Amblyopia, otherwise known as
lazy eye, is a
disorderA disease or medical condition isan abnormal condition of an organism that impairs bodily functions, associated with specific symptoms and signs...
of the
visual systemThe visual system is the part of the central nervous system which enables organisms to see.It interprets the information from visible light to build a representation of the world surrounding the body...
that is characterized by poor or indistinct
visionVisual perception is the ability to interpret information and surroundings from visible light reaching the eye. The resulting perception is also known as eyesight, sight or 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.
The problem is caused by either no transmission or poor transmission of the visual image to the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
for a sustained period of dysfunction or during early
childhoodChildhood is a broad term usually applied to the phase of development in humans between infancy and adulthood.-Age definition of a child:...
. 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.
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. "Lazy brain" is a more accurate term to describe amblyopia. The term "lazy eye" is imprecise because it is also a
laymanA "layman" is a person who is a non-expert in a given field of knowledge. The term originated from "laity", but over the centuries, shifted in definition....
's term for
strabismusStrabismus is a condition in which the eyes are not properly aligned with each other...
, particularly
exotropiaExotropia is a form of strabismus where the eyes are deviated outward. It is the opposite of esotropia. People with exotropia often experience crossed diplopia. Intermittent exotropia is a fairly common condition. "Sensory exotropia" occurs in the presence of poor vision...
.
Physiology
Amblyopia is a developmental problem in the brain, not an organic problem in the eye (although organic problems can induce amblyopia 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. HubelDavid Hunter Hubel was co-recipient with Torsten Wiesel of the 1981 Nobel Prize in Physiology or Medicine, for their discoveries concerning information processing in the visual system; the prize was shared with Roger W. Sperry for his independent research on the cerebral hemispheres...
and
Torsten WieselTorsten Nils Wiesel was a Swedish co-recipient with David H. Hubel of the 1981 Nobel Prize in Physiology or Medicine, for their discoveries concerning information processing in the visual system; the prize was shared with Roger W...
won the
Nobel Prize in Physiology or MedicineThe Nobel Prize in Physiology or Medicine is awarded once a year by the Swedish Karolinska Institute. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in Physics, Chemistry, Literature, Peace, and 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". The maximum critical period in humans is from birth to two years old.
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 perceptionDepth perception is the visual ability to perceive the world in three dimensions. Although any animal capable of moving around its environment must be able to sense the distance of objects in that environment, the term perception is reserved for humans, who are, as far as is known, the only beings...
. Amblyopes may suffer from poor spatial
acuityVisual acuity is acuteness or clearness of vision, especially form vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
, low sensitivity to
contrastContrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...
and some "higher-level" deficits to vision such as reduced sensitivity to
motionMotion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs...
. These deficits are usually specific to the amblyopic 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
autostereogramAn autostereogram is a single-image stereogram , designed to create the visual illusion of a three-dimensional scene from a two-dimensional image in the human brain...
s.
However perception of depth from monocular cues such as size,
perspectivePerspective, in context of vision and visual perception, is the way in which objects appear to the eye based on their spatial attributes; or their dimensions and the position of the eye relative to the objects...
, and motion parallax is normal.
Types
Amblyopia can be caused by
deprivationDeprivation may refer to:* Poverty* Relative deprivation* Sleep deprivation* Emotional Deprivation Disorder* Maternal deprivation...
of vision early in life by vision-obstructing disorders such as congenital
cataractA cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light...
s, by
strabismusStrabismus is a condition in which the eyes are not properly aligned with each other...
(misaligned eyes), or by
anisometropiaAnisometropia is the condition in which the two eyes have unequal refractive power; that is, are in different states of myopia , hyperopia or in the extreme, antimetropia , the unequal refractive states cause unequal rotations thus leading to diplopia and...
(different degrees of
myopiaMyopia , also called nearsightedness or shortsightedness, is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed....
or hypermetropia in each eye).
Ambylopia can also occur physiologically after tobacco or alcohol consumption.
Strabismic amblyopia
StrabismusStrabismus is a condition in which the eyes are not properly aligned with each other...
, sometimes erroneously also called
lazy eye, is a condition in which the eyes are misaligned. Strabismus usually results in normal vision in the preferred sighting (or "fellow") 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 (
diplopiaDiplopia, commonly known as double vision, is the simultaneous perception of two images of a single object. These images may be displaced horizontally, vertically, or diagonally Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object. These images...
), since the two eyes are not fixated on the same object.
Children's brains, however, are more
neuroplasticNeuroplasticity is the changing of neurons, the organization of their networks, and their function via new experiences. This idea was first proposed in 1892 by Santiago Ramón y Cajal the proposer of the neuron doctrine though the idea was largely neglected for the next fifty years...
, and therefore can more easily adapt by
suppressing imagesSuppression of an eye is an adaptation by the patient to eliminate the symptoms of disorders of binocular vision such strabismus, convergence insufficiency and aniseikonia. This adaptation occurs in the brain, eliminating double vision by ignoring the images of one of the eyes...
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 an
eyepatchAn eyepatch is a small patch that is worn in front of one eye. It may be a cloth patch attached around the head by an elastic band or by a string, or an adhesive bandage. It is often worn by people to cover a lost or injured eye, but it also has a therapeutic use in children for the treatment of...
over the dominant eye or pharmacologic penalization of it. Penalization usually consists of applying
atropineAtropine is a tropane alkaloid extracted from deadly nightshade , jimsonweed , mandrake and other plants of the family Solanaceae. It is a secondary metabolite of these plants and serves as a drug with a wide variety of effects. It is a competitive antagonist for the muscarinic acetylcholine...
drops to temporarily dilate the
pupilThe pupil is an opening located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils...
, which leads to blurring of vision in the good eye. This helps to prevent the bullying and teasing associated with wearing a patch, although application of the eyedrops is more challenging. The ocular alignment itself may be treated with surgical or non-surgical methods, depending on the type and severity of the
strabismusStrabismus is a condition in which the eyes are not properly aligned with each other...
.
Refractive or anisometropic amblyopia
Refractive amblyopia may result from
anisometropiaAnisometropia is the condition in which the two eyes have unequal refractive power; that is, are in different states of myopia , hyperopia or in the extreme, antimetropia , the unequal refractive states cause unequal rotations thus leading to diplopia and...
(unequal refractive error between the two eyes). Anisometropia exists when there is a difference in the
refractionRefraction is the change in direction of a wave due to a change in its velocity. This is most commonly observed when a wave passes from one medium to another...
between the two eyes. The eye which provides the brain with a clearer image (closer to 20/20) typically becomes the dominant eye. The image in the other eye is blurred, which results in abnormal development of one half of the visual system. Refractive amblyopia is usually less severe than strabismic amblyopia and is commonly missed by
primary care physicianA primary care physician, or PCP, is a physician/medical doctor who provides both the first contact for a person with an undiagnosed health concern as well as continuing care of varied medical conditions, not limited by cause, organ system, or diagnosis....
s because of its less dramatic appearance and lack of obvious physical manifestation, such as with strabismus. 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 and
patchingAn eyepatch is a small patch that is worn in front of one eye. It may be a cloth patch attached around the head by an elastic band or by a string, or an adhesive bandage. It is often worn by people to cover a lost or injured eye, but it also has a therapeutic use in children for the treatment of...
or penalizing the good eye.
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
astigmatismAstigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...
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
opaqueOpacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
, such as is the case with
cataractA cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light...
s or
corneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43...
l scarring from
forcepsForceps are a handheld, hinged instrument used for grasping and holding objects. Forceps are used when fingers are too large to grasp small objects or when many objects need to be held at one time while the hands are used to perform a task. The term forceps is used almost exclusively within the...
injuries during birth. These opacities prevent adequate visual input from reaching the eye, and therefore disrupt development. If not treated in a timely fashion, amblyopia may persist even after the cause of the opacity is removed. Sometimes, drooping of the
eyelidAn eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...
(
ptosisPtosis is an abnormally low position of the upper eyelid. The drooping may be worse after being awake longer, when the individual's muscles are tired. This condition is sometimes called "lazy eye", but that term normally refers to amblyopia. If severe enough and left untreated, the drooping eyelid...
) 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
hemangiomaA hemangioma of infancy is a benign self-involuting tumour of endothelial cells . In most cases it appears during the first days or weeks of life and will have resolved at the latest by age 10...
that blocks some or all of the eye.
Treatments
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
topicalIn medicine, a topical medication is applied to body surfaces such as the skin or mucous membranes such as the vagina, anus, throat, eyes and ears....
atropineAtropine is a tropane alkaloid extracted from deadly nightshade , jimsonweed , mandrake and other plants of the family Solanaceae. It is a secondary metabolite of these plants and serves as a drug with a wide variety of effects. It is a competitive antagonist for the muscarinic acetylcholine...
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.
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.
Clinical trials and experiments
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 therapyVision therapy, also known as visual training, vision training, or visual therapy, is a broad group of techniques aimed at correcting and improving binocular, oculomotor, visual processing, and perceptual disorders." Vision therapy encompasses a wide variety of non-surgical methods which some...
) were four times as likely to show a two line improvement on a standard 11 line
eye chartA Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862.-Description:...
than amblyopic children who did not receive treatment. Adolescents aged 13 to 17 showed improvement as well, albeit in smaller amounts than younger children.
Vision therapyVision therapy, also known as visual training, vision training, or visual therapy, is a broad group of techniques aimed at correcting and improving binocular, oculomotor, visual processing, and perceptual disorders." Vision therapy encompasses a wide variety of non-surgical methods which some...
programs are occasionally partially effective on motivated adults, at least in the short term.
A recent study, widely reported in the popular press, has suggested that repetitive
transcranial magnetic stimulationTranscranial magnetic stimulation is a noninvasive method to excite neurons in the brain: weak electric currents are induced in the tissue by rapidly changing magnetic fields...
may temporarily improve contrast sensitivity and spatial resolution in the affected eye of amblyopic adults. These results await verification by other researchers.
Virtual reality computer games where each eye receives different signals of the virtual world that the player's brain must combine in order to successfully play the game have shown some promise in improving both monocularity in the affected eye as well as binocularity.
In another recent clinical trial conducted in a hospital in China and coordinated with a research program at
University of Southern CaliforniaThe University of Southern California is a private, nonsectarian, research university located in the University Park neighborhood in Los Angeles, California, USA...
, 28 of 30 patients being treated for amblyopia showed dramatic gains with some developing 20/20 vision. The treatment involved only the use of basic computer desktop software and vision training exercises.
See also
- Eyepatch
An eyepatch is a small patch that is worn in front of one eye. It may be a cloth patch attached around the head by an elastic band or by a string, or an adhesive bandage. It is often worn by people to cover a lost or injured eye, but it also has a therapeutic use in children for the treatment of...
- Convergence insufficiency
Convergence insufficiency or Convergence Disorder is a sensory and neuromuscular anomaly of the binocular vision system, characterized by an inability of the eyes to turn towards each other, or sustain convergence.-Symptoms:...
- Diplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object. These images may be displaced horizontally, vertically, or diagonally Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object. These images...
- Orthoptist
- Stereoblindness
Stereoblindness is the inability to see in 3D using stereo vision, resulting in inability to perceive stereoscopic depth by combining and comparing images from the two eyes...
- Vision therapy
Vision therapy, also known as visual training, vision training, or visual therapy, is a broad group of techniques aimed at correcting and improving binocular, oculomotor, visual processing, and perceptual disorders." Vision therapy encompasses a wide variety of non-surgical methods which some...
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