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Pinholes and presbyopia: solution or sideshow?

2019; Wiley; Volume: 39; Issue: 1 Linguagem: Inglês

10.1111/opo.12594

1475-1313

W. N. Charman,

Corneal surgery and disorders

The gradual decline with age in the amplitude of accommodation, leading to presbyopia at the age of about 40 years and the complete absence of active accommodation after the age of about 55 years, has always presented problems for those lucky enough to survive into middle age and beyond. They resent the fact that they no longer possess their youthful ability to see clearly over a wide range of distances. The introduction of spectacles in the 13th C provided, for the first time, a practical solution to these difficulties. Later spectacle lens developments, starting with bifocals and proliferating into many different types of multifocal and progressive lens, have allowed the differing visual needs of many presbyopes to be adequately satisfied. Nevertheless, spectacles do have optical, cosmetic and other limitations which cause some presbyopes to wish for spectacle-independence. The search has therefore continued for improved alternative methods for overcoming the visual difficulties associated with presbyopia. The world population of presbyopes exceeds 1 billion,1 with more than 30 million of these in the UK, so that the potential market for any method which successfully meets the needs of even of small fraction of this population is huge. Contact-lens (CL) methods have figured prominently among the various newer approaches that have been explored. Many of the optical concepts pioneered by the CL workers, such as monovision, simultaneous-image lenses and modified monovision, have been transferred to presbyopic refractive surgery techniques and intraocular lens designs.2, 3 It is, however, surprising to find that, in recent years, variants of the very old technique of using a circular stenopaic aperture ('pinhole') to increase ocular depth-of-focus (DOF) and hence allow reasonable imagery over a range of object distances in the absence of any active accommodation have re-emerged as serious contenders as aids for presbyopes who have low levels of distance ametropia. This pinhole effect, by which a reduction in the effective pupil diameter reduces the diameter of the retinal blur circle formed for any error of focus and hence improves the clarity of the associated retinal image, is, of course, widely used in the clinical pinhole test to determine whether reduced acuity is due to refractive error or amblyopia.4, 5 Theoretically, for an aberration-free eye the DOF would be expected to vary with the inverse square of the pupil diameter but the DOF found in practice is increased by the monochromatic and chromatic aberrations of real eyes, particularly at larger pupil diameters. Figure 1 shows recent data for through-focus acuity at two pupil diameters when aberrations are corrected.6 Evidently acuity declines more slowly with defocus when the pupil diameter is smaller, although optimal acuity is better with larger pupils due to the greater diffractive blur associated with smaller diameters.7 The value of the DOF derived from plots like Figure 1 depends upon the criterion used to assess it, e.g. the dioptric range over which acuity is 0.1 logMAR or better. Figure 2 shows some typical experimental measurements of the pupil-dependence of DOF for normal aberrated eyes. Values differ between studies due to the differences in the blur criteria and methodology used but it is clear that DOF in dioptric terms increases rapidly as the pupil diameter is reduced below 2 mm. However, although 'pinhole' corrections can undoubtedly give many near-emmetropic presbyopes adequate visual acuity at both distance and near under some conditions, can they really be recommended as the ideal solution for the visual needs of the older patient? Might patients equipped with such devices sometimes suffer from vision which is degraded to an extent which could cause them to be put at risk? To answer such questions, it is helpful to consider the history of devices in which some form of pinhole is placed in the optical path between the object and the retina. It is interesting that although the earliest workers recognised that pinholes increased ocular DOF, they felt that stenopaic spectacles and similar devices were of most value for those who suffered from high levels of irregular astigmatism or intraocular scattered light, as a result of such problems as corneal scarring, keratoconus or multiple opacities.13, 14 Daza de Valdés, in his 1619 'Uso de los Antojos' (The Use of Spectacles) suggested, for example, that his stenopaic spectacles, in which the 'lenses' were opaque disks containing a series of small circular holes across a horizontal diameter (Figure 3), were particularly suitable for albinos.14 Much later, Donders15 made similar recommendations, and also advocated pinhole use to improve the acuity of high myopes. He pointed out, however, that pinhole spectacles possessed the disadvantage that they caused a loss of retinal illuminance and visual field, the latter shrinking as the vertex distance increased. Helmholtz summarised the enhancement of DOF associated with viewing through a small aperture by noting that 'A screen with a narrow opening in it may be placed in front of the eye to enable it to get a distinct view of an object for which the eye is not accommodated. In such a case the cone of rays entering the eye has a very small aperture and a correspondingly small cross section anywhere along its route, so that the blur circles on the retina will be small also.'16 One criticism that can be levelled against these early authors is that they make no mention of the effects of diffraction. Following these earlier studies, the concept of advocating the use of pinholes on a daily basis to improve vision for some patients seems to have become progressively less popular among eye-care professionals, probably largely because of the associated problems of light and field loss, and the poor cosmetic appearance. Their use was still being advocated in the early 20th C by some ophthalmologists17 but by 1970 Duke-Elder and Abrams13 were describing stenopaic spectacles as being 'almost obsolete'. Nevertheless, outside mainstream optometry and ophthalmology, a steady market in 'pinhole spectacles', which were promoted as being suitable for a wide range of ocular disorders, including presbyopia, by their sellers and Bates method practitioners, continued to flourish, as it still does today. Typically these spectacles now have pinholes about 1 mm in diameter arranged in a triangular lattice of side length about 3 mm across the surface of each opaque plastic lens. Perhaps surprisingly, only a few serious attempts to study their effects on vision have been made.18-20 A new approach in the use of pinholes was pioneered by Ziller, working at the Zeiss School, Jena, who in the 1930s designed a glass scleral CL with a pinhole aperture and an opaque periphery, reasoning that DOF would be increased and near vision would be improved.21 This idea was revived by Freeman, who in 1952 made customised scleral prescription CLs incorporating a coloured artificial iris with a small artificial pupil. He suggested that a pupil diameter of 2 mm gave a DOF which was sufficient for most presbyopes while avoiding excessive restriction of the visual field.22 Field restriction with pinholes is less of a problem with CLs than with spectacle lenses: the artificial pupil is axially much closer to the natural pupil, so that vignetting effects between the artificial and natural pupil are reduced. Pinholes were also among the various other artificial-pupil CLs considered by Wesley.23 Other opaque shell lenses with small artificial pupils were used by Freeman in cases of albinism, irregular astigmatism and polycoria.22 At a later date, following suggestions by Bier24 and others, Abadi & Papas25 were able to demonstrate that soft contact lenses of similar basic design could improve contrast sensitivity in tyrosinase-negative albinos, and occasional reports consider cases where opaque soft lenses with pinhole apertures have been helpful in cases of corneal scarring.26 Quantitative aspects of the vignetting and field restriction that occurs with CLs of this type were investigated theoretically by Carkeet.27 Curiously, Freeman's work with presbyopes attracted few immediate followers, and it was not acknowledged by Miller and Johnson28 when they explored pinhole techniques in 1977. Their own valuable study was inspired by the hope that the method might be applicable to the problems of aphakes, and offer an alternative to the use of high-power positive lenses, with their unwanted bulk, magnification and distortion effects. Miller and Johnson's results, however, had a much wider potential applicability, particularly to the possible role of pinholes in presbyopia correction. Demonstrating that the dioptric range over which acuity was better than 6/12 (20/40, 0.3 logMAR) increased as the pupil diameter was reduced over the range 2.0–0.5 mm, they found that with a 1.5 mm diameter pupil at least 6/12 vision was achieved with up to 3D of refractive blur. They noted that, at best focus, diffractive blur was just noticeable with a 1 mm pupil but not with a 2 mm pupil This agreed with the conclusions of Lebensohn, who found that the pupil diameter which offered the best compromise between maximising the DOF while minimising the effects of diffraction was 1.3 mm.29 Miller and Johnson admitted, however, that the use of an artificial pupil which is smaller than the natural pupil has an adverse effect on resolution and other aspects of visual function at low ambient light levels,30 and that, if the artificial pupil is placed in the spectacle plane, there is a substantial loss in visual field. Like Freeman22 they found that this field loss was reduced when a 1.5 mm pupil was incorporated in an otherwise opaque soft CL rather than being placed in the spectacle plane, although it was still significant. Importantly, they then confirmed that the visual field was even less affected if a clear soft CL containing only a black annulus (in their study the inner and outer diameters were 1.5 and 4.5 mm) was used instead of a simple pinhole in an otherwise opaque lens. They also suggested that the use of a miotic might yield benefits similar to those of small artificial pupils. Large-scale commercial exploitation of these ideas had to await the arrival about a decade ago of the Kamra corneal inlay (Acufocus, www.acufocus.com). When implanted in the cornea at a depth of about 200 microns, this essentially duplicates the optical effect of Miller and Johnson's opaque-annulus CL. In the case of the inlay, the annulus inner and outer diameters are 1.6 and 3.8 mm respectively (Figure 4a). The inlay possesses an advantage over small-aperture CLs, which are prone to decentration and movement, in that the annulus can be stably positioned more exactly with respect to the visual axis and natural pupil. Such positioning is important if optimal vision is to be obtained.31, 32 The inlay is usually implanted monocularly in the cornea of the non-dominant eye of the near-emmetropic presbyope with the other, dominant eye being left unchanged. The emmetropia may be natural or the result of an earlier LASIK or similar procedure.33 Typically, patients have ages 45–60 years, mean spherical errors between +0.50 and −0.75 D and cylindrical errors ≤0.75 D, and would normally have reading additions of +1.00 to +2.50 D. Residual accommodation may help to improve near performance of the younger phakic patients. Clinical reports show that the devices improve binocular near and intermediate vision to a level which is adequate for most tasks without compromising distance vision.34-37 Reading performance is improved and there is a high degree of spectacle independence for most tasks.35, 38 Clinical and theoretical studies suggest that best performance would be expected if the small-aperture eye had a refractive error of about −0.75 D,39-41 so that best use can be made of the full available DOF to improve near vision. Laboratory studies demonstrate that, in near vision, binocular acuity closely follows that of the eye with the smaller pupil.42 Although, as expected theoretically,43, 44 the inlay causes a small loss in visual sensitivity across the visual field, effects are minor.36, 44 Kamra-type corneal inlays are primarily intended for use with phakic patients but have also been used in patients with monofocal IOLs.45 A diffractive variant of this type of inlay, which theoretically would enhance both DOF and retinal image illuminance, has been proposed.46 The use of small-aperture optics has recently been extended to intraocular lenses.47 The IC-8 monofocal IOL (www.acufocus.com) (Figure 4b) uses the same principle as the Kamra inlay, the embedded annular mask (1.36 mm inner diameter, 3.2 mm outer diameter) being 15% smaller than that in the Kamra inlay to give the same ocular entrance pupil dimensions as in the case of the inlay.48, 49 The IOL is again implanted monocularly in the non-dominant eye with a target refraction of −0.5 to −0.75 D, the dominant eye receiving a monofocal distance correction. Results of the IOL and Kamra inlay appear to be broadly similar. Through-focus measurements in eyes implanted with the devices show that there is a considerable enhancement over the DOF for the preoperative eyes. LogMAR acuity remains better than 6/12 equivalent (0.3 logMAR) within a roughly ±2 D range about best focus when measured monocularly (Figure 5). In a return to the use of simple pinholes to overcome problems associated with irregular corneas, a small-aperture supplementary intraocular implant has been introduced (Xtrafocus, Morcher GmbH, www.morcher.com). Essentially this is a black, opaque, 6 mm diameter, meniscus-shaped diaphragm with a 1.3 mm diameter central aperture (Figure 4c,47). This is implanted in the ciliary sulcus of pseudophakic eyes in a piggyback configuration. The acrylic material is infra-red transparent, to allow the fundus to be visualised with an infra-red image converter. The device was originally introduced as a way of improving vision in eyes having irregular corneal astigmatism or severe light sensitivity after penetrating keratoplasty.50, 51 It has since been suggested that it might be useful for presbyopic patients who have already been implanted with multifocal IOLs but who find that their vision is poor. Since the pinhole is very close to the natural pupil, it has negligible effect upon the extent of the visual field although sensitivity is, of course, lost because of the reduced retinal illuminance.52 Rather than using some form of artificial pupil, several authors have recently re-explored the idea put forward by Miller& Johnson,28 of using the small pupils created by topical instillation of suitably-formulated miotic eye drops.53-56 Abdelkar demonstrated significant improvements in binocular near acuity when a mixture of carbachol (2.25%) and brimonidine (0.2%) drops was instilled monocularly in the non-dominant eye, the near vision achieved being good enough to allow his emmetropic presbyopes to dispense with their glasses during the working day: no changes in distance acuity occurred.54 Similar improvements in near vision have been found by later authors, using more complex drug mixtures and binocular drug instillation. Some of these authors postulate that factors other than reduced pupil diameter may contribute to the observed improvement in presbyopic near vision53, 55, 56 but as yet no direct evidence for such effects, e.g. in terms of measurable changes in ocular aberrations or partial restoration of objective accommodative changes in ocular power, has been offered. Discomfort following drop instillation was experienced by some patients in some studies and long-term side effects remain to be fully explored. Since the miotic natural pupil provides the aperture stop of the eye, there is no effect on the visual field although, as with artificial pupils, retinal illuminance is reduced. It can be seen that there is substantial enthusiasm for applying the small-pupil approach to the problems of presbyopia, This enthusiasm is mainly supported by the standards of vision recorded at distance, intermediate and near, as assessed by measurements of high-contrast visual acuity and contrast sensitivity function under photopic or high mesopic clinical testing conditions (typical test luminances 85 and 3 cd.m−2, respectively). How do the results obtained compare with alternative methods of correction? As an example, Figure 6 shows through-focus acuity from two studies36, 37 which used very similar operating and monocular measurement conditions (photopic, 85 cd m−2) for eyes with either Kamra inlays or one of two types of diffractive multifocal IOL (AcrysSof IQ ReSTOR +3.0 D, Alcon Laboratories, www.myalcon.com; AMO Tecnis +4.0 D, Johnson & Johnson Vision, www.surgical.jnjvision.com). Pre-operatively, with no artificial pupil, acuity degrades fairly rapidly and symmetrically with the magnitude of the defocus. Results with the Kamra corneal inlay are very similar in the two studies, the defocus curve being comparable in shape to that for the preoperative eyes but broadened as a result of the small-aperture of the inlay. Acuity falls steadily away as defocus is increased and has fallen to about 0.5 logMAR (about 6/18 equivalent) for a chart distance of 1/3 metre. In contrast, the two essentially-bifocal diffractive IOLs produce defocus curves which are asymmetric about zero defocus. Distance acuities are similar to that in the pre-operative case and there is excellent acuity at their designed near-focus. Although the extended DOF associated with the small aperture results in slightly better intermediate vision,37 the multifocal IOLs offer much superior near vision at their optimal near focus. If the refraction of the Kamra eye is slightly myopic, vision at near can be modestly improved. However, it seems reasonable to infer that the Kamra inlay or related small-aperture devices, while helpful to those who are undemanding in their near-vision requirements, are unlikely to satisfy by themselves the needs of those who wish to be spectacle-free and to carry out critical near tasks which demand maximal levels of near acuity. One obvious consequence of the use of small-aperture methods is that the restriction in effective pupil diameter reduces retinal illuminance. Does this cause any problems, particularly when task luminances are low? This concern is likely to be greater in phakic presbyopes, whose ocular transmittance is already substantially reduced in comparison with that of young adults, primarily as a result of greater absorption and light scatter within the crystalline lens. Lenticular light loss rises only slowly through early presbyopia but typically increases rapidly after the age of about 60 years with the onset of early cataract development, although there is substantial individual variation.57, 58 As examples of the theoretical magnitude of the light losses involved with the use of small apertures, Figure 7 plots the relative transmittance (i.e. the area of the artificially-restricted pupil divided by that of the unobstructed natural pupil) for axial viewing when either a simple 1.6 mm diameter pinhole or an opaque annulus with inner and outer diameters 1.6 and 3.8 mm (the dimensions of the Kamra corneal inlay) is used. Natural pupil diameters under any specific lighting conditions are affected by age, the individual, the task and other factors,59, 60 but Table 1 gives some representative values obtained under various viewing conditions for two presbyopic age groups.61 It can be seen from Table 1 and Figure 7 that under representative photopic indoor and outdoor levels of illumination, when typical natural pupil diameters are 3.8 mm), (e.g when night driving, see Table 1), Figure 7 suggests that a simple 1.6 mm diameter pinhole causes a very substantial loss in retinal illuminance, this loss exceeding one log unit when the natural pupil diameter is > 5 mm. Thus vision under dim conditions would be severely compromised in an eye wearing a simple pinhole of this diameter. In contrast, the relative transmittance of the annular Kamra-type stop rises at larger pupil diameters and never falls below about 0.2 (Figure 7), since additional light can reach the retina from the clear area of the pupil outside the opaque annulus. However this is at the expense of a reduction in the quality of the retinal image of near objects, as rather than each axial object point appearing as a simple blur circle, the latter is surrounded by a blur annulus corresponding to the zone between the outer diameter of the annulus and the edge of the larger natural pupil. This reduces the effective DOF provided by the inlay.41 It is, however, important to recognise that effects are different if the more peripheral areas of the visual field are considered. With contact lenses or corneal inlays, because of the vignetting associated with the depth of the anterior chamber which separates the artificial and natural pupils, the retinal areas corresponding to the peripheral field receive no light when a simple pinhole is used, whereas with an annulus retinal illuminance in the periphery approaches that found with the unobstructed natural pupil.44 If, however, the stops are placed close to the natural pupil, as with the IC-8 or Xtrafocus intraocular devices, there is negligible vignetting and the relative transmittance may be almost constant across the visual field. Thus no absolute peripheral scotoma is produced with the simple circular aperture of the Xtrafocus or the annulus of the IC-8, although visual sensitivity is reduced across the entire visual field. In general, the light losses intrinsic to the use of small apertures would appear to make it undesirable to use them bilaterally. This raises an additional question, however, as to whether in monocular use the differences in the quality and illuminance of the retinal images in the two eyes may cause problems. One obvious possibility is that stereo acuity might be affected. Comparison of pre- and post-implantation of distance stereo performance for patients with Kamra inlays, using a conventional stereo screener, suggests that this is not the case.62 Stereo acuity was found to be unchanged, unlike the situation with surgically-created monovision, where stereo acuity was found to be adversely affected.63 It is not clear, however, whether the Kamra result would still apply at lower luminance levels than those used in the screener, or at other, shorter, distances when the differences between the two retinal images might be greater. At near, for example, the dominant quasi-emmetropic eye without the small-aperture device has a retinal image with higher illuminance but greater blur, so that visual acuity in that eye is markedly reduced. Stereopsis is relatively robust against modest differences in retinal illuminance64 but is degraded by reduced monocular visual acuity of this type.65 There is also concern about the dynamic aspects of binocular vision when the retinal illuminances differ in the two eyes. It is well known that such a situation may induce the Pulfrich effect, which results in the distortion of the apparent path of moving objects. This distortion occurs as a result of the interocular differences in visual latency which occur as a result of the differing retinal illluminances.66, 67 Plainis et al. used measurements of monocular and binocular pattern-reversal visual evoked potentials (VEP) to demonstrate the existence of these visual latency differences under conditions where subjects wore contact lenses with either a circular or an annular artificial pupil on the non-dominant eye, the other eye having its natural pupil.68 Observation of a pendulum bob swinging in a frontal plane under these conditions caused all subjects to experience a Pulfrich effect, the path of the bob appearing elliptical in depth.69 Continued wear of the lenses over a period of a week showed no evidence for any adaptation effect which might cause the Pulfrich effect to disappear,70 although there is limited evidence that partial adaptation might occur over longer periods.71, 72 Trindade found that about 10% of his Xtrafocus patients saw a Pulfrich effect.51 Such distortions in perception may lead to hazard in some everyday tasks, such as driving or moving through a congested environment.73-75 The attractive feature of the small-aperture approach is its optical simplicity. However, it is clear that that, while the associated increase in DOF can usefully improve presbyopic acuity performance over a range of distances, this approach is unlikely to satisfy the needs of those with visually challenging near tasks. It is notable, too, that spectacle near additions typically continue to increase beyond the age of 55 years, when active accommodation ceases.76, 77 This is presumably because older phakic presbyopes make more use of shorter viewing distances, to compensate for the age-related decline in their acuity,78-80 and implies that small-aperture optical devices, with their limited DOF, may be more useful for the younger phakic presbyope. It may be that this is not a problem for pseudophakes, whose lenses are free from the absorption and scattering of the older crystalline lens. In any case, if small-aperture monovision gives inadequate near vision, this can easily be improved by wear of spectacles with paired positive lenses of appropriate power. This is not the case for modified monovision involving some form of multifocal, where the retinal image is always of reduced contrast in the multifocal eye, whatever additional spectacle lens is used. In general, as discussed earlier, small-aperture optics would be expected to be less useful when light levels are low. Little quantitative work has been carried out on this aspect, although good levels of subjective satisfaction have been reported by patients with Kamra inlays for both day and night driving.36 If problems arise with small-aperture corneal inlays, it is claimed that they are relatively easy to remove without any deleterious after–effects but explantation of small-aperture IOLs is a more invasive procedure. The possible long-term role of pharmacological treatments for presbyopia remains uncertain.53-56 Abdelkader's study,54 in which the vision of matched groups of presbyopic emmetropes aged 43-56 years who received monocularly either a miotic or a placebo was compared, suggests that the small pupil produced by the miotic led to safe improvements in near vision. Other studies,53, 55, 56 using more complex drug mixtures, are more ambiguous in their results. More work is needed to clarify the possible utility of this general approach. To summarise, the 'pinhole' approach, in its many variants, is neither a complete solution for the problems of presbyopia nor an over-hyped irrelevance. Like many other CL and surgical approaches, it offers useful improvements in vision to the emmetropic presbyope who wishes to be spectacle-free and to be able to perform distance and near visual tasks which are reasonably well-lit and are not too taxing. As noted earlier, however, most clinical studies of those with monocular small-aperture optics have been conducted at photopic and high mesopic illumination levels and there is a need for more work to be carried out at lower mesopic and scotopic levels to determine whether inter-ocular differences in retinal illuminance and imagery then become more important. Finally it is worth emphasising that many of the problems experienced by presbyopes with near tasks under normal interior lighting levels of a few hundred lux can often be alleviated by increasing the task illuminance with some form of localised lighting.81, 82 Near vision then improves through natural pupillary miosis and increased retinal illuminance. Emeritus Professor W. Neil Charman worked for many years in the Optometry Department of the University of Manchester Institute of Science and Technology, this now being integrated into the University of Manchester. He has a long-term interest in Visual Optics, much of his more recent work being focused on the problems of accommodation and presbyopia.