This is part 6 of the vision therapy series and will focus on phorias, tropias, and fusional vergence dysfunction disorders and their therapy regimens.
If you have not yet read the previous portions of the vision therapy series we highly recommend you review that article first as some things discussed in that article will not be rediscussed here.
Part 1: A Basic Introduction to Vision Therapy (Part 1 of the VT Series)
Part 2: The Visual Pathway (Part 2 of the VT Series)
Part 3: Eye Accommodation Disorders
Part 4: 7 Eye Exercises for Accomodative Disorders (Part 4 of the VT series)
Part 5: Vergence Disorders
What is a Phoria?
A phoria is a misalignment of the eyes so that their natural resting point is not perfectly aligned. It is only seen when fusion is broken—i.e. one eye is covered or when the two eyes are looking at different targets (accomplished via prism lenses, red/green glasses, or Maddox rod).
When the two eyes are not aligned properly, we see double. To compensate for small variations in alignment, the brain has the fusional vergence system in which positive and negative fusional vergence may be used to compensate for a phoria and keep an image single and clear, even if the eyes are not perfectly aligned.
Phorias are often overcome through the fusional vergence system and thus may go unnoticed. These situations are considered to be a “compensated phoria” as the brain is able to overcome the eyes’ misalignment and see a target as clear and single.
When the fusional vergence system is not strong enough to overcome a phoria, a person may begin to see double images and experience eye strain and fatigue when trying to overcome the phoria. These situations are called “decompensated phorias”.
Decompensated phorias occur when the phoria is abnormally high or the fusional vergence system is too weak. This can change over time—a once compensated phoria can become an decompensated phoria. This situation often occurs as we tire, after spending extended periods of time doing near work, and as we age.
The natural posture we all aim to have is called “orthophoria”. This occurs when the eyes are perfectly aligned–equally and accurately converging and diverging to keep images at a variety of distances clear and single.
Esophoria occurs when the eyes’ natural posture is to sit inward, or point slightly toward the nose.
Exophoria occurs when the eyes’ natural posture is to sit outward, or point slight toward the ears.
Vision therapy to alleviate the symptoms of a phoria focuses on expanding the fusional vergence system.
If a person has a high esophoria, he or she will need to work on improving his or her negative fusional vergence range.
If a person has a high exophoria, he or she will need to work on improving his or her positive fusional vergence range.
What is a Tropia?
Unlike phorias, tropias occur during binocular conditions—i.e. when the two eyes should be working simultaneously to view a target.
Another name for a tropia is strabismus or “eye turn”. This occurs when one, or both eyes, do not align properly to view a central target.
Just like in phorias, an eso-tropia means the eye(s) is sitting inward, an exo-tropia means the eye(s) is sitting outward, a hyper-tropia means the eye(s) is sitting upward, and a hypo-tropia means the eye(s) is sitting downward.
Tropias can be tricky because they can present in a handful of different ways.
A tropia can be constant or intermittent.
In constant circumstances, the eye always sits in a deviated position (outward, inward, up, or down). This eye will only move to a normal position to pick up fixation when the opposing eye is covered.
In intermittent circumstances, the eye sometimes sits in its deviated position, but will also sit in the normal position on occasion. Patients with intermittent tropias will often notice that their eye “drifts” from center into its tropic posture.
A tropia can also be unilateral, bilateral, or alternating.
A unilateral tropia would be a tropia that only occurred in one eye—for example a left exotropia would mean that the left eye sits in an outward direction, either constantly or intermittently.
A constant bilateral tropia would be a tropia that occurs in both eyes all the time—this circumstance is rare and would likely only be seen in circumstances of extraocular muscle restriction or paralysis.
An alternative tropia would be a tropia that changes eyes—the right eye might be troping one day and the left eye might be troping the next. An example would be an alternating exotropia—sometimes the right eye sits in an outward position while the left eye is centered and has fixation, and other times the left eye sits in an outward position while the right eye is centered and has fixation.
Tropias are a bit more complicated than phorias. Intermittent and alternating tropias can sometimes be overcome with vision therapies. Constant tropias may benefit from vision therapy but are not quite as responsive to therapy as they tend to be more deeper set.
In some circumstances, tropias cannot be overcome with vision therapy and may require corrective surgery. This is easier when performed prior to age 7 when the visual system is still developing.
Tropia vision therapy focuses on training the extraocular muscles—depending on the degree of the tropia. In some cases, the processes of converging and diverging only need some fine-tuning that can easily be accomplished via therapy. Other times, in larger tropias, the objective of therapy is simply to get gross, basic motor movements of the tropic eye.
What is Fusional Vergence Dysfunction?
Fusional vergence dysfunction occurs when the eyes are well aligned (orthophoria), but negative and positive fusional vergence ranges are decreased, resulting in fatigue, eyestrain, headaches, and lack of concentration.
In these circumstances, the eyes just feel tired all the time! This is because the eyes are constantly working to fuse images, there’s no buffer from the fusional vergence system.
Individuals with fusional vergence dysfunction have both reduced positive and negative fusional vergence ranges. The only treatment option for fusional vergence dysfunction is vision therapy centered around increasing the amplitude of both vergence ranges.
Vision Therapy Exercises for Phorias, Tropias, and Fusional Vergence Dysfunction
Fusional vergence and fine-tuning extraocular muscle movements go together. Therefore, exercises targeted to help phorias, tropias, and fusional vergence dysfunction often go hand-in-hand and are just slightly modified to focus on the problem at hand.
Basic exophorias will focus on activities to improve positive fusional vergence, basic esophorias will focus on activities to improve negative fusional vergence, and fusional vergence dysfunction patients will work on both!
Exotropes will focus on strengthening both eyes’ ability to accurately converge together as a set, whereas esotropes will focus on strengthening both eyes’ ability to accurately diverge together as a set.
Stick-In-Straw Exercise
This exercise is an introductory exercise used to gain awareness of what it feels like to converge.
The patient holds a stick in one hand while the therapist holds a straw. The patient is then asked to insert the stick into the straw using one swift motion.
This activity requires binocular vision and depth perception to accurately get the stick into the straw. Therefore, it will engage both eyes’ muscles and fusional vergence system.
Brock String Activity
This activity can be used to work on both negative and positive fusional vergence as well as fine tuning convergence and divergence.
It involves a string with colored beads on it. The string is attached at one end to a table or door. The other end of the string is held up to the tip of the patient’s nose.
The patient is asked to look at one of the colored beads. He or she should report that the bead is single and that the string is forming a X shape from the bead.
Looking at beads farther away (closer to the table or door) works on negative fusional vergence whereas looking at the beads closer to the patient’s nose works on positive fusional vergence.
Over time, your therapist will make this activity more difficult by asking you to “walk” your point of focus smoothly from one bead to another.
Barrel Card Activity
This activity is used to work both on fusional vergence as well as helping to fine tune the eyes’ ability to converge.
A double-sided card with colored barrels is held by the patient against his or her nose. One side has green barrels, the other side has red barrels. The right eye will see one colored barrel whereas the left eye will see the other colored barrels—depending on which way the card is held.
The therapist will ask the patient to focus on the farthest set of barrels until they see a “mixture” of red and green barrels forming an “X”.
If he or she reports only seeing red or green barrels, it means they are suppressing, or “closing off” one eye and only seeing monocularly.
The therapist will offer suggestions and coaching to help overcome this suppression, strengthening the eyes.
Lifesaver Cards Activity
This activity is used to help strengthen a patient’s convergence/positive fusional vergence and divergence/negative fusional vergence.
It involves a card with green and red colored circles on it. The card is held 40 cm from the patient. The objective of the task is to focus on an area between the red and green circle to create a fused, 3-D third circle.
The only way this task can be accomplished is through using the appropriate amount of both positive and negative fusional vergence.
The amount of fusional vergence can be altered depending on the difficulty of the card. As fusional vergence is strengthened, the therapist can make the activity more difficult to continue to improve the range of which the patient can complete the activity.
Aperature Rule Activity
In this activity, a special instrument called an aperature is used along with specially calibrated picture cards.
The patient is asked to rest their nose at the opening of the instrument and look at the picture card. Different picture cards will require different amounts of positive and negative fusional vergence to fuse the image of the picture card.
The therapist will help coach the patient to fuse the image, creating a single, clear, 3-D image.
Over time, the picture card is changed, altering the requirement to keep it clear, single and 3-D, and thus train the fusional vergence system.
Eccentric Circles Activity
This activity is similar to the lifesaver card activity, except it focuses on improving fusional vergence amplitudes in free space.
Two sets of circles on a card are held in front of the patient. Holding the card closer will work on divergence (negative fusional vergence) whereas holding the card farther away will work on convergence (positive fusional vergence).
Using the vergence system, the patient is asked to focus on fusing the two circles into a third, 3-D circle. Once the 3-D circle is created, the patient is asked to ignore the 2 peripheral circles and focus on the 3-D image he or she has created for 10-15 seconds.
This activity can be made more difficult by increasing the time of focus or by changing the working distance of the target.