Strabismus & Amblyopia


By: Alan P Pearson OD MEd PhD FCOVD
©2017 Vision Clinics of Development & Learning


Strabismus means that the two eyes are not aligned or coordinated together as a team.  One eye can be turned inward, outward, or up/down relative to the other eye.  The dominant eye is usually the eye that is fixated or pointed directly at the object of regard, whereas the strabismic eye is the eye that is turned and uncoordinated with the dominant eye.   With good binocular alignment of the two eyes, the brain can fuse the images together for the experience of single binocular vision, usually with a 3D depth experience, whereas with strabismus the experience may be double, or the brain suppresses (ignores) some vision information from the turned eye to avoid the confusion of double vision.

Binocular vision involves precise fine-motor alignment of the eye muscles so that the line of sight of each eye points at the same object. With precise alignment the brain can then fuse the images from each eye into a single image. Binocular vision allows us to appreciate 3D depth. When the two eyes are not aligned, we may experience double vision.

binocular vision

Outward eye turns are called exotropia or divergent strabimus.

Inward eye turns are called esotropia or convergent strabismus.

Up/Down eye turns are called hypotropia or hypertropia.

Amblyopia is often a result of strabismus, but it can also be the result of other eye conditions such as anisometropia (a large difference in optical power between the two eyes).   Amblyopia is commonly called "lazy eye."   The public often confuses "lazy eye" with strabismus or eye turns, but the term actually means something different than an eye turn.   Amblyopia is the condition where an eye cannot see sharp and clear, even if the eye is looking through the best eyeglass lens.  Amblyopia is when the brain has not yet developed or learned how to see clearly.  In amblyopia there isn't anything wrong with the eyeball or the eye optics, but when the visual signals from the eyes get back to the brain, the brain has not developed the neural connections to make good use of the signals, and thus the person has poor discrimination skills with the amblyopic eye.

DIAGNOSING STRABISMUS & AMBLYOPIA

Regular routine eye exams emphasize eye health and eye optics but usually include some tests that can identify the presence of strabismus or amblyopia. In order to function well, the eyes need  to be healthy and any need for eyeglasses taken care of first.  With healthy eyes and necessary eyeglasses, a Functional Vision Evaluation will take an in-depth assessment of binocular vision, eye focusing, and eye tracking in the context of the visual challenges the individual faces in development, school, work, or recreation. A Functional Vision Evaluation looks at abilities to maintain single and clear vision over time when involved in dynamic tasks.  The extent of a strabismus or amblyopia can be measured and the prognosis for treatment can be determined. 

Only an eye doctor (Optometrist or Ophthalmologist) has the tools to perform a Functional Vision Evaluation and diagnose strabismus or amblyopia. Not all eye doctors have the interest or training, however, to perform a Functional Vision Evaluation. The following directories can be helpful in locating a doctor who can perform a Functional Vision Evaluation:

College of Optometrists in Vision Development:  www.covd.org

Optometric Extension Program Foundation:  www.oepf.org

Neuro-Optometric Rehabilitation Association:  www.nora.cc

TREATMENT OF STRABISMUS & AMBLYOPIA

When amblyopia is present, it is important to first take care of any optical problems that could be the cause or contributor to the amblyopia.  This means learning to wear eyeglasses, even if one can see 20/20 in the better eye without eyeglasses.   If there is a big optical imbalance between the two eyes, then it may be impossible to get both eyes focused at one time, therefore, the individual only will focus enough to clear the better eye, leaving the other eye unfocused and less developed.  The eyeglasses can balance the optics so both eyes are in focus at the same time, and this provides the best opportunity for the equal development of visual abilities between the two eyes.  

When there is a difference in eye optics between the two eyes, there is another issue to consider.  While standard eyeglass lenses can balance each eye's optical focus, the images that result maybe different sizes.  Imagine if you have two projectors shining the same picture on a screen.  If the zoom is exactly equal between the projectors then the pictures will perfectly overlap.  But if one projector is zoomed in a bit, and the other projector is zoomed out a bit, then the images are two different sizes and you will not be able to overlap the two pictures.   Conventional, or standard eyeglass lenses often have this problem of different zoom between the two eyes even though they correct the focus in each eye.  This makes it much more difficult for the brain to fuse the two images together into one perception.   To fix this problem, specialty lenses that correct both the focus and the size of the images are necessary.  At Vision Clinics of Development & Learning we are able to prescribe and deliver the Shaw Lens to address this need.

The next most common approach to treatment of amblyopia is using an eye patch.  For several hours a day, the better eye is patched forcing the individual to use and develop vision in the amblyopic eye so it is more equal with the dominant eye.  While eye patching can be effective in some situations, it does have limitations.  For example, when wearing the eye patch on the better eye, the amblyopic eye is forced to improve, but as soon as the patch is taken off most individual simply revert to using the dominant eye.   Even if you patch several hours a day, the rest of the day is spent practicing monocular, one eyed viewing with the dominant eye.  Eye patching alone doesn't usually result in any development of two eye coordination.  If strabismus (eye turn) is the cause for the amblyopia then the eye patching will not resolve or treat the strabismus.

An eye patch is kind of all or nothing, but it doesn't have to be that way.  There are many other options available that only partially block vision in the better eye, and these approaches allow for and encourage the simultaneous development of binocular two eye coordination.   Only an eye doctor can evaluate, prescribe, and monitor the appropriate type of partial occlusion.

A common approach for strabismus treatment is surgery.  In strabismus surgery some eye muscles are removed, the eye ball twisted slightly and then the muscles are sewn back on.  Notice that this is just a physical restructuring of the eyeball and muscles.  Such a physical restructuring doesn't have any immediate impact on the ability of an amblyopic eye to see more clearly, and it doesn't have any immediate resolution of images to be fused into one perception for depth awareness.  Binocular vision is a skill that takes time to develop.   Therefore, while strabismus surgery can often make the eyes look straight cosmetically, the individual might not have any better functional binocular vision.  Sometimes, the individual is very adapted to the position of the eyes being in their turned positions so after a strabismus surgery they just learn to put their eyes back in the same turned position they were to start.

At Vision Clinics of Development & Learning we advocate more active vision therapy for the treatment of strabismus and amblyopia.   First, any eye optics issues need to be taken care of with eyeglasses or contact lenses.  Second, if there is a need for the specialty lenses that correct the optical zoom in addition to optical focus we will recommend them.  Third, if there is amblyopia we try to improve balance between the eyes with partial occlusion if possible.  Then, we assess where the patient is on a binocular vision continuum from totally monocular (like the patient is missing one eye) and high functioning binocular vision (average to above average depth perception and two eye coordination skills).   From this starting point we design a treatment plan that will move the patient along this continuum towards better two eye coordination and depth perception.  In Strabismus and ambloypia treatment there has to be high motivation in the patient and/or family support as it is not possible to do everything in the clinic.  Consistent stimulation and practice of prescribed home activities is essential for good success. 

NOTES

There are many other visual dysfunctions that can occur simultaneously with strabismus and amblyopia   For example, reflex problems, visual-vestibular problems, fixation and orientation problems, rhythm and timing problems, body knowledge/coordination problems, and visual perception and processing problems.  Some of these problems will need to be addressed before strabismus and amblyopia treatment is appropriate. Some of the problems can be addressed simultaneously with the strabismus and amblyopia treatment.  The eye doctor will determine the best strategy, sequence, and emphasis in the treatment plan. 


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