A heterophoria only requires treatment if it is causing symptoms or impaired performance, if the binocular status is likely to deteriorate if it is not treated, or if the condition might in the future need treatment and would be more effectively treated now ( Evans, 2010 ). A heterophoria that is causing symptoms or impaired performance or is breaking down to a strabismus is called a decompensated heterophoria . If it is decompensated, the evaluation should identify which factors have caused it to become so. In general, it is a heterophoria that has been fully compensated but becomes decompensated that gives rise to symptoms. After identifying the factors that cause the heterophoria to decompensate, the management consists of removing or relieving as many of them as possible.
Some heterophoria can be secondary to an active disease, pathological process, or recent injury. This type will be called ‘pathological’ heterophoria. It is usually incomitant. In some directions of gaze, it may even break down into a strabismus and double vision occurs. As already explained ( Chapter 2 ), some parts of the routine eye examination are particularly important in the detection of pathological deviations, and these assume more significance in the total evaluation when pathology is suspected. These aspects are summarised in Table 15.1 and the detection of incomitant deviations is covered in Chapter 17 . At this stage, the emphasis is on nonpathological heterophoria.
Factors Affecting Compensation
As most people have some degree of heterophoria, it is obviously important to decide which cases require treatment. That is to say, it is necessary to distinguish compensated from decompensated heterophoria. If the heterophoria is compensated, there is no need to evaluate it further. If it is decompensated, further evaluation is required to see which of the classifications describes the appearance presented, which may help to reveal the reason for the decompensation and the appropriate treatment.
The factors that influence whether a heterophoria is compensated can be broadly classified under three headings: the size of the heterophoria, sensory fusion, and motor fusion. These are illustrated schematically in Fig. 4.1 , which is derived from Fig. 1.1 . It is important to identify and remove as many as possible of the decompensating factors. The factors listed in the next section may contribute to heterophoria becoming decompensated, particularly if there is a marked change in them. In the list, factors 1 (a–d), 2, and 3 are motor factors and 1 (e) and 4 and 5 are sensory factors.
Stress on The Visual System
Excessive use of vision under adverse circumstances
Work held too close to the eyes for long periods. A comfortable working distance depends on the amplitude of accommodation, and therefore on the patient’s age. As the amplitude decreases with age, stress on accommodation and convergence can occur if a proper working distance is not adopted. The near distance can also present stress in early presbyopia ( Pickwell, Jenkins, & Yekta, 1987 ). Prolonged use of computer display screen equipment, which is typically further out than the usual reading distance, can cause a deterioration in the near point of convergence and near point of accommodation ( Gur, Ron, & Heicklenklein, 1994 ).
A sudden increase in the amount of close work. This can occur with a change in the workplace, or students nearing examination time.
Increased use of the pursuit reflexes: for example, playing or watching ball games, or reading when travelling.
Tasks which dissociate accommodation and convergence. Several features of virtual reality displays can disrupt the normal relationship between convergence and accommodation ( Wann, Rushton, & Mon-Williams, 1995 ) and cause stress on the visual system ( Mon-Williams, Wann, & Rushton, 1993 ) associated with symptoms ( Morse & Jiang, 1999 ). Similar effects can occur with 3-D displays, when symptoms occur in a minority of users ( Fortuin et al., 2011 ) but are more likely in those with subtle binocular vision anomalies ( Lambooij, Fortuin, Ijsselsteijn, Evans, & Heynderickx, 2010 ). One cause of this motor visual stress is inappropriate vertical gaze angle ( Mon-Williams, Plooy, Burgess-Limerick, & Wann, 1998 ), but visually-induced motion sickness (VIMS) also contributes to symptoms with 3-D displays ( Howarth, 2011 ).
Inappropriate illumination or contrast ( Pickwell, Yekta, & Jenkins, 1987 ). Visual conditions in the home or workplace are sometimes inappropriate, involving too little or too much illumination or contrast or glare. Night driving conditions may produce long periods of looking into a dark field with very reduced fusion stimulus at a time when the patient is fatigued. Reduced illumination does not influence the degree of heterophoria ( Kromeier, Schmitt, Bach, & Kommerell, 2001 ), but presumably reduces sensory fusion and perhaps also fusional reserves.
Accommodative anomalies . Because of the relationship of accommodation and convergence, anomalies of accommodation can put stress on binocular vision. The additional accommodation required by an uncorrected hypermetrope to permit clear vision or the high accommodative effort in incipient presbyopia are examples of this. Both conditions may show decompensated phoria until the appropriate refractive correction is given.
Imbalanced and/or low fusional reserves. Where there is stress on binocular vision, the fusional reserves are often found to be imbalanced and/or low. It is not known if this is a cause of the stress or the result of it, but the fusional reserves of individuals are known to vary from time to time. This is related to Sheard’s and Percival’s criteria, described later.
Refractive error. Refractive errors, such as astigmatism and anisometropia (and sometimes myopia), can make fusion more difficult due to image blur ( Irving & Robertson, 1996 ), particularly if it is unequal between the two eyes ( Wood, 1983 ), and contribute to decompensation of the phoria. However, in normal subjects, binocularity is only slightly affected by blur, reduced contrast ( Ukwade & Bedell, 1993 ), and induced anisometropia from monovision contact lenses ( Evans, 2007a ).
Visual loss. A visual impairment involving a portion of the visual field (e.g., in macular degeneration or glaucoma) will reduce the amount of matching binocular field from each eye and hence impair the sensory fusion lock. This will increase the likelihood of a heterophoria becoming decompensated. A distortion in the visual field can interfere with central fusion ( Burian, 1939 ) and this might produce symptoms, including diplopia ( Steffen, Krugel, Holz, & Kolling, 1996 ).
Stress on the Well-being of the Patient
Poor general health. A deterioration in the patient’s health can result in decompensation of the phoria ( Pickwell & Hampshire, 1984 ). This is particularly true if other decompensating factors are also present.
Worry and anxiety. It is helpful to know if there are major worries that may contribute to the binocular vision symptoms, even if the problems themselves are not visual. If the situation is temporary, as with a student’s pre-examination stress, this may affect the type or the timing of treatment. For example, a student approaching examinations may be prescribed prismatic spectacles to temporarily correct an anomaly that might usually be treated in the first instance with exercises.
Old age. This can be important for decompensation of near phoria. Presbyopic patients can respond to eye exercises but frequently require ‘top up’ exercises ( Wick, 1977 ). In some cases, prism relief may be required. Distance heterophoria varies little with age ( Palomo, Puell, Sanchez-Ramos, & Villena, 2006 ).
Emotional and temperamental problems. Psychological difficulties and personality problems are difficult to assess during an eye examination, but they may be relevant factors. The treatment of psychological difficulties lies outside the scope of eyecare practitioners, although it may be necessary to take such difficulties into account. This is a useful reminder that we are not dealing just with eyes, but with people.
Adverse effect of alcohol and pharmacological agents. Alcohol decreases convergent and divergent fusional reserves ( Watten & Lie, 1996 ). Alcohol and some prescribed and abused drugs can cause patients to become relatively esophoric at distance and exophoric at near ( Rosenfield, 1997 ). Some drugs reduce the amplitude of accommodation and can therefore affect binocular vision indirectly.
Traumatic brain injury (TBI). TBI, even when mild, can be associated with diffuse axonal injury (DAI) resulting in abnormal vergence and accommodation, hyperphoria ( Doble, Feinberg, Rosner, & Rosner, 2010 ), and other eye movement deficits ( Thiagarajan, Ciuffreda, & Ludlam, 2011 ).
In deciding if heterophoria is compensated, the results of all parts of the eye examination need to be considered, but some sections or tests are particularly important. Sometimes the routine eye examination may also suggest that supplementary tests should be carried out to assist the evaluation. The following parts of the routine or supplementary tests are particularly useful in assessing heterophoria:
Measurement of the degree of heterophoria
Partial dissociation tests
Fixation disparity tests
Diagnosis of Decompensated Heterophoria
Many optometric procedures, including the careful taking of symptoms, have been proposed as useful methods of diagnosing whether a heterophoria is decompensated. These will now be discussed and, at the end of this chapter, and of the next chapter, conclusions will be drawn about which tests are the most useful.
Symptoms are usually present in decompensated heterophoria ( McKeon, Wick, Aday, & Begley, 1997 ). Less commonly, suppression develops to such an extent that symptoms are avoided. There is no set of symptoms that is pathognomic of heterophoria, and the symptoms that are sometimes associated with decompensated heterophoria can also be caused by other problems. It can, however, be said that in the absence of symptoms and of suppression, any heterophoria is compensated, at least at that point in time. When symptoms are present, the practitioner must decide if these are due to the heterophoria or to some other cause. It is only by considering the symptoms together with the other findings that the total picture enables the diagnosis of decompensated heterophoria. In general, symptoms from decompensated heterophoria are associated with some particular use of the eyes for prolonged periods, and these symptoms are lessened or alleviated by resting the eyes. It follows that, in general, the symptoms will be less in the morning and increase during the day. In heterophoria, they are more frequently associated with near visual tasks.
Decompensated heterophoria can give rise to the symptoms detailed here, which are summarised in Table 4.1 . The symptoms can be broadly classified into three categories: visual perceptual distortions (numbered in Table 4.1 and below as 1–3), binocular factors (4–6), and asthenopia (7–10).
Blurred vision. Uncorrected refractive error may put stress on the accommodation–convergence relationship, which results in decompensated heterophoria. Conversely, in other cases, where there is no refractive error, high degrees of phoria can influence accommodation resulting in blurred vision. Some patients interpret small amounts of diplopia as blur.
Diplopia. In heterophoria, any diplopia is intermittent and is worse after prolonged use of the eyes, particularly for concentrated tasks. The diplopia that accompanies a pathological deviation is usually more sudden in onset and is less often associated with any particular use of the eyes for lengthy periods.
Distorted vision. In some cases of decompensated heterophoria (and in binocular instability, discussed in the next chapter), the precise binocular alignment may be variable. This can be seen during the fixation disparity test: even if the patient does not experience diplopia, there may be a transient fixation disparity with the visual axes showing a variable misalignment of several minutes of arc. This may cause the patient to perceive visual perceptual distortions ( Gibson, 1947 ), such as letters or words moving, flickering, or jumping. They may see shapes or patterns on the page and may skip or omit words or lines. This condition needs to be differentially diagnosed from sensory visual stress (Meares-Irlen Syndrome; see next section).
Stereopsis problems . Occasionally there are difficulties in depth perception reported by the patient; e.g., in ball games, pouring liquids into receptacles.
Monocular comfort . A patient may notice that vision is more comfortable if one eye is closed or covered. This can be due to photophobia, but also seems to be associated with heterophoria problems, especially divergence excess ( Chapter 8 ). Patients, especially children, may adopt an abnormal head posture when they are reading (e.g., lay their head on the page) so that their nose is acting as an occluder to give monocular vision.
Difficulty changing focus . Patients may report that distance vision is blurred immediately following prolonged periods of close work. This can also be a sign of a myopic shift.
Headache. Rabbetts (2007) stated that horizontal heterophorias tend to give frontal headaches. These frontal headaches are said to occur, in exophoria, during concentrated vision but, in esophoria, at other times, possibly the day after concentrated work. A survey found that the commonest symptom in children consulting an optometric clinic was headache (8%) and for a quarter of these cases the headache was commonly associated with study or reading ( Barnard & Edgar, 1996 ). One study suggests that 10% of an unselected university student population report headaches associated with studying ( Porcar & Martinez-Palomera, 1997 ).
Aching eyes. The patient says that the eyes hurt after intensive use of the eyes for demanding visual tasks at the relevant distance. In heterophoria, this is usually a dull pain, and is therefore described by the patient as an ache, sometimes saying that the eyes ‘feel tired’.
Sore eyes . The patient may describe a feeling of soreness.
General irritation . The difficulty in maintaining comfortable single vision may result in the patient reporting a feeling of irritability or of nervous exhaustion.
|Visual perceptual symptoms
|10. General irritation
Sensory Visual Stress
Sensory visual stress was previously called ‘scotopic sensitivity syndrome’ or Meares-Irlen syndrome ( Evans, 2001a ) and is also known as pattern-related visual stress or visual stress . Visual stress is also used to describe visually stressful conditions relating to binocular vision anomalies ( Yekta, Pickwell, & Jenkins, 1989 ; Wilmer & Buchanan, 2009 ), which is more motor in origin. Therefore, the word sensory may be more appropriate when describing the form of visual stress described here.
Sensory visual stress is controversial, but some evidence indicates it is a visual processing anomaly associated with a hyperexcitability of the visual cortex. The condition appears to be prevalent in people with autism ( Ludlow, Wilkins, & Heaton, 2008 ), and affects approximately 20% of people with dyslexia ( Evans & Allen, 2016 ) and some individuals with migraine ( Wilkins, Patel, Adjamian, & Evans, 2002 ) and epilepsy ( Wilkins et al., 1999 ). Sensory visual stress is alleviated with individually prescribed coloured filters and the hue and saturation of the required tint seems to vary from one sufferer to another and sometimes needs to be highly specific ( Wilkins et al., 1994 ). It is believed tinted lenses redistribute the excitation in the cortex so as to avoid local regions that are hyperexcitable ( Wilkins, 2018 ). Diagnostic criteria have been suggested following a Delphi study ( Evans, Allen, & Wilkins, 2017 ). The condition is characterised by reports of asthenopia and visual perceptual distortions: sufferers typically perceive words appearing to move, shimmer, or blur and report similar effects from mid-spatial frequency striped patterns ( Evans & Stevenson, 2008 ).
In a small minority ( Monger, Wilkins, & Allen, 2015 ) of cases of sensory visual stress ( Case Study 4.1 ), the situation is further complicated by a heterophoria that may be decompensating ( Evans, 2005b ), or by binocular instability ( Chapter 5 ) ( Evans, Wilkins et al., 1996 ). This can make the differential diagnosis difficult, as it may not be clear whether the unstable visual perception from sensory visual stress is causing the binocular vision anomaly, or whether the binocular vision anomaly is a primary cause of the symptoms. The diagnostic criteria outlined in the next chapter can be helpful in these cases and significant optometric anomalies are corrected before colour is used ( Evans, 2018a, Evans, 2018b ). This is important because accommodative ( Drew, Borsting, Stark, & Chase, 2012 ) or binocular ( Evans & Allen, 2016 ) anomalies could explain some reports of a benefit from coloured filters owing to effects on longitudinal chromatic aberration.
Boy, aged 8, with specific learning difficulties.
After reading for 20 mins: words ‘jump around on the page’, trouble following the line, and eyestrain. Skips or omits words or lines and light sensitive. No headaches.
Normal: ocular health, visual acuities, refractive error (low hypermetropia), accommodative function. Large decompensated exophoria at near.
Given eye exercises for convergent fusional reserves.
Follow-up 2 Months Later
Exercises done but make eyes hurt and symptoms unchanged. Ocular motor status improved and exophoria now compensated. Tested with coloured overlays and showed consistent response, so issued coloured overlay and significant improvement in reading speed.
Follow-up 3 Months Later, Returned for Testing with Intuitive Colorimeter
Overlay definitely helps: less ‘hurting eyes’ and less movement of text. Consistent response to testing with Intuitive Colorimeter and Precision Tints. Precision Tints prescribed.
Follow-up 9 Months Later
Precision Tints used voluntarily for reading, writing, etc. No symptoms when wearing tinted glasses; symptoms without glasses unchanged. Refraction, ocular motor tests, ocular health, and visual fields all normal. Colorimeter checked and new tint prescription found which further improved perception. This was prescribed.
Follow-up 24 Months Later
No symptoms when wearing tints. Reading and spelling greatly improved. Refraction, ocular motor tests, ocular health, visual fields all normal. Colorimetry checked and no change to tint required. Advised yearly checks.
In this case, correction of the ocular motor problem had no effect on symptoms, which seem to originate from sensory visual stress. The tint initially changed, but then stabilised.
In some cases, symptoms may only be completely alleviated by correction of any ocular motor anomaly in addition to coloured filters ( Evans, 2001a ). Indeed, it is interesting to note that in some of the most thorough studies of vision therapy for decompensated convergence weakness exophoria, 58% of adults ( Scheiman, Mitchell, Cotter, Kulp et al., 2005 ) and 56% of children ( CITT, 2008 ) were still symptomatic after 12 weeks of intensive vision therapy. A factor analysis of the symptoms of visual discomfort concluded that symptoms may be attributable in some cases to sensory visual stress and in others to binocular vision anomalies or refractive errors ( Borsting, Chase, & Ridder, 2007 ). It seems likely that the best approach to alleviating symptoms that may be attributable to ocular motor anomalies or sensory visual stress is an holistic approach, in which all potential causes of the symptoms are detected ( Evans, 2018a ). A flow chart for the investigation of visual factors for people with (suspected) dyslexia is given in Appendix 4 .
Nearly 8% of the population suffer from migraine ( Bates, Blau, & Campbell, 1993 ) and a literature review ( Harle & Evans, 2004 ) found claims that migraine can be triggered by low convergent fusional reserves, decompensated near exophoria, and hyperphoria. However, this review noted that the scientific evidence supporting these claims is weak. A study found that people with migraine have a slightly higher than usual prevalence of heterophoria, aligning prism, and impaired stereoacuity ( Harle & Evans, 2006 ). This research indicates decompensated heterophoria is unlikely to be a cause of migraine in all but exceptional cases. Although correcting decompensated heterophoria did not decrease the prevalence of migraine, it was found to decrease symptoms of pain and need for analgesia in some patients with migraine ( Harle, 2007 ). Common sense advice is for practitioners to ask patients about any association between migraine, or other headaches, and visual tasks. It often helps if patients keep a diary of their headaches, including activities before the headache starts, and the diary in Appendix 11 can be issued to patients for this purpose. If specific visual tasks trigger headaches, including migraine, attention should be paid to the refractive and binocular status at the relevant test distance(s).
It has been claimed that a device, SightSync, is effective for prescribing prismatic corrections as a treatment for chronic daily headache, or ‘migraine’ ( Miles, Krall, Thompson, & Colvard, 2015 ). These authors only presented a single group observational study and it is therefore unclear whether this intervention is more than a placebo.
A double-masked placebo-controlled trial revealed that some patients with migraine experience a visual trigger in the form of lights or patterned stimuli (including lines of text) and that this trigger can be treated with individually prescribed coloured filters, in a condition related to the sensory visual stress described earlier ( Wilkins et al., 2002 ). This subgroup of migraine patients are more prone to binocular vision anomalies ( Evans, Patel, & Wilkins, 2002 ) and can be identified with the headache diary in Appendix 11 .
The method of performing the cover test is described in Chapter 2 . Here, we are concerned with evaluating the results. In heterophoria, three things should be noted during the cover test:
Direction of phoria . The direction of the recovery movement will indicate how the eye was deviated under the cover before its removal, and hence the type of phoria. For distance fixation, most patients show little or no movement. For near fixation, the average patient becomes gradually more exophoric from the mid-twenties and has about 6Δ of physiological exophoria by the age of 65 years ( Freier & Pickwell, 1983 ). Obvious departures from this usual state may be decompensated, depending on other factors.
Magnitude of phoria . The larger the amount of heterophoria present, the more likely it is to be decompensated. However, quite small departures from the normal degree are sometimes decompensated, and sometimes high degrees are compensated.
Quality of recovery . The speed and ease of recovery are a good guide to the degree of compensation. A quick, smooth recovery is likely to indicate compensated heterophoria, whereas a slow, hesitant or jerking recovery movement usually accompanies decompensation. A schema for grading the quality of cover test recovery movements in heterophoria is given in Table 2.4 .
It will be seen that all three of the aforementioned aspects of the recovery movement to the cover test need to be considered in deciding if the heterophoria is compensated.
Refraction and Visual Acuity
Because of the accommodation–convergence relationship, there is an association between esophoria and uncorrected hypermetropia in young patients. When the patient can accommodate to compensate for hypermetropia and thereby achieve clear vision, the extra accommodation brought into play will induce increased convergence. Usually this will show as esophoria and there will be an unusual stress on binocular vision, sometimes resulting in decompensation. This will be exaggerated in near vision when the amount of accommodation required may be a large proportion of the patient’s total amplitude. In such cases, the degree of heterophoria is usually less with the refractive correction, and the clinical signs of decompensation will be less apparent. In hypermetropic cases with exophoria, the correction may make the decompensation worse. This is not always the case: in some patients with low uncorrected hypermetropia, correction of the refractive error may sharpen the retinal image which, through aiding sensory fusion, improves the ability to compensate for an exophoria ( Fig. 4.1 ).
In myopia, if exophoria is present, the refractive correction usually assists the compensation. Sometimes the first sign of a refractive change towards myopia is decompensation of an exophoria, often at near. Myopia onset can be associated with a near esophoria and that is discussed in the next section, on myopia development.
The effect of the refractive correction on the heterophoria should always be considered. Correction of refractive errors may reduce blur and so improve sensory fusion ( Carter, 1963 ), even if these refractive errors are relatively small, such as low astigmatism.
Although there are methods of binocular refraction which do not require the use of an occluder, such as the Humphriss immediate contrast method ( Humphriss & Woodruff, 1962 ), many refractive methods occlude each eye in turn. When both the monocular refractions have been completed, the occluder is removed and the eyes are free to resume binocular vision. In compensated heterophoria, this is done promptly, and the binocular acuity is found to be slightly better than the best monocular acuity ( Jenkins, Abd-Manan, Pardhan, & Murgatroyd, 1994 ). The patient will usually report a slight subjective improvement. However, in decompensated heterophoria, the increase in binocular acuity over monocular is less than usual, unless an appropriate prism is prescribed. This effect occurs at distance ( Jenkins et al., 1994 ) and near ( Jenkins, Abd-Manan, & Pardhan, 1995 ).
The removal of the occluder may therefore be regarded as an important part of the assessment of compensation. The patient is asked to look at the best line of Snellen letters which was seen monocularly, the occluder is removed, and the patient is asked if the line is better or worse. In most cases, it will be better, and the binocular acuity can be recorded. Where there are binocular vision problems, the patient may report that the appearance is not quite so good or may hesitate and blink a few times before comfortable binocular vision is restored. In some cases, diplopia may be reported, and the patient may have to make a convergent movement to look at a near object before binocular vision can be obtained. These reactions are subjective correlates of the objective observation of poor recovery during the cover test and suggest decompensated heterophoria. Where binocular difficulties are suggested at this stage, particular attention to this aspect is indicated in the rest of the eye examination.
Some research has found myopia onset to be associated with increased accommodative lag and one hypothesis is that increased accommodative lag leads to hyperopic defocus resulting in axial length growth ( Wildsoet et al., 2019 ). An interesting finding is that the increased accommodative lag is associated with, and preceded by, an increased AC/A ratio ( Mutti et al., 2017 ). The increased AC/A ratio starts to occur 4 years before the onset of myopia. Mutti and colleagues hypothesised that compromised accommodation requires an increased effort needed per dioptre of accommodative output, which could also explain the finding of increased prevalence of esophoria in children who become myopic ( Mutti et al., 2017 ).
When the onset of myopia is associated with a near esophoria, there is some evidence that myopia progression can be slowed by multifocals and this is discussed further in Chapter 6 .
Measurement of the Dissociated Heterophoria
Indications for Measuring the Dissociated Heterophoria
It has long been recognised that dissociation test results do not relate to symptoms ( Percival, 1928 ), except for the case of cyclo vertical heterophoria. Vertical heterophoria, if consistently 1Δ or more, often requires correction ( Sheard, 1931 ). High degrees of heterophoria can be compensated and low degrees decompensated ( Yekta, Jenkins, & Pickwell, 1987 ). As early as 1954, Marton suggested that the size of prism to eliminate a fixation disparity might be more closely related to symptoms than the dissociated heterophoria ( Yekta et al., 1987 ). Tests which use this fixation disparity principle are described later in this chapter and are better uses of clinical time than dissociation tests. However, dissociation tests are sometimes useful to monitor changes in the magnitude of a heterophoria over time. They are also valuable for detecting cyclovertical deviations which are difficult to see on cover testing.
In accommodative esophoria, a hypermetropic correction reduces the magnitude of the esophoria. Measurement in these cases may give an indication of the likely effect of wearing the glasses. If the heterophoria is reduced by the glasses, it is likely that it will become compensated by wearing the glasses without any other treatment. However, other tests for compensation (e.g., Mallett unit) may give similar indications.
One occasion when it may be useful to carry out a dissociation test is to quantify the relationship between the dissociated heterophoria and the opposing fusional reserves (see later). One limitation of dissociation tests is that, with a large slightly paretic heterophoria, the eye may make a secondary movement of elevation in abducting or adducting. This is not likely to be a problem with a fixation disparity test.
Methods of Assessing the Dissociated Heterophoria
Dissociation tests may be carried out at 6 m and at the reading distance and several methods are available. The Maddox rod (multiple groove) consists of a series of very-high-power cylindrical elements which blur a spot of light into a streak. When placed before one eye, the Maddox rod produces this streak, which cannot be fused with the spot seen with the other eye at the same time. The eyes are therefore dissociated and take up the heterophoria position. The amount of the deviation can be noted by the patient subjectively as the separation of the spot and streak judged by a tangent scale (Thorington test), or by the power of prism required to restore the streak to the central position where it appears to pass through the spot. Clear Maddox rods may be preferable to coloured ones, which might influence accommodation.
In another technique (von Graefe’s), dissociation is achieved by using a prism that is too large to be fused whose axis is orthogonal to the direction of phoria which is to be measured. For example, to measure the horizontal phoria, a 10Δ base up or down would be placed before one eye which would cause the object of regard to become vertically diplopic. Horizontal prisms would then be introduced and varied until the two diplopic images were vertically aligned: the magnitude of the horizontal prism to achieve this would equal the horizontal phoria. The Howell phoria card is a tangent scale to be used with a vertical prism in a variation of the von Graefe method ( Wong, Fricke, & Dinardo, 2002 ).
For near vision, the same sorts of method may be used, or the phoria measurement made with a Maddox wing test. This employs septa to dissociate one part of the field from that seen by the other eye. The measurement is read by the patient where an arrow seen by one eye points to a tangent scale seen by the other. A disadvantage of the Maddox wing test is the fixed distance the test uses. More stable results are obtained if a smaller than usual print size is used ( Pointer, 2005 ).
When the heterophoria is measured it is not just the degree of phoria that needs to be assessed, but also the stability of the phoria should be noted ( Chapter 5 ). For example, in the Maddox wing test the amplitude of movement can be recorded in addition to the median position of the arrow (e.g., recorded as 4Δ XOP ± 2Δ).
Most subjective methods of measuring heterophoria have limitations which make them unreliable on some patients. The degree and duration of dissociation and the stimulus to accommodation may vary, so that different techniques produce different results ( Schroeder, Rainey, Goss, & Grosvenor, 1996 ). The 95% confidence limits of most tests are approximately 2–5Δ ( Schroeder et al., 1996 ). A comparison of the interexaminer repeatability of dissociation tests ( Rainey et al., 1998 ) found that the Thorington test was the most reliable with 98% confidence limits of ±2.3Δ and compared well with the reliability of the cover test (98% limits ±3.3Δ). The Howell phoria card interexaminer repeatability is ±3.5Δ for continuous presentation, which had better repeatability than flashed presentation ( Wong et al., 2002 ). Less repeatable results are obtained with a refractor head (phoropter) than with a trial frame ( Casillas & Rosenfield, 2006 ).
The measurement obtained with these methods should be taken only as one factor in helping to evaluate the heterophoria. Although dissociation tests are time-honoured procedures, it is doubtful they are the best way of spending time in a routine eye examination.
The fusional reserves represent the amount of vergence (motor fusion) which can be induced before fusion is compromised leading to blurred and/or double vision. Fusional reserves can be measured with rotary or variable prism devices (ramp stimulus) or, most commonly, with a prism bar (step stimulus; Fig. 4.2 ). The patient is asked to look at a target (see later) and the prism power introduced and slowly increased until the patient reports that the print blurs or doubles. The prism is then reduced until single vision (not necessarily clear) is recovered. The prism power at which these occur is noted and recorded as the fusional reserve to blur (the relative convergence or divergence), break, and recovery. This may be carried out with prism base-in (divergent reserves), with prism base-out (convergent reserves), or (for vertical fusional reserves) base up and base down. Measurements can be taken for distance and for near vision. In young children, and unreliable patients, the break and recovery points should be checked by observing the eye movements.