Vergence Disorders (VT Part 5)

by Aug 30, 2021

This is part 5 of the vision therapy series and will focus on different disorders of the vergence system. To best understand the vergence system, it is best to dive in a little deeper into understanding how the eye muscles work and what vergence really is.

It may be helpful to review part 1 and part 2 of the VT series as some topics discussed in those articles will not be reviewed here.


Understanding the Extraocular Muscles

There are 6 extraocular muscles for each eye with the main responsibilities of upward/downward movement, inward/outward movement, and intorsion/extorsion rotation to keep images clear and focused even when you are moving about and looking in various directions.

The ultimate goal of vision is to keep the object of interest focused on the macula of the retina. Therefore, the extraocular muscles are constantly being fine-tuned and making small contractions or relaxations to keep your vision crisp and centered.

Upward movement of the eye is called elevation whereas downward movement of the eye is called depression.

Inward movement of the eyes (movement toward the nose) is called adduction whereas outward movement of the eyes (movement toward the ears) is called abduction.

Intorsion and extorsion entail the eyeball itself rotating slightly to the left or right to keep images in focus.

Each of the extraocular muscles has a primary function as well as secondary functions. Therefore, they all must work together and be properly aligned to maintain clear and singular vision. If even just one extraocular muscle out of the twelve total muscles is slightly mis-calibrated—it can create double vision, eye strain, and fatigue of the visual system.

Here is a list of each of the six extraocular muscles and their primary and secondary functions:

  • Superior Rectus: Elevation, adduction, and intorsion of the eye.
  • Inferior Rectus: Depression, adduction, and extorsion of the eye.
  • Lateral Rectus: Abduction of the eye.
  • Medial Rectus: Adduction of the eye.
  • Superior Oblique: Intorsion, depression, and abduction of the eye.
  • Inferior Oblique: Extorsion, elevation, and abduction of the eye.


What is Vergence?

Since we have two eyes, they must work together to view an object and create binocular vision.

When we look at the combined visual input from the two eyes is where things can get a little complicated.

The extraocular muscles are mirror images between the two eyes. This means to look right or left, different muscles are used with each eye.

When the same muscles are used by both eyes, we get a vergence movement. Vergence occurs when both eyes look inward (convergence) or both eyes look outward (divergence).

Convergence is accomplished through contraction of both medial rectus muscles. Divergence, on the other hand, is accomplished through contraction of both lateral rectus muscles.

Our eyes naturally converge when looking at a target up close (i.e. a near target) and diverge when looking at a target far away.

How much the eyes converge or diverge really depends on what and where we are looking. It is a very fine-tuned system.

When the eyes do not accurately align when looking at a given target, we see double. This is because the two eyes are seeing the image individually and not fusing the overall picture to see a single image.

This sounds like an almost impossible task—to have each eye calibrated just perfectly to allow us to see single and clearly at all distances. Thankfully—our eyes get a little help through a function called the fusional vergence system.

Fusional vergence gives our eyes a little bit of leeway in regards to the accuracy of the muscle movements. If the eyes are converged or diverged “close enough” to accurately look at the target at hand, the brain can use the fusional vergence system to “make up the difference” and fuse the images.

Just like we have the ability to converge and diverge, the fusional vergence system can move in one of two ways—positively (positive fusional vergence) or negatively (negative fusional vergence).

The greater the fusional vergence system ranges, the less precise our eye movements need to be.

Generally speaking, positive fusional vergence helps to fuse an image when looking at a target up close (converging), whereas negative fusional vergence helps to fuse an image when looking far away (diverging).

In other words, someone whose eyes might be inset (converging too much) may still be able to see a single clear image if his negative fusional vergence system is strong enough to pull the image outward to fuse it.

If that last sentence confused you, do not worry, we’ll chat a little more about this concept in the next article in regards to vision therapy for vergence disorders.


What are Vergence Problems?

There are several different major vergence disorders, here are some of the most common types:

Divergence Insufficiency: The inability for one or both eye(s) to divergence appropriately to a given target.

Convergence Insufficiency: In inability for one or both eye(s) to converge appropriately to a given target.

Basic Esophoria: When the eye(s) natural posture is to sit inwards (converge) more than average when no target is given.

Basic Exophoria: When the eye(s) natural posture is to sit outward (diverge) more than average when no target is given.

Divergence Excess: When one or both eye(s) diverge too much when looking at a given target.

Convergence Excess: When on or both eye(s) converge too much when looking at a given target.

Fusional Vergence Dysfunction: When a person’s fusional vergence system is lower than normal, resulting in the inability to fuse images appropriately and the creation of double vision.

Exotropias (strabismus): When one or more eye muscles does not function appropriately, resulting in the eye(s) drifting outward.

Esotropia (strabismus): When one or more eye muscles does not function appropriately, resulting in the eye(s) drifting inward.

As you can see, in comparison to accommodative disorders, vergence disorders are a bit more complex.

Having said this, we’ll be breaking down these disorders into further detail and discussing examples of vision therapy exercises into more than just one article.

The next article (VT Series #6) will cover phorias, tropias, and fusional vergence dysfunction. The following (VT Series #7) will cover convergence and divergence insufficiencies. The third and last vergence disorder article (VT Series #8) will cover convergence and divergence excess.

In the meantime, we’ll discuss the different testing aspects used to help determine which vergence disorder an individual has.


Testing for Vergence Disorders

The following tests are examples of more common tests used to determine binocular vision disorders. This is not all-inclusive, but shows a good example of what to expect during a binocular vision evaluation.

To begin any vergence testing your doctor will want to look at the posture of the eyes to determine if it is a weak muscle problem or something more complex.

Your doctor may conduct a cover test to look at the natural postures of the eyes to determine if there is a phoria or tropia. This test is done both when looking at a distance target as well as when looking at a near target.

Your doctor will cover one eye as you focus on the target of choice and watch for small eye movements. He or she will then cover the other eye, and repeat this process several times. The amount of movement observed can be measured with a tool called a prism bar.

Other good tests that can be used to determine the natural resting position of the eyes include the Von Graefe phoria test and Maddox rod test.

In each of these tests, the eyes are dissociated, meaning you will see two separate images.

In Von Graefe testing, special lenses will be placed in front of one of the eyes and you will be asked to state when the two images you are seeing are lined up either vertically or horizontally.

Your doctor will then be able to measure and quantify your eye’s natural resting point.

In Maddox Rod testing you will hold a red lens over one eye and be asked to stare at a light source. The light source will appear white through the uncovered eye, and the covered eye will see a red line.

Depending on your eye’s posture, the red line and white light may or may not align. Depending on the location of the red line in reference to the white light, your doctor will be able to measure and quantify your eye’s natural resting position.

Another test that is important for vergence disorder diagnosis is called the AC/A ratio.

AC/A ratio is the amount of accommodative convergence for every diopter of accommodation.

AC/A ratio is important because the accommodation system and the vergence system are linked. The more you accommodate, the more the eyes converge.

A normal AC/A ratio is between 3-5/1.

A low AC/A ratio could indicate divergence insufficiency or convergence insufficiency.

A high AC/A ratio could indicate convergence excess or divergence excess.

AC/A ratio is determined in one of two ways. The first method is called the gradient AC/A ratio. It involves comparing the results from your Von Graefe Phoria Test to your results of the Von Graefe Phoria test when looking through a lens (+1.00 or -1.00).

The second method of determining AC/A ratio is called the calculated AC/A ratio. In this test, your doctor will need to use an equation and your results from the distance and far Von Graefe phoria test to determine your AC/A ratio.

In addition to phoria testing and AC/A ratio determination, the last major test used to diagnosis vergence disorders is through determining a person’s fusional vergence ranges.

There are many different ways to determine fusional vergence ranges. One of the more common ways is through manipulating special lenses called prism lenses. You will be asked to focus on a target. Prism lenses will be added in front of one of your eyes.

You will be asked to focus on the target and keep the image of it single and clear. Whenever the image breaks into two separate images, you tell your doctor and he or she records the number. This is done in all directions to determine your positive and negative fusional vergence ranges in both the horizontal and vertical meridians.

Your doctor can then take all this information and analyze it to determine what the underlying vergence disorder is, and then assign the appropriate vision therapy exercises.


Our eye doctors at Wilmington Family Eye Care in Wilmington, DE excel in prescription of glasses, contact lenses and the diagnosis of a variety of eye diseases. Call our optometrists at 302-299-1286 or schedule an eye exam appointment online if you would like to learn more about vergence disorders. Our eye doctors, Drs. Daniel Baruffi, Joseph Goldberg, Karen Darrell and Patricia Jones provide the highest quality optometry services and eye exams in Wilmington, Delaware and its surrounding areas.


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