|
|
Given the recent pace of change, it would be a brave act to try and predict the shape of eye care services in 10, let alone 60 years. However, there are a number of drivers emerging which perhaps give us reason to be optimistic about the future.
Undoubtedly technology will play a major part in shaping the future of our profession. Auto-refractors, subjective refractors, automated fundus analysis systems are technologies which already exist and will undoubtedly impact on the future role of optometrists. Commercial pressures and the internet are already impacting on the delivery of eye care services.
While these developments could be viewed as threats to the conventional role of optometrists, a number of other factors are emerging which provide new opportunities for optometry. Foremost among these are the challenges of meeting the visual and ocular needs of an ageing population.
The previous article in this series (08.06.12) gave an excellent review of the changing demographics in the UK. Changes in fertility, mortality and patterns of migration have led to unprecedented changes in the size and shape of the population in the UK. The most striking trend at present is the increase in the proportion of elderly people that make up the population. In 1901, just 5 per cent of the population was over the age of 65. Today, this has risen to approximately 16 per cent and is predicted to rise to 20 per cent by 2021 and 25 per cent by 2041.
EU initiative
Against this background, 2012 has been designated the European Year of Active Ageing and Solidarity between Generations by the European Union. The initiative is intended to 'raise awareness of the contribution that older people make to society' and 'seeks to encourage policymakers and relevant stakeholders at all levels to take action with the aim of creating better opportunities for active ageing and strengthening solidarity between generations'. The three main strands of this initiative are:
? To provide better employment opportunities for older people
? To 'ensure greater recognition of what older people bring to society and create more supportive conditions for them'
? To encourage independent living by providing appropriate support and adapting the environment.
The ageing population introduces a range of social and economic challenges, not least, the provision of medical care. In addition to the normal gradual decline in function with advancing years, age is a risk factor for most diseases. Therefore, in general, the level of medical support required increases with age. It follows that an expansion in medical capacity will be required to meet the increasing demands of an ageing population, particularly among the disciplines that are well-used by the elderly. Optometry and ophthalmology are certainly in this category.1
Furthermore, providing eye care for older people is more than just detecting and treating disease. As people live longer there will be a desire (and perhaps a need) to continue working for longer and this will provide new challenges in terms of optimising the visual performance of the ageing visual system. Even beyond retirement, good vision is essential for maintaining independence, mobility and general quality of life into old age.2 Despite the availability of state-funded eye care for the elderly in the UK, recent evidence suggests that many elderly people do not avail themselves of the services.3 The reasons for this are varied and include concerns about cost, a lack of awareness of the services available and the perception that poor vision is an inevitable consequence of ageing.
Optometry is in the front line in terms of ensuring the visual welfare of the elderly. Uncorrected or residual refractive error is by far the most common cause of poor vision among the elderly and this is firmly in the domain of optometry.
Optometrists also play an important role as the gatekeepers for secondary and tertiary eye care, referring patients when appropriate and managing their care after treatment. With the predicted increase in the number of older people in the population and the parallel increase in the prevalence of eye disease, optometry is ideally positioned to fill the vacuum left by ophthalmology as it struggles to meet the demands for cataract surgery, anti-VEGF injections and whatever new treatments lie around the corner.
Pleasingly, there seems to be growing support for a competency-based model of service provision where authorisation to perform a function or procedure is based on the ability to demonstrate proficiency rather than professional grouping. It seems likely that the increasing demand for eye care services coupled with the cost effectiveness of a competency-based model for service delivery will see optometry taking on an increasing role in the diagnosis and management of eye disease (perhaps at the expense of routine refraction).
To meet the challenges that lie ahead, optometrists will need to be proficient at examining the elderly and have a good understanding of the changes in visual performance associated with normal ageing and age-related eye disease. In this brief review, the principal changes that occur in normal, healthy, ageing eyes will be outlined.
The ageing visual system
The quality of visual perception is contingent on the integrity of the entire visual system - the optical system of the eyes, the retina, the visual pathway and the hugely complex neural network which takes on the task of transforming information received from the eyes into the rich perception we enjoy.
Therefore, any consideration of the effects of ageing on visual performance must take into account changes in the entire visual system. However, while the anatomical changes in the visual areas of the ageing brain are reasonably well documented, the complexity of the neurology makes it very difficult to judge the potential impact of these changes on visual performance. Therefore, this article will be mainly limited to a consideration of the impact of changes occurring within the eye.
The ageing retinal image
The first stage in perception is the formation of the retinal image. Any degradation of the optics of the eye with age will have a corresponding impact on visual performance. The quality of the retinal image is determined by the tear film, the cornea, the pupil, the crystalline lens and, to a limited extent, the aqueous and vitreous humors. The principal effects of age on the retinal image are:
? A reduction in retinal illuminance
? Degradation of optical quality
? An increase in light scatter.
Reduction in retinal illuminance
The young cornea and crystalline lens are remarkably transparent to wavelengths within the visible spectrum. However, the absorption of the lens and to a much lesser extent the cornea, increase with age, particularly towards the blue end of the spectrum - a characteristic that accounts for the apparent yellowing of the lens.4 In certain types of cataract, these effects are more marked and occur at an earlier age.
The reduction in retinal illuminance caused by the increasing absorption of the crystalline lens is exacerbated by the fact that pupil size tends to decrease with age. The combined effect of these factors can be to reduce the retinal illuminance by a factor of 10 or more.4
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. However, the reduction in retinal illuminance does become disadvantageous at lower light levels when, for example, an older person may experience the relatively poor vision associated with scotopic or mesopic vision while their younger counterpart may still be enjoying the colour vision and good acuity associated with photopic vision. This loss of retinal illuminance can often be compensated for by a corresponding increase in artificial lighting.
Degradation of optical quality
Changes in refractive error are common with age. Where the refractive changes are 'regular', vision may be improved by prescribing appropriate spectacles or contact lenses. However, changes in the structure of the cornea and lens may result in a reduction in optical quality which may not be readily corrected (Figures 1 and 2). A number of studies have reported an increase in higher order aberrations in the ageing eye.5-9
Increase in light scatter
The ocular media of a young eye are remarkably transparent. However, as the eye ages, the transparency of the crystalline lens in particular is reduced and this leads to an increase in scattered light within the eye. Forward light scatter occurs when the light hits disorganised fibres or large protein molecules within the lens. 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.10
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. In these cases, a measurement of low contrast visual acuity can provide valuable information about the quality of vision (Figure 3).
The ageing retina and brain
The degradation of the retinal image is compounded by a reduction in the resolution and sensitivity of the retina. The loss of photoreceptors and other retinal neurons, the accumulation of waste products like lipofuscin and drusen, coupled with sclerosis of the retinal and choroidal vasculature, inevitably results in some reduction in the efficacy of the retina as a sensor. Cell loss has also been reported in the lateral geniculate nuclei and the visual cortex. Devaney and Johson11 report a 54 per cent reduction in the number of cortical cells mapping the macula between the ages of 20 and 87. The impact of these neural changes depends on the function of the affected cells and can range from a loss of sensitivity in the case of retinal cells, to a subtle reduction in the ability to process complex information in the case of higher order cortical neurons.
Changes in visual performance with age
Visual acuity
Most studies report a decline in visual acuity (VA) with age although there is some dispute as to when the decline commences. The commonly held belief among clinicians is that visual acuity remains stable throughout adulthood until the age of approximately 50 years, after which it shows a more or less linear decline. This belief is largely based on the reviews of Pitts and Weale.12,13
Others have suggested that the apparent stability of visual acuity up to middle age is a truncation artifact resulting from the use of Snellen charts with a minimum letter size of 6/5 of larger. Using logMAR charts with letter sizes down to -0.3 logMAR (6/3) it is possible to demonstrate that healthy young individuals can usually achieve visual acuities of better than 6/5 and that there is some decline in VA from the third decade onwards.14
Contrast sensitivity
While high contrast visual acuity remains the most popular index of visual performance among clinicians, the contrast sensitivity function (CSF) has been shown to provide a more comprehensive assessment of visual function.
Early studies of the effects of ageing on contrast sensitivity produced conflicting results. However, more recent studies have tended to find a preferential loss of sensitivity at high and medium spatial frequencies.15 There are a number of candidates to explain this loss, including the reduction in retinal illuminance, an increase in light scatter and neural changes in the retina and possibly cortex. In practice, the loss is probably attributable to a combination of these factors in different proportions depending on the exact nature of the ageing changes.15
Dark adaptation and absolute threshold
The human visual system is capable of functioning over a range of light levels exceeding 10 log units (10,000,000,000:1). In view of senile miosis and the reduced transmittance of the lens in particular, it is not surprising to find that the absolute threshold tends to increase (sensitivity decreases) with age.16 Whether or not this provides a complete explanation has been the subject of some debate.17,18 Studies which have attempted to compensate for the losses in retinal illuminance report that a component of the loss is likely to be neural in origin.19 Whatever the cause, it is important to note that the absolute threshold of an 80 year old is likely to be about 2 log units (100 times) less than that of a 20-year-old.
It also takes rather longer for the visual acuity of older people to recover following exposure to a bright light. A clinical implementation of this test, the photostress test, involves directing an ophthalmoscope light onto the macular region for an adaptation period and then recording the time taken for visual acuity to recovery to the pre-adaptation level. It is reported that the recovery time increases with age and also significantly with age-related maculopathy.20-22
Colour vision
Changes in the ocular media and pupil size with age are likely to have some impact on the perception of colour.23 Colour discrimination is known to be affected by changes in retinal illuminance, particularly at lower light levels. Therefore, some reduction in colour discrimination may be expected purely on the basis of the reduced retinal illuminance in older eyes. Furthermore, the spectral absorption tends to increase more for shorter wavelengths than longer wavelengths giving the crystalline lens a 'yellow' tint. The yellowing of the lens will distort the spectral distribution of the light falling on the retina and preferentially reduce the input to the blue cones. This will tend to diminish the brightness of blue objects and in the absence of any corrective mechanism, would result in the visual field having a 'yellow tinge'.
However, 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, the perception of the colour of objects remains remarkably constant. It is likely that these same mechanisms are used to negate the effect of the yellowing of the lens.24
The second factor to consider is light scatter. Forward light scatter casts a veil of light over the retina, reducing the luminous and chromatic contrast of the retinal image (Figure 3).25 In terms of colour vision, this reduces the saturation of colours and can result in a reduction in colour discrimination. It also explains the observation that 'colours look so much more vivid', often expressed by patients following cataract extraction.
The final factor to consider is the impact of neural changes in the visual system. A number of studies have reported a preferential loss in sensitivity of the short wavelength sensitive (blue) cones, while others suggest there is an equivalent loss of sensitivity affecting the medium (green) and long wavelength sensitive (red) cones.26,27
It is likely that changes in colour vision with advancing age are a consequence of age-related changes in the absorption of the ocular media and changes in neural sensitivity at the receptoral or postreceptoral level.
Changes in colour vision are not usually apparent using tests designed to screen for congenital red/green colour vision defects such as the Ishihara test. However, changes 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.28
Visual fields
Clinicians will be very familiar with the need to make an allowance for an age-related decline in sensitivity when assessing visual fields. The literature provides conflicting data regarding the onset and rate of this decline. Some authors report a more or less linear decline in sensitivity at a rate of approximately 0.6 db per decade. Spry and Johnson29 report that the decline is nonlinear, showing a small decline in the early decades of life which accelerates particularly from the seventh decade onwards. The decline tended to be greater in the periphery and the superior hemifield.
This age-related reduction in sensitivity is likely to be partly attributable to the reduction in pupil size and increased absorption of the lens, but there is also good evidence that neural changes play an important role. Another possibility is that an accelerated loss of sensitivity in older age groups may reflect an increased proportion of subclinical pathology among the more aged members of the presumed normal population.29
Binocular vision and stereopsis
Accurate and steady fixation is a pre-requisite for binocular vision and stereopsis. While there is some evidence that fixation is less stable in older subjects under scotopic conditions,30 under photopic conditions, accurate fixation seems to be maintained into old age.
There is some evidence for deterioration in eye movement control with age. Pursuit eye movements tend to show reduced gain (greater lag)31,32 while the peak velocity of saccadic eye movements declines and the latency increases.33
Several studies have shown that the distribution of distance heterophoria is independent of age.34-38 However, after the age of 20 heterophoria for near vision has been reported to tend towards exophoria.34
Most studies report a decline in stereopsis with age although the extent of this decline seems to depend on the test used.39-41 This is probably attributable to the reduction in retinal illuminance, the deterioration in visual acuity and perhaps the loss of cortical cells.42
Summary
The dramatic increase in the proportion of older people in the population will pose a number of major challenges over the next few decades. In general, older people are more likely to require refractive correction and treatment or ongoing management of pathology. Furthermore, if people are to remain in work and active/independent for longer, they will require optimal vision correction and perhaps some environmental adaptation to compensate for the decrement in visual performance. The inevitable conclusion is that the demand for eye care services is set to increase. Eye care professionals will need to adapt to meet this demand.
It also seems likely that the scope of optometry will increase as ophthalmology struggles to cope with the increasing demand coupled with the relentless development of new treatments. With a competency-based model of service delivery, optometry is well placed to take on the ophthalmological 'middle ground'. While there are many political hurdles to overcome, an increasing role for community optometry is attractive in terms of accessibility and cost-effectiveness.
To prepare for this evolving role, optometrists will need to be familiar with the normal structural and functional changes affecting the ageing visual system and have the tools to measure and optimise visual performance.
Changes in visual performance are related to a combination of factors. The increased absorption of the ocular media and senile miosis combine to reduce the amount of light reaching the retina. Changes in the transparency and optical quality of the media result in degradation in retinal image quality. Changes in the retina and the visual pathway result in some loss of luminous, contrast and colour sensitivity. Changes in the visual areas of the brain impact on the hugely complex task of forming the cascade of neural information into a meaningful perception.
The onset and rate of decline of visual performance has been the subject of much debate. One of the difficulties with research in this area is the question of what constitutes a normal, healthy, aged eye. Few eyes which have survived 60 years are completely free from all signs of deterioration, degeneration or past or present disease. Should this normal 'wear and tear' be considered as part of the normal ageing process? Furthermore, there are large variations in the effects of ageing between individuals, resulting in a general dispersion of measured function with age.43 This makes it difficult to differentiate between 'normal' and 'abnormal' function - there are almost always some older persons who function better than younger ones.
However, there is a general consensus that even in the absence of overt disease, there is a decline in visual acuity and contrast sensitivity with age. The rate of dark adaptation slows and absolute sensitivity decreases. Recovery after light adaptation takes longer and colours become less saturated (Figure 4). Other aspects of visual function such as flicker sensitivity and motion perception tend to decline as does the ability to make higher-order discriminations mediated by extrastriate areas of the brain.
While little can be done to delay or reverse these changes at present, their effects can often be reduced by a combination of optimising the refractive correction and making various compensations to the visual environment.
Therefore, returning to the undergraduates, I believe there is good reason to be optimistic about the future of 'optometry' although it will not be optometry as I have known it. ?
References
A list of references and further reading is available from mike.hale@rbi.co.uk
? David Thomson is professor of Optometry and Visual Science at City University