Uncompensated binocular issues could lead to the onset and progression of myopia.
With this Accommodation Flexibility test, we evaluate whether a patient can accommodate enough to see clearly at near, relax that accommodation enough to see clearly in the distance, and how quick and flexible that change in accommodation is.
To establish whether a patient has an accommodation spasm, reduced accommodation reserve or reduced accommodation facility.
If the patient has a reduced facility, additional tests like Positive and Negative Relative Accommodation, Lag of Accommodation and Accommodation Reserve should be done. If the problem is limited to just the facility, it will affect the patient’s ability to quickly and effectively copy from a board or overhead projector, into a book or notes on table.
With this test, we assess whether a suppression is present. When our binocular system is compromised, the brain has the ability to suppress all or parts of one eye to eliminate double vision.
A suppression could be permanent or interment, complete or partial. By dissociating the system, so that both eyes are open but not seeing the other eye’s image, we can evaluate which eye is suppressing.
Our test is called Binocular Acuity, as we measure the smallest target size that can be identified correctly, with both eyes open, but in the absence of fusion. With big, easy targets, one may easily miss smaller partial or interment suppressions. With this method, we get a little bit more information about the magnitude and persistence of the suppression.
When a suppression is present, it means that the visual system is compromised, and additional binocular vision testing is necessary.
Suppression could be due to a number of factors, which include amblyopia, strabismus, convergence insufficiency, anisometropia and/or aniseikonia.
Suppression tests can be utilised throughout a therapy treatment plan to assess how the patient is progressing in improving binocular vision and reducing suppression. With me and vision therapy, the patient can reduce their suppression significantly and even eliminate it.
Here we measure the smallest difference in depth that can be detected, and is measured in seconds of arc.
We need two eyes, working together with perfect coordination, to be able to perceive depth, or appreciate three dimensional vision.
Like with a suppression, reduced stereo acuity is a sign of underlying binocular issues. If you are suppressing, you will have reduced stereo. But you could also
have reduced stereo acuity in the absence of suppression, due to issues like anisometropia, phoria and/or reduced fusional ranges.
If a patient can not perceive any depth, there is a total break down of Binocular Vision. If Stereo acuity is reduced, the system is compromised. On the other hand, if a very small acuity is achievable, there is good coordination and teamwork between the two eyes.
The fusion compulsive reflex (which controls the extra- ocular muscles) strives to align our eyes so that images of the object we are looking at, will fall on corresponding renal points (even in the absence of binocularity).
When a phoria is present, this reflex is not accurate, and the visual system needs feedback from the other eye to guide alignment. The fusional reserves (convergence and divergence) then assist in achieving union, in order for the image to appear ‘single’.
If we dissociate the two eyes, this deviation becomes apparent, and we can measure it. To test a phoria, we measure the amount of prism needed to align the images (from the patient’s two eyes) .
When a phoria is present, the visual axes can still achieve union, but at the cost of some effort by the fusional reserves (convergence and divergence). If the fusional ranges can achieve this task without too much strain, the phoria can be regarded as compensated.
When a phoria is present, it is important to assess fixation disparity and fusional reserves to determine how well the phoria is compensated, and at what cost.
With a basic phoria (without fiction disparity or fusional reserve issues), prism is not considered the best form of acon. Instead alignment and vergence therapy is recommended.
If a phoria is not completely compensated, we can measure the uncompensated residual deviation, called the associated phoria.
In the presence of binocular feedback (like a fusion lock), the actual distance between the two images (seen so close together that they look single), is called the Fixation Disparity.
Basically the same as above, but the deviation between the two images are here measured in the amount of prism needed to align them (not the actual distance between them).
Fixation Disparity can easily be converted to Associated Phoria and visa versa.
When a phoria is present, but not fully compensated, an image could still appear ‘single’ when both eyes are open (with binocular feedback). Fusion is possible, even if there is still a small misalignment, as long as the misalignment is small enough to fall on corresponding retinal areas, known as Panum’s Areas of Fusion.
But if a Fixation Disparity or Associated Phoria is present, the visual system is really struggling. In addition to eye exercises, prism is recommended (depending on degree of the issue).
As already mentioned above, the fusional reserves (convergence and divergence), assist in maintaining binocular single vision. If a phoria is present, the fusional ranges assist with achieving fusion, to compensate the phoria.
With this test, we can measure, how much reserves we have left, after this compensation.
If we have reduced fusional reserves, even if we can achieve alignment, it is at a great cost. Our visual system has to work really hard to achieve single
vision, which could lead to fatigue, eye strain, and even the onset of visual problems like myopia.
The Accommodative Convergence to Accommodation Ratio is calculated using results from:
With this ratio we can calculate how many diopters of convergence is needed for every diopter of accommodation.
How is eye posture affected when accommodative demand changes.
A high AC/A ratio means that the convergence response is greater than the demand. This patient over converges (more than the actual demand).
A low AC/A ratio means that the convergence response is smaller than the demand. This patient under converges (less than the actual demand)