What is Amblyopia?
Also known as ”lazy eye”, it is the medical term for poor visual development, usually in one eye and usually occurring in the early stages of life. In most cases, it is caused by lack of use of one eye when the brain ”favors” one eye over the other. Amblyopia is essentially a disorder of the brain cells that control the vision in one eye and not a problem with the eye itself.
Many people with amblyopia, especially those who only have a mild form, 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 related visual disorders, most notably poor depth perception. Amblyopia may 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. In this in disease 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 auto stereograms. However, perception of depth from monocular cues such as size, perspective, and motion parallax remains normal.
The Causes of Amblyopia
It may be caused by various conditions that interfere with normal vision in either eye or prevent the eyes from working together. In some cases, the conditions associated with amblyopia may be inherited. “Lazy eye” is typically caused by one of the following three conditions: A misaligned eye, known as strabismus.
A great disparity of refractive power (focus) in the eyes, causing one eye to be always in focus while the other eye may always be out of focus.
A cloudy obstruction of vision, such as a cataract, in one eye. Amblyopia occurs most commonly with misaligned or crossed eyes strabismus. In this case, the brain turns the misaligned eye off to avoid double vision, and the child uses only the better eye.
Crossed or misaligned eyes may be straightened in a number of ways:
- eye exercises
- special glasses
May be treated before or after the eyes are straightened. A great difference between the refractive power of the eyes is the second most common cause of amblyopia. When one eye is more nearsighted, farsighted, or astigmatic than the other, the brain will ”turn off” one of the eyes and it will become amblyopic. This is the most difficult cause of amblyopia to detect, as it requires careful measurement of vision and there is no physically apparent defect in either of the eyes. Less common causes of amblyopia include droopy eyelid (ptosis) and any number of diseases that cause a clouding of any one of the eye’s naturally clear tissues or other loss of vision. These diseases include corneal diseases (keratoconus, Peter’s anomaly, corneal scar or injury), infantile glaucoma, eyelid or orbital tumors, and a cataract in either eye. Amblyopia caused by a cataract is often the most severe form of amblyopia, as it typically occurs in infancy and requires strenuous treatment in the early months of a child’s life. A variant called organic amblyopia also can affect adults, though this is less frequent, and can be caused by exposure to toxic substances, diabetes, and medication.
The Diagnosis of Amblyopia
A child may not even be aware that one of their eyes is not always working with the other. If the child does not have an obvious misalignment or other abnormality, parents will rarely be able to tell something is wrong. A eye specialist detects amblyopia by checking for a difference in vision between the two eyes. Since it is often difficult to measure vision in young children, a doctor will often estimate visual acuity by watching how well a baby follows objects with one eye when the other is covered. If one eye is amblyopic and the good eye is covered, the baby may attempt to look around the patch, try to pull it off. Poor vision in one eye does not always mean a child has amblyopia. Vision can often be improved by prescribing glasses. An ophthalmologist will also examine the interior of the eye to see if other eye diseases like cataracts, tumors, or inflammations may be causing the decreased vision.
Treatment the weak eye must be forced into use. This is usually accomplished by patching or otherwise covering the health eye for specific periods of time during the day over weeks or even months. Even after vision has been restored in the weak eye, periodic patching may be necessary over a period of years to maintain the improvement. In some cases, glasses may be prescribed to correct refractive errors and patching may not even be necessary. Occasionally, amblyopia can be treated by blurring the vision in the healthy eye with special eye drops or lenses, forcing the weaker eye into use. If the amblyopia is caused by strabismus, the amblyopia is usually treated before the eyes are straightened by surgery. Patching is often continued even after the eyes are straightened. If amblyopia is caused by a cataract or other abnormality that causes a cloudiness of normally clear tissues, surgery or other therapy is required to correct the problem before the amblyopia may be dealt with. The weak eye must be made stronger in order to see normally. Patching must be done correctly, according to the doctor’s specifications. If not done correctly or overdone, patching can result in a loss of vision in the stronger of the two eyes. While your doctor can prescribe treatments for amblyopia, in the end it is up to you and/or your child to carry out his instructions and make sure they are not over- or under-done.
Clinical trials and experiments
The best outcome is achieved if treatment is started before age 8, research has shown that children older than age 12 and some adults can show improvement in the affected eye. Children from 9 to 11 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. Adolescents aged 13 to 17 showed improvement as well, albeit in smaller amounts than younger children. It is uncertain whether such improvements are only temporary, however, particularly if treatment is discontinued.
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.
A recent study, widely reported in the popular press, has suggested that repetitive transcranial magnetic stimulation may temporarily improve contrast sensitivity and spatial resolution in the affected eye of amblyopic adults. These results await verification by other researchers.
The Development of Normal Vision
Newborns are able to see, but as they use their eyes during the first months of life, vision improves. During the first few years of childhood, the visual system changes quickly and vision continues to develop. If, during this time, one eye sends a clear image to the brain while the other sends a dull or double image, the brain will stop paying attention to the eye that sends the less sharp image and will simply shut the eye off. This will effectively stop (or at least stunt) the development of the brain cells that control vision in this eye. After enough time, these cells may actually begin to shrink. After the first nine years of life, the visual system is generally fully developed and usually cannot be changed (though some studies of amblyopia treatments have shown improvement in children as old as 12). It is extremely important for children to develop equal vision in both eyes in order for them to function normally as a child and also as they grow. Good visual acuity in both eyes is vital for children to succeed in school, sports, or any other activity that requires good hand-eye coordination, depth perception, or clear vision. When children with untreated amblyopia grow to be adults, their choice of career may be limited and, if they were unlucky enough to lose vision in their one good eye, they would be legally visually impaired. For these reasons and more, it is extremely important for parents to have their child’s eyes examined by the family doctor or an eye care professional as early as possible. Vision can be tested in infants and very young children and it is recommended that a child be tested before their fourth birthday, at the latest.
When amblyopia caught in early stages and treated properly, the success rate of recovery and improvement of vision is extremely good. Success depends on how severe the problem and how old the patient is when treatment is begun. If amblyopia is discovered after early childhood, treatment may not be entirely successful. The early, full medical eye exams to catch amblyopia (or other problems) is important because gives children a chance at healthy vision.