The Dr. Bill Takeshita Foundation

Last Modified: Sat, 18 Aug 2007

View other Common Causes of Vision Impairment

Amblyopia is the condition where the eyes are healthy but the clarity of sight does not correct to 20/40 or when there is a difference in the best-corrected acuity of the two eyes when measured with glasses. Amblyopia is caused by the lack of normal stimulation to the visual centers of the brain. Vision is a learned and developed skill that requires precise focusing of light on the retina along with subsequent stimulation to the visual centers of the brain. Interruptions to this experience during the first 7 years of life can severely hinder the development of clear sight and the visual acuity may range from 20/40 to 20/200 (ability to read a 4 inch letter from 20 feet). Amblyopia is a stable condition and does not cause total blindness. Patients with amblyopia may also experience problems finding objects or seeing as clearly when there are many visual distractions in the background. This phenomenon is called contour interaction and can cause students to lose their place when reading or have difficulty locating a math problem on a crowded sheet of math problems.

Children born with moderate to high refractive errors such as farsightedness (hyperopia), nearsightedness (myopia), and/or astigmatism are more prone to having blurred sight that cannot be corrected with glasses. This condition is called refractive amblyopia. When one eye has a higher refractive error than the other eye, light rays do not focus clearly on the retina of the eye with the high refractive error. The brain ignores the vision of this eye and uses the vision from the eye with the lower refractive error because the image is sharper. If glasses or contact lenses are not prescribed at a very young age, refractive amblyopia will develop. In cases where both eyes have a high refractive error, both eyes can develop amblyopia because neither eye can send a sharp image to the visual centers of the brain.

Amblyopia can also occur when the eyes do not work together as a team. When one eye crosses inward (convergent strabismus) or turns outward (divergent strabismus), the brain tends to use one eye and ignore the other. Consequently, the visual cells of the brain that normally receive information from the misaligned eye do not develop maximally, resulting in blurred sight. This is called strabismus amblyopia and generally occurs in one eye. When both eyes alternately cross or turn outward, the brain takes turns using either the right or left eye. Subsequently, the visual cells that correspond to the right and left eyes both receive stimulation and thus amblyopia does not develop. Strabismus amblyopia can be successfully treated when detected and treated at an early age with a program of patching, glasses, and vision therapy. Strabismus surgery is an effective treatment to straighten the crossed or turn eye but does not treat the blurred sight. Patching is necessary to stimulate the visual cortex after surgery.

Amblyopia can also develop if the eyelids do not open fully, resulting in the lack of normal stimulation to the visual centers of the brain. This form of amblyopia is called amblyopia ex-anopsia, meaning that amblyopia has developed as a result of light not entering the eyes. Eye surgery is often recommended to open the eyelid to allow light to enter the eye . In cases where surgery cannot be performed, specialized taping of the upper eyelid along with eye drops to prevent the eye from drying out is recommended.

Recommendations

• All infants should have their vision examined by an eye care professional before the age of six months. Amblyopia is preventable and pediatric eye examinations are strongly recommended.
• Infants who have been prescribed glasses must wear them at all times.
• Patching therapy of the stronger eye must be performed daily along with activities to promote the development of vision in the weaker eye. Play games that will involve small details such as inserting pegs into a pegboard, stringing beads, and playing with blocks. All activities must be performed under adult supervision. For more details, please see Developing Your Child’s Vision.
• Infants with strabismus amblyopia will benefit from a consultation by a strabismus surgeon and an optometrist who specializes in vision therapy. The surgical alignment of the eyes followed by vision therapy is often the best way to treat amblyopia.
• Students with amblyopia in both eyes will benefit from sitting in the front portion of the classroom to help them to read the board. Copies of handouts and lessons written on the board will be very helpful.
• Students with visual acuity of 20/200 in each eye will benefit from having access to low vision aids such as bioptic glasses, hand held telescopes, and video magnification systems to improve their ability to read print.
• Students may benefit from using a typoscope or cardboard cutout that displays a single line of text to help them to maintain their place while reading and doing homework. Software programs such as the J Bliss Scanning software can scan text and display a single line of text on the computer screen.
• Students with amblyopia who have been prescribed a patching program or the use of atropine eye drops should not perform therapy during school. In most cases, they will not have sufficient sight to read details and this may interfere with their academic development.
• For more information, please see Low Vision Rehabilitation