June 21, 2006
Amblyopia is a condition of visual impairment in which one eye cannot see as well as the other. In professional terms, the bad eye sees at least two lines on the chart worse than the good eye. The popular term for amblyopia is "lazy eye."
I suppose it's descriptive, except that it makes it sound like that eye has a choice about what it sees. The key factor in amblyopia is that no vision correction can bring that eye up to the clarity of the other. Just because one eye is worse than the other without glasses, it doesn't mean that a patient has amblyopia; it's only amblyopia if the best correction results in worse vision than the other eye.
There are two general types of amblyopia physiological and functional. Physiological amblyopia has a detectable defect in the eye's structure that prevents good vision. An eye with functional amblyopia is perfectly healthy. I'll be discussing the causes and treatment of functional amblyopia.
The bottom line cause of amblyopia is non-use. If someone does not use a body part, it never develops. If an eye is never used to its full capability, it will never learn to see well. There are two main reasons for this that we find. The first is an eye muscle imbalance, which causes one eye to be misaligned. When this condition begins at birth, which it usually does, the patient's brain suppresses the images from the misaligned eye rather than see double. If the eye turn is constant, so will the suppression of images to that eye. Over time, the amblyopia will become more and more profound. If the eyes are re-aligned by means of surgery, the eye will remain amblyopic unless the surgery is performed at a very early age.
The second reason for non-use occurs with eyeglass prescriptions (or refractions in my lingo) for the two eyes. Usually one eye requires a much stronger lens to correct vision than the other. Again, if this is not discovered and corrected at an early age, amblyopia will develop.
How do we treat amblyopia? The most effective method of improving the bad eye is to patch the good eye. The more it is used, the better the visual acuity will become. However, this technique is only of value when the underlying cause is treated. When the eye muscle imbalance is the cause, surgery to align the eyes is often the best course. In other cases, eyeglass correction is the most effective way to go. Many times, eye exercises and visual training (called orthoptics) are essential as well. Orthoptics teaches the eyes to work with each other.
What if amblyopia is untreated? The patient will be using one eye for the vast majority of their visual tasks, and will continue to suppress images from the other eye. Now, this may surprise you: in their day-to-day activities, there is little or no detriment. People often think there is no depth perception with one eye. This is not true. The brain uses a phenomenon called parallax, in which more distant objects appear to move less than closer objects. (Try covering your eye and looking at near and distant objects while moving your head.) Binocular depth perception, or stereopsis, is only significant within about 16 inches. I don't consider the inability to enjoy 3-D movies to be a major handicap (this requires steropsis). However, very close tasks as well as the use of binocular instruments would be an obvious problem. The greatest potential problem would arise if the patient ever lost sight in the good eye. The most important reason we have two eyes is to have a spare, and this alone is more than enough reason to treat amblyopia proactively.
Dr. David Eilbert, Optometrist
North Fork Optical Center
PO Box 1419
Mattituck Center, Main Rd
Mattituck, New York 11952