In the first of a two-part discussion concerning what influences measurements with the Mallett unit, Rajula Karania and Bruce Evans look at the theory behind fixation disparity and its measurement
The Mallett fixation disparity test is a widely used diagnostic tool within UK optometry. The purpose of this study was to evaluate for the first time the effect of test instructions on the results of this test in a clinical setting. Previous research indicates that the Mallett unit is a useful tool for detecting symptomatic heterophoria at near, having both good sensitivity and specificity. However, the significance of precise instruction technique has received little attention.
Our research was conducted as a two-part study. In part one, we surveyed and observed the current method of use of the Mallett fixation disparity test by practitioners and then, in part two, we compared results obtained with this 'standard' method of questioning with the more 'specific' method of questioning that has recently been suggested in the literature.1
The results demonstrate that the testing method is important: patients need to be asked not just whether the Nonius strips are aligned but also whether one or both of the strips ever move. In this respect, the authors feel that perhaps further research is indicated to investigate the significance of precise test instructions in other subjective optometric and orthoptic tests. In these two articles we will describe our findings and their implications.
An introduction to fixation disparity
Fixation disparity is a minute ocular misalignment under conditions of binocular single vision and, in the UK, is usually detected using the Mallett fixation disparity unit. This instrument creates natural viewing conditions, whereby the patient's binocular system is fused using both central and peripheral fusion locks. This allows the examiner to determine the minimum prism power that eliminates the fixation disparity: the associated phoria or aligning prism. The spherical power that eliminates the fixation disparity, the aligning sphere, can also be determined.
During normal binocular vision the object of regard stimulates retinal elements in each eye, which give rise to the same subjective visual direction so that the person perceives one image of the object. These elements have conventionally been described as corresponding retinal points or elements.2 The term 'corresponding retinal points' is really a misnomer since this is a point to area correspondence. Panum, in 1858, described these areas as Panum's fusional areas. They are elliptical with their long axes horizontal and increase in size in the periphery.3
Panum's fusional space is the spatial counterpart of Panum's fusional area and is contained within two curved surfaces, one on each side of the horopter surface.4 All objects lying within this three-dimensional space will fall in Panum's areas and will therefore be seen singly. Panum's area allows for some imprecision in eye movements without the person experiencing diplopia.5 Objects lying outside Panum's fusional space may appear in physiological diplopia.
Fixation disparity occurs when the images of a binocularly fixated object do not stimulate corresponding retinal points but still fall within Panum's fusional areas, the object thus being seen singly.6 Fixation disparity allows fusion and binocular single vision without precise bifoveal fixation.7
The angular extent of fixation disparity is a measure of the degree to which the images have slipped.8 The conventional view is that fixation disparity typically measures between 5-10 minutes of arc and rarely exceeds 10 minutes of arc.9 The cone density is greatest at the fovea and therefore here there is little room for error of fixation. Consequently, Panum's areas at the fovea are likely to be small10 and early estimates11,12,13 have shown them to be between 6-15 minutes of arc.10
Away from the macula, beyond a visual angle of 5º, Panum's areas measure approximately 6 per cent to 7 per cent of the angle of eccentricity.14 This is in direct relation to the decreasing cone density in more peripheral regions of the retina. This in turn makes the peripheral visual field more tolerant of larger degrees of disparity and less likely to undergo adaptations like suppression to avoid diplopia.15 Although the variation in size of Panum's area is described in terms of retinal eccentricities, they are in reality a cortical phenomenon.16 Ogle et al17 suggested that the magnitude of fixation disparity depends on the strength of innervations to the extra ocular muscles during fusion. This in turn is related to the magnitude of heterophoria, the strength of compensating fusional vergence, and the complexity of detail of the visual target. Fixation disparity plays an important role as a feedback stimulus to maintain a particular level of vergence innervation.18,19 The finding that the apparent dimensions of Panum's areas vary with different testing conditions and target parameters may help to explain studies indicating that Panum's areas may be much larger than the conventional view.20,21,22,23
The stress theory of fixation disparity and the importance of the fusion lock
Mallett interpreted fixation disparity as a sign of stress when the fusional reflexes are incapable of maintaining perfect superimposition of normally corresponding receptive fields24,25 and there is considerable evidence to support this view.
Joshua and Bishop26 found that the binocular response from optimally superimposed receptive fields is greater by about 45 per cent than the sum of uniocular responses. Fixation disparity, as detected with the Mallett unit, is associated with a reduction of the binocular visual evoked potential27 and of binocular visual acuity.28,29,30,31 Similarly, fixation disparity is associated with a reduction in stereo-acuity.32,33,34 Fixation disparity can be induced in normal subjects under abnormal test conditions such as reducing the ambient lighting to mesopic levels35,36 or reading at an unusually close working distance.37 Computer users who prefer longer than average viewing distances have abnormal fixation disparity curves and are more likely to experience symptoms if forced to work at a normal viewing distance.38
Several authors have concluded that fixation disparity tests should include both foveal and peripheral fusion details.39,40,41,42,43,44 The lack of a central fusion lock destabilises45 and increases46 fixation disparity and under these unnatural conditions fixation disparity is therefore a less useful indicator of visual stress.1
Instruments for measuring fixation disparity
Typically, studies have tended to measure fixation disparity using the method of moving monocular markers (bars or lines) until they are aligned in a binocular vernier alignment task. The amount through which the line has to be moved is translated into an angular measurement of fixation disparity. However, instruments that measure the angular fixation disparity tend not to have a central fusion lock so that the monocular marker lines are directly adjacent. This facilitates the vernier alignment task, but leads to higher levels of fixation disparity and less stable results than when a central fusion lock is present.46 Indeed, fixation disparity measurements are affected by a number of target design characteristics47,48 and by inclination of gaze.49
The Sheedy disparometer50 and Wesson card,51 neither of which are widely used in the UK, are examples of instruments that measure the actual fixation disparity. These devices can be used to plot the forced vergence fixation disparity curves showing the effect of horizontal prisms on fixation disparity. Ogle et al52 first described these as four classic types and their clinical relevance was evaluated by Sheedy and Saladin.50 Yekta et al found that the central slope of the forced vergence disparity curve was not significantly associated with symptoms.53
The Mallett Unit
The Mallett unit is perhaps the most commonly found orthoptic instrument in optometric practices in the UK. It detects fixation disparity and determines the prismatic power (associated phoria/aligning prism) or spherical lens power (aligning sphere) that eliminates the fixation disparity1: the Mallett unit does not measure the angular fixation disparity.
The Mallett distance (Figure 1) and near (Figure 2) fixation disparity tests were designed so that the only disturbances to normal binocularity are the monocularly-viewed Nonius markers54 and the reduction of ambient lighting by the cross-polarising visor. To allow for this reduction in illumination, the ambient lighting should be increased. The original distance test (Figure 1, left-hand figure) employed a target that could be rotated for investigating vertical and horizontal heterophoria independently.55 This was later modified (Figure 1, right-hand figure) so that both horizontal and vertical heterophoria could be assessed without adjusting the test and this was shown to produce similar results to the original design.56 Mallett55 explained some design details by which the distance chart differs from the near unit. On the distance Mallett unit, the Nonius test lines are 18.3 and 5.2 minutes of arc in length and width respectively.
The angular separation of the lines is 20.6 minutes of arc. For the near Mallett unit, the Nonius strips are approximately 26 minutes of arc in length, nine minutes of arc in width, with a separation of 30 minutes of arc at 40cms.56 Ukwade found that fixation disparity is most repeatable when measured with a central fusion lock and Nonius separation of 20 minutes of arc or less.57
Mallett stated that any displacement of one or both strips is important, however small the displacement may appear to the patient.58 The angle of the actual fixation disparity is not very strongly correlated with the magnitude of the aligning prism,59 as would be expected owing to the different types of fixation disparity curves.60
A small number of people perceive a noticeable misalignment of the two Nonius strips without the polarised filters in place. This may be a true 'alignment error', which some people experience on vernier tasks61,62, or may indicate an unreliable patient.1
There are other instruments that measure aligning prism such as the American Vectographic Slides,63,64 the Zeiss Polatest,65,66 Bernell test lantern46, Borish near point card,67 and RMS rotary near point card.68 However, the present paper concentrates only on the Mallett unit since this is the most commonly used fixation disparity test in the UK.
Relationship of fixation disparity and aligning prism with symptoms
The measurement of aligning prism is considered to be a useful clinical test in the investigation of binocular stress but should not be considered in isolation from other test results and must be related to the presence or absence of symptoms.69 Mallett70 claimed that patients with decompensated heterophoria all have fixation disparity with his test; and conversely the absence of fixation disparity demonstrates the adequacy of the fusional reserves to cope with whatever heterophoria may be present. It might be appropriate at this stage to consider the symptoms of decompensated heterophoria, which Evans1 classified into four categories: visual symptoms (blur, diplopia, distorted vision); binocular difficulties (difficulty with stereopsis, improved comfort with monocular viewing, difficulty changing focus); asthenopia (headache, aching eyes, sore eyes); and referred (general irritation). These symptoms are non-specific in that they can arise from other causes, which is why clinical tests are necessary to confirm the diagnosis of decompensated heterophoria.
Mallett70 stated that, in rare cases, decompensated heterophoria might be present without any associated symptoms: if there is deep foveal suppression in one eye as a sensory adaptation to avoid symptoms, lack of critical visual tasks, or if the patient is being tested with a different/new refractive correction. These cases, where decompensated heterophoria is not synonymous with symptomatic heterophoria, are said to be rare affecting only 6 per cent of a clinical sample that exhibited a fixation disparity with the Mallett Unit.70
Studies of fixation disparity tests in the detection of symptomatic heterophoria
Sheedy and Saladin71,72 performed two studies of the relationship between asthenopia and various diagnostic criteria for horizontal imbalances, including fixation disparity. However, they used the Sheedy disparometer, which does not have a good foveal fusion lock.
Yekta and Pickwell73 used a Mallett Fixation Disparity Unit which was modified such that the angular fixation disparity could be measured as well as the aligning prism. They found a low correlation between the magnitude of the heterophoria and fixation disparity. They found no significant difference in heterophoria between symptomatic and asymptomatic subjects, but the symptomatic participants did have significantly higher degrees of fixation disparity.
This agrees with the suggestion of Mallett70 and Sheedy and Saladin72 that fixation disparity is a better indicator of decompensated heterophoria than the degree of heterophoria. Yekta et al69 again found no significant relationship between symptoms and heterophoria, but found that both fixation disparity and aligning prism for near vision are associated with symptoms.
They concluded that, compared with heterophoria, fixation disparity and aligning prism are better indicators of decompensation of binocular vision at near and that they are more useful tests to incorporate in the routine optometric examination.
Jenkins et al74 investigated the relationship between aligning prism at near and symptoms. Their data, summarised in Figure 3, suggest that if an aligning prism of 1? or more is taken as indicating a test fail for the Mallett test, then the sensitivity would be 75 per cent and specificity 78 per cent for detecting symptomatic heterophoria in pre-presbyopes.
Pickwell et al75 investigated the relationship between Mallett aligning prism and symptoms. They were unable to demonstrate any useful relationship for distance vision but found that aligning prism can be very useful in the detection of decompensated heterophoria at near.
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? Rajula Karania was postgraduate research optometrist at City University and is specialist optometrist at Moorfields Eye Hospital. Bruce Evans is visiting professor at City University and director of research at the Institute of Optometry