Virtual Issue Editorial: Presbyopia - grappling with an age-old problem.

Abstract

This editorial fronts the first virtual issue on Presbyopia for Ophthalmic & Physiological Optics. Virtual issues are a collection of papers from previously published issues of the journal that are brought together in a single, online issue. All the OPO papers in this virtual issue are free to access at http://www.wiley.com/bw/vi.asp?ref=0275-5408&site=1 Millodot1, in his “Dictionary of Optometry and Visual Science” defines presbyopia as “A refractive condition in which the accommodative ability of the eye is insufficient for near vision work, due to ageing”. While the onset of presbyopia for any individual is influenced by factors such as arm-length and the nature of their near visual tasks, it typically occurs when the subjective amplitude of accommodation is 3-4 D, at an age in the mid-forties in temperate countries. However, as was first systematically charted by Donders2, the amplitude of accommodation starts to decline well before adulthood is reached. Thus presbyopia is only the last phase of a slow progressive loss in accommodative efficiency, which began long before. This decline, which in its later stages appears to be approximately linear with age for the individual3, continues beyond the onset of presbyopia, so that by the mid-fifties most people have almost no ability to change the power of their eyes, although they still possess a small residual subjective amplitude of accommodation due to depth-of-focus effects. With life expectancies over much of the developed world approaching 80 years, most phakic individuals are likely to spend almost half their life as presbyopes and more than a quarter of it with virtually no active accommodation at all. Hence there is a huge demand for spectacle, contact lens, surgical and other strategies to enable the presbyope to see clearly over a range of distances, and much of the work of optometrists is directed towards meeting this need. In the less-developed world, uncorrected presbyopia remains a major cause of visual disability4, 5. What would we ideally like achieve in relation to the “correction” of presbyopia? Although the invention of spectacles in the 13th century introduced a brilliant and highly successful form of correction, few would deny that spectacles and other current methods of presbyopic correction have real limitations in terms of the visual performance achieved, ease of use and, for some patients, appearance. Further development is therefore required to minimise these problems. Beyond this, a more ambitious goal would be that the “ideal” correction should, if applied at the natural onset of presbyopia, be capable of restoring to pre-presbyopic levels the dioptric range within which accurate focus can be smoothly and rapidly achieved. It should also be able to maintain this range throughout the remaining decades of the life of the individual, without any further intervention, with the eye always being emmetropic at the lower end of the range. Schor6 suggests that a minimum target for the achievable subjective amplitude of accommodation should be 5.00 D. These focus changes should occur “naturally” and be appropriate to the convergence movements of the eyes, which probably implies that at least some of the components of the natural accommodation system must be utilised, if these are still functional, as is the case for, e.g., the ciliary muscle7, 8. Lastly the presence of the correction should be invisible to the outside observer. Evidently at present no method of correction approaches this goal, although designers of “accommodating” intraocular lenses have attempted to achieve it, on the assumption that increasing lenticular rigidity is the main cause of presbyopia. Their efforts have so far failed to lead to convincing success but it may be that in future a method based on the regeneration of the crystalline lens, using endogenous stem cells, offers greater potential9. A helpful supplementary strategy would, in principle, be delay the onset of presbyopia by reducing the rate of decline in the natural amplitude of accommodation with age, perhaps by ocular exercises, diet, drugs or other means: as yet there is little evidence that this is possible. To understand the origins of presbyopia, and perhaps to develop more radical methods for its correction, we need to understand the structure and action of the accommodation system, and their changes with age. The possible importance to accommodation of changes in the shape of the crystalline lens was recognised in the 17th C by Kepler and Descartes. In the same century, Scheiner clearly demonstrated with his double-pinhole experiment that an active change in the optics of the eye must be involved. Subsequent studies in succeeding centuries revealed the importance of other ocular structures in relation to the lenticular changes10. Nevertheless, Duke-Elder and Abrams in 1970 could still write “There are few subjects upon which so many diverse opinions have been entertained as the mechanism of accommodation; nor is everyone by any means agreed upon it today”11. Nearly 50 years later, and in spite of numerous sophisticated and revealing studies, these statements still remain largely true. Why should this be so? By as early as the 18thC workers had demonstrated that the thickness, weight and mechanical characteristics of the lens changed as age increased12, and the important roles of the ciliary body and zonule were, of course, recognised in the 19thC10. Since then, the dimensional, optical and mechanical characteristics of the lens as a function of age have been explored in progressively more detail. Dynamic optometers have allowed the performance characteristics of the accommodative system at different ages to be determined under a variety of stimulus conditions. The newer techniques of, e.g., Scheimpflug photography, optical coherence tomography, ultrasound biomicroscopy and magnetic resonance imaging have given deeper insights into many of the changes with age and accommodation in the forms and positions of the lens and ciliary body. Nevertheless much still remains to be understood about some elements of the accommodative system, their roles, and their changes with age. For example, the arrangement of the zonular fibres is of great importance, since they must transmit forces from the ciliary body to the lens, but they are difficult to visualise in vivo and their function in accommodation is still in dispute, as also is the role of the fibres which anchor the ciliary body to the choroid. The changes with accommodation and age in the gradients of refractive index within the lens have still to be fully established13, even though these have an important effect upon the lenticular power, and the mechanical characteristics of the lens also remain the subject of lively debate14, 15. Less controversial is the mainstream correction of presbyopia. Steady progress continues in refining the design of conventional presbyopic spectacle and contact lens corrections, although the utility of corrective lenses of continuously-variable power remains to be clarified16. An interesting issue is that, although accommodation is lost in the mid-fifties, conventional presbyopic adds increase in power beyond this age (e.g. Pointer, 1995a,b,c)17-19. Several authors have argued that this is because higher adds allow shorter working distances to be used, so that the angular subtense of near tasks can be increased to compensate for the age-dependent decline in visual acuity that occurs in later life3, 20, 21. As noted earlier, current invasive surgical techniques suffer from a variety of limitations22. In particular, the performance of “accommodating” intraocular lenses, supposedly designed to actively vary their power in response to the continuing activity of the ciliary muscle in presbyopes, has so far failed to provide a focusing ability which adequately mimics that of the younger eye6. This virtual issue of Ophthalmic and Physiological Optics includes 40 papers from the journal which discuss aspects of presbyopia, its effects and its treatment. During the period from which papers are taken (mainly 2011-2017 inclusive) one special issue was devoted to presbyopia (January 2016, vol 36, issue 1)23 In the past, one of the major barriers to a full understanding of accommodation and presbyopia was the difficulty of visualising the fine structure of the zonule and its attachments, both in vitro and in vivo. These difficulties are gradually being overcome through the use of techniques such as scanning electron microscopy, ultrasound biomicroscopy and optical coherence tomography. Two papers by Croft and her colleagues identify, first, a new class of zonular fibres which could be involved in the fine tuning of accommodation and, second, a strand that attaches to the posterior vitreous zonule insertion zone and the posterior lens equator24, 25. These may well be of relevance to the design of accommodating IOLs although, in discussing the interaction of these structures with the lens capsule and vitreous, and their impact on the accommodation process, the authors caution, slightly ominously, that “It may just be that the system is more complex than previously thought”. In an interesting 2.5 year longitudinal study, Laughton et al.26 explore the changes in accommodation response and ocular biometry that occur during incipient presbyopia (age at entry 34-43 years). The results are interpreted as supporting Helmholtz's theory of accommodation, which suggests that accommodative decline is primarily due to an increase in lens stiffness A further aspect of interest is the possibility of refractive change before, during and after the onset of presbyopia. In a longitudinal study of changes over several years in the refraction of 39 myopic patients during the age range 40-50 years, Pointer & Gilmartin27 find that, although in general refractions were stable, about 20% of the patients experienced a relatively abrupt myopic shift in refraction of between -0.50 and -0.75D in the years preceding the time at which a reading addition was thought to be desirable. They speculate that, among a variety of other possible causes, this might be due to changes in axial length. From the clinical point of view, one major parameter in relation to accommodation and presbyopia is the amplitude of accommodation. León et al.28 determine the reliability of dynamic retinoscopy (DR) for the measurement of amplitudes, comparing the results with those for the more widely used push-down and minus-lens methods. They conclude that DR has the advantages of offering better repeatability and of being less affected by depth-of-focus, so that the measured objective DR amplitudes are smaller than the subjective estimates. The same group has gone on to explore the age dependence of the amplitude as measured by the same three techniques29. The results support their earlier reliability findings. Peak DR amplitude is found to be around 9 D at the age of about 12, with <0.5 D of accommodation after the age of 55. Like Wolffsohn et al.30, they suggest that conventional views on the amplitude of accommodation needed for sustained near work may need revision Although the amplitude of accommodation is commonly used as the primary index of performance, other aspects of accommodation also change with age, notably the slope (gain) of the steady-state response/stimulus curve. Aldaba et al.31 measure these changes in slope using an objective double-pass system and find that in younger adults the slope remains almost constant up to the age of 35 and declines steadily thereafter. Almutairi et al.32 also measure the response/stimulus curve, together with the pupil diameter, of subjects between the ages 27 and 60 years. They find that, although a gradual reduction with age in accommodative amplitude occurs over the full age range, response gain remains constant up to the age of about 40, after which it begins to fall markedly as age increases. They therefore suggest that reduced near retinal image quality in early presbyopes is caused by a combination of both low amplitude and low gain (i.e. that accommodative lags are high) and that this may explain the apparent acceleration of symptoms and near-add prescriptions during the mid to late 40s. Developments in the correction of presbyopia by either spectacle, contact lenses and by surgical methods are reviewed by Charman16, 22. Both current and proposed possible future methods, mainly surgical, are described. In an interesting paper, Barbero & Portilla33 explore the way in which power and magnification are linked in progressive addition lenses (PALs) and consider their results in relation to the problems of distortion and blur experienced by some wearers of such lenses. Radhakrishnan & Charman34 present measurements of the optical performance of examples of some current commercial variable-power spectacles and conclude that these might be useful to at least some presbyopes. An area of continuing development in the contact lens world is that of multifocal lenses. Bradley et al.35 examine theoretically the performance of concentric 2- and 5-zone simultaneous-image bifocals and conclude that diffraction and pupil variation have substantial effects upon the retinal image. Five-zone rather than 2-zone designs may give more consistent performance, as pupil size varies with light levels and convergence amplitude. Xu et al.36 show that in multifocal designs there is little benefit from the inclusion of secondary spherical aberration. A similar theme is pursued by Rio et al.37. Twenty-seven observers viewed through-focus simulations of images produced by 2- and 8-zone concentric bifocals, the various sets of simulated images being affected by different decentrations, aberrations and pupil diameters. Like Bradley et al.35, the authors find that the lens with the larger number of zones is more robust against pupil diameter changes. They also show that the ocular aberrations of the contact lens wearers have more effect on the images than the small decentrations that occur in clinical use. This conclusion is strengthened by later work by the same group38, which includes some simulations for current commercial multifocal designs. Charman39 points out that, although pupil diameter is known to affect visual performance with various types of multifocal contact and intraocular lenses lens, there is a paucity of data on the changes in the presbyopic pupil as a function of light level and other conditions and that more systematic study of these changes is needed. Plainis et al.40 explore experimentally the through-focus performance of a commercial aspheric design with three different adds and show that depth-of-focus (pseudoaccommodation) is usefully enhanced in comparison with a single-vision correction, particularly when the pupil is small and vision is binocular. Like others, they note that the coupling of the wearer's spherical aberration with the power profile of the lens influences the effectiveness of the lens. Madrid-Costa et al.41 carry out a broadly similar study with two further designs but in addition compare the binocular vision achieved under photopic and mesopic conditions. They note the successful enhancement of depth-of-focus with both designs but emphasise that the relative performance of different designs depends upon the lighting conditions. Multifocal designs are also widely used for intraocular lenses (IOLs). Catalayud et al42 describe a useful set-up which allows IOL optical image quality to be assessed over a range of object positions. One relatively popular surgical method for improving the near vision of presbyopes is the implantation of a small opaque annulus (e.g. the Kamra inlay) in the cornea of one (usually the non-dominant) eye. Under photopic conditions, this restricts the diameter of the ray pencils entering the eye and hence enhances depth-of-focus and near vision. One possible disadvantage is that the effective anisocoria creates an imbalance in the retinal illuminances in the two eyes. This may cause a Pulfrich effect, leading to distortions in the perception of relative movement and the creation of possible hazard in some environments and tasks. VEP measurements by Plainis et al.43 confirm that in reduced aperture monovision, intraocular differences in visual latency can occur. However, one well–known criticism of pinhole contact lenses, that they reduce the effective visual field, does not appear to be valid with small annular opaque stops, presumably because oblique ray pencils can enter the cornea outside the opaque area to pass through the natural pupil44. One future possibility for increasing the depth-of-focus given by a contact lens or other type of correction is by modifying the phase and amplitude profile across the pupil, the use of the Kamra's annular opaque stop being a very simple example of this. Arines et al.45 explore the possibilities of using a cubic phase mask and show both theoretically and experimentally that it may be possible to obtain acceptable images of objects at distances between 6.0 and 0.4 m. The goal of producing an “accommodating” IOL is more challenging. To date all designs have rested on the twin assumptions that all the components of the accommodation system remain active throughout life and that accommodation can be achieved by replacing the excessively stiff natural lens by an artificial lens whose contribution to the overall power of the eye can somehow be changed through the action of the ciliary body, zonule and capsule. Two of the possible designs of accommodating IOL which have been produced commercially consist either of a single element which can be moved along the axis of the eye, or a two-element lens in which the axial separation of the components is varied. The required movements are achieved by capsular implantation of the IOLs combined with lens haptics or spring separators which deform appropriately as the zonular tension on the capsule changes with the form of the still-active ciliary muscle. Although in practice performance of these IOLs has been disappointing, Ale et al.46 have explored the magnification that would accompany the movements of idealised IOLs of this type. They note particularly that dynamic aniseikonia may occur in cases where IOLs of differing design or power are implanted in the two eyes. It is well known that uncorrected presbyopia is a cause of visual impairment in many parts of the world. Marmamula et al.47 provide further evidence for this problem and find that only about 11% of the presbyopes in a South Indian population possess appropriate corrections. A further study by the same group48 of weaving communities in the same area, who were largely involved in relatively challenging near visual tasks, revealed a higher rate of spectacle correction among the presbyopes but the prevalence of functional presbyopia (near vision worse than N8) was still 35%. While lack of spectacle correction is less of a problem in the developed world, it still occurs and Shickle & Griffin49 explore some of the reasons why older adults in England fail to get their eyes examined. Several papers consider the impact of presbyopic corrections on gait. Chapman et al.50 argue that magnification effects associated with lens wear affect judgements of the position and size of raised surfaces, such as steps, and that hazard may therefore occur when prescriptions are changed or not worn. Black et al.51 conclude that refractive blur from multifocal spectacle corrections can have an adverse effect on stepping accuracy, this being indirectly supported by Supuk et al.52. However Elliott et al.53 find that while this may be true with full-addition bifocals and PALs, gait with an intermediate-addition PALs (addition reduced by +1.5 D compared with the full reading addition) is similar to that when single-vision distance corrections are worn. Patients with this intermediate addition are also able carry out a range of near-vision tasks over short periods. Progress in this field has been steady, rather than spectacular, with advances being made in small steps rather than giant strides. After several centuries of study, the problem of presbyopia still offers many challenges! Emeritus Professor W Neil Charman worked for many years in the Optometry Department of the University of Manchester Institute of Science and Technology. He has a long-term interest in various aspects of visual optics, originally focusing mainly on ocular aberration and accommodation but, with advancing years, moving on to include presbyopia and its correction. He is a co-editor, with Ionnis Pallikaris and Sotiris Plainis, of the recent book “Presbyopia: Origins, Effects and Treatment”.