Assessment of binocular vision and accommodation

6.2.1

The evidence base: When and how to detect and assess strabismus and phoria

As the detection of strabismus and phoria is a fundamental part of any assessment of the binocular vision system, a cover-uncover test assessment should be part of every eye examination. It is used to detect these two conditions in the habitual state (i.e., with and/or without glasses in the pre-refraction part of the eye examination). A limitation of the cover test is that it requires a good view of your patient’s eyes, so that it is not possible to accurately perform the test in a phoropter or trial frame with reduced aperture lenses. For this reason, when subjective refraction indicates that a non-strabismic patient’s refractive correction has altered substantially, changes in the oculomotor status are typically estimated using subjective tests such as the modified Thorington or Maddox rod and wing tests ( section 6.11 ) rather than using the cover test.

A limitation of the cover test is that even experienced clinicians cannot detect very small deviations (up to 2–3 ^Δ ). Thus when no movements are detected on the objective test, the alternating cover test can also be run in a subjective fashion, where the patient is asked to say if the target being viewed appears to move when the cover is transferred from one eye to the other. The subjective test is particularly useful for identifying small vertical deviations, which can otherwise be missed and can cause significant visual problems.

6.2.2

Procedure for the cover test

The online 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 6.10 show a variety of cover test movements with strabismus (exotropia, esotropia, hypertropia, alternating) and heterophoria (exophoria, esophoria, Hering’s movements and Phi movements). Videos 6.11 and 6.12 show heterophoria measurement using a prism bar. A summary of the procedure is shown in Box 6.1 .

• 1.

  Prior to performing the test, be aware of possible strabismus from indicators in other parts of the eye examination, which could include simple observation of your patient; symptoms of diplopia when tired; an ocular or family history of an eye turn, patching or strabismus surgery; premature birth; reduced visual acuity in one eye suggesting possible amblyopia, and strabismic risk factors such as anisometropic hyperopia.

• 2.

  Similarly, be aware of possible signs and symptoms of decompensated heterophoria, such as symptoms of blurred vision, headaches, or asthenopia at distance and/or near; poor reading ability (or reading avoidance); poor progress at school. It is important to note that a lack of symptoms does not mean that the binocular system is normal as the patient may have suppression.

• 3.

  Keep the room lights on and, if necessary, use localised lighting so that you can see your patient’s eyes easily without shadows.

• 4.

  Explain the purpose of the test to your patient: “I am now going to find out how well your eye muscles work together.”

• 5.

  Use the following targets:

  • (a)

    For the distance cover test, isolate a single letter of a size one line larger than the patient’s worst visual acuity. For example, if monocular visual acuities are 105 VAR (6/4.8, 20/16 or 1.25) and 90 VAR (6/9.5, 20/32 or 0.63), use an 85 VAR letter (6/12, 20/40 or 0.50) as a target. The patient must be able to see the letter easily with both eyes, but it should be a target that requires accurate fixation and stable accommodation. If you cannot present an isolated letter, ask the patient to look at a letter at the end (or beginning) of a line. If the monocular visual acuity in either eye is 75 VAR (6/19, 20/63 or 0.32) or worse, use a spotlight for fixation.

  • (b)

    For the near cover test, a fixation stick should be used that contains letters or pictures of various sizes ( Fig. 6.1 ). A single letter of a size one line larger than the patient’s near visual acuity of the poorer eye should be chosen. The fixation stick should be held at the patient’s typical near working distance. This may be at an intermediate distance (e.g., 60 cm) if you wish to assess the binocular status at a distance corresponding to the distance from which the patient views a computer screen.

    

• 6.

  Sit directly in front of your patient, at a distance of 33–40 cm away, so that you are close enough to be able to critically note eye movements. Make sure you do not block the patient’s view of the target.

  • (a)

    For the distance cover test, make sure that the patient has their head held straight with the eyes in the primary position of gaze.

  • (b)

    For the near cover test, make sure the patient’s eyes are in a slight downward gaze to replicate their viewing position when reading.

• 7.

  Instruct your patient: “I would like you to look at the letter* at the other end of the room (or the letter * on this stick). If the letter moves when I cover one eye, please follow it and try to keep it as clear as possible at all times”.

• 8.

  Perform the cover-uncover test to look for a strabismus ( Box 6.1 , Fig. 6.2 ):

  • (a)

    Place the cover before the left eye and observe the right eye. Repeat this procedure two or three times. If the right eye moves when the left is covered, then a strabismus is present in the right eye. You should allow the eye time to take up fixation, which may be as long as 2–3 seconds. If the eye moves OUT to take up fixation, then in the binocular situation it must have been directed inwards and so an ESOtropia is present. If the eye moves IN to take up fixation, an EXOtropia is present. If the eye moves UP to take up fixation, then in the binocular situation it must have been directed downwards and so a HYPOtropia is present. If the eye moves DOWN to take up fixation, a HYPERtropia is present.

  • (b)

    Repeat the cover-uncover test by placing the cover over the right eye and look for a strabismus in the left eye. Once again, repeat the procedure two or three times. If neither eye moves when the other is covered, there is no strabismus and you should go to step 8 below.

  • (c)

    In a unilateral strabismus, when the deviating eye is covered and then uncovered, the ‘normal’ eye will continue to fixate and will not move. Reduced visual acuity (e.g., caused by amblyopia) will often be present when there is unilateral strabismus. For this reason, eyes with strabismus and amblyopia may not take up fixation immediately when the normal eye is covered, particularly if the visual acuity is poor in the deviating eye. Give them time to fixate and actively encourage them to do so. Note and record any fixation instability or tremor (nystagmus).

  • (d)

    If strabismus was detected, repeat the test to confirm your diagnosis.

  • (e)

    Estimate or measure the size of the strabismus. The lowest prism power that eliminates movement of the strabismic eye to take up fixation using the cover-uncover test indicates the strabismus size. The base direction should be IN for exotropia, OUT for esotropia, DOWN in front of a hypertropic eye, and UP in front of a hypotropic eye. Although it does not matter which eye receives the prism, it is easier to place the prism in front of the fixating (i.e., the non-deviating) eye as you then have an unobstructed view of the movement of the strabismic eye.

  • (f)

    If a strabismus is present, it is not appropriate to search for a phoria. You should record your result and move on to the next test. The terminology used to describe different types of strabismus is provided in Table 6.1 , and examples of how to record the results from cover testing are given in Table 6.2 .

    Table 6.2

    Examples of recordings from the cover test

    Abbreviation	Description
    With Rx: NMD @ D or N	No movement detected (hence deviation <2–3 Δ ) during distance and near viewing wearing appropriate refractive correction.
    N without Rx: <3 Δ SOP (Phi)	Unaided, at near, a small esophoria (<3 Δ ) is present but not seen; it is reported subjectively.
    D with Rx: ∼4 Δ XOP	A small exophoria, estimated to be 4 Δ , is present during distance viewing with appropriate refractive correction.
    8 Δ SOP @ N with Rx, slow rec.	An esophoria with slow recovery, measured to be 8 Δ , is present on cover/uncover testing, during near viewing with appropriate refractive correction in place.
    ∼4 Δ R/L @ D & N, with and without Rx	Right hyperphoria, estimated to be 4 Δ , is present during distance and near viewing. The deviation is unchanged by refractive correction.
    Int (50%) ∼10 Δ RSOT @ N without Rx	Intermittent right esotropia is present about 50% of the time during near viewing without refractive correction and estimated to be 10 Δ .
    8 Δ R hyper T @ D & N, with and without Rx	Constant right hypertropia, measured with a prism bar to be 8 Δ , is present during distance and near viewing with and with refractive correction.
    25 Δ Alt XOT 4 R/L @D without Rx	Constant alternating exotropia of 25 Δ with a much smaller vertical component (R hypertropia) during distance viewing without refractive correction.

  

• 9.

  If no strabismus was found, now search for phoria using the cover-uncover and/or the alternating cover test ^{, , ,} (see Box 6.1 , Fig. 6.2 ).

  • (a)

    Cover-uncover: Place the cover before the left eye and hold it there for ∼2 seconds. Then remove the cover and observe the response of the previously covered eye. If the eye changed its position when covered, the recovery movement of the eye will be opposite to that which took place behind the cover. For example, in EXOphoria the eye moves IN when the cover is removed as it drifted out (away from the nose) behind the cover. To confirm your observations, repeat the cover/uncover test by covering and uncovering the right eye. If there is a phoria, the movements seen on cover-uncover test will generally be very similar in magnitude irrespective of whether the right or left eye is covered.

  • (b)

    The alternating cover test: Place the occluder before one eye for 2–3 seconds and then transfer it quickly to the other eye. Again keep the occluder in front of the eye for 2–3 seconds and then repeat the cycle. The patient must not view the target binocularly at any time, and thus rapid movement of the cover between the eyes is required. If there is a deviation of the eyes, it will be seen as a re-fixation eye movement when the cover is transferred from one eye to the other. As with the cover-uncover test you are observing the recovery movements of the eyes and these are the opposite of the eye movements when they are covered so that the eyes will move outwards in esophoria and inwards in exophoria. It is important to stress that, unlike the cover-uncover test, the alternating cover test cannot distinguish between a phoria and a strabismus. Thus the pattern of eye movements on an alternating cover test will not differ between a patient with exophoria compared with a patient with exotropia. The advantage of the alternating cover test is that phoria movements are generally much more obvious compared with those seen during cover-uncover test. For this reason, whereas the cover-uncover test is used in all patients, the alternating cover test is only used in patients without a strabismus.

  • (c)

    Estimate or measure the magnitude of the phoria. Deviations can be measured by placing prisms of increasing power in front of one eye until no movement is observed during the alternating cover test. The prism is normally placed in front of one eye only. Base-in prism power is used to measure EXOphorias and base-out to measure ESOphorias. A prism bar is most conveniently used for this purpose, although estimates made by experienced clinicians can be in good agreement with measurements made using prism bars.

  • (d)

    During a cover-uncover test in a patient with a phoria, observing the latency, speed, and smoothness of the fusional recovery movement can give clues as to the strength of the fusion reflex. The movement should be smooth and fast. Poor fusion reflexes are slow and hesitant, often with jerky movements. Sometimes, although an eye has deviated behind the cover, there is no recovery when the cover is removed and a strabismus that was not there initially is now present (see Box 6.1 ). Note that the speed and smoothness of the movements seen during the alternating cover test hold little diagnostic value. This is because binocular vision is suspended during the alternating cover test. Judgements about the speed and smoothness of recovery are more valuable during the cover-uncover test because, unlike in the alternating cover test, the movements are taking place to restore binocular vision following the removal of the cover.

  • (e)

    Subjective cover test and phi movements: If, using the alternating cover test, you cannot detect any heterophoria and the patient can provide good subjective responses, continue to perform the alternating cover test and ask the patient if the target appears to move when the occluder is switched from one eye to the other. Subjectively reported movements of the target are called ‘phi’ (pronounced as ‘fy’ as in ‘why’) movements. The type of deviation present can be inferred according to whether the target appears to move in the same or opposite direction as the cover. For example, esophoria will cause the target to move ‘against’ the movement of the occluder and an exophoria will cause the target to move ‘with’ it. In ‘with’ movements: for example, when the cover is moved from the left eye to the right eye, the target is also seen to jump to the right. Extracting the direction of movement perceived by the patient can be difficult and it is best not to ask whether the movement was ‘with’ or ‘against or to the left or right, as these are easily confused.’ Instead, ask the patient to indicate to you whether the target moves towards you (the clinician) or in the opposite direction as you move the occluder.

6.2.3

Adaptations to the standard procedure

• 1.

  When examining children: Pictures can be used to retain attention, but they should be of an appropriate size. Pictures (or letters) that are too large do not provide an accurate stimulus for fixation or accommodation, which is important for an accurate cover test. In order to check compliance with your fixation instructions, occasionally it is useful to move the stick a little to one side. If the eyes are seen to follow the target, you can be confident that the patient is looking at the target.

• 2.

  When examining older patients: In presbyopic patients, cover testing needs to be conducted with their multifocal or varifocal lenses, and the target should be held in the correct location to ensure clear vision. Alternatively the appropriate correction should be provided by the phoropter or trial frame and trial case lenses. During near cover testing, it can sometimes be difficult to see an older patient’s eyes because of drooping upper lids and the downward gaze needed to view the fixation target with multifocal spectacles. First, asking the patient to tilt their head back slightly may improve the visibility of their eyes. If the problem is owing to drooping upper lids, they may need to be gently held up. In this case, you should ask the patient to hold the fixation stick. Finally, if it is otherwise not possible to see the patient’s eyes sufficiently well, ask the patient to hold their multifocal spectacles up slightly and to view the fixation target with their head erect and looking straight ahead with their eyes in the primary position of gaze. In this case, you should note that the near cover test has been performed in primary gaze rather than the preferred, slight downward gaze.

• 3.

  If a strabismus is suspected in a patient with equal visual acuity in the right and left eye: (see online video 6.6 ). When visual acuity is the same or very similar in the two eyes, investigate the possibility of an alternating strabismus. With an alternating strabismus, the right eye will exhibit the strabismus when the left eye fixates during the cover test and the left eye will exhibit the strabismus when the right eye fixates. The difficulty with diagnosing an alternating strabismus is that the strabismus movement only occurs during the first run of the cover-uncover test. This is because the preciously deviating eye has now become the fixating eye. Thus, when the cover-uncover assessment is repeated a second and third time, the eye being observed does not move. In this scenario, the strabismus will become apparent again during the first cover-uncover assessment of the other eye. When asked to view binocularly after completion of the cover-uncover test, some patients with an alternating strabismus will continue to fixate with the eye that fixated the target during the last iteration of the cover test procedure. In some cases there is no preferred fixating eye, whereas in others, there is a definite preference for fixation with one eye over the other and, although the non-preferred eye might continue to fixate for a short period (e.g., a few seconds) after the cover has been removed, fixation then switches back to the preferred eye. Some patients with alternating strabismus can switch the eye that is fixating if you ask them, and some may even switch fixation as the occluder approaches the fixing eye, so that they can be very confusing to diagnose.

• 4.

  In patients with an abnormal head posture (head turn or tilt): Ask the patient to straighten their head position before testing commences. If the abnormal head position is a permanent feature for a particular patient, the cover test should be carried out with the head in the habitual (i.e., turned/tilted) position and again when the head is straightened. If the deviation differs markedly with adjustment of the head position, it is likely that the head is being turned/tilted to address an underlying binocular vision issue. This can be further investigated if the head is tilted/turned in the opposite direction to the direction that the patient typically exhibits. If the deviation becomes even more pronounced, an incomitancy is certainly present ( section 6.16 ) and you can conclude that the abnormal head posture is linked to a binocular vision condition rather than to another, non-visual cause.

6.2.4

Recording

Examples are given in Table 6.2 .

• 1.

  Write ‘cover test’ or ‘CT’ and record separately for distance (D) and near (N).

• 2.

  Indicate the Rx, if any, the patient was wearing during testing.

• 3.

  Record NMD (no movement detected) if this was the case. NMD is preferred to ‘orthophoria’ or similar, as even experienced clinicians cannot detect eye movements less than 2–3 ^Δ .

• 4.

  Heterophorias found using the subjective cover test, but not seen by you, should be recorded in the usual manner and followed by the term ‘phi.’

  • If strabismus or phoria is detected, then record:

• 5.

  The size of the deviation (if estimated, precede your result with the symbol ‘∼’).

• 6.

  The laterality of the strabismus (right, left, or alternating recorded as R, L, or ‘Alt’; Table 6.1 ).

  Table 6.1

  Cover test strabismus diagnoses

  Unilateral: the deviation (strabismus) is only ever present in one eye (e.g., esotropia of the right eye).	Alternating: the deviation can exist in either eye. Only one eye deviates, but it can be the right eye or the left eye.
  Constant: the deviation is present all of the time.	Intermittent: The deviation is present only some of the time (e.g., an esotropia may appear only when the patient is tired).
  Comitant: The size of the strabismus does not vary with the angle of gaze.	Incomitant: The size of the strabismus varies with the angle of gaze; the strabismus may only be present in one gaze direction or it may not be present in some gaze directions.
  Distance: The strabismus is present when viewing distant targets (e.g., whilst watching TV).	Near: The strabismus is present when viewing near targets (e.g. when reading).
  Affected by refractive correction: The presence and size of strabismus is not affected by the wearing of appropriate refractive correction.	Not affected by refractive correction: The presence and size of strabismus is affected by the wearing of appropriate refractive correction (e.g., the strabismus is smaller or absent altogether when spectacles are worn).
  Unilateral constant: One eye deviates all of the time and it is always the same eye. Unilateral intermittent: The deviation is only present some of the time. When it is present, it is always the same eye that deviates. Alternating constant: There is always a deviation present but the deviating eye can be the right eye or left eye. Alternating intermittent: The deviation is not always present. When the deviation is present, the deviating eye can be the right eye or left eye.

• 7.

  The direction of the strabismus or phoria ( Table 6.2 ).

• 8.

  The type of deviation using P for phoria and T for tropia.

• 9.

  If the tropia is intermittent rather than constant ( Table 6.1 ), include an estimate of the percentage of time that the eye deviates.

• 10.

  Record phoria recovery movements on the cover-uncover test that were slow, hesitant, and/or jerky. This is of particular relevance when the heterophoria is large (i.e., when there is more chance it may be decompensated). Examples of cover test recordings are given in Table 6.2 .

6.2.5

Interpreting the results

Hering’s law states that the innervation to synergist muscles of the two eyes is equal. This would imply that the eyes would always move by equal amounts (in the same direction in version movements and in the opposite direction in vergence movements). The common cover test response, in which the fixating eye remains still and the previously covered eye, when the cover is withdrawn, moves to restore fusion thus contravenes Hering’s law. Hering’s law would predict that when one eye is uncovered, both eyes would make a version movement equal to half the deviation, and then both eyes would make an equal fusional (vergence) movement, to restore bifoveal fixation. This response does occur in some patients and it is important that it is not confused with heterotropic movements ( Fig. 6.3 ). Note that heterotropic cover test movements are in one direction and take place when the cover is introduced to the other eye, whereas when they occur, Hering’s law movements have the appearance of a ‘wobble’ and take place when the cover is removed from the other eye (see online videos 6.7 and 6.10 ).

Most children show no movement on the cover test at distance and either no movement or a just visible exophoria at near. There appears to be little information regarding cover test results for normal adults in the research literature. Textbooks suggest that the majority of adults will also show either no movement or a just visible exophoria or esophoria (up to about 4 ^Δ ) on the distance cover test. At near, a small amount (3–6 ^Δ ) of exophoria is considered normal (physiological exophoria) and this is likely to increase with age. As even experienced clinicians cannot detect very small eye movements (up to 2–3 ^Δ ), small hyperphorias may be missed with the cover-uncover test, although they are likely to be more obvious during the alternating cover test, particularly of the subjective (phi phenomenon) type. It is important to note that any vertical phoria detected is abnormal and is likely to need correction.

The movements made by each eye are usually similar in heterophoria. In cases where the heterophoria movements are more obvious in one eye than the other, suspect uncorrected or residual anisometropia, particularly in pre-presbyopic anisometropes with hyperopia.

6.2.6

Most common errors

• 1.

  Not positioning yourself appropriately to allow a clear and unimpeded view of the patient’s eyes.

• 2.

  Blocking the patient’s view of the distance target.

• 3.

  Covering and uncovering the eyes too rapidly (less than ∼2 seconds) so that the eyes do not have time to make the movements consistent with the deviation that is present.

• 4.

  Arriving at your diagnosis too quickly. Repeat the test two or three times to confirm your diagnosis.

• 5.

  Using a fixation target that is too large, so that accommodation and fixation are not precisely controlled.

• 6.

  Not moving the occluder in an appropriate fashion. For example, not swiftly transferring the cover from one eye to the other during an alternating cover test with the result that binocular vision is not fully suspended.

• 7.

  Mistaking a Hering’s law movement for a strabismus ( Fig. 6.3 e and f).

• 8.

  Failing to record information about the speed and/or smoothness of recovery in patients with a heterophoria during cover-uncover test. Conversely, recording information about recovery in patients with strabismus.

• 9.

  Not realising that the cover test results can change dramatically when the viewing distance is altered (e.g., from distance to near) or whether or not glasses are worn.

6.3.1

The evidence base: When and how alternative tests for strabismus should be used

In very young children, who may be unable to maintain fixation for long enough to allow the cover test to be performed, other tests to detect and strabismus can be used. The Hirschberg test compares the position of the corneal reflexes of the two eyes. It is quick and easy to perform, and requires little co-operation on the part of the patient, but it can really only be performed at near and intermediate distances. The penlight target provides a poor stimulus to accommodation and the test is more useful for detecting the presence of strabismus than for measuring its size, with even experienced clinicians obtaining results that differ by up to 10 prism dioptres. The Krimsky test extends the Hirschberg test by using prisms to equalise the apparent positions of the corneal reflexes in the two eyes, but is similarly inaccurate for measuring strabismus size. The Bruckner test relies on a comparison of the brightness of the retinal reflex in the two eyes. In the presence of a strabismus the reflex can be brighter and whiter in the deviating eye compared with the reflex from the darkly pigmented macular area of a normally fixating eye (see online video 6.13 ). The test can be useful in identifying which is the strabismic eye but it is prone to false–positive findings, indicating that a strabismus is present when it is not and hence the usefulness of the Bruckner test is controversial. ^, Given their limited accuracy, the cover test ( section 6.2 ) should be used in preference to these tests as soon as the child can co-operate with the cover test requirements. The tests described in this section are typically performed without spectacles, although a comparison with and without spectacles can also be made, for example in patients with significant hyperopia.

6.3.2

Procedure: Hirschberg and Krimsky

(See online video 6.14 ).

• 1.

  Keep the room fully illuminated. Additional use of localised lighting is recommended so that the patient’s eyes can be easily seen without shadows.

• 2.

  Hold a penlight horizontally 40 to 50 cm from the patient with the light directly in front of the patient’s face and aimed at the bridge of the patient’s nose. The back of the penlight should be very close to the tip of your nose so you are viewing from directly behind it.

• 3.

  Ask the patient to look at the light with both eyes open. Young children will automatically tend to look toward the bright light, but may need a little encouragement.

• 4.

  Note the location of the corneal reflex in each eye individually. To do this you should briefly cover each eye in turn; you can do this with the palm of your hand. Remember that the reflex is frequently decentred about 0.5 mm nasally with respect to the centre of the pupil because angle kappa is normally positive.

• 5.

  Now compare the location of the corneal reflexes in the two eyes as the patient views the light with both eyes open. The eye that has the its corneal reflex in the same position as in the monocular test is the fixing eye. The location of that reflex should be considered the reference position.

• 6.

  If a strabismus is present, the corneal reflex of the other eye will have shifted in a direction opposite to the strabismus. For example, in the case of an ESOtropia, the corneal reflex will be displaced temporally on the patient’s cornea relative to the position of the reflex in the fellow eye.

• 7.

  Hirschberg: Estimate the magnitude of the deviation from the displacement of the reflex in millimetres (mm) relative to the reference position using the approximation of 1 mm = ∼22 ^Δ .

• 8.

  Krimsky: Use a prism bar in front of the fixating eye and vary the prism power to achieve symmetrical positioning of the reflexes in the two eyes.

6.3.3

Procedure: Bruckner test

(See online video 6.13 ).

• 1.

  Turn down the lights so the room is dimly lit.

• 2.

  Hold a penlight 1 m from the patient with the light aimed at the bridge of the patient’s nose. The back of the penlight should be very close to the tip of your nose.

• 3.

  Ask the patient to look at the light with both eyes open. Young children will automatically tend to look toward the bright light but may need a little encouragement.

• 4.

  Compare the colour and brightness of the fundus reflexes. Differences in the apparent brightness of the fundal reflexes can signal the presence of a strabismus.

6.3.4

Recording Hirschberg, Krimsky, and Bruckner test results

Note the test used, the eye that deviates, and the direction of the strabismus. For example:

• Hirschberg @ 50 cm: No Strab; Hirschberg @ 50 cm ŝ Rx: ∼10 ^Δ RSOT; Krimsky @50cms: 15 ^Δ L XOT. Bruckner @1 m: L SOT.

6.3.5

Interpreting Hirschberg, Krimsky, and Bruckner test results

For the Krimsky and Hirschberg tests, the displacement of the reflex indicates the type of strabismus: nasally displaced reflex indicates exotropia; temporally displaced indicates esotropia; superior displacement indicates hypotropia, and inferior displacement indicates hypertropia. In visually normal eyes, both corneal reflections are usually displaced slightly nasally relative to the pupil centres because of the separation between the pupillary and visual axes (quantified by angle kappa). Thus if there is a large angle kappa, it may result in the misdiagnosis of strabismus or failure to detect a strabismus. Interestingly, patients with exotropia appear to have higher angle kappa values when compared with those who are esotropic. All of this raises questions about the reliability of the results of Hirschberg and Krimsky tests. For the Bruckner tests, any brightness difference may indicate the presence of at least a moderate sized strabismus, although the brightness difference gives no indication of its type or size. Also, when interpreting the results of the Bruckner test, remember that there are other causes of interocular brightness differences, including cataract and retinoblastoma.

6.3.6

Most common errors using Hirschberg, Krimsky, and Bruckner tests

• 1.

  Hirschberg and Krimsky: Basing your decision on the absolute position of a single reflex relative to the pupil centre rather than on a comparison of the relative locations of the corneal reflexes in the two pupils.

• 2.

  Not viewing the patient’s eyes from a position that is directly behind the penlight for the Hirschberg and Bruckner tests or from directly in front of the deviating eye in the case of the Krimsky test.

• 3.

  Placing too much trust on the sensitivity and accuracy of these tests.

• 4.

  For the Bruckner test, not realising that interocular differences in reflex brightness may be caused by factors other than strabismus.

6.4.1

The evidence base: When and how to assess incomitancy

A test to detect and, where present, classify incomitant strabismus is part of all first eye examinations of a patient. It is typically part of the ‘entrance tests’ that are performed subsequent to case history and prior to refraction. Congenital incomitant strabismus is caused by a developmental problem in the anatomy or functioning of one or more of the six extra-ocular muscles or their nerve supply. Acquired incomitant strabismus can occur from conditions such as diabetes, hypertension, multiple sclerosis, thyrotoxicosis, temporal arteritis, or tumour and can be the first sign of the underlying disease and it is therefore essential to distinguish recent-onset from long-standing incomitant strabismus. Missing the signs of these conditions represents a significant cause of malpractice claims in the United States. Signs and symptoms that can differentiate between new and old ocular muscle palsies are given in Table 6.3 .

The motility test (or broad H test) is currently the simplest method of evaluating a deviation in the six diagnostic positions of gaze. It is relatively quick and easy to perform and requires only a pen torch and an occluder.

6.4.2

Procedure: Motility test

See online 6.15, 6.16, 6.17 of the motility test being used to detect incomitant deviations.

• 1.

  Be attentive to the possibility of incomitant strabismus (particularly recent-onset) via indicators from the case history (see Table 6.3 ).

• 2.

  Keep the room lights on and illuminate the eyes without shadows. Explain the test to your patient: “This test checks whether all your eye muscles are working well to move your eyes together.”

• 3.

  Ask the patient to remove their spectacles, if worn. Spectacles can make observation of the eyes more difficult, and the frame may hide the fixation target. In addition, in peripheral gaze, diplopia can be induced by the prismatic effect produced by anisometropic spectacles and the ‘jack-in-the-box’ effect of myopic spectacles, particularly with small, modern frames. Sit directly in front of the patient so that both eyes can be clearly viewed.

• 4.

  The target used is not critical as long as the patient can easily see it, although a penlight is particularly useful because it allows you to observe the corneal reflexes and it will indicate when the light has moved from the binocular field into the monocular field, when one of the corneal reflexes will disappear. A small picture on a fixation stick may be used when examining children.

• 5.

  Instruct your patient: “Please watch the light/picture and follow it with your eyes. Keep your head still. Tell me if the light/picture appears doubled at any time or if your eyes feel particularly uncomfortable or painful in any of the positions. Don’t worry if the light/picture appears blurred.” Patients sometimes mistake diplopia for blur so this can be a very important distinction to make in your instructions.

• 6.

  Shine the penlight at eye level towards the patient from a distance of approximately 40 cm. First ask the patient if the light appears single in the straight-ahead position. If it appears single, start to move it in an arc with the bridge of the patient’s nose at the centre, so that as the light is moved, it continues to be aimed at the bridge of the nose. For each gaze direction, move the penlight to the edge of the binocular field. The loss of the corneal reflex will indicate to you that you have moved into the monocular field. Note that at the limits of movement, even those with normal ocular motility can feel uncomfortable and end-point nystagmus may be visible.

• 7.

  If a strabismus is present in the straight-ahead position, no diplopia may be reported in that gaze direction owing to strabismic suppression. This may mean that the patient will not report diplopia in any direction of gaze, even if there is marked incomitancy. In such situations, objective judgements about relative eye positioning are more important than the subjective responses from the patient.

• 8.

  Move the penlight into the six diagnostic positions of gaze by moving the target in a cross or broad H formation. Either type of movement is acceptable. During downgaze, you may need to ask the patient to open their eyes widely or to hold up the patient’s eyelids to gain a view of the eyes and corneal reflexes. Transfer the target/penlight from your left to your right hand when switching between the patient’s right and left visual fields.

• 9.

  Carefully look for any misalignment of the eyes in all positions of gaze (the corneal reflexes can help you in this). Also determine whether the movements of the eyes are smooth and accurate (see pursuit eye movements, section 6.17 ).

• 10.

  If the eye movements were smooth and accurate with no reported diplopia, and both eyes appeared to the practitioner to track the moving target, the test is complete and the results can be recorded.

• 11.

  If, in one or more gaze directions, the two eyes appear not to be looking at the pen torch from your position directly in front of the patient, a useful tip is to switch your viewing position to directly behind the pen torch as the patient looks in this gaze direction. This will enable you to ascertain whether both eyes are in fact looking at the light, and if not, which eye is not adopting the appropriate position.

• 12.

  Note and record the position of the eyes when any of the following occur:

  • (a)

    The patient reports any diplopia.

  • (b)

    The patient reports pain or discomfort in one or more positions of gaze that exceeded that experienced in other gaze directions.

  • (c)

    Any underaction or overaction in one eye.

  • (d)

    Jerky or inaccurate pursuit eye movements.

  • (e)

    The size of the palpebral apertures differs between the right and left eyes and/or varies as a function of the direction of gaze.

• 13.

  Locate the gaze direction that yields the greatest diplopia because this indicates the field of action of the affected muscle(s). This can be difficult for some patients as similar separations of the doubled images may be reported in two or more directions of gaze.

• 14.

  Establish whether the doubled images are horizontally or diagonally separated. Pure horizontal diplopia does arise (e.g., along the horizontal meridian), but purely vertical diplopia does not arise because of the secondary actions of the elevating and depressing extra-ocular muscles. Thus diagonal separation is found most commonly in cases where one (or more) of the oblique or vertical recti muscles is affected.

• 15.

  When diplopia is reported, briefly cover each eye in turn to identify which eye is seeing which image. When the eyes are elevated, the eye that sees the higher image is seen by the eye which is physically lower than in its fellow; if there is an underaction this is the eye with the underacting muscle. Alternatively, it could be that there is an overaction in which case the fellow eye has risen too high. Similarly, when the eyes are looking down, the eye seeing the lower image is seen by the eye with the underacting muscle if there is an underaction present; if not, the other eye has an overacting muscle and it has descended too far. Similarly, when the eyes are looking right or left, the eye that sees the image that is further to the right or left, respectively, represents the eye that has moved less from the primary position. A cover test ( section 6.2 ) performed in this direction of gaze can be used to confirm the diagnosis.

• 16.

  If an incomitancy is observed, repeat the testing monocularly (assessment of ductions) to help discriminate between paretic and mechanical incomitancy.

6.4.3

Recording motility findings

If the ocular movements appear full and no diplopia is reported in any position, a normal result has been obtained and may be recorded using the acronym SAFE (or FESA). This indicates that the ocular motility movements were (S) Smooth, (A) Accurate, (F) Full, and (E) Extensive. For a patient with strabismus, normal motility can be recorded as ‘no incomitancy detected’ if the size of the strabismus did not change objectively in different directions of gaze.

In cases where you detect an incomitancy or where diplopia is reported by the patient in some directions of gaze, but not others (or more in some than in others), record a cross/H-pattern to clearly indicate where diplopia was indicated by the patient and where it was maximal. Fig. 6.4 shows the six cardinal diagnostic positions of gaze, showing the Yoke muscles that principally maintain the eyes in these positions. Record any apparent underactions or overactions, clearly stating which eye and in which gaze direction this was observed. Increasingly, incomitancies are recorded using a 9-point scale. Using this system, overactions and underactions are recorded on a basic template in the primary field of action of each muscle. Underactions are recorded as negative numbers on a scale from −1 to −4, where −4 represents the greatest underaction. Similarly, overactions are recorded using positive numbers on a scale from +1 to +4, where +4 represents greatest overaction. For example, underactions that are scored as −4 indicate that the eye is unable to move at all from the primary position into the field of action of that muscle. Overaction of a horizontal rectus muscle is graded according to the amount of cornea covered by the canthus; in extreme overaction (+4), half of the cornea is concealed. This diagrammatic representation also provides a useful way of signalling the presence of an A or V pattern, restrictions of movements as well as other ocular movement abnormalities (e.g., up- and down-drifts, up- and down-shoots).

If a head tilt/turn is present, it should be noted and you may find it useful to perform the motility in the head-straight and head-tilted/turned conditions so that the results can be compared. The movements observed during a motility test may conform to one of the so-called alphabet patterns. For example, if the deviation is significantly (>15Δ) more convergent (i.e., more eso or less exo) in upgaze than in downgaze, this is referred to as an A-pattern. Similarly, the term V-pattern describes a situation where the deviation is significantly more divergent (i.e., more exo or less eso) in upgaze than in downgaze. Note that incomitancies of this nature do not always conform strictly to the A and V patterns. For example, some patterns may be more correctly described as Y- or inverted Y-patterns.

6.4.4

Interpreting motility results

The results of motility testing are relatively straightforward to interpret if there is a problem with the lateral or medial rectus muscles. A movement to the patient’s right along the horizontal meridian will examine the right lateral rectus and left medial rectus. Eye movements to the patient’s left will assess their left lateral rectus and right medial rectus.

The clinical interpretation of motility test results is more complicated when diplopia is experienced on upwards or downwards gaze, because there are four extra-ocular muscles that help to elevate (the right superior rectus, RSR; left superior rectus, LSR; right inferior oblique, RIO; and left inferior oblique, LIO) and four that depress the eyes (the right inferior rectus, RIR; left inferior rectus, LIR; right superior oblique, RSO; and left superior oblique, LSO). By having the patient look in the various directions of gaze, a clinical determination of affected muscle(s) can be made because the eyes will appear most misaligned (and the diplopia noticed by the patient will be maximal) when the eyes move into the field of action of the affected muscle. Diplopia experienced in peripheral gaze can be caused by an underaction or overaction of one or more muscles. An underaction could be caused by a mechanical restriction or a muscle palsy/paralysis. An overaction will occur in the non-paretic eye if the paretic eye is fixating the target.

Duction (monocular) testing (step 16) helps to differentiate between an incomitant deviation owing to paresis/paralysis (underactions are less obvious monocularly) and one caused by mechanical restriction (underactions are similar monocularly and binocularly). To aid or confirm a diagnosis of an underaction/overaction, or to measure the extent of the deviation and to assess the degree of incomitancy, further tests such as the 9-point cover test are required ( section 6.16 ).

6.4.5

Most common errors with motility testing

• 1.

  Allowing the patient to turn their head towards the target. The head should remain in the straight-ahead position so as to fully test the ocular motility.

• 2.

  Not using a penlight. This makes it difficult to determine when the target is entering the monocular field.

• 3.

  Relying too much on the patient to report doubling and not paying enough attention to symmetry of corneal reflexes and the appearance of the relative positioning of the eyes.

• 4.

  Moving the target too quickly or too slowly.

• 5.

  Moving the target in a straight line rather than an arc, so that increasing unequal angular demands are made of the two eyes as the target is moved into a peripheral position of gaze.

• 6.

  Not elevating the top eyelid when viewing the eye movements in downgaze.

6.5.1

The evidence base: When and how to measure NPC

NPC is often included in the ‘entrance tests’ of a typical eye examination as part of the assessment of the binocular vision system. The test determines convergence ability in the patient’s habitual state (i.e., with their glasses if worn). It is used with cover test results to screen for convergence insufficiency, which has a relatively high prevalence, and is a treatable condition. Note that a lack of symptoms does not in itself indicate good convergence because a remote NPC can be associated with suppression or with the avoidance of near work.

The NPC is a quick and easy test to perform. It requires no special equipment and it provides a very repeatable result. It is the standard test for convergence ability and it provides both subjective and objective information. In patients who are presbyopic the choice of target does not seem important, but in those who are pre-presbyopic there may be a difference in NPCs measured with accommodative and non-accommodative targets. Although such differences may not be clinically significant in patients with normal vision, Scheiman et al. suggest that individuals with convergence insufficiency show more remote break and recovery NPCs with a penlight compared with when an accommodative target is used. The NPC can be measured without glasses or with the distance or near correction, depending on when symptoms occur.

6.5.2

Procedure: NPC

(See online video 6.18 ).

• 1.

  Seat the patient comfortably with their head erect and eyes in a slightly downward gaze. Sit directly in front of the patient so that you have a clear view of the two eyes.

• 2.

  Keep the room lights on. If necessary, position additional lighting to illuminate the patient’s eyes and/or the target without causing glare.

• 3.

  Explain the measurement to the patient: “This test determines how well your eyes can turn in to follow a close object.”

• 4.

  Position the target at a distance of ∼50 cm directly in front of the patient slightly below the midline. A target with fine detail should be avoided because otherwise patients (particularly presbyopes) often confuse blur with diplopia. In adults, the tip of a pen can be used. A medium sized, coloured picture on a fixation stick can be used with children.

• 5.

  Instruct your patient: “Please keep looking at the pen/picture as I move it towards your eyes. Let me know as soon as it becomes doubled—not blurred but doubled. Try really hard to keep it single. Don’t worry if you feel your eyes pulling.”

• 6.

  Make sure that the patient is looking at the target with both eyes. It can be useful to move the target to the side slightly to check that the patient is maintaining fixation.

• 7.

  Slowly but steadily move the target towards the bridge of the patient’s nose. The speed should be such that it takes 5–10 seconds to move the target from 50 cm to the bridge of the patient’s nose.

• 8.

  Observe the patient’s eyes for loss of convergence. Measure the distance the target is from the eyes when one of the eyes loses fixation by flicking outwards (objective NPC) and/or the patient reports diplopia (subjective NPC).

• 9.

  If the target becomes doubled (subjective NPC) more than 10 cm from the bridge of the nose, encourage the patient to make an extra effort to make the target single again. Moving it away slightly will help this before the target is again moved towards the patient. When single binocular vision is re-established, advance the target again towards the patient.

• 10.

  If a patient reports diplopia at 7 cm or above yet both eyes appear to be converging to the target, the patient may be confusing diplopia with blur. Check this by covering one eye and asking the patient if the target is still double. Continue to move the target in until the objective NPC is found.

• 11.

  Once the NPC has been reached, slowly move the target away from the patient’s eyes and ask when the target becomes single again. Measure this point and record it as the recovery NPC point. Repeat the test. If the patient can keep the target single to their nose, this is recorded as ‘to nose’ and thus a recovery point is not measured.

• 12.

  If the history indicates that the patient requires prolonged and/or excessive convergence in a specific position of gaze, repeat the procedure under those circumstances.

6.5.3

Recording NPC

NPC: Record the break and recovery NPC points in centimetres from the bridge of the nose. Record the break point first, followed by the recovery point. Distinguish between objective and subjective NPC measures. Examples are given in Table 6.4 . If the subjective NPC is much larger than the objective NPC, it is likely that the patient has confused blurring with diplopia and the objective NPC should be recorded. If the patient reports that the target is still seen singly when the eyes are seen to be misaligned, suspect suppression and investigate further.

6.5.4

Interpreting NPC results

Normative NPC values show considerable variation between studies. One source of variation arises from the fact that, although some patients can converge, they may not do so on clinical testing. This makes it important not to overinterpret one abnormal test result and to consider results in combination ( section 6.18 ). Scheiman et al. suggest a clinical cut-off value of 5 cm for the NPC break and 7 cm for the NPC recovery with either an accommodative target or a penlight in children and adults. Children and adults should be able to converge to within about 7.5 cm and recovery should return within 10.5 cm. A recent study in children by Menjivar et al. recommends an NPC-break cut-off of 7 cm or above as potentially problematic. Thus NPC values 7 cm or above suggest possible convergence insufficiency and should be investigated further. This investigation should include jump convergence , , distance and near heterophoria ( sections 6.2 and 6.11 ), near fusional reserves ( section 6.12 ), and near fixation disparity ( section 6.13 ). Given the reported high prevalence of accommodative insufficiency in children with convergence insufficiency, tests of accommodation should also be conducted in these patients. The effect of correcting refractive error on convergence ability should be assessed.

Instead of a failure of the eyes to converge, it is possible that diplopia will be reported because of overconvergence. This is rarely encountered, but when it does arise it suggests that the patient may have an abnormally high accommodative convergence to accommodation (AC/A) ratio ( section 6.14 ) or significant uncorrected hyperopia. This should be recorded, and additional investigations should be carried out.

6.5.5

Most common errors (NPC)

• 1.

  Relying on subjective NPC measures. Objective estimates should also be gained from careful observation of the eyes as they converge.

• 2.

  Carrying out the test once only; the test should be carried out at least twice to gain an impression of sustained and repeated convergence ability.

• 3.

  Moving the target too rapidly can lead to over-estimation of convergence ability. Moving the target too slowly could cause the patient to lose interest. This is particularly true in children, especially if the target does not capture the child’s attention.

• 4.

  Not encouraging the patient enough to keep the NPC target single (particularly in children).

• 5.

  Testing the eyes in primary gaze instead of slight downward gaze.

• 6.

  Testing individuals who have a strabismus at near.

6.6.1

The evidence base: When and how to measure suppression

The Worth 4-dot test, which is often included in the ‘entrance tests’ of a typical eye examination as part of the assessment of the binocular vision system, determines the presence of suppression in the patient’s habitual state (i.e., with their glasses if worn).

The Worth 4-dot test is widely available and relatively cheap; it easy to use and can be used to assess fusion or reveal suppression at distance and near. It provides a rather imprecise indication of whether or not suppression is present, because a patient with unstable but functionally useful binocular vision may exhibit a suppression response on the test. Similarly, Worth 4-dot testing at near may incorrectly suggest that no suppression is present because of the large angular size of the lights when viewed at near. A major disadvantage of the test is that luminance of the red and green targets can vary widely between tests, as can the transmission characteristics of the red and green goggles, with the result that the test outcome can vary depending on whether the goggles are used in the standard format (red goggle in front of the right eye) or reversed. Another disadvantage of the test is that a patient with constant strabismus and abnormal retinal correspondence might achieve a normal result. For these reasons, a positive test result does not guarantee normal binocular vision. Because of this, the test result is only useful when considered alongside cover test results ( section 6.2 ). Another test of suppression is provided by the Mallett unit which uses Polaroid filters to provide different information to the two eyes, although this equipment is primarily used to assess fixation disparity at distance and near ( section 6.13 ).

6.6.2

Preparation for suppression test

• 1.

  Prior to performing a suppression test, be aware of possible suppression associated with conditions such as anisometropia, amblyopia, strabismus, unilateral eye disease, reduced visual acuity in one eye, no diplopia reported during NPC testing despite an objective failure to converge, and so on.

• 2.

  Explain the test to your patient: “This test checks whether you are using both eyes at the same time to see.”

6.6.4

Procedure: Mallett unit suppression testing

• 3.

  Do not allow the patient to see the Mallett unit until the Polaroid filters are worn over the eyes. The filters should be placed over any spectacles worn for that particular test distance.

• 4.

  For distance testing, suppression is present if the two monocular markers are not simultaneously visible to the patient. Cover each eye in turn and ask the patient to confirm seeing one of the markers (the red lines that are above and below the X of the OXO, section 6.13 ; Fig. 6.6 ). Then, with the eyes uncovered, ask the patient how many lines they can see. If two lines are not seen, it can be helpful to occlude the non-suppressed eye briefly. Occasionally, the patient will perceive two lines following this brief occlusion.

  

• 5.

  For near viewing, the same procedure is used. The patient should wear near correction, if appropriate. Now the monocular markers are green. In addition to the markers, one of the displays on the wheel of the near unit contains letters of different sizes. Whilst viewing through the polaroid filters, some letters are seen only by the right eye, some are seen only by the left eye, and the remainder are visible to both eyes. Thus if the patient reads all of the letters there is no suppression. The small letters provide a more detailed check on suppression than that offered by the markers. If one eye is suppressed the letters presented only to that eye will be omitted from the patient’s verbal reading of the letters. The clinician needs to be familiar with the letters that are presented to each eye and to both eyes. The letters on the left side of each line are presented to the left eye, those near the end of the line are visible only to the right eye, and those in the middle are presented to both eyes. In cases of mild suppression, the patient may read the larger letters but begin to omit letters on the smaller lines.

6.6.5

Recording suppression results

For the Worth 4-dot test, record the normal perception of four dots at 6 m and 40 cm as: ‘W 4-dot: 4-dots seen, DV and NV’ or similar. If suppression is found, indicate which eye was being suppressed and whether the suppression is continuous or intermittent. Indicate whether suppression was found at both distance and/or near, in both light and dark room conditions. Indicate whether the evidence of suppression was intermittent or constant. If diplopia is found, indicate the direction of deviation suggested.

For the Mallett-unit suppression test with the monocular markers, it is only necessary to record the presence of suppression when it interferes with fixation disparity assessment ( section 6.13 ).

For the letter suppression test on the near Mallett unit, record ‘no suppression’ if all letters were read. If letters were suppressed, record for example ‘suppression of RE on lowest two lines’ or, for example, ‘all RE letters suppressed.’

6.6.6

Interpreting the results of suppression tests

If a patient without strabismus sees all four dots on the Worth dot test, this is a normal test result. Note that absence of suppression does not mean that binocularity is necessarily normal. If a patient with strabismus sees four dots with the test, then this indicates that they have abnormal retinal correspondence (ARC). If the response is suppression of the right eye (i.e., the response is ‘three green dots’) or suppression of the left eye (i.e., the response is ‘two red dots’) (see Fig. 6.5 ), then there is a suppression scotoma larger than the angular subtense of one of the four dots. Because the dots on the distance target have a smaller angular subtense than those on the near target, suppression is found more frequently for distance viewing than for near. If the patient achieves fusion in the dark but not in the light, this indicates a shallower level of suppression as compared with the situation where suppression is present in both the dark- and light-room conditions.

The monocular markers on the Mallett unit provide a fairly gross test of suppression, meaning that if both markers cannot be perceived simultaneously, there is marked suppression. The letters on the near unit offer a more fine-grained check of suppression, although a disadvantage is that there is no quantitative scale to record the depth of suppression. One possible outcome is for larger letters to be perceived by both eyes but for the smaller letters to be suppressed by one eye. However, this is a fairly unusual result as suppression tends to be either present or absent.

6.6.7

Most common errors with suppression tests

• 1.

  Not performing the tests with the patient’s optimal refractive correction in place.

• 2.

  Asking the patient the leading question “Can you see four dots?” (Worth 4-dot test) or “Can you see the two lines?” (Mallett unit).

6.7.1

The evidence base: When and how to measure stereopsis

Stereopsis is often included in the ‘entrance tests’ of a typical eye examination in children because the presence of stereopsis strongly suggests roughly equal and good visual acuity at near in the two eyes and the absence of strabismus and/or amblyopia. ^, However, stereopsis testing has an important role to play in the visual assessment of all age groups ( section 3.11 ). There is growing evidence, for example, that stereoacuity levels are linked to the level of fine motor skills.

Distant stereoacuity tests, such as the Frisby-Davis Distance test and Distant Randot test, are not in common clinical use and are not discussed here. The TNO, Fly, Randot graded-circles, and Randot Pre-school tests may be difficult to use with young children because they may not be happy to wear goggles, although children from the age of about 3 years can usually be tested. ^, For younger children (6 months to 4 years) it is best to use tests that do not require goggles to be worn, such as the Frisby test, or if this test cannot be used, the Lang test. The Frisby test has the advantage that it provides the only test of real depth; all of the others create depth effects by artificial means; for this reason the Frisby is popular amongst many clinicians. Its drawback (monocular clues to depth) can be minimised with careful administration of the test.

The main advantage of the TNO test is that monocular cues are completely eliminated. The patient is required to describe the shape of the raised figure and because this shape is only seen if stereopsis is present there is no possibility of ‘cheating’ so you can be certain that stereopsis is present if the correct answers are given. The same is not necessarily true for the other tests because of monocular cues (e.g., Titmus Fly, Randot Circles test) and/or because head tilts or viewing from an oblique angle (e.g., Frisby) can help the patient to achieve a result that is not reflective of the genuine presence of stereopsis. However, by carefully following the correct procedures, this drawback should not be critical for the non-TNO stereotests. A disadvantage of the TNO is that the transmission characteristics of the red and green lenses may lead to different contrast levels being experienced by the patient. In some patients this can lead to a different test result depending on which way the goggles are worn (i.e., red before right eye or green before right eye). The Stereo Fly test is popular with children, although the Fly can frighten nervous or timid youngsters. The Randot graded-circles and Randot Pre-school tests operate on the same principle as the Fly test in that they use Polaroid spectacles to provide the disparate stimuli. Even when the polaroid spectacles are worn, a monocular, alert patient could identify which is the ‘odd one out’ by observing which of the circles is slightly displaced from the centre see ( Fig. 6.7 ). This disadvantage can be overcome to some extent by asking the patient, carefully and without leading them, what is odd about the target they selected, or whether the target seen in depth lies in front of or behind the other animals/circles. The target seen in depth is usually seen in front of the others, but by turning the book upside-down the target in depth is seen behind the other animals/circles. Other more recent versions of polaroid-based stereo tests include the Randot graded-circles and the Randot Pre-school tests. These tests have the advantage that they feature at least some material which, like the TNO, is constructed on a random dot principle, and thus which requires stereopsis in order to be able to detect the depth effects. The newer Randot Pre-school test has the added advantage that it has been validated on large sample sizes.

6.7.2

Procedure: TNO stereo test

This test works using a random-dot principle and red-green goggles.

• 1.

  Explain the test to your patient: “I am now going to test your 3-D vision.” Place the red-green goggles over the patient’s habitual correction. For patients who are presbyopic, appropriate near correction should be worn.

• 2.

  Hold the booklet at about 40 cm, angled to be parallel to the plane of the patient’s face.

• 3.

  Keep the room lights on. Additional lighting over the patient’s shoulder can be used to illuminate the booklet if required.

• 4.

  For a general screening test, the first four plates are useful because the disparity is large and provides a qualitative assessment of stereopsis. If the patient has a short attention span, it is advisable to present Plate III alone because this gives a good early qualitative indication if stereopsis is present. Find out if the following images can be seen:

  • Plate I: In this plate there are two butterflies, one can be seen monocularly, whereas the other can be seen only if stereopsis is present ( Fig. 6.8 ). Ask the patient: “How many butterflies can you find on this page?” and “Can you point to them?”

    

  • Plate II: There are four discs and two can be seen without stereopsis. Ask the patient: “How many circles do you see?” and then “Which is the biggest?”

  • Plate III: Four ‘hidden’ shapes (circle, square, triangle, and diamond) are arranged around a central cross that is visible without stereopsis. Ask the patient: “Can you find a cross/square/triangle/circle/diamond? Can you point to it?” This plate is very useful with children, because they like to find and name shapes. You will need to remember the correct locations of the shapes in order to verify the accuracy of their responses because they are only visible with the goggles.

  • Plate IV: This is a suppression test and consists of three discs. When viewed through the goggles, one disc is seen only with the right eye, one is seen only by the left eye, and one is seen binocularly. Ask the patient: “How many circles can you see on this page?”

  • Plates V to VII: These plates present disparities from 480 to 15 seconds of arc. At each disparity level, two discs with a sector missing are presented in different orientations ( Fig. 6.9 ). Using the demonstration on the left of the display, ask the patient: “In each of these squares there is a cake with a piece missing. Can you find the cake and point to the piece that is missing?”

    

• 5.

  If the patient is hesitant about an answer, allow them plenty of time to view the test plate. If only one of the two tests for each stereo level is called correctly, allow them a second attempt at the incorrect one, but if called incorrectly again, or if the patient does not volunteer an answer, record the result as the previous correctly identified stereo level.

• 6.

  Record the test (TNO) and the patient’s stereoacuity in seconds of arc.

• 7.

  In patients achieving a poor test result, it can be useful to repeat the test with the red-green goggles reversed to ensure that stereoacuity is not underestimated.

6.7.3

Procedure: The frisby test

The Frisby test is a test of sensitivity to real depth using perspex sheets containing contoured figures ( Fig. 6.10 ). One element of the contour that is the shape of a circle is printed on one side of the sheet, whereas the remainder are printed on the opposite side. The thickness of the plate thus generates the real depth effect. No goggles are needed and the patient has to select the square that contains the circle in depth.

• 1.

  Explain the test to your patient: “I am now going to test your 3-D vision.” In the case of younger children you could say “We are going to play a game where you have to find the ball.”

• 2.

  In patients who are presbyopic, the test should be properly positioned for near-point viewing and appropriate near correction should be worn.

• 3.

  Keep the room lights on. Additional lighting over the patient’s shoulder can be used to illuminate the test, if required, but make sure there are no reflections from overhead or any additional light sources from the perspex plates because these could interfere with the visibility of the target.

• 4.

  Hold the thickest of the three perspex plates (6 mm) a distance of 40 cm from the patient and angled so that it is parallel to the plane of the patient’s face. The sheet should be held against the white background card that is part of the box provided with the test. Because monocular cues can be provided with movement of the plate or of the patient’s head it is very important that the plate is displayed squarely and the patient’s head kept still to minimise parallax effects.

• 5.

  Ask the patient to point to the square that is the ‘odd one out’ (older children and adults) or ‘to point to the square that contains the ball.’ If the patient answers correctly, you should ask why it’s the odd one out; the patient may volunteer at this point, that they can see a ‘shape’ or a ‘ball’ or a ‘circle.’ To establish that they are seeing in depth, you should ask if the ball is in front or behind the rest of the ‘pattern’ or ‘background’ or ‘picture.’ All of this additional information, if provided, is very positive and strongly suggests that the patient is seeing real depth.

• 6.

  You can tell which is the correct plate by using your sense of touch. One of the buttons at the four corners has a flat top and this signals the location of the circle, and the side (front/back) of the Perspex on which the circle is printed. This avoids having to look at the sheet to tell if the response was correct. Encouraging the patient (especially children) is a useful way to ensure continued co-operation and thus to gain a reliable measure of the stereoacuity.

• 7.

  Repeat step 5 whether or not the response was correct. With the thickest plate, next turn and flip the plate so that the target circle now occupies a new position. Sometimes patients will get the answer correct the second time because they are more sensitive to crossed versus uncrossed disparities, or vice versa. This would manifest itself when the patient shows that they can detect the location of the circle more easily when it is, for example, in front rather than behind the surrounding contours.

• 8.

  If the patient is correct on two successive occasions, move to the intermediate plate (3 mm) and repeat steps 5 to 7.

• 9.

  If, using the intermediate plate, the patient is correct on two successive occasions, move to the thinnest plate (1.5 mm) and again repeat steps 5 to 7.

• 10.

  From 40 cm, the best level of stereoacuity that can be assessed for using the Frisby test is 85 seconds of arc. If you wish to measure the stereopsis (as opposed to just establishing that it is 85” or better), a longer viewing distance is used in combination with the thinnest plate. The lid of the box presents a table which can be used to determine the stereoacuity for different test distances.

6.7.4

Procedure: Stereo Fly test/Stereo Butterfly test/Randot Graded-circles test/Randot pre-school test/Random dot ‘E’ test

These tests operate on similar principles to one another in that they use crossed polaroid filters to present slightly different aspects of the same object to each eye. The vectograph consists of two superimposed, similar patterns that are polarised at right angles to each other. Some aspects of each pattern are identical, whilst for others, small crossed and uncrossed disparities are introduced. When the patterns are viewed with Polaroid goggles, the patterns are seen in depth if stereopsis is present. Although these tests all differ from one another, they contain many similar aspects and are run procedurally in a very similar fashion. The following is a description for the Stero Fly test. After this some additional points relating to the other tests are listed.

• 1.

  Explain the test to your patient: “I am now going to test your 3-D vision.”

• 2.

  Ask the patient to hold the booklet at about 40 cm angled so that it is parallel to the plane of the patient’s face.

• 3.

  Keep the room lights on. Additional lighting over the patient’s shoulder can be used to illuminate the booklet if necessary.

• 4.

  If you are measuring stereopsis in children, first show them the fly (see Fig. 6.7 ). Ask the patient to wear the polaroid goggles (you could refer to these as ‘magic glasses’ to younger children to make the test more of a game). Note the patient’s reaction and ask them to pinch the wings of the fly. A positive test result is indicated if in attempting to touch the wings, the child pinches the air a few centimetres above the chart.

  

• 5.

  Cartoon animals: Ask the patient to look at the top row of animals (see Fig. 6.7 ) and to tell you which is the odd one out. Then ask the patient why this one appears to differ from the others. If the patient volunteers that it is different because it is closer to them (or because it stands out) this is a strong indication that stereopsis is present. If there is any doubt that the patient may know the answer that was expected (e.g., sibling tested previously when the child was present), turn the test upside down and the figure that appeared in front should now appear behind. Repeat this for the two lower rows of animals.

• 6.

  Circle patterns (also known as the Wirt test, see Fig. 6.7 ): Starting at the top array of circles, ask the patient which one of the four circles (top, bottom, left, or right) is the odd one out. Check the test card to ensure that the patient gave the correct answer and, as with the cartoon figures, ask why it appears different. Continue with this process until the patient cannot tell which is the unique circle (‘odd one out’) or until they give a wrong answer. The stereo level measured with the test is the smallest disparity that could be correctly detected.

• 7.

  Record the result in seconds of arc.

• 8.

  For the Randot graded-circles test, the graded circles element works in a similar fashion to the Wirt circle test described above in step 6, except that there are 10 versions of a 3-circles test and one of the three contains depth. This test features disparities from 400” to 20.” The cartoon figures are identical to those in the Stereo Fly test (400” to 100”). This test differs from the Stereo Fly test mainly in that it offers six geometric forms created using the random dot principle (500” to 20”). This element of the test can be administered by asking the patient to name the shapes that they see or, in the case of younger children, to perform a matching task where the same shapes are made available on a separate, printed sheet.

• 9.

  For the Stereo Butterfly test, the same Wirt circle and animal cartoon tests as in the Fly test are presented. The only difference is that the butterfly is created using random dots and offers a test of gross stereopsis (2500” to 1200”).

• 10.

  The Randot Pre-school test is designed for children from the age of two. All of the figures in depth are generated using the random dot principle and the test takes the form of a matching test in which the child matches the 2-D pictures on the left side of the booklet with the 3-D/random-dot figures on the right hand side. The disparity range is 800” to 40”.

• 11.

  The Random dot ‘E’ test. In this test, which is suitable for children aged 3 years and above, the patient is asked to distinguish between a raised E and a non-stereo target.

6.7.5