Vision through cataract

A recent patient explains how her vision has deteriorated over the past year: 'The funny thing is that under some conditions I can still see quite well and yet at other times I feel that I am practically blind. I don't like going out on my own nowadays because I feel that I cannot see well enough to cross the road safely especially when it is sunny.'
'What is the lowest line of letters that you can read on the chart?'
To my surprise my patient descends the chart with surprising ease, finally admitting defeat at 6/9+1. Is her vision really as bad as she is claiming? Can I refer her with such good visual acuities?
Approximately 5 per cent of the population in the 55-64 age group have cataracts causing a reduction in visual acuity to 6/9 or worse. This figure rises to over 40 per cent for those over the age of 75.1 With the anticipated increase in life expectancy, the prevalence of cataract in the population is also likely to increase.
The initial management of patients with cataracts, including the decision to refer for surgery, is usually the responsibility of optometrists. It is vital, therefore, that optometrists have the skills to assess a cataract and a good understanding of the nature of the visual degradation associated with different types of lens opacities.

Classification of cataracts
A cataract occurs when the fibres within the crystalline lens become disorganised or the cytoplasm within the fibres degrades. This causes an increase in light scattering at the irregular interfaces between regions of different refractive index. These changes within the lens inevitably produce some reduction in the quality of the retinal image, although the exact nature of the effect on vision will depend on their type, 'density' and location within the lens.
There are three main morphological types of age-related cataract: cortical, nuclear and posterior subcapsular (Figure 1). According to Brown and Hill,2 cortical cataract is the most common in the UK (63 per cent), followed by nuclear (41 per cent) and posterior subcapsular (24 per cent). Mixed cataract (more than one morphological type) occurs in approximately 30 per cent of eyes.
Cortical cataracts are cuneiform or wedge-shaped 'opacities' found in the anterior or posterior lens cortex. Cortical cataracts form in the periphery of the lens behind the iris (often in the inferonasal quadrant) and progress towards the centre of the lens as they develop. Vision is only affected when the spokes start to impinge on the pupil area.
Nuclear cataract occurs when there is an increase in light scattering in the nucleus of the lens. This is often associated with an increase in yellowing (or brunescence).
Posterior subcapsular cataract (PSC) occurs when abnormal epithelial cells and extracellular granular and fibrillary material accumulates at the posterior pole of the lens in front of the posterior capsule. The location of PSCs (in the centre of the pupil and close to the nodal point) means that relatively minor aggregations can have a significant effect on vision.

Effects of cataracts on vision
The crystalline lens plays an important part in forming the retinal image. It follows that any change in the optical quality of the lens within the pupillary zone is likely to have some deleterious effect on vision. The nature and severity is strongly dependent on the position, extent and physical characteristics of the disturbances in the lens. However, the degradation of the retinal image can usually be attributed to one or more of the following:

A change in transmittance
A change in the refractive properties
An increase in forward light scatter.
Change in transmittance
The young crystalline lens is remarkably transparent to wavelengths within the visible spectrum. However, the absorption of the lens increases with age, particularly towards the blue end of the spectrum - a characteristic that accounts for the apparent yellowing of the lens. This is thought to be a result of the accumulation of chromophores, particularly in the nucleus of the lens. In certain types of cataract, these effects are more marked and occur at an earlier age.
Another cause of decreased transmittance in cataract is increased light scatter caused by disruption of the regular structure of the fibres. The type of scattering is determined largely by the size and irregularity of distribution of the scattering particles. Scattering of light back out of the eye is known as backward scattering, while light scattered onto the retina is known as forward scattering.
The principal effect of backward scattering is to reduce the amount of light that reaches the retina. Forward scattering, on the other hand, reduces the contrast of the retinal image and is therefore of more significance in terms of vision (see below). However, it is backward scattering that is assessed when viewing a lens section with a slit-lamp biomicroscope. Unfortunately, the amount of backward scatter is not always a good guide to the amount of forward scatter and so the appearance of a lens section does not always correlate well with the quality of vision.
Whatever the mechanism, decreased transmittance of the lens causes a relative reduction in retinal illuminance. While this can be a disadvantage when light is scarce, it has surprisingly little effect on visual performance under normal photopic conditions, thanks to the remarkable dynamic range of the retina. It may be concluded that in most cases the reduction in the transmittance of the lens that occurs in eyes with cataracts is not the principal cause of degraded vision.
However, the 'yellowing' of the lens that can occur with certain types of cataract results in reduced sensitivity to blue light (Figure 2). This may lead to some difficulty in differentiating between certain colours, which may be problematic for some tasks. It is interesting to note the change in the colours used by artists as they became older - most notably Claude Monet.3 Like Monet, patients often report a striking change in colour perception after cataract surgery.
Does this yellowing of the lens result in patients perceiving the world tinged with a yellow hue? Probably not. The visual system has an elegant mechanism for maintaining the perceived colour of objects constant, despite changes in the overall hue of the retinal image. This mechanism has evolved to counteract the large changes in the spectral power distribution of daylight throughout the day. Despite the fact that daylight is far 'bluer' at midday than at dawn or dusk, our perception of the colour of objects remains remarkably constant.
The ability of the visual system to adjust its colour balance can be demonstrated by staring at the cross in the centre of Figure 3 for about a minute and then looking back at the cross in the centre of Figure 2. You will find that the two images of Trafalgar Square appear to be approximately the same colour for a few seconds before gradually returning back to their true colours. It is likely that these same mechanisms are used to negate the effect of the yellowing of the lens.
Changes in colour vision can be detected using tests that are capable of detecting the reduced sensitivity to shorter wavelengths, such as the Farnsworth-Munsell Hue test, the D15 and the City University test.

Changes in the refractive properties
Changes in refractive error are common in cataract, in particular increasing myopia and astigmatism. Where the refractive changes are regular, vision may be improved by prescribing the appropriate spectacle or contact lens correction.
However, changes in the structure of the lens may result in pockets of material with higher and lower refractive index. This may result in local refractive and prismatic effects causing 'blurring' of the retinal image which cannot be corrected adequately with a spectacle lens and in some cases may cause monocular diplopia (Figure 4). Generally, changes in the refractive properties of the eye cause a reduction in visual acuity. Therefore, where this is the principal feature of a cataract, high contrast VA usually provides a reasonable indication of a patient's overall quality of vision.

Forward light scatter
One of the main effects of cataracts is to increase the amount of intraocular light scatter. Forward light scatter occurs when the light hits disorganised fibre membranes or lens proteins. Pencils of light passing though each point of the lens spread out into a cone, the width of which is determined by the size and density of the scattering source. This causes diffusion of the retinal image and results in a loss of luminous and chromatic contrast; in other words, objects look less black and white and colours become desaturated (Figure 5).
While this may have a dramatic effect on the quality of vision in the 'real world', it often has surprisingly little effect on high contrast visual acuity (VA) measured in the consulting room. This can be explained by the fact that VA is relatively insensitive to changes in contrast - a 50 per cent loss in the contrast of the retinal image may only produce a one-row decrease in VA when looking at a high contrast chart. Therefore, high-contrast VA does not always give a reliable indication of the quality of vision of patients with increased scatter.
Alternative methods for assessing vision
In the early 1960s, techniques that had been applied for many years to assess the characteristics of electronic and optical systems, started to be applied to the visual system.4 The technique involves investigating the response of the system to sine waves of various frequencies and gives rise to a function known as the Modulation Transfer Function (MTF). The MTF provides a powerful description of the characteristics of a system and can be used to predict the response of the system to more complex inputs.
Applied to the visual system, the technique involves measuring the minimum contrast required to detect sine wave gratings of various spatial frequencies. The resulting Contrast Sensitivity Function (CSF) (Figure 6) provides information about how well a patient sees over a range of spatial frequencies and therefore provides a more comprehensive assessment of visual function than VA.
Visual acuity can be thought of as a measure of the highest spatial frequency that can be detected at a high contrast and therefore provides a powerful description of a patient's visual capability (Figure 9).
As such, VA is sensitive to conditions that produce a loss of contrast sensitivity (CS) at high spatial frequencies. However, it is less good at detecting conditions that produce a loss of sensitivity at lower spatial frequencies.
Until recently, the measurement of CS required bulky and expensive equipment. The advent of chart-based systems (notably the Vistech5 and Pelli-Robson charts6) opened up the possibility of testing CS in optometric practice. However, the charts tended to be bulky and difficult to illuminate and have not been widely used outside research laboratories.
However, recent developments in computer and display technology have opened up new opportunities for vision assessment in the consulting room. Modern PCs, coupled with flat panel displays, are capable of generating an almost infinite variety of test stimuli on a single screen. Coupled with appropriate software such as Test Chart 2000, tests which were previously confined to research laboratories can now be applied in the clinic (Figure 7).
While computer-based test charts are capable of generating a vast range of test stimuli, the measurement of a CSF using sine waves requires very accurate calibration and extremely precise control of the screen luminance. Claims that this can be achieved on computer-based systems without regular calibration should be treated with caution.
An alternative and more satisfactory approach is to use a letter contrast test (similar to the Pelli-Robson) or to measure low-contrast visual acuity.
The Pelli-Robson chart consists of rows of letters of the same size but decreasing contrast (Figure 8). The chart is usually viewed from one metre, from where the letters equate to 6/275, giving an indication of CS just below the peak of the curve at 0.5-2 cycles per degree (Figure 9).
Another approach is to measure VA at lower contrasts, eg 10 per cent. This method provides a second point on the CSF.7
Combining a measurement of Pelli-Robson contrast sensitivity or low-contrast visual acuity with a measure of high-contrast acuity provides an estimate of two points on the CSF, which is usually adequate to predict the shape of the entire CSF and therefore provides a powerful description of a patient's visual capability (Figure 8).
effect of cataracts on CSF
Changes in the transmittance or refractive properties of the lens will tend to preferentially reduce CS at high spatial frequencies (Figure 10). If this is the case, high-contrast VA will provide a reasonable indication of functional vision.
However, cataracts that produce an increase in forward light scatter may also produce a loss in CS at lower spatial frequencies (Figure 11).8 The spatial frequency dependency of the loss will depend on the nature of the scatter. Large angle scatter tends to cause a loss of CS at all spatial frequencies. Narrow angle scatter tends to affect CS for higher spatial frequencies more than the low.
Because scatter tends to produce a greater loss in sensitivity at low and intermediate spatial frequencies than, for example, refractive error, patients with increased light scatter tend to have a poorer standard of functional vision than patients with refractive errors, even if they have the same high-contrast VA.
For example, consider the CSF for a patient with a refractive error and a patient with increased light scatter (Figure 12). These two patients would have approximately the same high-contrast VA but the patient with increased light scatter is likely to have poorer vision in the 'real world'. Measurement of Pelli-Robson CS or low-contrast VA would be helpful in this case.
In summary, the effect of cataracts on the CSF depends on the position and nature of the lens 'opacities'. For some patients with cataracts, the loss is primarily at high spatial frequencies in which case high contrast acuity tends to correlate well with the patient's reported visual disability. However, when there is also a loss of sensitivity at lower spatial frequencies, high-contrast VA tends to underestimate the degree of disability. In these cases, measurements of CS can be useful.

Disability glare
Increased forward light scatter becomes particularly troublesome when there are large differences in the light level between different parts of the visual field at the same time; for example, when the sun is low in the sky or when viewing oncoming car headlamps at night.
The light from the bright source is scattered by the lens and forms a veil of light over the retina, thus reducing the contrast of the underlying image (Figure 14). This is known as disability glare. Patients with increased light scatter in the eye, therefore, have particular difficulties under these conditions. However, this aspect of a patient's vision may not be apparent under the subdued lighting conditions found in a consulting room.
Various tests have been devised to assess glare sensitivity.9 The simplest of these involves placing a bright light source next to or around a letter chart. Patients with increased scatter will tend to record poorer acuities with the glare source than without, particularly for low contrast letters.
The Brightness Acuity Tester (BAT) provides a more elegant solution. It consists of an internally illuminated hemispherical bowl with an aperture in the middle. The patient holds the instrument to their eye and views the test chart through the hole. This provides a bright uniform glare source that can be used in conjunction with letter and CS tests.
While most patients show a reduction in VA or CS in the presence of a glare source, some actually show an improvement. This usually occurs in patients with peripheral lens 'opacities' where the pupillary constriction produced by the glare source removes the opacities from the pupil area.
In summary, glare testing can provide the clinician with useful information about patients' 'outdoor' vision and provide valuable evidence to justify the referral of patients who report significant functional vision impairment in bright conditions and yet record good VAs in the consulting room.
Near vision
The quality of near vision is normally closely related to the quality of distance vision for most patients (assuming appropriate refractive correction is provided). However, this is not always the case for patients with cataracts. Patients with cortical cataracts may have better vision for near tasks on account of the fact that the near miosis may remove the cortical spokes from the pupil area. Conversely, patients with central posterior subcapsular cataracts may find that their vision is significantly worse for near tasks. Therefore, measurements of visual function at near may be of value when justifying referral for such patients.

Potential vision tests
It is easy to assume that a cataract is the sole cause of a patient's poor vision. However, cataracts mainly affect older people and co-morbid ocular disease is common among this group. When assessing a patient's suitability for cataract surgery, the presence of co-morbid eye disease must be taken into account. However, it is important to bear in mind that a patient may appreciate an improvement in the overall quality of vision even if their VA is not improved.
Unfortunately, the detection of co-morbid eye disease is often hampered by the poor view of the fundus obtained through cataracts. A much better view is obtainable using indirect ophthalmoscopy. A careful analysis of symptoms, history and family history can give a clue as to the likelihood of co-existing eye problems. Pupil dilation may also help but in many cases it is impossible to predict the patient's potential vision after surgery using conventional clinical tests.
This has led to the development of a number of potential vision tests (see excellent review by McGraw et al).10 These range from simple techniques to observing various retinal entoptic phenomena such as the Purkinje tree or 'flying corpuscles', to complex instruments which attempt to bypass the degraded optics to produce a retinal image, such as the Potential Acuity Meter (PAM) and various interferometers.
The PAM uses a single channel Maxwellian view projection system to image a letter chart onto the patient's retina. The Maxwellian view arrangement ensures that the light source is imaged in the plane of the patient's entrance pupil thereby ensuring that a very narrow beam of light passes through the lens. This allows the examiner to select a relatively clear portion of the lens, thereby minimising the effects of the cataract. While this test can provide useful information about retinal function, it is dependent on there being a small 'window' in the cataract. Furthermore, if that window happens to be towards the periphery of the pupil, off-axis aberrations are likely to compromise results to some extent.
Interferometers employ two Maxwellian view optical systems to project two coherent beams of light onto the retina. Light arriving on the retina from the two sources will interfere to produce a grating pattern on the retina. Because this is an interference pattern, it is not subject to the degradation that affects images formed in the normal manner. Varying the separation of the two beams changes the spatial frequency of the grating allowing a measure of grating acuity to be obtained. Interferometers are also dependent on there being some clear areas in the lens and so are not suitable for assessing patients with dense cataracts. This is unfortunate as it is these patients for whom assessment is most important.
Clinical opinion as to the value of these instruments varies considerably and they are not used widely for the management of cataract patients in the UK.
A number of other tests have been shown to provide valuable information about retinal function in cataract patients. Various hyperacuity tasks such as vernier acuity and displacement thresholds are surprisingly immune to image degradation and may have a role in this respect. A module for assessing vernier acuity is included in Test Chart 2000.

Binocular effects
While cataracts tend to affect both eyes eventually, they often appear in one eye before the other and progress at different rates. There is a tendency to assume that the patient will see as well as the better eye and cataract surgery may be delayed on the basis that the patient's binocular VA is good. However, there is some evidence that binocular CS is reduced below that of the good eye in patients with uniocular cataracts.11 There is also likely to be some loss of stereopsis and possibly a Pulfrich effect and these factors should be taken into account in the management of such patients.

Summary
The initial management of patients with cataracts is usually the responsibility of optometrists. While cataract surgery is generally successful, there is still a degree of risk and the entire process places physical and emotional demands on the patient, which must be balanced against the potential benefits. The decision to refer for surgery should generally be based on the patient's assessment of whether their reduced vision is significantly interfering with their desired lifestyle. However, the case is strengthened if test results can be provided which confirm the patient's description of their vision. Distance and near VA is often adequate in this respect. However, for cases where the patient's symptoms appear to be disproportionate to the standard of vision measured using high contrast VA, additional tests such as contrast sensitivity, low contrast acuity and glare sensitivity can provide valuable information.


References
1 Klein BEK, Klein R, Linton KLP. Prevalence of age related opacities in a population: the Beaver Dam Eye Study. Ophthalmology, 1999; 99: 546-552.
2 Brown NAP, Hill AR. Cataract: the relationship between myopia and cataract morphology. Br J Ophthalmol, 1987; 71: 405-414.
3 Ravin JG. Monet's cataracts. JAMA, 1985; 254: 394-399.
4 Campbell FW, Robson JG. Application of Fourier analysis to the visibility of gratings. J Physiol, 1968; 197: 551-566.
5 Ginsburg AP. A new contrast sensitivity vision test chart. Am J Optom Physiol Opt, 1984; 61: 403-407.
6 Pelli D, Robson J, Wilkins A. The design of a new letter chart for measuring contrast sensitivity. Clin Vis Sci, 1988; 2: 187-199.
7 Regan D. Low contrast letter charts and sine wave grating tests in ophthalmological and neurological disorders. Clin Vis Sci, 1988; 2: 235-250.
8 Hess R, Woo G. Vision through cataracts. Invest Ophthalmol Vis Sci, 1978; 17: 428-435.
9 Elliott DB, Bullimore MA. Assessing the reliability, discriminative ability and validity of disability glare tests. Invest Ophthalmol Vis Sci, 1993; 34: 108-119.
10 McGraw PV, Barrett BT, Whitaker D. Assessment of vision behind cataracts. Ophthal Physiol Opt, 1996; 16: 26-32.
11 Pardhan S, Gilchrist J. The importance of measuring binocular contrast sensitivity in unilateral cataract. Eye, 1991; 5: 31-35.


Further reading
Elliott DB. Assessment of patients with age-related cataract. Ophthal Physiol Opt, 1998; 18: S2, 51-61.
Elliott DB. Evaluating visual function in cataract. Optom Vis Sci, 1993; 70: 896-902.
Elliott DB, Situ S. Visual acuity versus letter contrast sensitivity in early cataract. Vis Res, 1998; 38: 2047-2052.
Brown NAP. The morphology of cataract and visual performance. Eye, 1993; 7: 63-67.
Latham K. Who uses contrast sensitivity in optometric practice? Ophthal Physiol Opt, 1998; 18: S1, 2-13
Princeton Nadler M, Miller D, Nadler DJ (eds). Glare and contrast sensitivity for clinicians. Springer-Verlag, 1990.

David Thomson is professor of optometry and visual science at City University. w.d.thomson@city.ac.uk