Assessment of Vision Behind Cataracts

Ophrhal. Physiof. Opt. Vol. 16, Suppl. 2, pp. S26-S32, 1996 Copyright Q 1996 The College of Optometrists. Published by Elsevier Science Ltd Printed in Great Britain 0275-.5408/96 $15.00 +O.OO

Assessment of vision behind cataracts

Paul V. McGraw*, Brendan T. Barrett* and David Whitaker”

Department of Optometry, University of Bradford, Richmond Road, Bradford BD7 1 DP, West Yorkshire, UK

Introduction therapy is implemented to treat estab- ophthalmoscopic view of the fundus. lished retinal/neural disease. If the Clinical assessmentof the effect of Causes of reduced visual function can cataract is the limiting factor determin- lenticular opacification on vision is be grouped into three broad categories, ing visual function between visits, highly subjective and unreliable. In pre-retinal (i.e. optical), retinal/neural, then progression of retinal/neural addition, the presence of identifiable and a combination of the two. It is disease may go unnoticed, giving the retinal pathology does not preclude now widely accepted that one of the ophthalmologist a misplaced confidence improvement in visual acuity following penalties of longevity is the eventual in the treatment regime. Conversely, cataract surgery. Over the past three development of cataracts, which scatter any progression in the density of the decades, the problem of assessing light and reduce visual performance. cataract between visits may erroneously retinal/neural function in the presence To date, the generally accepted form lead the clinician to conclude that of ocular media opacities has attracted of treatment is the surgical removal of therapy directed at the retinal/neural considerable attention. The large the cataractous lens and compensation disease is ineffective. number of techniques which have for its refractive power by either Cataract extraction and insertion of been developed and the voluminous spectacles, contact lenses or, more an intra-ocular lens has become an body of literature in this field are commonly, by insertion of an intra- extremely successful procedure, with indicative of the difficulty posed by ocular implant lens. There are a number approximately 90% of patients achiev- this problem. of clinical investigative techniques ing a post-operative acuity of 6112 or To assessthe functional capability employed by practitioners to detect better (Cataract Manage-ment Guide- of the macula in the presenceof media and evaluate lenticular opacification. line Panel, 1993). When cataract opacities, a test must be employed Similarly, retinal/neural disease, in surgery is completed without significant which has one or more of the follow- isolation, is readily identified using a complications, the success of the ing features: does not require the number of established investigative surgery depends upon two main factors: ocular systemto produce a high quality techniques. Considerable problems the degree of optical degradation image; projects an image through the exist, however, when optical and produced by the cataract, and the opacity; or bypassesthe optical system retinal/neural defects co-exist, since functional capabilities of the retina and of the eye. This review will be divided the clinician is faced with the arduous its neural system. Presumably cataract into three broad categories. The first task of attempting to quantify the surgery, a group of procedures with section will contain basic clinical tests. relative contribution of each factor to significant risks, would not be employ- These rudimentary techniquesprovide the observed loss in vision. This ed if the degree of optical degradation qualitative rather than quantitative in- problem is confounded since the like- is extremely small or if retinal function formation. The second category des- lihood of cataract and retinal/neural is severely impaired, since the surgery cribes techniques which require addi- disease co-existing increases rapidly will be of little benefit. The most tional dedicated instrumentation and with age (Courtney, 1992). common cause of ‘unsuccessful’ sur- are most commonly used by ophthal- A related problem is faced by the gery remains underlying retinal/neural mologists. The final category describes ophthalmologist when patients present disease. However, assessment of two relatively new approachesto the with dense media opacification and retinal integrity in the presence of problem. While these are essentially dense opacification is often problematic still research methods, both have *MCOptom due to the absence of an adequate recently made important inroads into

S26 Assessment of vision behind cataracts: P. V. McGraw et al. S27 the clinical domain. For a more grid; the latter group comprised more detailed treatment of this area the than a quarter of the patients in the interested reader should consult a study conducted by Bernth-Petersen comprehensive review by Hurst and (1981). Douthwaite (1993). Tests using entoptic imagery Basic clinical tests Entoptic images arise from optical The earliest attempts to investigate phenomenawithin the eye. Most types retinal/neural function behind ocular occur when the retina is stimulated media opacities include Maddox-rod with light in someunusual way. These orientation discrimination, colour dis- imagesare sufficiently novel that their crimination, two-point light discrimi- description by a naive patient is per- nation, the light-projection test and suasive evidence that a significant Figure 1. Purkinje vascular shadow. Comberg’s inter-ocular brightness test level of macular function exists (Brodie, (Comberg, 1938). For a review of 1987). However, the perception of may be produced by illuminating the these methods see Fuller and Hutton many entoptic phenomena does not globe either trans-sclerally (Brodie, (1982). While quick and simple to depend on fovea1 function alone 1987), or through closed eyelids with perform, these techniques are of (Guyton, 1987). Hence, description the patient looking up (Fuller and limited predictive value. However, of the image by a patient with ocular Hutton, 1982). The image arises from they do remain useful in locations media opacities cannot be taken as a the suddenillumination by the oblique where more modern methods are un- guarantee of normal fovea1 function. light source of photoreceptors which available (Minkowski, 1983). Equally, the inability of a cataract are normally in the shadow of the patient to describe an entoptic pheno- retinal vessels. To prevent image menon does not necessarily imply a fading, the light source must be con- Trans-illuminated amsler grid poor post-surgical visual prognosis; tinuously moved. This results in a This is a modified version of the there may simply be poor communi- constant shifting of the position of the standard Amsler test, first described cation between clinician and patient, vascular shadowson the retina. Astute by Miller, Lanberts and Perry (1978). or even an inability of the patient to observers will identify not only the The trans-illuminated grid is identical articulate such a novel sensation shadowsof the retinal vasculature, but in size to the standardAmsler test, but (Brodie, 1987). also the avascular fovea1 area and the has 1 mm holes located at the inter- The diagnostic value of entoptic optic disc area. section of the horizontal and vertical imagery may be improved if the test is The advantagesof using this test to grid lines. The grid is mounted on a first administered to the cataractous evaluate retinal/neural function behind light box containing two 15watt neon eye, and subsequently to the normal media opacities include the ease and tubes, and contains a 4mm fixation fellow eye. If the entoptic images are speed with which the test may be hole at its centre. Patients with media perceived in the fellow eye, but not in carried out. Other advantages are opacities, who often cannot see the the cataractous eye, the visual prog- that this test may prove useful in lines on the standard Amsler grid, can nosis is less favourable. patients with densecataracts, and the be consideredas having normal macular There are many types of entoptic general reliability of a positive test function if all the lines joining the phenomena.These include Maxwell’s result when the image is reported retro-illuminated holes are perceived spot, Haidinger brushes, the Purkinje without the need for prompting from as being straight; metamorphopsiasor vascular shadow and the blue-field the clinician (Minkowski, 1983). How- scotomas are suggestive of abnormal entoptic phenomenon (also known as ever, 20% of patients with normal macular function. The predictive ability the Flying Corpuscle Phenomenon). posterior segments and 616 acuity of this technique has been investigated Purkinje vascular shadow and blue- cannot seethe Purkinje entoptic pheno- by Bernth-Petersen (198 1) who found field entoptic phenomenaare the most menon, even after leading remarks that the technique correctly identified suitablefor investigating retinal/neural from the examiner (Goldmann, 1972). reduced retinal/neural function in function behind ocular media opacities The possibility of leading or confusing more than two-thirds of patients with (Brodie, 1987). the patient is not the only disadvantage cataract and co-existent macular The Purkinje tree is an image of the of this test. Since the macroscopically pathology. However, a significant retinal vasculature seen in fluctuating visible vesselsdo not closely approach disadvantage of the test is the high positive and negative contrast, and the fovea, the Purkinje tree may be proportion of patients with dense is the most familiar entoptic image seenby patientswith macular pathology opacities who are unable to see the (Figure / ). This entoptic phenomenon (Minkowski, 1983; Goldmann. 1972). S28 Ophthal. Physiol. Opt. 1996 16: Suppl. 2

Blue-field entoptoscopy, another en- classified as a normal result (Sinclair a Snellen chart is projected onto the toptic test, will be discussed in the et al., 1981). The result is equivocal retina (6” visual field), and the patient next section. if the patient reports that portions is instructed to read out the letters. of the visual field are without cor- The patient is usually dilated. It is Tests requiring additional puscles, and an abnormal result is important that the patient is optically instrumentation recorded if few or no corpuscles are corrected to ensure optimal clarity of seen(Figure 2). The diagnostic value the chart when projected onto the Blue--eld entoptic phenomenon of the test is enhanced if the fellow fundus. Essentially, this instrument is eye is also examined. Comparison similar to using a pin-hole in conjunc- Like the Purkinje vascular shadow, between the eyes in terms of speed, tion with a Snellen chart, the differ- this phenomenon is generated via the number, and distribution of the ences being that the reduced retinal retinal vasculature. It arises when corpuscles may then be made (Loebl illumination and increased diffraction the retina is stimulated with short- and Riva, 1978). associatedwith the conventional pin- wavelength visible blue light. The blue Studies suggest that the blue-field hole are avoided. Advantages of using light passes through the translucent entoptoscope is only slightly more a letter chart are that the task of white blood cells, and stimulates the accurate than the Purkinje entoptic test identifying letters is universally used relatively dark-adapted photoreceptors in predicting good macular function in and understoodby both clinicians and located beneaththe capillaries(Guyton, cataract patients. But it is considerably patients, and that a threshold can be 1987). In essence, the observer is more accurate in predicting poor identified rapidly. viewing his or her own white blood macular function (Sinclair et al., The potential acuity meter has a cells flowing in the perifoveal retinal 1979), becausethe blue-field entopto- tendency to overestimate the post- capillaries. Individual white blood scope commentsmore specifically on operative acuity level in eyes with corpuscles are seen as bright dots central visual function than the relative!y moderatecataract and certain with elongated tails, and normally Purkinje test, which covers approxi- pathological conditions suchas cystoid between 25 and 40 corpuscles are mately 30” of the central visual field macular oedema and macular holes. seensimultaneously (Loebl and Riva, (Fuller and Hutton, 1982). One possible explanation may be 1978). The series of accelerations that the retinal illumination of the and decelerations of the corpuscles projected test chart is higher than a Potential acuity meter occurs in tandem with the cardiac standardSnellen chart (Guyton, 1987). cycle. There are two commercially This test uses a single channel Max- Another disadvantage of this test is available devices facilitating the wellian view projection system to that often the only clear portion of phenomenon’s observation. The first image a small light source in the the cataractous lens is located peri- is a slit-lamp attachment, and is known plane of the patient’s entrance pupil pherally. Projection of the beam as the flying corpuscle viewer (Adams (Minkowski er al., 1983). The position through such regions createsproblems and Shock, 1986). The second, the of this highly localised beam can with longitudinal chromatic aberrations blue-field entoptoscope, provides a then be manipulated by the examiner and off-axis aberrations.Several studies self-contained high intensity light so that it is projected through a clear have reported that the instrument source and 430-nm filter (Sinclair et or relatively clear portion of the beam is unable to penetrate dense al., 1979). The higher intensity light cataractous lens. An aerial image of cataracts (Minkowski et al., 1983; source of the blue-field entoptoscope enablesthe image to be seenby patients with dense cataracts. The uniform blue field of the entop- toscopesubtends approximately 20” at the patient’s eye, and is divided into quadrants by means of a graticule. Testing is carried out monocularly and is usually, but not necessarily, performed following pupillary dilation. No refractive correction is necessary. The patient isasked whether corpuscles are visible in all four quadrants, and is Normal Equivocal Abnormal also askedto estimate the total number seen. Visibility of 15 or more cor- Figure 2. Blue-field entoptoscope responses: normal, equivocal and abnormal puscles in patients with cataract is (see text for details). Assessment of vision behind: P. V. McGraw et al. s29

Christenbury and MacPherson, 1985)) can be altered (90”, 180”, 45” and 1971). The predictive abilities of restricting its clinical utility. 135”) and the patient’s task is to clinical interferometers have been the identify the orientation. The highest focus of much research. However, spatial frequency at which the orien- clinical opinion as to the value of Inter$erometers tation can be correctly identified is these instruments in predicting post- taken as a measure of the retinal/ operative visual function varies con- These instruments also employ Max- neural resolution. The grating acuity siderably (Faulkner, 1983; Guyton, wellian view optical systems to project can then be converted to a Snellen 1986). Interferometers also suffer from two coherent beams of light through equivalent to form the predicted post- the problem that they do not penetrate the patient’s pupil. After traversing operative acuity level. dense cataracts, restricting their use to the pupil, the beams interfere construc- The clinical interferometers, like the mild or moderate levels of opacifica- tively and destructively, to produce an potential acuity meter, also have a tion. This is rather unfortunate, since interference pattern on the patient’s tendency to overestimate the post- it is patients with dense cataracts for retina. An example of the interference operative visual acuity. Several studies whom assessment of retinal/neural pattern seen by a patient is shown in have found poor agreement between function is most important. Figure 3. The spacing between the interferometric acuity and letter acuity black and white bars (spatial frequency) (Gstadler and Green, 1971). This may of the interference pattern can be be partially explained by the fact Electrophysiological tests adjusted by manipulating the spacing that, in general, grating acuity tends between the two beams. The greater to be higher than Snellen acuity, Roth the electroretinogram (ERG) and the beam separation the finer the inter- particularly in patients with posterior visually evoked potentials (VEP) are ference pattern produced on the retina. segment pathology (Faulkner, 1983) useful for assessing retinal/neural The orientation of the grating pattern and amblyopia (Gstadler and Green, function in the presence of cataract. The major drawback to the ERG is that it reflects only gross retinal function, and patients with some forms of macular disease may show a normal ERG response. For this reason, the VEP is the more useful test as it primarily reflects macular function. Pattern VEP responses, where the visual stimulus is a checkerboard, are degraded by the presence of cataract. Therefore, flash VEPs are favoured in pre-operative evaluation. A high intensity flash, modulated at a frequency of approximately lOHz, is presented, and the second peak in the waveform is analysed. One advantage of the flash VEP over many of the previously described techniques is its ability to penetrate very dense cataracts. However, due to the cost of the equipment and the expertise required to perform these procedures they are not used routinely in preoperative assessment.

Research based tests Hyperacuity tasks Figure 3. An example of the interference fringes seen by a patient. This pattern The term ‘hyperacuity’ is used to was generated by a Rodenstock retinometer (reprinted with permission from describe a set of spatial thresholds Hurst and Douthwaite, 1993). which exhibit clear superiority over s30 Ophthal. Physiol. Opt. 1996 16: Suppl. 2 conventional spatial thresholds such as visual acuity (Westheimer, 1975). The precision with which relative localis- ation can be performed is often of the order of 5 seconds of arc, and is achieved despite the limitations im- posed by both the visual pathways and aberrations produced by the optical media. Examples of hyperacuity include Vernier acuity, stereoacuity, bisection acuity and displacement detection, The resolving capacity of the eye relies on the ability to distinguish two objects as being spatially separate. As the retinal image becomes increasingly diffused over a larger retinal area due to hazy optical media or uncorrected refractive error, the two objects merge and the eye becomes unable to resolve them as separate. For this reason Figure 4. Circularly symmetric resolution and Vernier acuity targets which measures of visual acuity are of little have been successively low-pass filtered to mimic the effects of media value in patients with media opacities as opacification. it is impossible to distinguish between reductions in acuity caused by the A single measure of Vernier acuity a displacement threshold hyperacuity opacity and those caused by underlying provides little useful information as a task is shown in Figure 5. In this task, posterior pole pathology. As hyper- range of gap separations must be exam- the subject is required to indicate when acuity thresholds such as Vernier acuity ined to determine optimum threshold. movement of the central target, relative and displacement thresholds are ex- This leads to an increase in test times to the two stationary flanking targets, tremely resistant to the effects of retinal and a consequent reduction in the can just be detected. This method pro- image degradation produced by media clinical appeal of the test. Measures of vides useful clinical information in a re- opacities (Essock et al., 1984), the Vernier acuity have been modified to latively short time (2-3 mitt) (Whitaker threshold obtained should relate directly create the ‘hyperacuity gap test’. This and Deady, 1989; McGraw et al., to the functional capabilities of the is essentially a comparison of fovea1 1995). This technique has previously retina and its neural system. This can be Vernier thresholds with those of eccen- been used in patients awaiting cataract understood by considering that the tric retinal locations in the same subject. extraction and Nd:YAG laser capsulo- human visual system is able to calculate Since optimum Vernier threshold is tomy, and has been shown to offer a the ‘centre of gravity’ or centroid of the obtained at the fovea, measurement of good indication of the likely quality of retinal light distribution of the target Vernier acuity with increasing eccen- vision achieved following surgery. (Whitaker and Walker, 1988). When tricity, creates an inverted v-shaped For example, a patient presenting with this light distribution is diffused over a function, broadly similar in shape to dense opacification, a visual acuity of larger retinal area by the media opacity, that found for visual acuity. A lack of a the position of the centroid of the pronounced central peak in this function target remains un-altered, thus leaving is indicative of a fovea1 lesion. Simi- the judgement that the subject must larl y , marked asymmetry in the make relatively unaffected. A com- function is thought to be characteristic parison of positional acuity and of abnormal retinal/neural function. resolution is shown in Figure 4. Although this technique has been Many studies examining hyperacuity shown to be useful in evaluating visual thresholds in the presence of cataract function in the presence of cataract, have utilised Vernier acuity (Essock even when the opacification is dense, et al., 1984; Enoch et al., 1985). the procedure is time consuming (up to However, this type of hyperacuity 25 min) and the results can be difficult may not be ideal as Vernier threshold to interpret (Guyton, 1987). Figure 5. Schematic representation of varies as a function of the vertical sep- An alternative method is to use stimuli used to measure displacement aration between the stimulus features. displacement detection; an example of threshold hyperacuity. Assessment of vision behind cataracts: P. V. McGraw et al. s31

3/60 but with normal retinal/neural function, should obtain a low pre- opeative displacement threshold. As thesethresholds are highly resistant to the effects of optical degradation, the magnitude of the post-operative displacement threshold should be &&ar, and several studieshave found this to be the case(Whitaker and Deady, 1989; McGraw er al., 1995). On the other hand, a patient with denseopacification coupled with the coexistence of abnormal retinal/neural function should show raised displacementthresholds.

Contrast detection in noise The measurementof contrast perception in the presenceof visual noisewas first suggested by Kersten and col- leagues (1988). The principle behind Figure 6. The effect of adding luminance noise to an image. this approach is that the visibility of a target is reduced by the introduction of visual noise which acts as a mask. As the noise density is increased, higher contrast levels are required to maintain performance levels. The noise is produced by generating a pattern which contains random increments and dec- rements in luminance. The density of the noise is controlled by manipulation of the luminancediscrepancy (contrast) between the light and dark portions of the noise pattern. The effect of adding luminance noise is shown in Figure 6. This figure showsa child’s face without noise (left) and with noise (right). A plot of signal energy againstnoise density allows the determination of two important parameters. Firstly, the X- intercept (see Figure 7) representsthe internal visual noise, also termed equi- 2 4 valent noise. This measure can be Noise Density affected by changesin both optical performance and retinal/neural function. Figure 7. ‘Contrast detection in noise‘ function. 1 represents the function of an Therefore, this measurein isolation is ideal observer with no equivalent noise present; 2 is the response of an observer of little use to the practitioner attemp- with optical degradation; and 3 is the response of an observer with both optical ting to assessthe relative contributions degradation and retinal/neural pathology. of optical and retinal/neural dysfunction to the observed loss in visual function. of the slopeof the plot of signal energy have a value of 1 (100%). Reductions The important information for dif- against noise density, converted into in visual processing, found in patients ferentiating between optical and neural percentage terms, provides a measure with amblyopia and diabetic retino- dysfunction is contained in the gradient of the patient’s visual processingeffi- pathy, manifest as an increase in the of the function (Kersten et al., 1988; ciency. To achieve perfect visual pro- gradient. In contrast, a patient with a Gilchrist et a/., 1991). The reciprocal cessing,the slopeof the function should cataract and normal retinal/neural S32 Ophthal. Physiol. Opt. 1996 16: Suppl. 2

function, will show a gradient similar tools to help practitioners addressthis effectsin visual dysfunction. Invest. to that of a visually normal subject, des- difficult clinical problem. Ophthalmol.Visual Sci. 32, 1281. Goldmann,H. (1972). Examinationof pite the presenceof increasedequivalent the fundusof the cataractouseye. Am. noise. This is shown schematically in J. Ophthalmol.73, 309-320. Figure 7. Three hypothetical conditions References Gstadler,R. J. and Green,D. G. have been included on the graph: 1, (I971). Laserinterferometric acuity in Adams, D. F. and Shock,J. P. (1986). visually normal (ideal observer); 2, amblyopia.J. Ped. Ophthalmol.8, Predictionof postoperativevisual 25l-256. optical degradation only; 3, optical acuity in cataractpatients using the Guyton, D. L. (1986). Misleadingpre- degradationand retinal/neural dysfunc- Flying CorpuscleViewer. Ophthalmic dictionsof post-operativevisual acuity. tion. In the figure, the subjectwith only slug. 17, 509-512. Arch. 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