial acuity, low sensitivity to contrast and
reduced sensitivity to motion. These deficits are usually specific to the
amblyopic eye, not the unaffected "fellow" eye. Amblyopes can also suffer from
problems of stereo vision and may have difficulty seeing the three-dimensional
images in autostereograms.
Types
Amblyopia can be caused by deprivation of vision early in life, or by strabismus
(misaligned eyes), vision-obstructing disorders, or anisometropia (different
degrees of myopia or hyperopia in each eye).
Strabismic Amblyopia
Strabismus, sometimes known as lazy eye, usually results in normal vision in the
fixating eye, but abnormal vision in the strabismic eye due to the brain
discarding certain information. 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 plastic, and therefore can more easily
adapt by ignoring images from one of the eyes, eliminating the double vision
(suppression). 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 patching or pharmacologic
penalization (usually employing atropine drops to the dominant eye). The ocular
alignment itself may be treated with surgical or non-surgical methods, depending
on the type and severity of the strabismus.
Refractive Amblyopia
If anisometropia is present, refractive amblyopia may result. Anisometropia
exists when there is a difference in the refraction between the two eyes. The
eye with less refractive error provides the brain with a clearer image, and is
favoured by the brain. Those with this condition are more susceptible to the
development of amblyopia, which may be as severe as strabismic amblyopia.
Despite its severity, refractive amblyopia is commonly missed by primary care
physicians because of its less dramatic appearance and lack of obvious physical
manifestation, such as with strabismus.
Refractive amblyopia is diagnosed when there is a wide disparity in visual
acuity between the two eyes. It 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.
Form-deprivation and Occlusion Amblyopia
Form-deprivation amblyopia (Amblyopia ex anopsia) results when the ocular media
is 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.
One should also be wary of creating this type of amblyopia in a 'good' eye when
treating for amblyopia in the other eye – so-called "reverse amblyopia".
This type of amblyopia is treated by removing the opacity as soon as possible.
Treatment and Prognosis
Treatment consists of forcing use of the amblyopic eye, either by patching, or
by instilling topical atropine in the eye with better vision.
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 that the Bates Method can reverse amblyopia; this remains
controversial.
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