6.1.1 Observations and symptoms

6.1.2 Ocular history

The ocular history may indicate that the patient, or perhaps someone in the family, has a ‘weak’ or ‘lazy’ eye and/or strabismus. This should be followed up by asking if ‘patching’ or spectacles or any eye exercises have been prescribed or if ‘eye muscle’ surgery has taken place. Any positive response to these questions should lead to further questioning regarding the age when these interventions happened, when they stopped, their success and if the patient is still under any care for the amblyopia/strabismus, in which case a note should be made of who is providing the treatment and where it is being provided. If the latter is the case, you should be careful not to change an optical prescription or alter the current therapy in any way without permission/agreement from the other practitioner(s) currently treating the patient.

6.1.3 General medical history and family history

General health questions may indicate a systemic condition that can lead to an accommodation or binocular vision problem, such as diabetes, or systemic medications that can affect accommodation or binocular vision. When examining young children with or without a binocular vision abnormality it is useful to ask whether any member of the patient’s family has a strabismus or ‘lazy eye’ as there appears to be a hereditary link, particularly for esotropia.

6.1.4 Birth history

It is also useful to ask the child’s parent/carer about the pregnancy and birth history. There is a high prevalence of ocular abnormality, in particular strabismus, in children born prematurely, those with low birth weight or disorders of the central nervous system, and in children with significant birth complications (e.g. forceps delivery).¹ It is, therefore, recommended that the following questions be posed to the parent/carer during the case history examination: Was the child a full-term baby or were they born prematurely? What was the birth weight? (Less than 2500 grams or 5.5 pounds is a significant risk factor for strabismus, in particular esotropia).² Were there significant complications at the child’s birth? Is the child’s current and past general health good? Since birth, has the child been investigated or received treatment for any medical condition?

6.1.5 Binocular visual acuity

In cases where the acuities in the right and left eyes are similar or identical, it is usual to find that binocular visual acuity (VA) is ½ to one line better than monocular acuity.³ Of course, it is not possible to find this improvement if monocular VA equals the ‘bottom line’ of the Snellen chart you are using (section 3.2.2). When using a non-truncated chart, a binocular VA that is equal or worse than the monocular VA can indicate a binocular vision problem. A poor patient reaction to the restoration of binocular vision after an occluder has been removed following monocular subjective refraction can also indicate a binocular vision problem.

6.1.6 Retinoscopy and subjective refraction

Fluctuations in retinoscopy, retinoscopy results more than 1.00 D more positive than subjective refraction and/or fluctuations in subjective refraction suggest fluctuations in accommodation and/or latent hyperopia or pseudomyopia and should be investigated using assessments of accommodation (sections 6.9 to 6.11) and/or cycloplegic refraction (section 4.13).

6.1.7 Systemic and ocular health assessment

Previous or current systemic or ocular disease may explain signs or symptoms that are binocular or accommodative in nature. For example, diabetes or thyroid disease can lead to binocular vision problems. Similarly, particular signs or symptoms may prompt you to ask again about systemic health and/or to seek explanation within the eye/visual system. For example, a newly acquired divergent heterotropia and ptosis may be observed in a palsy of the 3rd cranial nerve and is suggestive of diabetes. Finally, cortical cataract and occasionally posterior sub-capsular cataract can generate diplopia that is monocular in origin (i.e. it persists even when one eye is covered).

6.2.1 Comparison of tests

The cover/uncover test is the only method by which an ocular deviation can be distinguished as either a heterotropia (also called a ‘tropia’, ‘strabismus’ or a ‘squint’) or a heterophoria. The test has the advantage of being an objective test (i.e. one that requires co-operation but no response from the patient) although the subjective response of the patient while performing the test can provide valuable additional information. The cover test provides considerable information about a deviation including its direction and size. In addition, the pattern of movements observed may enable you to form an opinion about the stability, constancy, laterality or control of a deviation. The test is quick and simple to perform. One disadvantage of the test is that even experienced practitioners cannot detect very small deviations (up to 2–3^Δ).⁴ Since even small vertical heterophorias can be clinically significant, it is likely that you would miss these if just using the objective cover test. However, small deviations of any variety may be identified using the subjective cover test. The only real disadvantage of the cover test is that it requires considerable practice before accurate observations can be made. In addition, it is vital to be systematic in your approach (Box 6.1). FIRST: Search for the presence of a heterotropia. If one exists, then by definition, a heterophoria cannot be present simultaneously. SECOND: If there is no heterotropia, search for a heterophoria using the alternating cover test and/or the cover/uncover test. THIRD: If no heterophoria is evident you should perform a subjective cover test.

An advantage for the alternating cover test in the assessment of heterophoria is that the deviation observed will normally be considerably larger (and therefore more obvious) than that which is apparent during the cover/uncover test. This is because binocular vision is suspended altogether during the alternating cover test whereas binocular vision is interrupted and then restored during the cover/uncover test. The deviation found on the alternating cover test is often referred to as the ‘total angle of deviation’ as compared to the ‘habitual angle’ that is measured during the cover/uncover test. It should also be noted that the alternating cover test can be used as a means of placing stress on the oculomotor system, and it can be useful to consider the total deviation as an indicator of the heterophoria when the patient is tired or at the end of the day. The alternating cover test can also be used to identify those heterophorias that can intermittently break down into a heterotropia. The alternating cover test cannot be used alone to evaluate whether a patient has a heterotropia as the pattern of eye movements observed during the alternating cover test in a patient with, for example, an exotropia will be indistinguishable from those observed in a patient with exophoria, etc. To detect a heterotropia, the cover/uncover test must be performed. However, the alternating cover test can be used to estimate the magnitude of any heterotropia found during the cover/uncover test.

6.2.2 Procedure

See online videos 6.1-6.13.

6.2.3 Adaptations to the standard procedure

6.2.4 Adaptation for older patients

During the near cover test, it can sometimes be difficult to see an older patient’s eyes due to 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 due 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 stick 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.

6.2.5 Recording

6.2.6 Interpretation

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 uncovered eye 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 should not be confused with heterotropic movements (Figure 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.⁶ 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.⁷ At near, a small amount (3^Δ to 6^Δ) of exophoria is considered normal (physiological exophoria) and this is likely to increase with age (exophoria measured with the Maddox wing increased from a mean of zero at age 20 to 5^Δ at 65).⁸ As even experienced practitioners cannot detect very small eye movements (up to 2–3^Δ), small hyperphorias will be missed with the objective cover test, and any hyperphoria that is detected will be abnormal.⁴

The movements made by each eye are usually similar in heterophoria. In cases where the heterophoria movement is greater in one eye than the other, suspect poor technique (and re-assess), uncorrected or residual anisometropia or incomitancy (section 6.15).

6.2.7 Most common errors

6.3.1 Comparison of tests

The Hirschberg test compares the position of the corneal reflexes (the first Purkinje images) of the two eyes that are formed by a pentorch. It is quick and easy to perform, and requires little co-operation on the part of the patient, but can really only be performed at near, the penlight target provides a poor stimulus to accommodation and it is relatively inaccurate. Choi and Kushner found that even experienced practitioners can obtain results that differ by up to 10 prism dioptres.⁹ This is because a deviation of just 1 mm is equivalent to ~22 ^Δ. The Krimsky test extends the Hirschberg test by using prisms to equalise the positions of the corneal reflexes in the two eyes. The Bruckner test relies upon 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 as compared to the reflex from the fixing eye due to fundal reflections from a deviating eye being greater than from the darkly pigmented macular area of a normally fixating eye. The usefulness of the Bruckner test is, however, controversial.^10,11 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.

6.3.2 Procedure: Hirschberg and Krimsky

See online video 6.15.

6.3.3 Procedure: Bruckner test

See online video 6.14.

6.3.4 Recording

Record the eye that deviates, along with the direction of the deviation. In your recording, make it clear that the observation was made using the Hirschberg, Krimsky or Bruckner techniques. For the Hirschberg and Krimsky tests, equal nasal displacement of the corneal reflexes in each eye indicates a non-strabismic patient. For example:

Hirschberg: No Strab; Hirschberg: ~11^Δ RSOT; Krimsky: 15^Δ L XOT. Bruckner: L SOT.

6.3.5 Interpretation

For the Krimsky and Hirschberg tests, a reflex located nasally to the reference point suggests an exotropia, a reflex located temporally to the reference point suggests an esotropia. Superior displacement of the reflex suggests a hypotropia, and inferior displacement suggests a hypertropia. It is important to realise that, in most patients, the corneal reflections will be displaced slightly nasally relative to the pupil centres. This displacement arises because a separation exists between the pupillary and visual axes. The angle between these axes is referred to as the angle kappa, which is given a positive value if the reflexes are nasally displaced. It is important to remember that a large angle kappa may result in the misdiagnosis of heterotropia or failure to detect a heterotropia. For example, a large positive angle kappa will simulate the presence of an exotropia. Similarly, an existing exotropia will appear larger than it actually is and small esotropias may escape detection. For the Bruckner tests, any brightness difference indicates 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 differences in brightness can be caused by factors other than strabismus including anisometropia and media opacities.

6.3.6 Most common errors

6.4.1 Comparison of tests

Because the cover test can be difficult to perform when using a phoropter or reduced aperture trial case lenses, all of the techniques described in this section offer advantages over the cover test for assessment of oculomotor alignment post-refractive correction. Also since the objective cover test can’t reveal small eye movements below about 2–3^Δ, the subjective tests are useful for checking for small vertical heterophorias that may be clinically significant.⁴

The Maddox rod test can be easily performed with a phoropter, trial frame or the patient’s own spectacles and with any test chart that contains a spotlight. It is widely used, easy for patients to understand, and can be performed relatively quickly. One drawback is that a spotlight represents a poor stimulus for accommodation and some clinicians consider that this limits the usefulness of the Maddox rod to the measurement of vertical heterophorias, which are assumed to be unaffected by accommodative changes. However, the Maddox rod should produce reliable assessments of horizontal heterophoria when used in patients with no accommodation, such as patients over the age of 60 or pseudophakes. The test should be carried out with the head held in the habitual fashion.

The modified Thorington technique is a very simple and quick technique that can be used in a phoropter, trial frame or free space. It produces the most repeatable results of the most commonly used techniques.12–14 The modified Thorington overcomes the Maddox rod’s problem of lacking an accommodative target by using a target of small letters or numbers (Figure 6.4). It is principally used at near, but Thorington cards are available for both distance and near. In view of its many advantages it is somewhat surprising that it is not more widely used at present. Normative data from large study populations of children have been published.¹⁵

The Maddox wing provides a simple and relatively fast technique for the measurement of heterophoria at near. However, the figures used on the scale are relatively large with the result that accommodation does not need to be precisely controlled. This may lead to overestimation of an exo-deviation, to underestimation of an eso-deviation or to variable results. There are claims that changing to smaller letters improves test reliability.¹⁶ In addition, the eyes may not be fully dissociated because the septum may allow peripheral fusion to occur. Finally, the instrument uses a standard, fixed centration distance between the lenses and a fixed testing distance of 25 cm and it would be very difficult to use with a phoropter.

The von Graefe technique is widely used and can be easily performed in a phoropter with a projector chart and no additional equipment. Unfortunately, it is the least reliable technique of those commonly available and its results correlate poorly with the cover test, especially in the case of horizontal phoria measures.12–14,17,18 This may result from variable amounts of prism adaptation, phoropter-induced proximal accommodation, a head tilt behind the phoropter leading to an induced vertical deviation or a reduction in peripheral fusion.¹² In addition, it is a relatively lengthy procedure, can be difficult for patients to understand and cannot easily be used with a trial frame. The technique does not appear to warrant its widespread use and other more reliable techniques such as the modified-Thorington or Howell card methods should ideally replace it.^12–14,17

The Howell card method provides a simple and quick technique that can be used in a phoropter, trial frame or free space and it can be used for measurement of horizontal phorias at distance or near. It cannot be used to measure vertical phorias. Although it appears to be popular, the method has not been subjected to many comparisons with other techniques but a study by Wong et al. suggests that the Howell phoria card method has a better inter-examiner repeatability than the von Graefe method.¹⁷

6.4.2 Initial procedure for all tests

6.4.3 Procedure: Modified Thorington test

6.4.4 Procedure: Howell cards

6.4.5 Procedure: Maddox rod

6.4.6 Procedure: Maddox wing

6.4.7 Procedure: Von Graefe’s method

6.4.8 Recording

6.4.9 Interpretation

Most people with normal binocular vision have some slight degree of heterophoria. Lyon et al. reported 25th to 75th percentiles for distance and near phorias of 0 to 1^Δ esophoria and 2^Δ exophoria to 1^Δ esophoria, respectively, in a large sample of first-grade children (aged ~5 years).¹⁵ In older children (4th grade, aged ~8 years), distance and near phorias measured with the modified Thorington method were of 0 to 1^Δ esophoria (distance) and 2^Δ exophoria to 2^Δ esophoria (near). Mean distance heterophoria in children and young adults is 1^Δ exophoria ± 1^Δ and mean near heterophoria is 3^Δ exophoria ± 3^Δ.¹⁹ In older adults, there is a tendency towards greater amounts of exophoria (physiological exophoria) and up to 6^Δ of exophoria is not uncommon.⁷ In adults and children, only about 0.5^Δ of vertical phoria may be considered normal, and in some patients even this amount can give rise to symptoms. Significant vertical phorias should be checked to make sure they are not due to a head tilt (unseen behind a phoropter) or abnormal head posture or a non-level trial frame or phoropter.

The heterophoria determined using the subjective tests and measured with the optimal refractive correction, should be compared with the corresponding cover test result measured using the patient’s spectacles. If there is no significant change in refractive error, the horizontal heterophoria measurements post-refraction should be similar to that found with the cover test. Vertical heterophorias would not be expected to differ pre- versus post-refraction, even if there is a substantial change in refractive correction. If a change in refractive correction has occurred this should lead to a predictable change in the horizontal heterophoria so that if the optimal correction shows an increase in plus/decrease in minus power from the patient’s spectacles, then an increase in exophoria or a decrease in esophoria should be expected. Similarly, an increase in minus/decrease in plus power could lead to a decrease in exophoria or increase in esophoria. The amount of change will depend upon the accommodative convergence/accommodation ratio (AC/A ratio, section 6.12). This can be particularly useful to students as it can help to monitor the accuracy of their cover test results, particularly when estimates of the heterophoria are made, rather than measurements with a prism bar.

6.4.10 Most common errors

6.5.1 Comparison of tests

The assessment of fixation disparity with the Mallett unit is quick and simple and gives the prism or spherical lens power that can be used as the starting point for correction of binocular problems. Jenkins et al. found that 1^Δ and 2^Δ of fixation disparity was associated with symptoms in pre-presbyopes and presbyopes, respectively, and it may be the best indicator that a heterophoria is decompensated.22,23 Mallett reported that the aligning prism corresponded to the decompensated portion of the heterophoria, and fixation disparity has also been shown to increase under binocular stress, such as working under inadequate illumination or too close a working distance, and at the end of a working day.^23,24

The Saladin and Wesson cards provide a means for establishing the shape of the fixation disparity curve, something that was originally possible only with the Sheedy Disparometer, a device that is no longer commercially available.25,26 As opposed to the aligning prism measure (which corresponds to just one point on the fixation disparity curve) provided by the Mallett unit, the Saladin and Wesson cards provide estimates of fixation disparity when different amounts of prism are introduced and from these measures the key components of fixation disparity curves (e.g. slope in central region, as well as the x- and y-intercepts) can be deduced.^25,26 The Saladin card is reported to have good test-retest reliability.²⁷

The fixation disparity approach has a number of significant disadvantages. One is that fixation disparity measures seem to be critically dependent on the method used to measure them. For example, results obtained with the Wesson and Saladin cards are not comparable, raising the possibility that the measures indicate more about the equipment than about the visual system they are testing.²⁵ The size and position of the binocular lock and monocular markers appear to exert an influence on the magnitude of the fixation disparity.²⁸ This is a problem given that many computer-based programmes offer different formats of the fixation disparity test. For this and other reasons, many remain unconvinced about the clinical relevance of fixation disparity and view it instead as a physiological phenomenon.²⁹ For example, if fixation disparity does reflect the decompensated portion of the heterophoria, the type of fixation disparity present should always match the direction of heterophoric deviation (e.g. an exo fixation disparity should only be present in a patient with exophoria). However, this is not always the case and it is estimated that one quarter to one third of individuals may have so-called ‘paradoxical fixation disparity’.^30,31 Nevertheless, others place much greater emphasis on its clinical significance and claim that fixation disparities have strong diagnostic significance. There are claims, for example, that the magnitude of fixation disparity is linked to the level of stereopsis that can be achieved by the patient and that the size of the aligning prism at near is inversely correlated with the fusional reserves, supporting the view that both measures may be indicators of decompensation of heterophoria.^32,33 Proponents of fixation disparity also argue that since the eyes are only minimally dissociated, the conditions of testing mimic those in habitual viewing to a much greater extent than is the case in heterophoria measurement.

In the UK, the Mallet unit is typically used to measure the fixation disparity at distance and near. The distance Mallet unit uses red monocular strips and a central fixation lock (OXO), but does not have a peripheral fusion lock (Figure 6.9). The near Mallett unit uses green monocular strips, as green is usually more sharply focused at near due to a slight lag of accommodation, a central fixation lock (OXO) and a surrounding paragraph of print providing a peripheral fusion lock. The near Mallet unit (Figure 6.10) also contains paragraphs of text of various sizes (typically N5 to N10), a retractable ruler, a near duochrome and targets that allow investigation of stereopsis and suppression.

Like the Mallett unit, the Wesson Fixation Disparity card and the Saladin Near Point Balance card use a polarisation method to render the monocular markers visible to the right or left eye. In the newer Saladin card, a penlight is held behind the card and is used to illuminate each circle. The practitioner asks the patient to identify the circle that contains the vertical lines which appear in horizontal alignment (horizontal fixation disparity) and the circle containing the horizontal lines which appear in vertical alignment (vertical fixation disparity). In the Wesson chart, the patient sees an arrow with one eye and the lines above it with the other eye; the task is simply to say to which line the arrow points.

6.5.2 Procedure: Mallett unit

6.5.3 Procedure: Wesson card

6.5.4 Procedure: Saladin card

6.5.5 Recording

If the monocular markers of the Mallett unit are not simultaneously visible to both eyes, record the eye that was being suppressed and whether the suppression was intermittent or constant.

If a fixation disparity was present, record the lowest amount of prism or spherical lens required to align the strips. If the fixation disparity was found in one eye only, this should be recorded. For example, ‘Mallett: No Fixation Disparity (FD) D or N; ‘Mallett, Dist: 2^Δ BI; Near: 1^Δ BI LE’.

For the Saladin and Wesson cards, the recording consists of each fixation disparity in minutes of arc corresponding to the prism power that was introduced. If one of the targets disappears during measurement, this indicates suppression and the eye that is being suppressed should be recorded.

6.5.6 Interpretation

Most patients will be able to simultaneously perceive the monocular markers on the distance and near Mallett unit, and usually they are aligned without the need for any prisms. It is important to remember that the prism power required to align the markers is not predictable from the magnitude of the fixation disparity (e.g. small fixation disparities are not always eliminated by low prism powers) and two patients exhibiting the same amount of fixation disparity may require very different prism powers to perceive the lines as aligned. Owing to prism adaptation it is advisable to leave the lowest prism power that neutralises the fixation disparity in place for a period of time (several minutes). If a slip re-appears after a period of time when the same prism power had initially neutralised the fixation disparity, you can be much less certain that prescribing this prism will prove beneficial. On the other hand, it is claimed that most patients with abnormal binocular vision which gives rise to symptoms do not adapt, or only partially adapt, to prisms.³⁵ The prism power that neutralises any vertical fixation disparities can be used to prescribe vertical prism.

For the Wesson and Saladin card, the horizontal fixation disparity data gathered are used to plot fixation disparity curves and from these curves four key characteristics are identified; they are the type (I, II, III or IV), the slope, and the x- and y-intercepts. A discussion of fixation disparity curves is beyond the scope of this chapter except to say that most asymptomatic patients have type I curves that feature shallow slopes in the central region, and most show low numerical values for the x- (aligning prism) and y- (fixation disparity in minutes of arc) intercepts. Scheiman and Wick maintain that fixation disparity curve method provides the best means for determining the amount of prism to prescribe.¹⁹

6.5.7 Most common errors

6.6.1 Comparison of tests

The near point of convergence is a quick and easy test to perform. It requires no special equipment and it provides a very repeatable result.⁴¹ It is the standard test for convergence ability. The Jump Convergence test has the advantage that it more closely reflects typical near viewing situations where fixation is continually switching between near targets; it is seldom in the real world that we would encounter a target that moves slowly and predictably towards us along the midline as with the NPC task. Early research suggested poor jump convergence was more closely linked to visual difficulties at near by comparison with a remote NPC.³⁶ The test is relatively easy to perform and can be used as an additional assessment in patients who show signs of convergence insufficiency, and in patients who show a normal NPC but whose symptoms suggest possible convergence difficulties. The disadvantage of the jump convergence test is that it has not been subjected to the same research evaluation as NPC. Consequently there is a lack of normative values for the test and a lack of evidence that the test can discriminate symptomatic from asymptomatic individuals.⁴²

The issue of whether NPC should be measured with an accommodative (e.g. a letter) or a non-accommodative (e.g. spotlight) target has received considerable attention. In presbyopic patients the choice of target does not seem important but in pre-presbyopes there may be a difference in NPCs measured with accommodative and non-accommodative targets.42–44 Although such differences may not be substantial in visual normals, Scheiman et al. suggest that individuals with convergence insufficiency show more remote break and recovery NPCs with a penlight compared to when an accommodative target is used.⁴²

6.6.2 Procedure: NPC

See online video 6.11.

6.6.3 Adaptation for older patients

For older presbyopes the target will typically blur (due to loss of accommodation) before the NPC is reached. Patients often report this blur as ‘doubling’, so a remote subjective NPC cannot be relied upon in older patients. It is better therefore to rely upon the objective NPC. Use of a non-accommodative target (e.g. tip of a pen) is preferred in older patients because blurring may not be as noticeable as with an accommodative target (e.g. letter).

6.6.4 Procedure: Jump convergence

6.6.5 Recording

6.6.6 Interpretation

Normative NPC values show considerable variation between studies. Scheiman et al. suggest a clinical cut-off value of 5 cm for the near-point of convergence break and 7 cm for the near-point of convergence recovery with either an accommodative target or a penlight in children and adults.⁴² Children and adults should certainly be able to converge to within about 7.5 cm and recovery should return within 10.5 cm.³⁷ An NPC larger than these figures suggests possible convergence insufficiency and should be investigated further. This investigation should include jump convergence, distance and near heterophoria, near fusional reserves and near fixation disparity. Given the reported high prevalence of accommodative insufficiency in children with convergence insufficiency, tests of accommodation should also be conducted in these patients.³⁷ The effect of any new refractive error or refractive change on these measurements should be assessed.

Instead of a failure of one eye to converge it is possible that diplopia will be reported and/or that both eyes are seen to no longer view the target because of over-convergence. This is rarely encountered but when it does arise it suggests that the patient may have an abnormally high AC/A ratio (section 6.12). This should be recorded and additional investigations should be carried out. Good, fast, smooth jump convergence should be observed to 10–15 cm.

6.6.7 Most common errors (NPC)

6.7.1 Fusional reserves

The measurement of fusional reserves is an important clinical test in the assessment of binocular vision status. Heterophorias are latent deviations that are corrected by the sensory fusion reflex. It is useful to know what proportion of the fusional reserves are required to correct the heterophoria.⁴⁵ It is thought that between one-third and two-thirds of the fusional reserves may be used without placing the system under undue stress. Positive and negative fusional reserves can be measured at distance and near by placing appropriate prisms before the eyes. Prism is introduced before the eyes until fusion breaks down and diplopia results.⁴⁶ Placing base-out prism before the eyes stimulates convergence and the amount required to produce diplopia is called the positive fusional reserve (PFR). Because the eyes are forced to converge, accommodation is stimulated (convergence accommodation) but cannot be maintained at the correct level for the target distance and therefore the target usually blurs before diplopia occurs.⁴⁷

Placing base-in prism before the eyes stimulates divergence and the amount required to produce diplopia is called the negative fusional reserve (NFR). When measuring NFR at near distance, a blur point is usually reported prior to diplopia as accommodation relaxes when the eyes are forced to diverge. However, it is unusual to obtain a blur point when measuring NFR at distance as accommodation is already at a minimum (provided the eyes are emmetropic or appropriate distance correction is worn) and cannot relax beyond this point.

6.7.2 Comparison of techniques

Risley or rotary prisms are an ideal method of changing the amount of prism before the eyes in a smooth manner and they provide repeatable results in young adults, although the results are reported to be less repeatable in children.41,46 Although phoropters typically feature rotary prisms, they have the disadvantage that they do not allow a view of the patient’s eyes. Fusional reserve tests in free space, typically using prism bars, more closely mimic natural viewing conditions and are particularly useful with young children as the eyes can be seen and an objective assessment of the fusional reserves can also be obtained. Objective fusional reserve estimates are very important, particularly in individuals in whom subjective estimates are often unreliable (e.g. young children).

6.7.3 Procedure

See online videos 6.16-6.17. NOTE: This description is for the measurement at 6 m. The technique can be applied for near by adjusting the trial frame/phoropter to the near centration distance and locating a fixation target at the appropriate distance.

6.7.4 Recording

6.7.5 Interpretation

Fusional reserves can be compared to normal data (Table 6.3) and several tables of comparison have been published in adults and children.^15,19,49 It is clear from these comparisons that a wide variety of ‘normal’ data has been published over the years. While you should have some awareness of values that can be expected at distance and near for the various measures (base in, out, etc.), it is desirable that each clinician obtain their own impression of the normative values (average and range) that they can expect using their own equipment and their own technique. The value of fusional reserve measures is greatest when considered not in isolation but when compared to the heterophoria measurements. A patient with an exophoria will use part of their PFR to correct the deviation. The measured PFR therefore represents the amount of fusional vergence in reserve to maintain single binocular vision. Similarly, a patient with esophoria will use part of their NFR to correct the deviation. Knowledge of the heterophoria size and of the magnitude of the opposing fusional reserves can be useful in the assessment of a patient’s binocular status specifically in relation to whether the heterophoria is likely to be giving rise to the patient’s symptoms. The proportion of the total fusional vergence used to correct the phoria can be determined. For example:

Therefore, ⅓ (9^Δ) of the total positive fusional reserves (27^Δ) are used to correct the phoria, which is within normal limits. This approach has been formalised in Sheard’s and Percival’s rules, which are used to compare the fusional reserves with the heterophoria and to indicate whether the phoria is likely to be decompensated now or to decompensate in the future under conditions of stress (e.g. around examination time in the case of students).

Sheard’s rule proposes that the fusional reserve blur point should be at least twice the size of the phoria. Sheard’s criterion works best for exophoric cases so that the PFR to blur should be at least twice the size of the exophoria in order for it to be compensated.⁵⁰ Sheard’s criterion further suggests that the prism required to correct a decompensated exophoria is:

Prism required = 2/3 exophoria – 1/3 PFR. Thus, for example if the exophoria is 6^Δ and the PFR is also 6^Δ. Sheard’s criterion suggests that a prism of 2^Δ base-in should be prescribed. Percival’s rule suggests that a patient should operate in the middle third of their binocular vergence range. Percival’s rule should only be used for near phorias as normal distance PFR and NFR are typically very unbalanced and Percival’s rule tends to work best for near esophoric cases.⁵⁰ Percival’s rule suggests that the PFR and NFR should be balanced and that one should not be more than double the other. Percival’s criterion suggests:

Prism required for esophoria = ⅓ total range – NFR. For example, if the PFR is 11^Δ and the NFR is 4^Δ, the prism required is 15/3 – 4 = 1^Δ BO.

6.7.6 Most common errors

6.7.7 Acceptable alternative technique: 20^Δ base-out test

See online video 6.18.

This technique is suitable for use in those patients who may not be able to co-operate with fusional reserve measurement (e.g. young children). Rather than introduce variable prism power and obtain responses from the patient regarding the blurring or doubling of images, this test relies upon qualitative judgements made by the practitioner in response to the introduction of a high-powered prism. Typically, a 20^Δ base-out is used (though in theory any prism power or direction can be employed) and the practitioner examines whether the eye behind the prism makes a swift and smooth movement in order to restore the image of the object of regard on the fovea and a swift recovery movement in the opposite direction when the base-out prism is removed. The test is repeated with the prism in front of the other eye. In principle the test is similar to 4 prism base-out test (section 6.13) but it is qualitatively much easier for the practitioner to establish whether the appropriate motor fusion response has taken place following the introduction of this high powered prism. A normal response on this test can allow the practitioner to generalise about the effectiveness of the motor fusion system and thus the ability of the visual system to maintain fusion throughout the day. A normal response on this test may be recorded in the following fashion: ‘20^Δ base-out overcome with either eye, and good recovery’. A positive response on this test (i.e. an appropriate motor fusion response) is a very strong indicator that peripheral fusion exists and thus the 20^Δ base-out test can prove useful in children who are too young to undergo formal sensory testing.⁵¹ Unfortunately the same is not true in reverse, because a negative result on the 20^Δ base-out does not guarantee that peripheral fusion is poor or absent.

6.8.1 Comparison of tests

This test requires little additional equipment and is straightforward to perform. The results of the test may explain symptoms not readily explained by other tests.⁵² Gall and colleagues reported that the combination of 3^Δ base-in and 12^Δ base-out prism flippers provides good repeatability and the best discrimination between symptomatic and non-symptomatic patients.⁵³ However, normative values have also been published for 8BO/8BI prism flippers.^49,53

6.8.2 Procedure

See online video 6.19.

The test can be carried out at any test distance, although it is normally carried out at near. If, however, symptoms are reported at a non-reading test distance, testing should be carried out at that distance. The patient should view a single isolated letter/target or a vertical line of letters; the letter/target should be ~1 line bigger than the smallest letters that can be read at the test distance. The patient should wear the habitual near correction for the test. You should sit down during the test so that you can observe the patient’s eyes as the prisms are flipped.⁵⁴

6.8.3 Results

Record the number of cycles achieved in the following format:

Vergence facility at 40 cm: 10 cycles/minute (12BO/3BI).

6.8.4 Interpretation

Normal values for this test (using 12 BO/3BI) are in the region of 15 cycles/minute.¹⁹

6.8.5 Most common errors

6.9.1 Comparison of tests

There are a variety of ways in which the amplitude of accommodation can be measured.56,57 One is to bring a target closer and closer to the patient’s eyes until it first blurs; this is called the push-up amplitude. Another is to start with the target directly in front of the eyes and move it away until it first becomes clear; this is the pull-away method. Some practitioners take an average of the push-up and pull-away values as the amplitude of accommodation because it provides a useful compromise between the slight overestimate of the push-up technique and the slight underestimate of the pull-away technique.⁵⁸ However, the subjective element that is a feature of push-up methods (where the patient reports first sustained blur) is best avoided altogether because of differences between patients in their understanding of ‘blur’ or in their interpretation of these instructions and because the letters get progressively bigger in angular size (and thus easier to see) as they are moved closer to the eyes.⁵⁹ The technique advocated here is the pull-away method. The advantage of the pull-away method is that the patient responds by naming the letter/target as soon as they can identify it rather than when they first notice the subjective impression of blur (as in the push-up method). In the pull-away method, you hold the fixation stick and place your thumb beneath an isolated 20/30 letter (or use an RAF rule, Figure 6.12; or an appropriately sized picture target in the case of young children). The patient should not know the identity of the target/letter before the test starts. There is a modification to the pull-away method which involves inserting a –4 D lens before the eye before the test is carried out. This modification has some advantages and is described below in section 6.9.6.

Another alternative involves using increasing amounts of minus spherical lens power until distance vision blurs (‘Sheard’s technique’). This method typically provides lower estimates of amplitude of accommodation than those provided by the push-up method and it can only be satisfactorily measured using a phoropter.⁶⁰ In addition, the minus lens method provides a less clinically relevant measure than the push-up or pull-away techniques, which provide direct measurements of the near point of clear vision.⁵⁹

6.9.2 Procedure: Pull-away amplitude of accommodation

6.9.3 Recording

Record the number of dioptres of accommodation for each eye. Examples:

Amp. of Accomm. (pull-away) R(OD) 8.50 D, L(OS) 8.50 D, BE(OU) 10.00 D.

6.9.4 Interpretation

Pull-away values tend to be lower than push-up values for the amplitude of accommodation. Normal values of monocular spectacle accommodation are shown in Table 6.4. If the measured amplitude is significantly (>1.50 D) lower than the age-matched normal values the patient may have accommodative insufficiency.⁵⁶ Binocular values of amplitude of accommodation are usually a little higher (1–2 D) than the monocular values as the convergence response helps to induce additional accommodation (convergence accommodation).⁶¹ If amplitude of accommodation is reduced to a level below 5.00 D in a patient aged over 40 years wearing optimal distance correction but who has difficulty reading, the patient is presbyopic. In children aged 4 to 11 years, Adler et al. found large intra-individual variation in measures of amplitude of accommodation and suggested that, in this age-group, the test may prove useful mainly as a pass/fail check for substantially reduced accommodative amplitude of less than 8 D.⁶¹

Anomalies of accommodation may be associated with a wide variety of conditions including various systemic and ocular medication (probably the most common cause), trauma, inflammatory disease, metabolic disorders such as diabetes and other systemic diseases.⁵⁸ Reduced amplitudes of accommodation have also been reported in children with Down’s syndrome and cerebral palsy.^62,63 Wick and Hall found that a battery of tests (amplitude, lead/lag of accommodation, accommodative facility and a cycloplegic refraction) was required to detect accommodative dysfunction, and that just because a patient had an adequate amplitude of accommodation did not mean that accommodative function was normal.⁶⁴

6.9.5 Most common errors

6.9.6 Acceptable alternative technique: Modified pull-away method

This is carried out precisely as described above except that a –4 D lens (or pair of –4 D lenses in binocular measurements) is placed in front of the eye before the test is started.⁵⁷ This has the effect of moving the point at which the letter/target is identified away from the eyes. Measurements are more repeatable because of the non-linear relationship between the distance (metres) and dioptric scales. Once the distance has been converted to dioptres, 4 D is then added to obtain the final result.

6.10.1 Comparison of techniques

The ±2.00 DS flippers test of accommodative facility can be performed rapidly with minimal additional equipment. Measures of accommodative facility may be useful in diagnosing binocular vision problems in symptomatic patients whose phorias and visual acuity are normal.⁵² It appears to have diagnostic value in that a reduced facility correlates with near symptoms and facility increases as symptoms are alleviated through treatment. Indeed, flippers can be part of the treatment. There is little justification for the use of the ±2.00 DS flippers other than they are the power traditionally used. Indeed, it may be that what is required is a range of flipper powers that relate to the patient’s amplitude of accommodation.⁶⁸ For example, for a young patient with an amplitude of 12.00 D, the ±2.00 DS represent only a 33% range of the amplitude, whereas they represent a 67% range of the amplitude in an older patient with an amplitude of 6.00 D. Yothers et al. suggest using an amplitude-scaled test for adults, which uses a test distance that requires 45% of the amplitude of accommodation to be exerted and a lens flipper range that is 30% of the amplitude.⁶⁹ For example, a patient with 7.00 D of accommodation would indicate the use of an approximate working distance of 32 cm (1/3.15, i.e. 45% of 7.00) and a flipper range of 2.10 (30% of 7.00) giving a flipper power of ±1.00 D.

Many authors recommend measuring the binocular accommodative facility with a suppression check (typically using polaroid glasses with the Bernell No. 9 vectogram). For appropriate comparison, the monocular measurements should be made with the same set-up except that one eye is now fully occluded. Alternatively, accommodative facility can be measured using standard near charts with binocular facility measured only if other tests indicate that the patient does not suppress at near. In this case, the clinical ‘pass’ values obtained using the polaroid system cannot be used for comparison. Some authors just test binocularly first and only measure monocular facility if the binocular results are reduced. If the binocular facility is reduced, but monocular facility values are within normal limits, then this suggests a dysfunction of the vergence system rather than of accommodation.

6.10.2 Procedure

See online video 6.20.

6.10.3 Recording

Record the number of cycles/minute for each eye and then for the binocular viewing condition; e.g. ‘accomm. facility: 10 cycles/min. binocularly (+2 D/–2 D)’

One cycle consists of clearing both the plus and the minus lenses. Record the lens powers that were used to measure accommodative facility because facility may need to be measured with lower lens powers (e.g. +1 D/–1 D) if the patient is a presbyopic adult or if +2 D/–2 D lenses can’t be cleared by the patient. If the patient can’t clear one of the two lenses, the recording should note this. For example, ‘accomm. facility: 5 cycles/min, fast initially, failed to clear +lens power after 30 secs)’.

6.10.4 Interpretation

The normative data reported in the literature are variable, possibly because data were gathered across a range of ages but reported as a grand average or because they were collected from unselected samples (e.g. samples may have included patients with symptoms and accommodative or vergence dysfunctions). For these reasons, published normative data cannot be completely relied upon and you are encouraged to have an impression of normative data for a range of age groups based upon your own measurements.⁶⁸ Suggested ‘clinical pass’ criteria in young adults are 11 cycles/minute (monocular). The task becomes more difficult with the polaroid system, so that a clinical pass binocularly is 8 cycles/minute.⁷¹ For children aged between 8 and 12 years, ‘clinical pass’ criteria are 7 cycles/minute (monocular) and 5 cycles/minute (binocular polaroids).⁷² A major disadvantage of the accommodative facility test is that there is no objective information available to you. In other words from observing the eyes, it is not possible for you to ensure that the patient understands and is complying with the test requirements. This is because changes in accommodation do not produce a change in the appearance of the eyes in the way that changes in vergence (e.g. during vergence facility test with prism flippers) do. You are therefore forced to rely exclusively on the subjective impressions of the patient during this test.

6.10.5 Most common errors

6.11.1 Comparison of tests

Accommodative lag and lead can be measured objectively using various dynamic retinoscopy techniques or subjectively using relative accommodation measurements or the binocular crossed-cylinder method. The latter two subjective measurements are more often used in the assessment of accommodation to help determine the tentative reading addition and are discussed elsewhere (section 4.14). Dynamic retinoscopy offers a quick, repeatable and valid means for establishing the accuracy of the patient’s accommodation system and it requires minimal extra equipment.⁷³ Both dynamic retinoscopy tests provide results that are less variable than the crossed-cylinder or near duochrome techniques.⁷⁴ As with most clinical techniques, practice is required in order to develop proficiency in carrying out the tests, especially in relation to the short time in which to make retinoscopy judgements. One study has suggested that the Nott technique provides more accurate estimates of the accommodative response as it does not require the introduction of supplementary lenses.⁷⁴

6.11.2 Procedure: Nott dynamic retinoscopy

6.11.3 Procedure: MEM dynamic retinoscopy

6.11.4 Recording

For the Nott technique, record the dioptric difference between the near chart and the position of the retinoscope when neutrality is observed. If the neutrality point is behind the near chart position, then there is a lag of accommodation. If the neutrality point is in front of the near chart position, then there is accommodative lead. For example, if the near chart is at 40cm and neutrality is observed at 57cm, then the accommodative lag is +2.50 D – 1.75 D = +0.75 D. It is useful to learn corresponding distances and dioptric values, such as 80 cm (1.25 D), 67 cm (1.50 D), 57 cm (1.75 D), 50 cm (2.00 D), 44 cm (2.25 D) and 40 cm (2.50 D).

For the MEM technique, record the dioptric value of the lens that produces neutrality. Positive lenses indicate a lag of accommodation and negative lenses indicate a lead of accommodation).

6.11.5 Interpretation

Typically the accommodative response to a target is slightly less than the accommodative stimulus. For example, a target positioned at 40 cm provides an accommodative stimulus of 2.50 D, but the normal accommodative response is slightly less, at about 2.00 D. The target remains clear due to depth of focus. Accommodative lags of 1.00 D or greater could be due to uncorrected (or insufficiently corrected) presbyopia and/or hyperopia or it can indicate a lack of accommodative amplitude or reduced accommodative facility in a pre-presbyopic patient. The lack of an accommodative lag or an accommodative lead can indicate pseudomyopia or accommodative spasm. It has been claimed that the MEM technique provides lags which are on average twice those found using the Nott method but most studies find results that are similar.58,76 In children with low- to moderate hyperopia for whom it is not clear whether they would benefit from refractive correction, there is a growing belief that assessment of accommodative accuracy using dynamic retinoscopy offers a means of identifying those who are likely to benefit from spectacle correction.⁷⁷ Specifically those with a large lag of accommodation are predicted to benefit more than those with smaller lags.

6.11.6 Most common errors

6.12.1 Comparison of tests

The modified gradient test allows a quick and reliable measure of the AC/A ratio using standard clinical equipment. This procedure allows proximal components of the response to be controlled as the test is performed at a fixed distance.⁸⁰ The modified gradient AC/A depends on heterophoria measures at only two points, which can lead to errors.⁸¹ The full gradient test overcomes this problem by measuring heterophorias with additional powers from +3 D to –3 D in 1.00 D steps and plotting a graph of lens power against induced phoria. The gradient of this line gives the AC/A ratio in Δ/D. The AC/A ratio can also be calculated from the information that is already available during a routine eye examination, specifically by comparing the distance heterophoria and near heterophoria (see section 6.12.6 below). This method has the disadvantage that proximal accommodation is present in one heterophoria measure (near) but not the other (distance). Also, the result is subject to error because only two measures of heterophoria are used in the calculation. Irrespective of the method used, the target viewed by the patient should require controlled accommodation as the ratio obtained has been shown to depend upon the fixation target.⁸²

6.12.2 Procedure: Modified gradient AC/A ratio

6.12.3 Recording

Use the following formula to calculate the AC/A ratio. Use positive numbers for esophoria and negative numbers for exophoria.

The calculation required to determine the AC/A ratio is the same irrespective of whether the heterophoria measurements were taken during near or distance viewing. For example, if a patient exhibits 6^Δ esophoria during distance viewing when –2.00 D lenses are added to his/her normal refractive correction but 2^Δ exophoria with the normal refractive correction in place, the AC/A ratio is calculated as:

6.12.4 Interpretation

Normally the AC/A is 3–5^Δ/D. A low AC/A ratio may, depending upon the distance heterophoria, result in convergence insufficiency exophoria. Similarly, a high AC/A ratio may lead to a problem of convergence excess esophoria. Knowledge of the AC/A ratio can be useful when determining plus lens power for the correction of decompensated esophoria. As the amount of convergence induced by 1.00 D stimulus to accommodation is known, it is possible to calculate the extra plus lens power required to reduce the esophoria to an acceptable level.

6.12.5 Most common error

Using a method of heterophoria assessment to determine the AC/A ratio, which is less reliable than other available methods (section 6.4).

6.12.6 Alternative technique: Calculated AC/A

The calculated AC/A = PD_cms + (n – d)/D, where PD = interpupillary distance measured in cm; n = near phoria; d = distance phoria; D = accommodation. Exophorias are negative and esophorias are positive;

e.g., PD = 6 and D = 2.5 (accommodation required at 40 cm), distance phoria is ortho and near phoria is 5 exo, AC/A = 6 + (–5/2.5) = 4^Δ:1 D.

Without using the above formula, it is of course possible to get an impression of whether the AC/A ratio is normal or not by comparing the near and distance heterophorias. For example, an exophoria that is much greater at near than at distance will be found in a patient with a low AC/A ratio. Similarly, esophoria that is greater at near than at distance suggests either a high AC/A ratio or significant uncorrected hyperopia. In cases where the near and distance phorias are the same, the formula above indicates that the AC/A ratio is given by the patient’s PD (in cm).

6.13.1 Comparison of tests

The Worth 4-dot test is widely available, relatively cheap, easy to use and can be used to assess fusion or reveal suppression at distance and near. It provides a rather coarse indication of suppression in the sense that other tests may reveal the presence of suppression when the 4-dot test suggests that none is present. This is particularly true for near 4-dot testing because of the relatively large angular size of the lights when viewed at near compared to distance viewing. Conversely, the rivalry produced by the red/green goggles may lead to dissociation even in a patient with useful or normal binocular vision so that the test can suggest the existence of suppression when none is present under habitual viewing conditions. The 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.⁸³ Another disadvantage of the test is that a patient with constant strabismus and abnormal retinal correspondence may achieve a normal result. A positive test result does not therefore guarantee the presence of normal binocular vision.

An advantage of the Bagolini lens test is that it creates minimal dissociation between the eyes and thus allows you to assess binocular status in conditions that very closely resemble the patient’s habitual viewing conditions. This is a particular advantage over the Worth 4-dot test which may provide results that do not apply in habitual viewing. Bagolini lenses offer the only method that is likely to be available to practitioners wishing to investigate retinal correspondence. The disadvantages are that, for whatever purpose the Bagolini lenses are used, the test is quite subjective and therefore there is a danger that the patient will be led to give the answer you expect. Also, the method offers a qualitative rather than a quantitative method for assessing suppression and retinal correspondence.

The 4^Δ base-out test is used as a test of suppression in the specific case of a suspected microtropia. Indeed, it is used in combination with tests of visual acuity, refraction, eccentric fixation, abnormal retinal correspondence (ARC) and stereopsis to confirm a diagnosis of microtropia. It differs from the Worth 4-dot and Bagolini lens tests in that it is entirely objective; the result does not rely upon a verbal response from the patient but rather is determined by a comparison of the pattern of eye movements that result when the 4^Δ (base-out) is introduced in front of one eye and then the other. This test thus requires little additional equipment and is quick and straightforward to perform. However, its repeatability is relatively poor and visually normal children can show atypical responses.⁸⁴ Other assessments of suppression are also available on the Mallett unit (section 6.5) and with some stereopsis tests (section 6.14).

6.13.2 Procedure: Worth 4-dot

Children who cannot respond verbally can be asked to touch the dots to indicate the number seen, and ‘touching four’ indicates normal flat fusion. There is some evidence to indicate that although the test will reliably detect suppression in this way, it is unlikely to differentiate between normal fusion and alternating suppression.⁸⁵

6.13.3 Procedure: Bagolini lenses

6.13.4 Procedure: 4^Δ base-out (BO) test

6.13.5 Recording

6.13.6 Interpretation

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”) (Figure 6.14) then there is a suppression scotoma larger then the angular subtense of one of the four dots. The dots on the distance target have a smaller angular subtense than those on the near target. Because suppression is more common for targets imaged in central vision, suppression is therefore found more frequently for distance viewing than for near. The size of the suppression scotoma can be estimated by moving the near target further away from the patient than the standard 40 cm until a response consistent with suppression is noted. The distance that the target is from the patient should be recorded. If the patient achieves fusion in the dark but not in the light, this indicates a shallower level of suppression as compared to the situation where suppression is present in both the dark- and light-room conditions.

If neither eye moves when the 4^Δ base-out prism is introduced before the eye with reduced VA but the expected pattern of movement is observed when the prism is placed before the fellow eye, this confirms the presence of suppression in the weaker eye. Along with reduced visual acuity, anisometropia, the presence of eccentric fixation, and degraded but measurable stereopsis, this result on the 4^Δ BO test confirms a diagnosis of microtropia.

If the patient reports seeing two orthogonal, continuous lines which intersect at the spotlight when using the Bagolini lenses, then there is no suppression. If one line is missing when the spotlight is viewed with both eyes open, one eye is being suppressed. You can establish which eye is being suppressed from the orientation of the line that is seen. Instead of a line being completely absent, part of a line may be missing. Typically, if part of a line is missing, it will be in the vicinity of the spotlight. Thus the patient might, for example, report a continuous line oriented at 45 degrees but a line oriented at 135 degrees which has a gap on either side of the centre (Figure 6.15) but which appears further away from the spotlight. If the 45 degree line should have been generated in the right eye, this report would be interpreted as evidence for central suppression in the right eye. If the left eye is now covered and the patient reports that the vertical line now runs continuously through the spotlight, this is powerful evidence that the right eye is being suppressed by the left eye in habitual viewing.

6.13.7 Most common errors