When you were young, your mother probably told you to stop crossing your eyes because if didn’t, they might get stuck that way. While there is no truth at all to this myth, it is possible that infants and young children can develop misaligned eyes, a condition that is called strabismus.
There are several types of strabismus. (See figure 1) If one eye turns inward it is called esotropia, while if an eye turns outward it is called exotropia. Strabismus can be constant, meaning that it is present at all distances and circumstances, or intermittent, which means it is present only some of the time. An example of intermittent esotropia would be if it was only seen during reading or fatigue. Sometimes, it is always the same eye that is misaligned, while in others the misalignment can alternate between the two eyes, and that is referred to as alternating.
The medical terms above are just meant to make communication between optometric physicians more clear and cause less confusion, however, the terms following will make it easier for parents to understand their child’s vision. Objects that are far away are called distant, or at distance, while books or reading materials, are near, nearpoint or at near. Thus, when an optometric physician refers to “distance vision” he or she is talking about things seen at least 20 feet away; “at near” or “near vision” refers to the vision for close-up, usually less than 16 inches from the eyes. There is no intent to confuse, but rather to make things more clear.
Parents cannot always tell which eye is turning and which isn’t, especially when the condition alternates or the misalignment is very small. For this reason, optometric physicians recommend that parents have their children’s eyes examined as early as possible, and at least by the age of three. This is especially important when there is a family history of any type of esotropia or exotropia.
Parents can be unnecessarily worried about these conditions because newborns may appear to have crossed eyes because the focusing and aiming systems are still undeveloped, and some infants have folds of skin near the nasal corners of their eyes which gives an illusion that the eyes are misaligned. These problems are only apparent, not real, and will diminish over time. In strabismus, however, these signs do not disappear and can become worse with age.
There are several different causes of strabismus, and the treatment will depend on diagnosing the underlying cause in each individual case. The subject here is concerning one particular type of strabismus, a special type called accommodative esotropia.
There are two main systems that must work together for the eyes to be properly aligned: convergence and accommodation.
The convergence system is responsible for aiming the two eyes at the same point in space. When looking at an object at distance, the eyes are usually almost parallel, looking straight ahead. When a person changes from looking at distance to a nearpoint object, the eyes must change their alignment from parallel to turn inward somewhat so there is no double vision. Each time the person looks from distance to near and back again, the eyes must change their convergence to see normally.
The accommodation system keeps the objects a person is looking at focused and clear. For a person to see an object clearly at distance, less focusing power in the eye is needed; when the person looks from distance to near, the muscles in the eyes must change to accommodate the change in the amount of focusing power required.
As noted above, these two systems must work together for optimal vision, and they are linked together in the brain, so that when the convergence increases, so does the accommodation, and vice-versa. In most cases, this results in quick and easy changes from distance to near for optimal vision.
Sometimes, however, there is a mismatch between the amount of accommodation and the amount of convergence, so that when the eyes accommodate for near, there is too much convergence. This results in double vision, fatigue, poor reading and other symptoms, and sometimes we see an intermittent esotropia, caused by this mismatch, which is referred to as accommodative esotropia.
This condition is usually found in people who have uncorrected farsightedness, which requires some amount of accommodation (focusing), even for objects at distance. That accommodation is present to provide clear distance vision, but when the person changes to a nearpoint regard, even more accommodation is required, which can result in too much convergence, which in turn leads to the esotropia.
Accommodation and convergence normally occur automatically, without any conscious thought or effort. Farsighted people who have no lens correction are always accommodating to compensate, to keep things as clear as possible.
Fortunately, treatment for this particular type of esotropia is relatively straightforward.
First: children should have a vision examination and evaluation before the age of three, or sooner if the parent notices an eye turn. Second: lenses to allow clear vision without accommodation, which reduces the amount of convergence, too. If the amount of esotropia is greater at the nearpoint, multifocal lenses are useful to achieve relaxation of accommodation at all distances. (Note: parents should not be dismayed by their child’s lens prescription being a multifocal; most children adapt to them without any difficulty at all.)
A lens prescription that relieves the accommodation and allows binocular vision must be worn full time, because when the lenses are removed, accommodation and its associated convergence will return.
Because most accommodative esotropia is only intermittent and either disappears or is at least largely diminished with a lens prescription, surgery is not usually a good option for treatment. If the eyes can work as a team at any distance at any time, binocular vision can then be strengthened to allow it to overcome the esotropia. The brain favors binocular vision.
Once the full lens prescription is in place, the best possible result is for the eyes to spontaneously rediscover their normal binocular vision and begin to work together immediately. However, if the esotropia has not been discovered early, it is possible that one eye might have developed amblyopia, which is a condition where the eye does not develop normal clear vision. In this case, more treatment will be necessary.
The goal of treatment for amblyopia is to strengthen the eye with decreased vision and allow it to partner with its fellow eye more easily. One way to help achieve this is with the use of a patch worn over the better eye, in order to force the other to work and begin to recognize clear vision.
Patching works to break up the adaptation the brain has already made, which saves the child from experiencing double vision. It is important to follow a routine of patching to get the best results. Formerly, patching was often prescribed for six or more hours per day, but more recent research has indicated that patching an eye for two hours a day is just as effective, and has the added benefit of being easier to achieve. Alternating the patches is also effective, again, to help the brain re-adapt itself to different conditions once a full lens prescription is being worn. Once vision in both eyes is roughly equal, the brain must then learn to “fuse” the two images into one.
Overall, accommodative esotropia, even in the presence of amblyopia, is considered to be one of the most easily treated types, because it is usually intermittent and the lens prescription itself is helpful in most cases. Once binocular vision has been re-established, long-term patching will probably not be required, and as the child gets older, contact lenses become another lens option.
