Sensory and Motor Testing
There are a few ways to detect strabismus (deviating eyes) on a patient depending on their visual acuity and whether or not they are verbal. Eyes can be deviated inward (esotropia), outward (exotropia), upward (hypertropia), downward (hypotropia), intorted (incyclotorsion), or extorted (excylcotorsion).
Hirschberg (H) Test
On infants or an individual with a poor seeing eye, it is useful to use the Hirschberg Test. The examiner is looking at the corneal light reflex.
A muscle light or penlight is used at 1/3 of a meter and patient is asked to fixate on the light. If the light reflex is centered in both eyes, the patient does not have a manifest misalignment. If the light reflex is displaced, it shows a manifest misalignment. The amount of decentration can be used to estimate the degree of the misalignment. 1mm of displacement is roughly 7 degrees or 15Δ. For example, the light reflex at the pupillary margin is about 2mm from the center (with a normal 4 mm pupil). This corresponds with 15 degrees or about 30Δ. The mid-iris region is about 4mm from the center of the pupil, which is about 30 degrees or 60Δ. A reflex at the limbus is about 45 degrees or 90Δ.
If the light reflex is displaced nasally, the patient has an exotropia. If the light reflex is displaced temporally, the patient is esotropic. It is important to note reflex is always slightly nasal, even in an orthophoric patient. It is also important to note that angle kappa can affect light reflex measurements. A positive angle kappa simulates an exotropia, a negative angle kappa simulates an esotropia.
Krimsky (K) Test
As in the Hirschberg test a light is shone from 1/3 meter and the patient is asked to fixate on the light. If the light reflex is decentered a prism is placed in front of the deviating eye to correct the deviation of the corneal light reflex. The strabismus measurement is equal to the amount of prism necessary to center the corneal light reflex on the pupil of the deviating eye. A base-in (BI) prism is used to neutralize an exotropia, a base-out (BO) prism is used to neutralize an esotropia. Base-up (BU) is used to neutralize a hypertropia. Base-down (BD) is used to neutralize a hypotropia.
Cover-Uncover (Single Cover) Testing
In patients who have a deviation and are able to fixate on a distance and near target, cover tests offer the most accurate measurement of degree of misalignment.
A cover-uncover test picks up manifest deviations (deviations that are always present).
The patient is asked to fixate on an object. A cover is introduced over the first eye for 1-2 seconds. As the eye is covered the uncovered eye is observed for movement. The cover is removed and again shift in fixation is noted. The same is repeated, covering and uncovering the contralateral eye.
The following observations can be made:
- No shift of either eye with cover of each eye: orthotropia, the patient has no manifest deviation
- Shift of one uncovered eye: unilateral manifest deviation (tropia)
- Shift of both uncovered eyes: alternating manifest deviation (tropia). 
Alternate Cover Testing
Alternate Cover Testing (ACT) is used to pick up any latent deviation on top of the manifest deviation. A cover-uncover test is done before moving on to alternate cover testing.
The alternate cover test (shifting cover from eye to eye) will bring out the phoric portion. Prisms are used in Alternate Prism Cover Test to measure the degree of the deviation.
On both of these testing methods, it is important to occlude each eye long enough so the patient has time to take up fixation with the uncovered eye as the occluder is shifted back and forth. This is especially true if one eye is amblyopic. An accurate way to measure strabismus correctly is to measure to reversal. 
Sensory tests determine the fusional status and binocularity of the patient.
Titmus or Randot
These tests are used the most often to quantify stereopsis, it can also be used to detect ARC (abnormal retinal correspondence). The level of dissociation is moderate. This test should be done on all patients who are bi-foveal and can be used as a quick screening to see if the patient is using two eyes together. Patients with deviations of 8Δ or less should also be tested as well patients with constant tropias, as finding of stereopsis will indicated that the deviation is actually intermittent. The test can also uncover malingering in a patient claiming severe vision loss or blindness in one eye. 
The patient is directed to put on polarized glasses. They are then shown a book with stereoscopic images (3-dimensional) and 2-dimensional images. The patient is asked to identify which pictures are stereoscopic. The book either has a titmus fly or a randot stereogram. The other side of the book has stereoscopic animals and circles. It is typical to ask the patient to identify the animals after the fly or shapes and the circles last.
The “titmus fly” is considered to be 3000 seconds of arc where as the “9th circle” is about 40 seconds of arc. Some could argue that the titmus fly gives off monocular clues that allow patients with reduced stereo to able to identify the image. One way to verify if the patient truly has stereopsis is to flip the book at a 90 degree angle, the image should appear 2-dimensional. Falsely good stereopsis is very common when using the titmus fly because it is the most commonly used stereopscopic test. Conditions such as strabismus and amblyopia can reduce stereopsis, however, some degree of stereopsis can be seen in patients with anisometropic amblyopia or small angle strabismus of 8 prism diopters or less (as noted in Park’s Monofixation Syndrome).  If a patient has reduced stereopsis, it would be beneficial to then test them with the Worth 4 Dot to confirm fusion status or if in fact they have a central suppression scotoma consistent with Monofixation Syndrome.
4Δ Base Out Test
The purpose of this test is to detect a small central suppression scotoma (or foveal suppression). The level of dissociation is mild. It can be used with patients who have decreased stereopsis but are orthophoric on cover testing. This test can also be used on patients with small to intermediate angle deviations, and with patients with Monofixation Syndrome.
Instruct the patient to look at a distance target. A 4-BO prism is quickly placed over the right eye and the examiner observes the movement of the left eye. A sudden displacement of the image onto the parafovea will cause re-fixation if the image is falling on corresponding points on a normal retina. The test is repeated on the left eye and the examiner observes the movement of the right eye.
When the test is negative, the patient is considered to be bifoveal. When the prism is over the right eye, the left eye moves out and in. When the prism is over the left eye the right eye goes out and in. There are two responses when there is a scotoma (microtropia). There is a scotoma on the right eye if the prism is over the right eye and there is no response bilaterally, vice versa for the fellow eye.
Bagolini glasses do not have dioptric power but have narrow striations running parallel in one meridian. This test is used to determine the direction of a pseudofovea, abnormal retinal correspondence (ARC), and suppression. This test also is able to tell the examiner if the patient has periphery-sensory fusion. This test is mildly dissociating so it will detect small ARC even in large angle strabismus. The Bagolini glasses also simulate life-like testing conditions, which makes it the least dissociating. A disadvantage to this test is that it is difficult for young children to appreciate because it requires reliable subjective responses.
If the examiner does not have access to a Bagolini Lens, Bagolini trial frames are placed at 135 degrees OD, 45 degrees OS. Patient is asked to look at muscle light or pen light.
When a patient sees only one line at a time, the patient is suppressing. If patient sees two lines in an “A” or “V” arrangement, the patient is diplopic. You may offset the deviation with a prism and if the patient reports an X this confirms NRC. The fusion response will always be some form of an “X” If the eyes are straight, and they see an X, this means NRC, if they are strabismic and report an X this means ARC.
Double Maddox Rod
This test is used to determine the presence of a cyclodeviation. This test is very dissociating. Clinically, it is useful to use this on patients who have a suspected 4th Nerve Palsy, or Thyroid Eye Disease.It is also important to note that patients will usually tolerate extorsion more than inctorsion. 
Orient two Maddox rods (one white and one red) at 90 degrees, so the striations are vertical, in trial frames. When shown to the patient with a muscle light at 1/3 of a meter, the patient should see a red line and a white line. Instruct the patient to turn the knobs to rotate the lenses until both lines are horizontal . For a 4th Nerve Palsy, it could be helpful to put the red rod on the eye that is paretic.  It can be useful for the examiner to put the glasses on themselves to show the patient the trial frame and demonstrate use of the knobs.
If the lines are horizontal with the rods , there are 0 degrees of torsion. If the lines aren’t parallel, there is torsion present. The patient should be able use the knob to rotate the line until it is parallel line. If the rod has to be rotated inward, the patient has intorsion. If the rod has to be rotated outward, the patient has extorsion. The trial frame has an axis marked, so the examiner can determine the amount of torsion.
A patient that have globe extorsion, sees intorsion of the horizontal lines (incyclopia). In the other words, the the nasal side of the horizontal lines are downward deviated towards his nose. The reverse is also valid, globe intorsion cause excyclopia and the nasal side of horizontal line is seen upward deviated. Please see figure 5 and the videos.
Worth 4 Dot
The purpose of this test is used to detect peripheral sensory fusion and foveal suppression. The level of dissociation is high. This test can be used for any verbal child with any angle of strabismus especially with reduced stereopsis.
This can be done with the lights on or the lights off depending on what level of dissociation is preferred. With the lights off, the depth of suppression can be measured. Place the red-green glasses on the patient, with the red glass on the right eye. Ask the patient to report how many lights they see at the distance and at near (1/3 of a meter).
Note: With the right eye patient should see two red lights and with the left eye, patient should see three green lights. The examiner can also measure the boundary of suppression by testing the patient at the end of the room and walking closer to the patient.
- Fusion at distance and near: Patient reports 4 lights. Fusion is only possible when the patient is bifoveal, which implies normal retinal correspondence.
- Suppression at near and distance: Patient sees 3 or 2 lights.
- Suppressing the right eye means the patient is not seeing the lights with the red glass; therefore they will report seeing 3 green lights.
- Suppressing the left eye means the patient is not seeing the lights with the green glass; therefore they will report seeing 2 red lights.
- Fusion at near, but suppression at distance: this is the classic response for Monofixation Syndrome, but this can also happen with patients who have a large deviation.
- Diplopia: Patient reports 5 lights. It is important to ask if the red and green lights are there at the same time. Patients can rapidly alternate suppression which can give a false diplopic response.
- If a deviation is off set by a prism, and the patient still reports diplopia, this would indicate ARC. 
Red Filter Test
The red glass test is used to detect the presence and type of diplopia a patient has. It can also be used to detect suppression, determine retinal correspondence, and to confirm the type of diplopia a patient has (crossed or uncrossed). This is a moderately disassociating test. Clinically, it is most useful to use this test on adults who complain of diplopia who seem orthophoric on cover testing. It is also a good way to fit adults with prisms. Testing children with the red filter can be difficult. 
Ask the patient to fixate on the white circle at the end of the room. Place the red filter on the patient’s preferred eye. The examiner directs the patient to fixate on the white circle. The patient will give answers of suppression, fusion, or diplopia.
The patient is suppressing if they report seeing one red circle, however, it can also be a fusion answer. This is why it is important that the patient is able to distinguish the red circle as seen through the filter only as opposed to a red circle on top of a white circle when they are fused. Patients who have alternating suppression should see the red and white light alternating. If the patient sees one red circle and one white circle simultaneously, they are diplopic. The location of the red circle in relation to the white circle will tell the examiner the type of strabismus or diplopia the patient has. For example, the exotropia patient has the red filter on the right eye and reports the red circle is to their left, this denotes crossed diplopia. An esotropic patient will report the red circle to their right, which is uncrossed diplopia. If the patient reports the red circle above the white circle, this denotes a hypotropia. If the red circle is below the white dot, it is a hypertropia. If the esotropic patient reports crossed diplopia, this could mean ARC (vice versa with an exotropia patient reporting uncrossed diplopia.) This is called paradoxical diplopia. Another indication of ARC is when a prism is used to neutralize the deviation, but diplopia still persists.
This test is used to determine retinal correspondence and also can be used to detecting dense suppression. This is the most dissociating test for ARC. It also directly determines the direction of the fovea of the deviating eye. It is most useful for adults that can monocularly fixate precisely. It is useful to check on adults pre-operatively, especially if they have a long-standing intermittent exotropia. It will work with patients with eccentric fixation. This test is also too difficult for children. 
First, extinguish the room lights and occlude the non-dominate eye, orient the light in the direction of the strabismus. The dominant eye should be stimulated first with the horizontal beam for horizontal strabismus and vertical for vertical strabismus (the non dominant eye would get the opposite beam). The patient is directed to fixate on the lighted bar for 10 seconds so that the fovea is stimulated. The patient is instructed to close their eyes and observe the white lines (positive after image). With the lights on, the patient can be instructed to look at the blank wall and blinks to see the black lines (negative after image).
If the patient reports “a cross,” the patient is fusing. This indicates NRC. If the patient reports one line, this indicates dense suppression. The After-Image test is designed to “break through” mild to moderate suppression. If the patient reports “Two lines, but no intersection in the middle,” they are diplopic.
Amblyoscope | Synoptophore
The amblyoscope is used to diagnose anomalies of binocular vision with targets that are presented to each eye. It gives the examiner detailed information of the sensory status of the patient. It can be used to detect and quantify suppression, determine retinal correspondence, measure the size and shape of a suppression scotoma, measure primary/secondary deviations, and determine fusional potential. Angle Kappa, Stereopsis, and Fusional amplitudes can also be measured on the amblyoscope. The level of dissociation can be little or highly dissociating depending on what the objective is. An amblyoscope exam can be given to any adult with a history of childhood onset strabismus pre-operatively, or if they need surgery at all, as with a cyclodeviation. It is also useful for adults who report diplopia, but have a hard time describing it. An amblyoscope exam on children can be difficult due to the need for reliable, subjective answers. It is important to do a full orthoptic work up before putting a patient on the amblyoscope. 
The patient is instructed to put their chin on the chin rest. Chin rest and IPD can be adjusted accordingly. The examiner should use the toggle switches to show the patient the slide illuminated to right and left eye. The exam will proceed as the examiner switches the toggle from the right eye to the left eye. This is representative of cover testing. The eye that is illuminated is the eye that is fixating. The examiner should be able to see the eye movements and be able to neutralize a horizontal deviation by using the arms at the side of the machine. There is a scale near the arms to note the size of deviation. Neutralizing a vertical deviation requires the use of the R/L Hyper knobs. Cyclo deviations are also neutralized using the appropriate knobs. The goal of the examiner is to have the patient fuse the fusion targets. Near deviations can be neutralized by placing a -3.00 sphere trial lenses in the lens holder in front of the eyepiece. This way, the patient has to exert 3 diopters of accommodation to make the objects clear. There are different slides that can be used. The red slides (pictured above) are simultaneous perception slides that are used to test subjective and motor angles. They are also known as Worth’s Grade 1 slides. Grade 2 slides are green and are used to test motor and sensory fusion. The yellow slides are known has stereoscopic vision slides and are used to test the presence of third grade binocularity.
The amblyoscope is useful to see subjective and objective angles. The objective angle (motor angle) is what is seen on cover testing with the light source. The subjective angle (sensory angle) is noted when there is no shift and the deviation is neutralized, the images should be superimposed. In this case, the subjective angle and objective angle are the same. If the images are not superimposed, the patient is asked to move the left arm until the images are superimposed. This is the subjective angle. If there is a difference between the subjective and objective angle, this is called the angle of anomaly, which indicates ARC.
All Images are Courtesy of the Ross Eye Institute and Kyle Arnoldi, CO COMT
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- Basic and Clinical Science Course Pediatric Ophthalmology and Strabismus. San Francisco: American Academy of Ophthalmology, 2011. P.75-77
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- Arnoldi, Kyle. Orthoptic Evaluation and Treatment. Pediatric Ophthalmology: Current Thought and Practical Guide. By M. Edward. Wilson, Richard A. Saunders, and Rupal H. Trivedi. Berlin: Springer-Verlag, 2009. P.113-40.
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- Arnoldi K, Reynolds JD: “Unmasking Bilateral Inferior Rectus Restriction in Thyroid Eye Diasese: Using Degree of Cyclotropia. Am Orthopt J 2015; 65: 81-86