|Vision and EyeCare FAQ|
Section 6: Binocular Vision Problems6.1 Strabismus (Turned Eye)
A squint or strabismus is a failure of the two eyes to look at the same object thereby preventing binocular vision. Human binocular vision develops during the first few years of life. Interruption to the motor, sensory or central components, for example nerve or muscle defect, can lead to sensory or central defect. Causes of many squints are not fully understood although the majority are either a hereditary factor or a responsible defect.
Such factors causing squints include;
+ ocular abnormalities that prevent good central vision, eg congenital cataract.
+ paresis of one of the eye's muscle
+ brain damage, eg cerebral palsy
+ large refractive errors, eg accommodative esotropia
Refractive errors are an important causative factor to strabismus since it can prevent clear vision - impeding development of the sensory mechanism, and affecting the normal relationship between accommodation and convergence. When the eye's require vision at a near distance there both a movement of the two eyes (convergence) and change in focus (accommodation).
A child that is hypermetropic (long sighted) has to accommodate more than is normally required for distance and near vision. Because of the linkage between accommodation and convergence the excess accommodation causes an excess convergence of the two eyes. The result, in a young child whose visual system is still developing is what is called a convergent squint.
When binocular vision is disturbed double vision results. Young infants are able to suppress one of the images to one eye to remove the double vision that occurs at the onset of strabismus. The effect of the suppression of the image causes the vision in the "squinting" eye to fail to develop normally. This is called AMBLYOPIA. There are other visual abnormalities that can develop as a result of squint.
The treatment for squint depends on the cause. Some essential treatments include:
+ search for ocular defects
+ refraction and glasses to remove any refractive errors
+ patching or occlusion of the eyes to prevent amblyopia
+ surgical adjustments to the muscles of the eye.
These treatments aim to remove amblyopia, restore binocular vision and if necessary cosmetic corrections to the appearance of the turned eye.
6.2 Amblyopia ("Lazy Eye")
Amblyopia is defined by Schapero et al. as the condition of reduced visual acuity which cannot be corrected by refractive means and is not attributed to structural or pathological ocular anomalies.
Acuities of worse than 20/30 (6/9) are considered to meet the criteria of amblyopia according to Griffins reference on Binocular Anomalies.
There are a variety of classifications of amblyopia, in general the categories are organic or functional. Examples of organic amblyopia include;
+ nutritional, e.g poor diet in the case of alcoholism
+ toxic, e.g methyl alcohol poisoning or salicylate poisoning
+ congenital, e.g bilateral or unilateral central scotoma at birth.
Functional amblyopia also has three classifications;
+ hysterical, e.g psychogenic causes giving central visual field defect
+ refractive, e.g uncorrected isometropia resulting in poor visual acuity development
+ strabismic, e.g long standing suppression in cases of strabismus
Commonly used therapy for amblyopia is occlusion or lens therapy in the case of refractive. The patching is associated with general to increasing eye-hand coordinated tasks to stimulate development of the amblyopic eye.
6.3 Vision Therapy
Common problems that require vision therapy include
+ accommodation insufficiency
+ accommodation excess
+ convergence insufficiency
+ convergence excess
Treatment is often by a combination of either lenses or prisms with or without convergence training.
The next level of 'therapy' is the tracking exercises, eye-hand coordination and similar coordination tasks.
Details of the training is beyond the scope of this FAQ and the interested reader/patient is recommended to seek professional examination
6.3.2 Bates Method (Articles references)
Vision therapy, especially people claiming improvement of vision through 'holistic' medicine can often lead into a *very* heated debate. The techniques generally describe some form of eye excerise associated with relaxation technique to improve the 'perception' of letters/images. There is limited to virtually no statistical studies/results indicating the success or failure of these methods.
Often the central theme is to "to getting the eyes to shift more rapidly, to get you centre of focus to hit directly on the fovea, and to reduce tension in the eyes so that the above can be accomplished".
As described by one internet reader (firstname.lastname@example.org) the method for improved sight involves :
1) KEEP YOUR EYES MOVING. Your eyes have to be fast to catch all the
2) GET LOTS OF SUNLIGHT. Don't be afraid of the sun. If it's too bright
3) WEAR A PATCH. If you do this even for as little as fifteen minutes,
4) STRETCH YOUR NECK. If your neck is cramped, then so are your eyes.
© Grant Sayer , email: email@example.com
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