Assessment of Binocular Vision

Abstract

Assessment of binocular vision parameters comprise of quantified evaluations of the parameters. It is essential that these parameters are objective as far as possible and should resort to the laws and norms of ocular motility and space perception. This document on evaluation of binocular vision parameters begins with a brief discussion on anatomy, physiology of the binocular system and the laws governing it. These are followed by a detailed description of tests to assess accommodation, vergence and sensory status of the eyes. The tests have been explained and each of them have been described as a stepwise approach too to make the understanding easy to follow and perform. A brief classification of neuromuscular ocular disorders and non-strabismic binocular vision disorders is included at the end.

1. Introduction

In the world of computers and mobile devices, visual demands have increased multiple folds. Our eyes which were not meant to be used the way they are being used today need further and detailed evaluations. The mechanisms and the parameters of accommodation, vergences and binocularity require appropriate evaluation and interpretation of the data. Analysis of the findings and extrapolation of the data become the starting point to initiate vision therapies. These finally result in the reduction in symptoms and improvements in parameters ascertained initially.

2. Anatomy and physiology of extraocular muscles

Humans have 6 extraocular muscles in each eye. Four recti muscles move anteriorly from the orbit and get attached to sclera near cornea. Two obliques muscles arise from the medial aspect of the orbit, from there they continue obliquely and laterally, and insert into sclera posterior to the equator, on the temporal part of the globe. Contraction of recti muscles, pulls the globe backwards and nasally, whereas contraction of oblique muscles pulls the globe forward and nasally [1].

Recti muscles:

The recti are flat and narrow in appearance. They attach to the globe with thin tendons.

The four recti are:

1. Medial rectus

2. Lateral rectus

3. Superior rectus

4. Inferior rectus

The four recti, superior oblique, and levator palpebrae superioris originate at the orbital apex and are arranged in a circular fashion around the orbital opening and called the annulus of Zinn. Insertion of medial rectus is closest to the limbus, followed by inferior rectus, lateral rectus, and superior rectus.

Oblique muscles:

Superior oblique originates at the apex of the orbit. It runs anteriorly parallel to the upper part of the medial wall of the orbit. There it goes through trochlea and turns laterodorsally at an angle of 54 degrees with respect to the pretrochlear part of the muscle. This, then becomes tendinous towards the distal end, passes beneath superior rectus and merges laterally with the sclera.

Inferior oblique is the shortest of all the extraocular muscles. It arises in the anteroinferior angle of the orbit, from there it continues backward, upward, and laterally. It passes between the floor of orbit and inferior rectus and insert in the posterior and external aspect of sclera.

Centre of rotation:

The eyeball is presumed to perform rotary movements around a center of rotation, which is assumed to be within the globe. Presumably, the line connecting middle of lateral orbital margins goes through center of rotation of the two eyes.

As the eyeball is presumed to have a fixed center of rotation for practical purposes, the globe rotates in three directions going through the center of rotation.

1. Anteroposterior (Y axis)

2. Vertical (Z axis)

3. Horizontal (X axis)

The plane in which the vertical and horizontal movements take place is called Listing’s plane, and is defined as the plane fixed in the orbit that passes through the center of rotation and the equator of the globe, with the eye in primary position.

Duction movements:

The rotation of one eye is called duction.

1. Rotation around Z axis: adduction/abduction

2. Rotation around X axis: elevation/depression

3. Rotation around Y axis: incycloduction/excycloduction

Movements around “Z” and “X” axes are called cardinal movements, and a combination of these cause oblique movements which are around “Y” axis.

Positions of the globe:

When a person is looking straight ahead with the body and head erect, it is called primary position.

Adduction, abduction, elevation, and depression are secondary movements and oblique positions are tertiary positions of the eye.

Actions of muscles:

1. Lateral rectus abducts and medial rectus adducts the eye from the primary position.

2. In primary position, superior rectus not only elevates but also rotates the eyeball causing incycloduction and adduction. Inferior rectus on the other hand, depresses the globe, excycloducts and has a slight adducting action.

3. Superior oblique causes incycloduction, depression and abduction whereas inferior oblique causes, excycloduction, elevation and abduction.

Versions:

These are binocular movements of the eyes in the same direction. These movements enlarge the field of view and help in bringing the object of interest on the fovea. Versions are fast movements and can be voluntary or involuntary.

Stimuli in the retinal periphery elicit saccadic eye movements, which are fast movements to place the object of regard on the fovea and keep it there. Pursuits track a moving object. The function of saccades is to correct the position error between target and fovea, whereas that of pursuit is to match eye velocity to target velocity.

Vergences:

These are binocular movements of the eyes in opposite directions. Vergence movements align the eyes to ensure and maintain binocular fixation and binocular vision. These are slow movements as compared to versions.

Convergence occurs when an object approaches the eyes and divergence occurs when it recedes. Vergence movements are made for fusion and are required for fusional vergences. Cyclovergences are compensatory adjustment movements to correct the relative position of eyes in the presence of vertical or cyclophorias.

Laws of ocular motility:

Contraction of extraocular muscles brings about movement of the eyeball. Muscles responsible for bringing about this movement are called agonists. Muscles that move the eyeball in the opposite direction to that of agonists are called antagonists. Medial rectus adducts the eyeball and lateral rectus abducts it. Hence medial rectus and laeral rectus are antagonists.

Two muscles that move the eyeball in the same direction are called synergists. Superior oblique and inferior rectus depress the eyeball, they are synergists for that, but for cyclorotation they are antagonists as superior oblique incycloducts and inferior rectus excycloducts the eyeball.

Considering binocular actions, two synergistic muscles moving both the eyes in the same direction are called yoke muscles. Right medial rectus and left lateral rectus cause levoversion of the two eyes. They are yoke muscles.

A pair of muscles in one eye can be yoked with a pair of muscles in the other eye. Elevators of one eye are yoked with the elevators of the other eye. Yoking also changes with the eye movements. The right medial rectus is yoked with the left lateral rectus for lateroversion, however, medial recti of both eyes are yoked for convergence.

Sherrington’s law of reciprocal innervation:

Whenever an agonist receives an impulse to contract, an equivalent inhibitory impulse is sent to its antagonist to relax. This law is applicable to all the striated muscles of the body and not extraocular muscles alone.

Hering’s law of equal innervation:

Whenever an impulse is sent to an extraocular muscle, corresponding muscles of each eye receive equal innervations to contract or relax. This is also called the law of motor correspondence. Unlike Sherrington’s law, Hering’s law applies only to extraocular muscles.

Isolated innervations to an extraocular muscle do not occur, nor can the muscles of one eye alone be innervated. Impulses for ocular movements are integrated and all ocular movements are associated.

3. Tests for binocular functions

Assessment of binocular functions involves examination of ocular motilities, presence of any kind of motility problems like inaccurate saccades or pursuits, speed deficits in motility etc. Binocular status must be assessed for alignment in all the cardinal directions of gaze [2, 3, 4].

3.1 Extraocular motilities

This test is done to check the ability to perform conjugate eye movements (i.e. both the eyes together).

A penlight is held directly in front of the patient’s eyes at about 30 to 40 cm. This is the “primary position” of the eyes. Then, the light is moved in eight cardinal directions. (It will resemble a figure “H” bisected by a vertical line).

Patient is instructed to follow the light with his eyes, without moving the head. He or she is also instructed to report if the light appears double in any direction or if any pain or discomfort is experienced. Examiner should note the smoothness, accuracy & extent of movements.

If the ocular movements are full, smooth, and accurate, then it is recorded as “FROM” (full range of movements). Otherwise, record the problem in movements. For example: jerky, nystagmoid movement in a specific gaze. If diplopia is experienced, it must be recorded in the specific direction of gaze and should be followed by diplopia charting.

Procedure in steps:

1. Patient is instructed to follow the light in different directions with his eyes and report if he or she experiences double vision, pain, or discomfort.

2. Penlight is held in primary position and then moved in different directions of gaze.

3. Note the corneal reflex, smoothness, and accuracy of movements.

4. If everything appears normal, then record it as “FROM” (full range of ocular motility), otherwise note the deficits.

5. If the patient complains of diplopia, note the direction in which it was observed by the patient and perform diplopia charting.

Muscle imbalances can cause turning of the eye/s in, out, up, down, or tendencies to do so. Eso is a turning in, exo is a turning out, hyper is a turning up, and hypo is a turning down. A phoria is a tendency to turn, while a tropia is a definite turning of the eye. Therefore, esotropia is a definite turning in, while an exophoria is a tendency to turn out. Hypotropia is a definite turning down, while hyperphoria is a tendency to turn up.

Strabismus is a condition in which the eyes are not aligned appropriately. It is also referred to as a squint or crossed eyes and may lead to Amblyopia. Strabismus and Amblyopia are the most common causes of visual impairment in children. Different tests are used to assess and quantify latent and manifest squint. Most of these tests are objective and do not require patient’s response.

3.2 Bruckner test

This test is usually performed in infants and preverbal children. It is to screen for binocularity by comparing the brightness of red reflex from the pupils of both the eyes.

A direct ophthalmoscope with a large spot of illumination is directed towards the patient’s eyes from a distance that is sufficient to cover both the eyes (usually about 1 meter), with the patient looking at the spot. Examiner views the red reflexes in both the pupils through the peephole of the scope.

Brightness of reflexes from both the pupils is compared. Equally bright reflexes indicate binocular fixation. If they are not equally bright, the brighter one indicates a non-fixating eye. This difference in brightness may be due to strabismus, anisometropia, anisocoria etc.

If the reflex appears unequal, record the eye from which the reflex appears brighter.

Procedure in steps:

1. A large spot of light from a direct ophthalmoscope is directed towards the patient’s eyes from approximately 1 meter.

2. The examiner views the reflex from the pupils through the peephole of the scope.

3. Reflexes are compared for brightness.

4. Observation is recorded as reflex appearing equal or unequal. If it is unequal, then record the eye which shows a brighter reflex.

3.3 Hirschberg test

This test is performed to determine the approximate position of visual axes at near, thus identifying strabismus and quantifying it approximately.

Penlight is directed towards the patient’s eye from about 50 cm, with the patient viewing it directly. The location of corneal reflex in both the eyes are viewed. If they appear centered, then the patient does not have strabismus, otherwise he or she has strabismus.

The size of strabismus can be estimated by measuring the distance between the expected and the deviated positions. One millimeter of deviation is equal to 22 prisms.

Strabismus can be approximated quantified in the following way:

1. If the reflex appears at the edge of the pupil, then the deviation is approximately 15 degrees.

2. If it is centered between the edge of the pupil and the limbus, then the deviation is about 30 degrees.

3. If the reflex is at the limbus, then the deviation is quantified to be approximately 45 degrees.

The relationship between the position of the corneal reflex and the type of deviation is as follows

1. If the position of the reflex is nasal to the centre of the pupil in the deviating eye, then there is exotropia.

2. If the position of the reflex is temporal to the centre of the pupil in the deviating eye, then there is esotropia.

3. If it is above the centre of the pupil, then it is hypotropia relative to the fixating eye.

4. If it is below the centre of the pupil, then there is hypertropia relative to the fixating eye.

Procedure in steps:

1. Light from a penlight is shown from about 50 cm with the patient viewing it directly.

2. Examiner looks for the corneal reflex.

3. Position of the corneal reflex is noticed, and if the reflex is not centered, then the strabismus is approximately quantified depending on the location of its position in the deviated eye.

4. Recording is accordingly documented.

The quantification can be measured by placing prisms in front of the deviating eye.

3.4 Krimsky test

This is an extension of Hirschberg test to quantify the amount of strabismus. On noticing the deviation, a prism with its apex towards the deviation is placed in front of the deviating eye. For example, in case of exotropia, a prism is placed with its base in, in front of the deviating eye.

The amount of prism is gradually increased until the corneal reflex is in the same relative position in the deviating eye as it is in the fixating eye.

In case of scarred cornea or very poor visual acuity in the deviating eye, an alternate procedure called modified Krimsky is used. The procedure is same as Krimsky, except that the prism is placed in front of the fixating eye.

To avoid parallax error, it is ideal for the examiner to view the corneal reflex with one eye closed.

Procedure in steps:

1. If a deviation is noticed on Hirschberg test, a prism is placed in front of the deviating eye, with its apex towards the direction of deviation.

2. The amount of prism is increased until the reflexes in both the eyes appear centered. This is the amount of strabismus that the patient has.

To identify and differentiate between phoria and tropia, cover – uncover test and alternate cover test is used. These tests help in differentiation as well as quantification of the squint.

3.5 Cover test

This test is performed to evaluate the presence of phoria or tropia. The cover test assesses the presence or absence of motor fusion. Motor fusion is responsible for bringing about alignment of the eyes.

This test is performed both for distance and near. When it is done at near, accommodation and accommodative convergence comes into play. The test is performed with the patient wearing the best correction for refractive error. For distance, the target is one line larger than the best visual acuity in the poorer eye. For example, if the patient has 20/20 in the right eye and 20/30 in the left eye, then, 20/40 line is isolated to perform the test. Target for near is an accommodative target held at 40 cm. For this too the line larger than the best visual acuity in the poorer eye is isolated for the test.

An occluder is held by the examiner to cover one of the eyes. The examiner is positioned in such a way that the patient’s view of the target is not obstructed, and the examiner is able to view any movements of the eyes.

3.6 Cover: Uncover test

This test, as mentioned before, differentiates between phoria and tropia. It also helps in differentiating between an alternating or unilateral tropia.

As the first step, fixating eye is covered by the occluder, with the patient fixating the target at distance. The occluder is held in front of the eye for a few seconds before the eye is uncovered. Examiner should look for any movement of the uncovered eye. After a few seconds of covering, the occluder is removed and the eye which has been uncovered is observed as soon as the cover is removed. Then, the other eye is covered, and similar observations are made.

If the patient has tropia, then the uncovered eye will take up fixation showing the presence of tropia. If there is no movement seen, then there may be phoria or the patient may be orthophoric.

When the cover is removed from the fixating eye, and the just uncovered eye shows a movement, it is indicative of the presence of phoria. This is seen because when both the eyes were uncovered, the patient was using fusional vergences to maintain alignment. However, on covering one of the eyes, the fusion was broken and the phoria gets uncovered.

If there is no movement seen on covering or uncovering either of the eyes, then the patient is orthophoric.

Cover – uncover test is also used to differentiate between alternating and unilateral tropia.

1. If the right eye is covered and the left eye takes up fixation and returns to the deviated position on uncovering right eye, then the patient has tropia in the left eye.

2. If, on uncovering the right eye, the left eye retains fixation, and the right eye takes up deviated position, then the patient has alternating tropia.

The same procedure is repeated for a near target at 40 cm. This provides information about phoria or tropia with the accommodation and accommodative convergence mechanisms acting. If a patient is orthophoric for distance but has deviation for near, then it is because of the accommodation/convergence mechanism.

3.7 Alternating cover test

The settings are same as in the cover test. Patient is required to fixate the line on the distance chart. Examiner covers the right eye with the occluder and after a few seconds quickly shifts it to the left eye. This procedure of alternately covering and uncovering is carried out several times, making sure that either of the eyes is kept covered to disrupt fusion.

After the direction of the deviation has been determined, magnitude of deviation can be measured by placing prism bar in front of either eye. After interposing the prism bar, the amount of prism is increased gradually until no movement of the eye on alternate cover test is seen. This is also referred to as prism bar cover test (PBCT). Alternate cover test is similarly performed for near, with the patient fixating a near line larger than the best visual acuity in the poorer eye.

3.7.1 Relationship between eye movement and the direction of deviation in the deviating eye when the fixating eye is covered

See Table 1.

	Direction of eye movement	Direction of deviation
1	In	Exo
2	Out	Eso
3	Up	Hypo
4	Down	Hyper

Table 1.

Interpretation of type of tropia based on the direction of eye movement.

Prisms required for neutralizing the movement:

1. For exo deviation, prism with its base in is used.

2. For eso deviation, prism with its base out is used.

3. For hypo deviation, prism with its base up is used.

4. For hyper deviation, prism with its base down is used.

Procedure in steps: (cover – uncover test)

1. For cover – uncover test, the fixating eye is covered with an occluder, while the patient is fixating an isolated line in the distance Snellen’s chart.

2. The examiner looks for movement of the eye which is not covered.

3. If the eye which is not covered moves, then the patient has a tropia.

4. If it does not move, then it could be a phoria or the patient may be orthophoric.

5. The covered eye is uncovered, and the examiner looks for movement of the uncovered eye immediately on uncovering it.

6. If a movement is seen, then the patient has phoria.

7. If no movement was perceived, then the patient is orthophoric.

8. If tropia was noticed, and on removal of cover, the eye goes back to its deviated position, then the patient has unilateral tropia.

9. If the uncovered eye retains fixation, and the phenomenon repeats on covering and uncovering the fellow eye, then the patient is said to have alternating tropia.

10. Test is repeated in the same way for a near target

Procedure in steps: (alternating cover test)

1. This test is to determine the magnitude of deviation seen in cover – uncover test on incorporation of prisms in front of the deviating eye.

2. Both the eyes are covered alternately, without leaving either of them uncovered to avoid fusion.

3. A prism bar in placed over the deviating eye, and alternate cover test is performed to see the movement of the eye.

4. The amount of prisms is gradually increased, until no movement of either eye is perceived on alternate cover test.

5. This is the amount of deviation of tropia or phoria.

6. The test is repeated for a near target and the deviation for near assessed.

4. Tests to evaluate sensory status

To avoid diplopia in strabismus or aniseikonia in clinically significant anisometropia, several sensory adaptations manifest. They are [2, 5]:

1. Suppresion

2. Anomalous retinal correspondence

3. Amblyopia

In suppression, the patient may have normal visual acuity in both the eyes, but one of the eyes (usually the deviated eye in strabismus) is suppressed to avoid diplopia. These patients do not have binocular vision though they may have normal monocular visual acuities.

For a complete binocular visual status, it is important to attain the three grades of binocular vision.

1. Simultaneous macular perception

2. Fusion

3. Stereopsis

When an object is imaged simultaneously on maculae of both the eyes, then a person is said to have simultaneous macular perception. In a patient who has strabismus, due to the deviation, simultaneous macular perception does not take place and the image falls on an extrafoveal point. Sometimes, in strabismus, an anomalous correspondence takes place between the macula of one eye and an extrafoveal point of the other eye. This, as mentioned before, is a sensory adaptation.

Fusion is the ability of the brain to form a single image by coordinating the movements of the two eyes so that the visual images fall on corresponding areas of the retinas of the two eyes. This can be sensory or supported by motor activity.

To assess fusion ability at different distances, a Worth 4 dot test is used. This test also shows if the patient is suppressing either of the eyes or has diplopia.

4.1 Worth four dot test

This test is performed at different distances. The target comprises of four colored dots. They are arranged in a diamond shape. Usually, red dot is at the top, white at the bottom and two green dots on the sides.

The patient wears his correction and red – green glasses are worn over it, with red in front of the right eye and green in front of the left eye. It is important to check the cancelation of the target colors with the filters. The patient is required to tell how many dots he sees, their colors and respective positions.

The test is similarly performed for different distances using a handheld Worth 4 dot flashlight.

Responses that may be given and their interpretation:

1. If the patient reports that he sees four dots, then he has flat fusion.

2. If the patient sees only two red dots vertically, then he is suppressing his left eye.

3. If the patient reports seeing three green dots alone, then he is suppressing his right eye.

4. If he or she reports seeing five dots, then ask the patient where the green dots (seen by the left eye) are located with respect to the position of the red dots (seen by the right eye). Based on the response, relationship of the visual axes can be determined.

5. If the red dots are to the right of green dots, then the patient has eso (uncrossed) deviation.

6. If the red dots are to the left of green dots, then the patient has exo (crossed) deviation.

7. If the red dots are above the green dots, then there is left hyper deviation.

8. If the red dots are below the green dots, then there is a right hyper deviation.

9. The distance at which the patient reports suppression, provides an idea about the size of the suppression scotoma. For example, if the reported suppression is at a very close distance from the patient, then the size of suppression scotoma is big as compared to a small suppression scotoma if it is reported at a far distance.

Procedure in steps:

1. The patient wears red – green glasses over the habitual correction, with red in front of right eye and green in front of left eye.

2. He or she is requested to look at Worth 4 dot test, projected at different distances.

3. Patient’s response about the number and relative position of different dots is elicited.

4. Based on the response, diagnosis is ascertained.

After the patient’s ability to fuse has been ascertained, it is important to measure the final grade of binocular vision – stereopsis. This is done to measure the patient’s depth perception through his or her ability to fuse stereoscopic targets.

There are different tests used to measure stereopsis. Most of them require the patient to wear red – green or Polaroid glasses. There are some tests like Lang stereo test, which do not require use of either of these glasses. These tests are primarily used for very young children.

Usually employed stereopsis tests are:

1. Randot test

2. Titmus fly test

3. Bernell stereo test

4. TNO test

In all these tests, varying degrees of depth is perceived on wearing red – green or Polaroid glasses. The patient reports the same and it is recorded in seconds of arc. The depth in the targets is created by changing the disparity between them.

4.2 Stereopsis

The patient wears either Polaroid or red – green glasses over the near correction. A stereo test booklet is presented to the patient at 40 cm and his or her attention directed towards the smallest set of targets. The patient is asked to identify which of the circles in the set of circles appears to be floating above the plane of the booklet.

If stereopsis is appreciated, then he or she is instructed to go to the next set of targets. Continue this procedure until the patient reports two consecutive wrong replies.

Record the stereopsis in seconds of arc (mentioned in the instruction booklet supplied with the test) for the last correct response obtained from the patient before two consecutive incorrect responses.

Procedure in steps:

1. The patient wears red – green or Polaroid glasses over the near correction.

2. Stereo test booklet is presented at a distance of 40 cm.

3. The patient is asked to identify the circle which appears above the plane of booklet, in a set of circles shown to him or her.

4. Test is carried on until the patient gives two consecutive incorrect responses.

5. Amount of perceived stereopsis is recorded in seconds of arc.

When the patient has a mild reduction in monocular visual acuity and slightly reduced stereopsis, then, a very small central scotoma is suspected. This central suppression could be secondary to a small angle strabismus.

To confirm or rule out the presence of a small central scotoma or a small central suppression, 4 prism base out test is performed.

4.3 Four prism diopter base out test

The patient wears his best correction for distance. He or she is required to fixate at an isolated letter, one line above the best visual acuity in the poorer eye, on the distance chart. The patient is instructed to fixate on the target and try to keep it single all the time. The examiner holds a 4 prism with its base out in front of the better eye. The fellow eye is watched carefully for any movement.

Normal response is an outward comitant movement of the fellow eye, followed by an inward refixation movement. The prism should be held in front of the eye for refixation movement to take place.

As the next step, prism is held in front of the fellow eye (poorer visual acuity). Normal response is an outward movement of the fellow eye, followed by an inward refixation movement. The prism should be held in front of the eye for refixation movement to take place.

Expected results:

1. Normal result indicating no suppression of either eye: When the prism is placed in front of the right eye, the left eye moves out (Hering’s law). The patient experiences diplopia, and to avoid that, an inner refixation movement takes place. Similar phenomenon is seen when the prism is placed in front of the left eye.

2. Abnormal result indicating suppression of the poorer eye: When the prism is placed in front of the better eye, the fellow eye moves outwards in consistence with Hering’s law. However, as the fellow eye has a small suppression scotoma, the patient does not experience diplopia and hence no refixation movement is seen. When the prism is shifted to the suppressing eye, it does not experience shift in retinal image, and hence no movement of either eye is elicited.

Procedure in steps:

1. The patient wears his or her best correction for distance.

2. He or she fixates an isolated letter, which is from one acuity line better than the acuity in the poorer eye.

3. A 4 prism with its base outward is placed in front of the better eye and the movement of the fellow eye watched.

4. Then the prism is shifted to the other eye, and again the movement of the fellow eye is looked for.

5. In case there is no suppression, then fellow eyes in both the cases show a movement outward, consistent with Hering’s law and then, an inward refixation movement is seen.

6. If the prism is placed in front of the suppressed eye, then no movement of either eye is seen.

7. When the prism is placed in front of the better eye (the other eye suppressing), then, an outward movement of suppressing eye is elicited (Hering’s law), but no inward refixation movement is seen as the patient does not experience diplopia.

Once different grades of binocular vision have been evaluated, tests to ascertain accommodative and vergence system are performed. Clear, comfortable binocular vision depends upon several skills. One should be able to align the eyes and maintain it for a certain period. The patient must have sufficient amplitude of accommodation and must possess the ability to sustain it appropriately and efficiently. The accommodation and vergence mechanism must interact properly.

On ascertaining these values, the examiner decides if the patient’s problems can be corrected with the spectacle or if he or she requires binocular vision training. Patient’s spectacle prescription influences the accommodation and vergence systems and vice versa. Based on the findings, the examiner must decide if the correction requires to be revamped or not.

5. Measurement of latent deviation

Latent deviation or phoria is seen when fusion is disrupted. Both the eyes maintain alignment with the help of motor fusion. Accommodation convergence mechanism plays a big role in maintenance of this alignment. Sometimes, there is a great stress on this system, which results in aesthenopic symptoms. By proper measurement of phorias and vergences, one can diagnose the cause for aesthenopia and then treat it with optical devices or vision training. There are several methods to measure phorias. A few of them are [2, 3]:

1. Maddox rod method

2. Modified Thorington method

3. Von Graefe technique

Phoria is measured both for distance and near. It is also evaluated laterally and vertically. Finally fusional vergences are measured to ascertain the entire vergence mechanism.

5.1 Maddox rod method

This method of phoria measurement is used for lateral (horizontal) and vertical phoria for both distance and near.

The patient wears his or her correction, and a red or white Maddox rod is introduced in the trial frame in front of his right eye. A spot of light is switched on at six meters and the patient is instructed to look at it.

Maddox rod should be aligned in the following way:

1. For measuring lateral phoria, grooves of Maddox rod should be aligned horizontally, so that the streak appears vertical.

2. For measuring vertical phoria, grooves of Maddox rod should be aligned vertically, so that the streak appears horizontal.

Prisms should be introduced in front of the right eye in the following way:

1. For measuring lateral phoria, sufficient base in prism is introduced in front of the right eye. (They may be handheld or prism bar). This is done to move the streak of light to the right of the spot of light.

2. For measuring vertical phoria, sufficient prisms with base up orientation are introduced in front of the right eye. (They may be handheld or prism bar). This is done to move the streak of light below the spot of light.

The patient is instructed to look at the spot of light and be aware of a streak and a spot.

For lateral phoria measurement, reduce the amount of base in prisms until the patient reports that the streak of light is bisecting the spot. Record the prism value and the base orientation.

For vertical phoria measurement, reduce the amount of base up prisms until the patient reports that the streak of light is bisecting the spot. Record the prism value and the base orientation.

Similarly, phoria is measured for near using a penlight held at 40 cm by the examiner.

Procedure in steps:

1. The patient wears the correction and the Maddox rod (oriented horizontally for horizontal phoria and vertically for vertical phoria measurements) is placed in front of the right eye.

2. Sufficient prisms with base in orientation (for lateral phoria) and base up orientation (for vertical phoria) are placed in front of the right eye.

3. A spot of light is switched on at six meters and the patient is asked to look at it.

4. In case of lateral phoria measurement, the patient sees a streak of light with the right eye, which is to the right of the spot (displacement is due to the prisms incorporated) and with the left eye he sees a spot of light.

5. For vertical phoria measurement, with the right eye, the patient sees a streak of light, which is below the spot (displacement is due to the prisms incorporated) and with the left eye he sees a spot of light.

6. Prisms are reduced by the examiner until the patient reports that the streak bisects the spot of light.

7. The test is similarly carried out for near, with the patient looking at the penlight held by the examiner at 40 cm.

8. Amount of prisms and their base orientations are recorded as the measurement of phoria.

5.2 Modified thorington method

This test is performed to check lateral and vertical phoria for near. Thorington card is used to perform this test. This card is held 40 cm from the patient. The separation of numbers is such that at 40 cm, the spacing between any two numbers is one prism. There is a hole in the center of the card, through which penlight provides a source of light.

The patient wears his correction, over which a Maddox rod is placed in the trial frame in front of the right eye. For measuring lateral phoria, Maddox rod is oriented horizontally, and the streak appears vertical to the patient.

The Thorington card is held 40 cm from the patient with penlight at the center of the card. He is instructed to look at the light and tell the examiner if the streak is to the right or to the left of the light. If the streak is to the right of the light, then patient has esophoria, and if it is to the left, then it suggests exophoria. The size of the phoria is determined by asking the patient to tell the number through which streak passes.

For vertical phoria, the same procedure as in the measurement of horizontal phoria is used with the Maddox rod oriented vertically and a Thorington card with vertical rows of numbers is used. In this case if the streak passes through the light, it is orthophoria. If it is above the light, then the patient has left hyperphoria and if it is below the light, then it is right hyperphoria.

Size and the direction of phoria is recorded accordingly.

Procedure in steps:

1. The patient wears his or her usual near correction with Maddox rod in front of the right eye.

2. Maddox rod is oriented horizontally for lateral phoria and vertically for vertical phoria measurements.

3. Thorington card is presented to the patient at 40 cm distance, with penlight shown through the centre of the card.

4. The patient reports the number which is bisected by the streak and also the location of streak relative to the light.

5. Same method is used to measure vertical phoria except that vertically oriented Thorington card is used, and Maddox rod is placed with its grooves oriented vertically.

6. Phoria for both horizontal and vertical orientation is recorded.

6. Tests for vergence

When an object is brought closer, both the eyes converge to look at it. There is a synkinetic triad, wherein when an object is brought closer, accommodation, convergence and pupil constriction takes place simultaneously [2, 3].

If the near point of convergence recedes, it leads to aesthenopic symptoms. NPC (near point of convergence) is a measure of convergence amplitude – the maximum ability to converge while maintaining sensory fusion.

1. NPC Break (Objective) – The point where the patient can no longer maintain fusion. At this point one eye is seen moving out.

2. NPC break (Subjective) – The point where the patient reports he sees double is the subjective break.

3. NPC Recovery (Objective) – The distance at which the patient regains binocular vision. This point is noted as the place where the two eyes realign. The recovery is farther out than the break.

4. NPC Recovery (Subjective) – The distance at which the patient reports that he or she can regain single vision.

6.1 Near point of convergence

The patient wears his near correction. Accommodative targets (reduced Snellen’s letters) of different sizes are required. The patient is asked to look at the target, which is shown to him or her from 40 cm. The examiner moves the target towards the patient, while observing his eyes for fixation.

The target is brought towards the patient until he reports doubling (subjective break point) or until the examiner sees one eye lose fixation on the target (objective break point).

Move the target away from the patient, urging him to report when the target appears single again (subjective recovery point) or the examiner notices realignment (objective recovery point).

Record the linear distance from the target to the spectacle plane for both break and recovery points.

Expected findings:

A break point of more than 5 cm is considered abnormal, and the recovery point is required to be within 7 cm.

Procedure in steps:

1. The patient wears his correction for near.

2. An accommodative target (reduced Snellen’s letter) is shown to the patient from 40 cm.

3. The patient fixates the target binocularly.

4. Target is brought closer to the patient until he reports doubling or the examiner notices the break in binocular fixation.

5. The linear distance between the target and the spectacle plane is measured. This is the near point of convergence (NPC).

6. The target is then moved away from the patient, until he reports seeing the target single or the examiner notices realignment. This is the recovery point.

7. Break and recovery points are measured and recorded.

A patient may have all the parameters like NPA, NPC, fusional vergences etc. within acceptable limits, but continue to have aesthenopic symptoms. In these cases, it becomes mandatory to check the patient’s ability to increase and decrease accommodation binocularly while keeping convergence demand constant. Under these conditions, changes in accommodative convergence are compensated by fusional vergence. Analysis of negative and positive relative accommodation play an important role in determining presbyopic addition too.

6.2 Horizontal fusional vergences at distance

To examine horizontal vergences at distance, appropriate distance correction is placed in the trial frame. An isolated letter is exposed on the Snellen’s chart, which is one line larger than the patient’s best corrected visual acuity in the poorer eye.

Risley prisms or a prism bar is introduced in front of either of the eyes. The patient is required to look at the target (isolated letter on the Snellen’s chart) and keep it clear. Inform the patient that prisms in front of his eyes will be changed, which, in turn, will move the target horizontally. The patient is required to keep the target clear and single and report if:

1. The target blurs.

2. The target becomes double and cannot be made single with effort.

To start the test base in prisms are incorporated in front of the eye. Base in prisms is tested before base out prisms as base out prisms affect accommodation and convergence and hence may affect base in prism findings.

The prisms are changed in front of the eye. Amount of prism in front of the eye is noted when the patient reports blur and break points.

After the break point is reached, decrease the prisms in front of the eye and ask the patient to report when he sees the target single. This is the recovery point.

Prism value for blur, break and recovery is noted. The procedure is repeated with base out prisms for distance.

There is no blur point reported for base in vergence testing for distance. This is because with ideal refraction, the accommodation is fully relaxed and there is no further accommodation which could be relaxed with base in prisms. However, if the patient reports a blur with base in prisms, it is indicative of improper refraction (over corrected myopia or under corrected hyperopia). In such a case refraction needs to be repeated.

Findings are recorded for distance testing, indicating the orientation of the prism with the obtained result. If no blur is seen, then an “X” is marked against it.

Expected findings

Morgan’s findings for adult, clinical population:

Distance BI: X/7/4

Distance BOUT: 9/19/12

Procedure in steps:

1. A line better than the best corrected distance visual acuity in the poorer eye is isolated.

2. The patient wears his normal correction for distance and Risley prisms are placed in front of either of the eyes.

3. Base in vergences are checked first, by increasing the base in prisms in front of the eye.

4. The patient is asked to report the blur and the break points.

5. The value of prism in front of the eye is recorded.

6. When the break point is recorded, prism in front of the eye is reduced, until the patient reports seeing the target single again.

7. This is recorded as the recovery point.

8. The procedure is repeated for distance with bout prisms.

6.3 Vertical fusional vergences at distance

This test measures patient’s vertical fusional vergences at distance. This test is done with base up or base down prisms.

Base down prisms induce supravergence and base up prisms induce infravergence. While testing vertical vergences, only two findings are expected:

1. Break: At this point, the patient cannot maintain binocular single vision as all the vertical vergences have been exhausted.

2. Recovery: After the break point has been reached, the examiner starts reducing the induced prisms until the image appears single once again. This indicates reduction in retinal disparity to a point where the patient accesses the vertical vergence system again.

There is no blur finding for vertical vergence testing as accommodation does not change during vertical vergence movements.

To examine vertical vergences at distance, appropriate distance correction is placed in the trial frame. An isolated letter is exposed on the Snellen’s chart, which is one line larger than the patient’s best corrected visual acuity in the poorer eye.

Risley prisms or prism bar is introduced in front of either eye. The patient is required to look at the target (isolated letter on the Snellen’s chart) and keep it single. Inform the patient that the prism in front of his eye will be changed, which, in turn, will move the target. The patient is required to keep the target single and report if the target becomes double.

Prisms with their base up are increased in front of the right eye until the patient reports a doubling of the image. This is the break point. On reaching the break point, base up prisms in front of the right eye are reduced until the patient reports seeing the target single. This is the recovery point.

Break and recovery points are recorded. The entire procedure is repeated with base down prisms in front of the right eye and the findings recorded.

Expected findings

Break: 3–4 prisms

Recovery: 1.5–2 prisms

Procedure in steps:

1. A line better than the best corrected visual acuity in the poorer eye is isolated.

2. The patient wears his normal correction for distance and prisms are placed in front of the eye.

3. Base up prisms are increased in front of the right eye.

4. The patient is asked to report the break point.

5. Prisms in front of the right eye are recorded.

6. After the break point is recorded, prisms are reduced, until the patient reports seeing single again.

7. This is recorded as the recovery point.

6.4 Horizontal fusional vergences at near

This procedure is like that done at distance. This measures the patient’s ability to use horizontal vergence to maintain binocular vision at near distance.

To examine horizontal vergences at near, appropriate near correction is placed in the trial frame. An isolated line is exposed on the near card, which is one line larger than the patient’s best corrected near visual acuity in the poorer eye.

A prism bar is introduced in front of the eye as in the procedure for distance. The patient is required to look at the target (isolated line on the near card) and keep it clear and single. Inform the patient that prisms in front of his or her eyes will be changed, which, in turn will move the target horizontally. The patient is required to keep the target clear and single and report if:

1. The target blurs.

2. The target becomes double and cannot be made single with effort.

Prism value in front of the eye is noted when the patient reports blur and break points. After the break point is reached, decrease the prisms in front of the eye and ask the patient to report when he sees the target single. This is the recovery point.

Prism value for blur, break and recovery is noted. The procedure is repeated with base out prisms for near. Findings are recorded for near testing, indicating the orientation of the prism with the obtained result.

Expected findings:

Morgan’s findings for adult, clinical population:

Near BI: 13 / 21 / 13

Near BOUT: 17 / 21 / 11

Procedure in steps:

1. A line better than the best corrected near visual acuity in the poorer eye is isolated.

2. Prisms are incorporated in front of the eye as in the distance procedure.

3. Base in vergences are checked first, by increasing the base in prisms in front of the eye.

4. The patient is asked to report the blur and the break points.

5. When the break point is recorded, prism in front of the eye is reduced until the patient reports seeing the target single again.

6. This is recorded as the recovery point.

7. The procedure is repeated for near with bout prisms.

A patient may be symptomatic despite having normal fusional vergences for both distance and near. In such cases, a patient’s fusional vergence facility at near should be checked. This tests the ability of the patient’s fusional vergence system to respond to disparity changes within a specific time frame.

6.5 Fusional vergence facility at near

To examine fusional vergence facility at near, appropriate near correction is placed in the trial frame. An isolated vertical line is exposed on the near card, which is one line larger than the patient’s best corrected near visual acuity in the poorer eye. 12 prisms base out and 3 prisms base in are used to check the facility of vergence.

The patient is asked to report when the target becomes single and clear. The prisms are flipped to 3 prisms base in position as soon as the patient reports clearing with the 12 prisms base out prisms.

The above-mentioned steps are repeated and the number of full cycles are noted in 1 minute. (One full cycle comprises of clearing both base out and base in prisms).

Number of cycles completed in 60 seconds are recorded.

Expected findings

Fusional facility at near: 15 cycles per minute.

Procedure in steps

1. A line better than the best corrected near visual acuity in the poorer eye is isolated.

2. The patient wears his normal correction for near.

3. A flipper with 12 prisms base out on one side and 3 prisms base in on the other side is introduced with the side having 12 prisms base out in front of the eyes.

4. The patient is asked to report as soon as the line appears single and clear.

5. Prisms are flipped to expose both the eyes to 3 prisms base in.

6. This constitutes one full cycle.

7. Number of cycles completed in 1 minute are recorded.

7. Evaluation of accommodation parameters

For a comprehensive binocular vision examination, it is essential to check the amplitude of accommodation, negative and positive relative accommodation and facility of accommodation. These are required as the accommodation convergence mechanism plays a big role in binocular vision. It is necessary to have these parameters within normal ranges to avoid binocular distress leading to aesthenopic symptoms [2, 3, 4, 5].

The amplitude of accommodation is age specific and decreases with age. There are several methods to calculate as well as predict the amplitude of accommodation.

Donder’s table is a very good reference to predict it.

Hofstetter’s formula is another way to predict amplitude of accommodation:

1. Minimum expected amplitude: 15–0.25 (age)

2. Average expected amplitude: 18.5–0.30 (age)

3. Maximum expected amplitude: 25–0.40 (age)

For a person who is 20 years old, according to Hofstetter’s formula:

1. minimum expected amplitude: 15–0.25 (20) = 10 D

2. Average expected amplitude: 18.5–0.30 (age) = 12.5 D

3. Maximum expected amplitude: 25–0.40 (age) = 17 D

For comfortable binocular vision, the amplitude of accommodation of the two eyes should be within one diopter of each other.

There are different ways to physically measure the amplitude of accommodation:

1. Push up method

2. Negative lens method

7.1 Amplitude of accommodation: Push up method

This procedure measures the ability of a patient’s crystalline lens to change focus when a near stimulus is provided. The patient’s distance correction is placed in the trial frame and the eye not being examined is occluded. His attention is directed towards a row of letters on a near card, one line larger than the best corrected near acuity. The patient is instructed to keep the letters clear. The card is slowly moved closer to the patient and the patient is asked to report when he experiences the first sustained blur. The distance between the card and the patient’s spectacle plane is measured in centimeters. This measured distance is called the near point of accommodation (NPA).

The linear distance thus measured is converted into diopters to obtain the amplitude of accommodation.

For example, if the near point of accommodation is measured to be 20 cm, then the amplitude of accommodation is 100 / 20 = 5 D.

Amplitude of accommodation is then measured for the other eye too. Push away is an alternative method usually used with young children, who may not understand the concept of blur. In this method, the card is moved away from close to the patient to a distance where the patient reports that the letters become clear for the first time.

NPA is recorded monocularly as well as binocularly, and then converted to the amplitude of accommodation.

Procedure in steps

1. The patient wears his or her usual correction for distance.

2. He is directed to a row of letters on the near card, which is one line larger than the best corrected near acuity.

3. The card is brought closer to the patient, until he or she reports a sustained blur.

4. The distance between the card and the spectacle plane is measured and is called the near point of accommodation.

5. NPA is converted to its dioptric equivalent to give the amplitude of accommodation.

7.2 Amplitude of accommodation: Minus lens to blur

This is a monocular method to measure the amplitude of accommodation. Minus lenses are used in this test to stimulate accommodation.

Distance prescription is placed in the trial frame and a near card is introduced at 40 cm. The fellow eye is occluded. The patient is asked to look at the line of letters which is one point larger than his near visual acuity.

Minus lenses are added – 0.25 D at a time, with the patient given enough time to clear the letters. Lenses are added until the patient reports first sustained blur. Minus lenses added over the distance prescription plus 2.50 D (accommodative demand for the distance of 40 cm) is the amplitude of accommodation. For example, if – 4.00 D is added over the distance prescription to obtain first sustained blur, then the amplitude of accommodation is 4.00 + 2.50 = 6.50 D.

Procedure is repeated for the other eye and the readings recorded. The amplitude of accommodation as determined by minus lens method is almost 2.0 D less than that measured by the push up method described previously.

Procedure in steps:

1. The patient wears his usual correction for distance.

2. He is directed to a row of letters on the near card, which is one line larger than the best corrected near acuity.

3. Minus lenses are added 0.25 D at a time until the patient reports first sustained blur.

4. Minus lenses plus 2.50 D, gives the amplitude of accommodation for that eye.

5. The procedure is repeated for the fellow eye and measurements recorded.

7.3 Negative and positive relative accommodation

For a non presbyope, the trial frame should have the best corrected distance prescription, and for a presbyope, it should have the patient’s near correction.

A near vision card is placed at 40 cm from the patient’s spectacle plane. He is required to look at one line larger than his best near visual acuity.

To assess patient’s negative relative accommodation (NRA), plus lenses are added binocularly, in steps of +0.25 D. Plus lenses are added until the patient reports first sustained blur. Plus lenses added are noted and this is the NRA value.

Remove the plus lenses until the initial stage of the test is reached (best corrected distance or near correction). On reaching this stage, positive relative accommodation test (PRA) is performed by adding minus lenses binocularly, in steps of – 0.25 D. These are added until the patient reports first sustained blur. Record the total minus lenses added, and this is PRA.

In this test, accommodation is relaxed during NRA and stimulated during PRA. As the test distance is kept constant (40 cm), no accommodative convergence takes place, and that is compensated by changes in fusional vergence.

Expected findings:

For a non presbyope: NRA: + 2.00 D (+/− 0.50 D); PRA: – 2.37D (+/− 1.00 D). For a presbyope, the values vary widely.

Procedure in steps:

1. The patient wears his correction for distance (non presbyope) or near correction (presbyope).

2. He is directed to a row of letters on the near card, which is one line larger than the best corrected near acuity.

3. The patient fixates the target binocularly.

4. Plus lenses are added in +0.25 D steps, until he reports first sustained blur (NRA).

5. Plus lenses are removed slowly and minus lenses are added in - 0.25 D steps until first sustained blur is reported by the patient (PRA).

6. These values are recorded.

To assess the dynamics of accommodative system over a period, accommodative facility testing is done. Patient’s ability to make rapid repetitive accommodative changes over an extended period is assessed. This is also known as the inertia of accommodation. Sustenance of accommodation could also be checked while doing this test.

7.4 Accommodative facility

This test is performed both monocularly and binocularly. Monocular test is purely accommodative, while the binocular test is an interaction between accommodation and vergence mechanism.

The patient wears his correction for distance and is required to look at the near card held at 40 cm. A line of letters one line larger than his best near visual acuity is isolated. Plus lenses of +/− 2.00 D flipper is introduced in front of the patient’s right eye, while the other eye is occluded. The patient is asked to report as soon as the line of letters clears out. The flipper is flipped to minus lenses and the procedure is continued for 1 minute. Clearing the target through a plus and a minus lens constitutes 1 cycle. Number of cycles performed in 1 minute are recorded. The procedure is repeated for the left eye.

For binocular assessment, flipper is introduced binocularly, and the number of cycles performed in 1 minute is recorded.

Expected findings:

Children (8 to 12 years): 5 cpm binocular, 7 cpm monocular.

Adults (up to 35 years): 10 cpm binocular, 11 cpm monocular.

Monocular findings should be within 4 cpm of each other for each eye.

Procedure in steps:

1. The patient wears his distance correction.

2. He is directed to a row of letters on the near card, which is one line larger than his best corrected near acuity.

3. The patient fixates the target monocularly.

4. A +/− 2.00 D flipper is introduced, and the patient is asked to report as soon as the target clears out.

5. Clearance of target through a plus and a minus lens constitutes 1 cycle.

6. The number of cycles performed in 1 minute is recorded.

7. Procedure is repeated for the other eye and also performed binocularly.

8. Classification of neuromuscular ocular disorders

Nonalignment of visual axes relative to each other is the most common neuromuscular ocular disorders. These are classified based on different criteria [3, 4]:

1. Tropia/phoria

2. Direction of deviation

3. Comitancy

4. Constancy

5. Vergence system

6. Fixation

7. Onset

8.1 Non strabismic binocular vision disorders

These are highly prevalent conditions, next in prevalence only to refractive errors. Patients suffering from these disorders usually present with the following symptoms:

Eyestrain

Diplopia

Headaches

Sleepiness

Blur

Tearing

Inability to change focus from distance to near or vice versa

Lack of attentiveness

To diagnose these problems, evaluation of binocular status as well as accommodation status is required.

Tests required for binocular status evaluation are:

Near point of convergence

Cover test for both distance and near

AC/A ratio

Vergence testing for distance and near

Vergence facility

Randot Stereopsis

Test required for accommodative status evaluation are:

Accommodative amplitude

Accommodative facility

MEM retinoscopy

NRA/PRA

All the above tests can be grouped into 3 basic categories:

1. Positive fusional vergence (PFV)

2. Negative fusional vergence (NFV)

3. Accommodation

For positive fusional vergence group (PFV), following direct and indirect tests need to be performed:

Step vergence / smooth vergence (phoropter)

Vergence facility

NRA

Binocular accommodative facility with plus

NPC

MEM retinoscopy

For negative fusional vergence group (NFV), following direct and indirect tests need to be performed:

Step vergence/smooth vergence (phoropter)

Vergence facility

RA

Binocular accommodative facility with minus

MEM retinoscopy

For accommodative group, following direct and indirect tests need to be performed:

1. Amplitude of accommodation

2. Monocular accommodative facility

3. MEM

4. Binocular Accommodative Facility.

5. NRA/PRA.

After performing the above mentioned tests, different syndromes are identified and managed accordingly:

1. Convergence insufficiency

2. Convergence excess

3. Fusional vergence dysfunction

4. Divergence excess

5. Basic Exophoria

6. Divergence Insufficiency

7. Basic Esophoria

8. Vertical Heterophoria

9. Accommodative insufficiency

   1. ill-sustained accommodation

10. Accommodative excess

11. Accommodative infacility