Cover Test

Binocularity is the degree to which both eyes can work together, turning in or out, using the extra ocular muscles, to fixate on an object at the same time fusing it into a single image and allowing the viewer to see the object in three dimensions. This is more difficult or less difficult depending on each eye’s natural resting point and the strength of the extra-ocular muscles. If testing indicates that a patient’s ability to achieve binocular vision is decreased the doctor may recommend treatment such as vision therapy or prism/PAL in specs.

The stereo testing which is completed during the workup is one way to check for binocularity. Another way is with the cover test which, unlike stereo testing, is completed at every exam. During the cover test, the patient will focus on a target; the doctor, with the occluder, will then cover alternating eyes and alternate covering and uncovering a single eye to determine the patient’s phoria/tropia (see below). The doctor will complete this test with the patient focusing on a target in the distance and again with the patient focusing on a target up close, as the findings may be different depending on how far away the fixation point is. The doctor will call out a series of numbers and letters (I.e. 6XP and 8XP, or 10 IAXT and 18 CLET) which quantify the phoria/tropia. The letters indicate the type of phoria/tropia, and the numbers indicate the amount of phoria/tropia. record these findings in the Workup tab in the boxes labeled Prism and Cover Test (numbers in the box labeled prism, letters on the Cover Test box on the right side, distance findings in the distance boxes, near findings in the near boxes).

Types of Phorias

If there is no phoria or tropia present, this is recorded as Ortho

The two types of Phoria are:

EP: Esophoria (eyes turn inward when covered or fatigued)

XP: Exophoria (eyes turn outward when covered or fatigued)

Types of Tropias and modifiers

There are four types of Tropias:

ET: Esotropia (eye turns inward when both eyes are uncovered)

XT: Exotropia (eye turns outward when both eyes are uncovered)

Hyper: Hypertropia (eye turns upward when both eyes are uncovered)

Hypo: Hypotropia (eye turns downward when both eyes are uncovered)

All tropias will have two modifiers:

I (Intermittent) or C (Constant)

and

R (Right eye), L (Left Eye), or A (Alternating Eye

Examples: a esotropia of the left eye that is always present would be recorded CLET (constant, left, esotropia) and an exotropia that alternates between eyes and is only present some of the time would be recorded IAXT (intermittant, alternating, exotropia)

Here are what some types of phorias and tropias look like during the cover test. Note that phorias and tropias appear similar; however, a phoria manifests when the eye is covered or fatigued whereas a tropia manifests when both eyes are uncovered.

The doctor may also test the Near Point of Convergence (NPC) by bringing a ruler or pen close to the patient’s nose and observing how well the eyes are able to turn inward to converge on the object. The NPC is how far away the object is when one eye breaks away and the patient is no longer able to fuse the object into a single image. The doctor will tell you a number to record in the box labeled NPC on the Binocularity tab (recorded in centimeters or inches depending on the doctor or record “nose” if the patient is able to converge to the tip of their nose).

Here is an example of what NPC testing could look like.

Other tests the doctor may use to measure binocularity include Prism Bar (Conv/Div), Von Graefe, and Maddox Rod. These findings are all recorded on the binocularity tab.

Convergence insufficiency and Convergence Excess

For patients with a significant phoria (esophoria or exophoria) and/or a decreased NPC and are symptomatic, the doctor may diagnose the patient with convergence disorder. For a patient with exophoria, this would be called convergence insufficiency. For a patient with esophoria, this would be called convergence excess. Please watch the following video on convergence insufficiency.

Amblyopia

Patients with a tropia, or eye turn, if left untreated, may develop a condition called amblyopia, or “lazy eye”. The eye turn itself is often referred to as a “lazy eye”; however, this is incorrect (an eye turn is called a tropia or strabismus). Amblyopia develops in young patients where one eye is either having difficulty working with the the to fuse an object into a single image (as with a tropia) or is much blurrier than the other (as with patient’s who have very different prescriptions between the eyes or a congenital cataract). The brain does not like processing the disparate images and starts to ignore the worse of the two eyes, using only the good eye. In time, if the underlying cause is left untreated the patient’s ability to process visual images in their bad eye can be permanently decreased. This means that even with the best correction in glasses, that eye may never be able to see with 20/20 vision. This is why it is very important the children get their eyes examined consistently from an early age, because while they may be able to see well at all distances, it may be because they are only using one eye. If caught early enough, amblyopia can often be prevented or treated with glasses, patching, vision therapy, or surgery depending on the underlying cause.

Please watch the following video on amblyopia.

Please watch the following video which illustrates how to chart different binocularity testing.

Please take the Checkpoint Quiz