Do you have trouble reading on your phone?

Many people have difficulty reading, using their phone, or using the computer. It might be convergence insufficiency!

Many people have difficulty reading, using their phone, or using the computer. If you do, you might have convergence insufficiency! 

More than 30% of people suffer from one or more binocular vision issues. So, how do you know if you do too? The answer is simple, ask your doctor to perform a cover test

What is a cover test?

A cover test can diagnose several types of binocular vision disorders, including strabismus, convergence insufficiency, and esotropia.

It is a quite simple test that doesn't require any expensive medical equipment.

The test involves having the patient focus on a near object. A cover is placed over an eye for a short moment then removed while observing both eyes for movement. The misaligned eye will deviate inwards or outwards. The process is repeated on both eyes and again with the patient focusing on a distant object.

Factors to be specificed during a cover test:

FACTOR POSSIBILITIES
Type of deviation Phoria or Tropia
Laterality Unilateral or Alternating
Magnitude In prism diopters (unit of angle)
Direction Eso, Exo, Hypo, Encyclo, Excyclo, or combination
Comitancy Comitant or Incomitatnt
Refractive correction  In diopters
Test distance In meters

Tropia vs Phoria

If you have a tropia, it means that your eyes are misaligned when both eyes are uncovered, and you have strabismus.

If you have a phoria, it means that your eyes are misaligned when one of the eyes is covered, and you have a vergence disorder. A phoria is also sometimes called a latent deviation.

Unilateral vs Bilateral

The unilateral cover test is performed by having the patient focus on an object then covering the fixating eye and observing the movement of the other eye. If the eye was exotropic, covering the fixating eye will cause an inwards movement; and esotropic if covering the fixating eye will cause an outwards movement. 

The alternating cover test, or cross cover test is used to detect total deviation (tropia + phoria).

Comitant vs Incomitant

If the onset of the strabismus was sudden, the doctor may also test for comitancy.

If it is comitant, then the magnitude of the tropia is the same in every gaze direction. Usually 9 gaze directions are tested. Comitant misalignment is characteristically found in patients with congenital or early-onset strabismus. 

If it is non-comitant then the magnitude changes based on the gaze direction.

Possible results of an objective cover test

A cover test will detect if your eyes are misaligned at near or far, if they are turned inward or outward, and if you have a tropia or a phoria.

RESULT DISTANCE DIRECTION TYPE
NEAR EXOTROPIA NEAR EXO TROPIC
DISTANCE EXOTROPIA DISTANCE EXO TROPIC
NEAR ESOTROPIA NEAR ESO TROPIC
DISTANCE ESOTROPIA DISTANCE ESO TROPIC
CONVERGENCE INSUFFICIENCY NEAR
EXO PHORIC
CONVERGENCE EXCESS NEAR ESO PHORIC
DIVERGENCE INSUFFICIENCY DISTANCE ESO PHORIC
DIVERGENCE EXCESS DISTANCE EXO PHORIC

Possible results of a subjective cover test

A subjective cover test is sometimes done to determine if you have anomalous retinal correspondance or ARC. ARC is when your weak eye learns to use a non-foveal (off center) part of your retina instead of the fovea (central part with the best vision). Patients with ARC can be more difficult to treat.

RESULT DISTANCE DIRECTION TYPE
NEAR EXOTROPIA NEAR EXO TROPIC +Anomalous Retinal Correspondence
DISTANCE EXOTROPIA DISTANCE EXO TROPIC +Anomalous Retinal Correspondence
NEAR ESOTROPIA NEAR ESO TROPIC +Anomalous Retinal Correspondence
DISTANCE ESOTROPIA DISTANCE ESO TROPIC +Anomalous Retinal Correspondence

I think I might have an eye problem, what should I do?

If you think you might have strabismus or a convergence disorder, you can find a doctor who can do a full binocular vision exam.

If you found this interesting, try this cool simulator by Faruk H. Orge, MD from the Case Western Reserve University School of Medicine:

Find a Provider phone reading convergence insufficiency cover test strabismus simulator

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