This is part 7 of the vision therapy series and will focus on vergence insufficiencies—Convergence Insufficiency and Divergence Insufficiency.
If you have not read the previous portions of the vision therapy series it is highly advisable that you check them out first as they cover basics not explained here.
If you will recall, convergence insufficiency is the inability for one or both eye(s) to converge appropriately to a given target.
This means the eyes do not like to move inward toward the nose and is problematic when trying to look at a near target.
In some circumstances, individuals with convergence insufficiency will also suffer from high exophoria or exotropia, meaning the eyes like to sit in an outward position.
Patients with convergence insufficiency will often complain of eye strain when looking at near objects (reading, writing, computer work, etc.), headaches, double vision at near, eye fatigue, and sometimes avoidance of near tasks all together.
In many cases, individuals with convergence insufficiency will notice that their symptoms worsen as they get tired or after attempting to do near tasks for a prolonged period of time.
Individuals with convergence insufficiency often have a low AC/A ratio and a low near point of convergence.
Therapy will aim at increasing positive fusional vergence ranges to help strengthen the fusional vergence system and make near tasks more tolerable.
Divergence insufficiency is the opposite problem of convergence insufficiency. In divergence insufficiency, one or both eye(s) are unable to diverge appropriately to a given target.
This means the eyes do not like to move outward toward the ears and is problematic when trying to view a distant target.
In some circumstances, individuals with divergence insufficiency will also suffer from high esophoria or esotropia, meaning the eyes like to sit in an inward position.
Patients with divergence insufficiency may complain of intermittent double vision (diplopia) when looking at something in the distance, frequent “wandering” of the eyes, headaches, motion sickness, and eye fatigue that comes and goes and can vary from day-to-day depending on how tired the individual is.
Individuals with divergence insufficiency often have a low AC/A ratio with a high near point of convergence.
Therapy will aim at increasing negative fusional vergence ranges to strengthen the fusional vergence system and help make average daily distance tasks more manageable with less frequent symptoms.
The exercises for divergence and convergence insufficiencies are very similar in structure, but fine-tuned to work on strengthening either negative fusional vergence or positive fusional vergence.
To begin therapy, many therapists will focus first on normalizing the accommodative system. In many individuals with vergence problems, accommodation will be worked on first to ensure it is equal between the two eyes.
Normalizing accommodation is important because the accommodative and vergence systems are tightly correlated. In fact, when the vergence system is struggling, the accommodative system often picks up the slack and becomes overworked, which can also play a role in some of the symptoms of eye strain, fatigue, and headaches.
Vision therapy exercises aimed at strengthening and normalizing accommodation can be found in Part 4 of this series.
From there, basic vergence exercises will be added in. Some of the most common introductory exercises for vergence insufficiencies include Brock String, Barrel Cards, Stick-In-Straw, and Lifesaver cards.
These exercises were discussed in detail in Part 6 of this series.
More advanced exercises for vergence insufficiencies include Vectograms, Tranaglyphs, and Stereoscopes.
A Vectogram is a tool which utilizes specialized imaging along with the fusional vergence system to create a 3-D object.
The patient places two transparent cards into the vectogram system. Each card contains a partial view of the complete image.
The patient will wear a pair of polarizing glasses (similar to what is worn in modern 3-D movies) to create depth within the image (stereopsis).
Between the polarizing glasses and the patient’s fusional vergence system, he or she will be asked to either converge or diverge to fuse the two partial images into one 3-D image.
If the patient is only using one eye—the image will not be formed correctly. Therefore, it forces the patient to use both eyes properly to see the image.
Vectograms use an approach called the SILO affect to work on increasing divergence or convergence ranges.
To increase convergence, the two cards will be pushed inward, making the image smaller (Small In for the SI in SILO).
To increase divergence, the two cards will be pulled outward, making the image larger (Large Out for the LO in SILO).
Increasing or decreasing the distance between the two cards and asking the patient to converge or diverge will work on the focusing system and expand the eyes’ abilities to converge or diverge—hence helping with an insufficiency problem.
Therapists can make this activity more advanced by increasing the amount the cards are moved (i.e. changing the target) or by using multiple vectograms and making the patient look from one to another and quickly adapting the vergence system appropriately to each target (this is called “jumping” vectograms).
Tranaglyphs are similar to vectograms, however instead of there being two different transparent image cards that need to be fused into one, there is one image card with a stereo image already pre-made onto the card.
This means that in a vectogram the amount of convergence or divergence needed to fuse the image depends solely upon how far apart the two cards are held, i.e. is “variable”. In a Tranaglyph, the amount of convergence or divergence needed to fuse the image is pre-determined, i.e. is “fixed”.
If you can remember the old newspaper comics that had a green image with a red image overlying it that would turn into a 3-D image if you put on red/blue 3-D glasses—tranaglyphs are made similarly.
In this exercise, the patient is asked to wear a pair of red/green glasses—one eye sees red and the other eye sees green.
When looking at the image on the card, patients will be required to use their fusional vergence system to fuse the red and green images into one 3-D image.
Different cards with different images will require different amounts of negative and positive fusional vergence.
Patients will be able to work on fusing the images of one card and move on to a more difficult card as their fusional vergence ranges expand.
A stereoscope is a tool used to help expand fusional vergence ranges once a baseline has been established.
The mirror stereoscope is one of the most commonly used types. It involves a specialty optical system and a uniquely created image book. The tool itself can be calibrated to work on increasing divergence or convergence, depending on what it is set to.
The patient looks into the stereoscope to see two images—the right eye will see one image and the left eye will see the other. For example, the right eye might see a lion while the left eye sees a cage.
In this example, the patient will be asked to either relax his or her eyes to put the lion in the cage (diverge) or stimulate his or her eyes to put the lion in the cage (converge).
Once the two images have been combined into one, the patient will be asked to hold the image in place for 10-15 seconds.
Once this can be repeated multiple times, the amount of required divergence or convergence can be increased, thus expanding the vergence range and strengthening the problem area.
Like the previously mentioned exercises, stereoscope exercises can also be made more complex as therapy advances.