Optical solutions for presbyopia in the ageing eyethe effect of the size and shape of the pupil

Résumé

Presbyopia is the natural progressive loss of the ability for focusing near objects that occurs with ageing. This ability is known as accommodation. This loss usually means a reduction of the quality of life, which is why its correction has been a matter of special interest in the past years. New correction methods have emerged on the market in the last decades with the intention of improving the quality of life of the presbyopic population. Nowadays, there is an increasingly social demand for tasks that require functional vision at a wider range of distances, such as those using computers or smartphones. This, together with significant demographic changes, places special value on its optimal correction. The global population is increasing every year especially due to the general life expectancy. This will cause an inversion of the actual population pyramid, and it is a phenomenon that is already happening throughout the world. With this, the number of presbyopic population is expected to reach high rates in the near future and presbyopia will become a global public health matter. Nowadays, several solutions are available for the correction of presbyopia. The treatments for presbyopia are currently corrective by means of optical elements or surgical refractive modification. The optical solutions are not independent, but their effectiveness is subordinated to physical, physiological and even psychophysical factors of the subjects, depending on the technique. Among the most commonly used solutions are spectacle correction, contact lenses (CLs) and intraocular lenses (IOLs). Spectacles provide a reliable method for the correction of presbyopia, its usage has been associated to a decrease in the quality of life due to the patient’s dependence to carry out daily life activities. Prescription of CLs is an alternative method to spectacle correction, although recent reports show that the rates of prescription are still low. This might be explained in part by a lack of clinical knowledge by CL fitters and the presence of a climate of mistrust due to the visual compromises of presbyopic designs, among other reasons. There are several approaches to achieve presbyopia correction by means of CLs. These options can be mainly divided into three categories: combination with spectacle correction, monovision and multifocal CLs (MCLs). As for MCLs, one of the most used approaches is based on the principle of simultaneous image formation. Here, the distance and near powers are placed within the pupillary area simultaneously for every gaze position. Thus, different images are formed at the same time on the retina. In this situation, the visual system centres the attention on the corresponding image for the desired observation distance and ignores the rest. The main aim of these corrections is to extend the range of functional vision. However, this carries with negative effects in visual performance. Image degradation occurs monocularly due to the superimposition of different images. This can especially occur for near objects with low contrast and in low lighting conditions, although it can improve over time. Poor vision and discomfort are the main factors that cause CL wearing discontinuation. IOLs are another widely used optical solution for the correction of presbyopia. Current available IOLs can be classified into two main groups, according to their role inside the eyeball: phakic or pseudophakic IOLs. The lenses of the first group act as supporting elements that are added to the optical system of the eye, but no extraction of the crystalline lens is performed. The second group is formed by those IOLs that replace the natural lens, mainly because of cataract formation. There are presbyopic solutions using both types of lenses. All kinds of multifocal IOLs (MIOLs) for presbyopia use the simultaneous image formation principle, either with refractive or diffractive technologies. Multiple MCL designs have been manufactured in an attempt to satisfy the visual requirements of the presbyopic population. However, manufacturers sometimes provide insufficient information, which makes difficult to properly evaluate the advantages and disadvantages of a particular design. In order to provide more information that can be highly useful for professionals, measurements of power profiles are performed. Power profiles are a powerful tool that could give practitioners a deeper understanding of the behaviour of the lenses and thus, it would facilitate the selection of the best option for each individual patient. In this situation, the rates of prescribing of MCLs could actually be increased. This knowledge is needed in order to achieve a proper fitting. The pupil dependence of simultaneous image MCLs is well known and it has been investigated by many researchers. Presbyopic patients usually carry out their tasks under different levels of illumination and thus with different pupil sizes. Besides, small pupils should be considered as an important and common situation because near activities are usually performed under photopic conditions and this, together with the accommodative reflex, leads to pupil constriction. Furthermore, presbyopic patients normally present smaller pupil sizes than younger population. Average pupil size in patients over 60 years is considered to be between 3.0 mm (photopic conditions) and 4.5 mm (mesopic conditions). Considering this, since there is a change of the power distribution depending on the aperture, it seems evident that the performance of these lenses depend on the pupil dynamics of the patients. Thus, differences in the pupillary response among subjects could lead to different refractive power behaviour even for those with the same distance refraction, addition or visual requirements. For all these reasons, the pupil size takes a significant role when studying the performance of a lens. As for the case of MIOLs, the pupil dependence has also been extensively investigated. Several studies of in-vitro evaluation of MIOLs have shown the relationship between the aperture size and the optical performance for different designs. The optical performance of an IOL is generally assessed at its centred position. However, it is also important to take into account that different factors can affect its final position inside the eye, such as, the implantation technique, asymmetrical implantation, asymmetrical capsular shrinkage, capsule fibrosis, rupture of the posterior capsule or zonular dialysis, etc. This results in decentration of the lens and/or tilt. This misalignment with respect to the visual axis can affect the optical performance of the lenses. Well-centred IOLs with an aspheric design intend to improve optical performance and contrast sensitivity by reducing spherical aberration. However, decentring can result in lower optical transfer function. Objective comparison of the imaging properties of MIOLs, as well as those of MCLs, and their association with the pupil size can be useful in order to select the most appropriate solution for each patient. Naturally, the pupil dependence is always thought in terms of the total diameter of a circular pupil. However, due to congenital or acquired conditions, the pupil may not be circular and it may have lost part of its functionality. Indeed, any disorder that physically causes any harm in the iris mechanisms or alter the iris innervation can result in an irregular shaped pupil. As an example, the iridal coloboma is a condition which arises early in gestation and it is associated with defective closure of the optic fissure. As a result, the choroid is not completely closed and thus the shape of the pupil is stretched. Also, posterior synechiae are presented when the iridal tissue adheres to the anterior capsule of the lens giving an irregular shape of the pupil as a result of ocular trauma or other ocular conditions that imply intraocular inflammation such as uveitis. Ocular trauma or other disorders may also lead to shape alterations such as muscle spasms that lead to segmental iris mydriasis. It has to be taken into account that eyes with irregular pupils due to ocular conditions can also present a decreased functional vision. It is important to recognize the iris structural abnormalities in order to detect the cause of abnormal pupil size, shape or pupillary function (reactivity). The shape of the pupil and the influence on the behaviour of multifocal optical solutions for presbyopia has not been previously investigated. Thus, if it has been mentioned that the pupil size has a significant role on the performance of multifocal solutions, it is thought that the shape of the pupil might also have additional implications in its optical behaviour even in terms of other limiting factors such as the light distortion effects. Although multifocal optical solutions for presbyopia are successful for a great number of people, they also present some limitations. Candidates for multifocal solutions need to accept a level of potential visual compromise, in exchange for an increased quality of life without spectacle dependence. The evaluation of night vision disturbances under dim light conditions has become a matter of special interest in the past years due to the increasing number of MCLs, corneal refractive surgery and IOL implantation. Under this concept, several phenomena are included, such as positive and negative dysphotopsia, glare, halos, starburst, arcs, etc. The term “light distortion” was suggested in order to incorporate all these phenomena. These disturbances are frequently reported by patients, but they are commonly described as subjective complaints. Beyond the use of subjective questionnaires, there is a need to characterize the size and shape of these light distortions. The evaluation of light disturbances of optical solutions for the correction of presbyopia under different conditions, including the pupil size or shape, can be very useful for the selection of the patient. The main goal of this thesis is to analyse existing optical solutions for presbyopia as a first evaluation for describing optical elements and then go further by assessing the influence of different sizes and also shapes of the pupil, since most of the current assessing methods have only taken into account a circular shape. Thus, this work gives special importance to those pupils that are not circular, and how this characteristic may influence the effectiveness of the current optical solutions for presbyopia in terms of image formation quality metrics and the induced light disturbances. Two main approaches are used as general methods in this thesis, which are explained in chapter 3, titled “General Methods”. The first one makes use of a commercially available optical device, the NIMO TR1504 device (Lambda-X, Nivelles, Belgium), and one experimental laboratory equipment, the “Light Disturbance Analyser” (LDA) (CEORLab, University of Minho, Gualtar, Braga, Portugal). However, one of the main limitations of these instruments is that they assume circular apertures. This is the reason why only by means of computational simulations (second approach) it has been possible to assess the influence of the shape of the pupil, since existing optical devices merely include circular apertures. Furthermore, computational methods allow to develop new metrics from data yielded by those devices. For this purpose, specific custom-made software has been developed. Specifically, one software based in Fourier-optics methods has been used for the simulations and calculations with different pupil sizes and shapes. This software generates a wavefront error map via the cumulative integration of a given power profile along the radial direction. Then, the pupil function is calculated with the definition of a pupil mask. The pupil function describes how light is affected when it is transmitted through an optical imaging system, such as a lens or the human eye. This information is used in order to characterize the optical elements under different conditions. A second custom-made software has been used in order to enhance the metrics provided by the LDA instrument for light disturbance characterization. The software makes use of the raw data of the LDA, but it makes an alternative analysis to that of the native software of the instrument. In this case, the information of the instrument is used for the calculation of the distortion at each meridian. This new adjustment generates new metrics in order to take into account the possible effects of the introduction of a non-circular shaped aperture. In chapter 4, which is titled “Assessment of multifocal contact lenses for presbyopia”, detailed information of new commercially available designs is given, which is of crucial interest for practitioners. Additionally, the effect of the pupil shape has been analysed and it is discussed as a possible factor to affect the optical performance of MCLs. This chapter is subdivided in section 4.1, which is titled “In-vitro evaluation of rigid gas permeable multifocal contact lenses with variable multifocal zone”, and section 4.2: “The effect of non-circular shaped pupils on the performance of multifocal contact lenses”. Specifically, in section 4.1, a new set of MCLs with variable multifocal zone is analysed. These lenses are a centre-distance rigid gas permeable lenses that are available with five distance-vision diameters (XS, S, M, L and XL) and two different additions: Type A (up to +2.00 D) and Type B (up to +2.50 D). The multifocal zone is located on the front lens’ surface, so the posterior surface can be designed as a function of the patient’s corneal shape. The results are given in the form of power profiles. It is shown that the amount of total addition achieved depends on the diameter of the distance-vision area. In other words, the bigger the distance vision area, the bigger the radius of the lens in order to get the same level of addition. The XS lens yields higher addition values in comparison with the XL lens design for a given pupil. With this, the XS and S designs seem to be aimed to favour near vision. The L and XL designs seem to favour distance vision. For this reason, patients who demand good distance vision might benefit from the L or XL designs, and those with high demand on near-vision tasks might benefit from the XS or S. The M design could be the best solution for those patients who require the same needs for distance and near vision. In section 4.2, theoretical approaches are considered and implemented by means of optical simulations in order to investigate different pupil size and shapes, using power profiles similar to those obtained in the previous section. Specifically, one center-distance bifocal, one progressive multifocal power profiles, together with a monofocal profile, were considered. Three pupils are used: one circular and two non-circular, including one elliptical and one irregular shape. The apertures were defined within a maximum diameter of 6 mm. Metrics based on the point spread function (PSF) and the optical transfer function (OTF) have been obtained, such as the diameter which gathers the 25% of light from the PSF centre (D25) or the visual Strehl ratio in the frequency domain (VSOTF). The VSOTF parameter has been previously used for describing visual performance because it shows a strong correlation with subjective methods. A threshold of 0.12 is usually set as the VSOTF value that corresponds to a visual acuity at which approximately half of the people present difficulties while reading (20/32 Snellen equivalent). Values above 0.12 are considered to be acceptable. The results of the through-focus analysis of the PSF are represented as a qualitative assessment of the light compactness and distribution. An irregular pupil causes an irregular shape of the PSF. However, by analysing these previous images, only a qualitative assessment can be done. A quantitative evaluation is desirable, since it can describe the variations numerically. For this purpose, the D25 is introduced. The D25 differences among pupils are greater for the distance vergence of the centre-distance bifocal profile. The results for near distance are similar for every pupil. For the circular and synechial pupil, the size of D25 is smaller for near than it is for distance vergence, whereas regarding the elliptical pupil the size it is smaller for distance but comparable to near vision. With regards to the centre-distance progressive lens, all pupils show similar results at far and better than those for the bifocal. The VSOTF curves show a change in the energy distribution with different pupil shapes. In some cases, the elliptical and synechial apertures yield better results in terms of compactness (D25) or VSOTF. This is due to the fact that the defined pupils are masking a part of the wavefront, since they were described within the circular 6 mm pupil. For this reason, the total area dedicated for each optical zone and whether it contributes to near or distance foci, should be considered. As a conclusion, the pupil shape affects the physical parameters of the analysed lens power profiles. This means that the effectiveness of the optical solutions for the correction of presbyopia might be altered with irregular pupil shapes. However, the clinical implications of these phenomena might differ from the real physical measurements, due to the influence of additional factors such as ocular wavefront aberrations or the neural adaptation process. Chapter 5 is titled “Assessment of multifocal intraocular lenses for presbyopia”. In section 5.1, “The effect of non-circular shaped pupils and decentration on the performance of multifocal intraocular lenses”, the analysis is centred on MIOLs. As well as with MCLs, the number of new MIOL designs is growing and the target public is also increasing. In this chapter, the effect of the pupil shape was analysed by means of optical simulations. It is also important to evaluate the effect of decentration on the IOL performance, since as it was previously mentioned, the in-the-bag IOL placement can result in a desalignment. Depending on the IOL design, some studies have shown that decentration can have an important effect on the optical quality provided by the lens. For this study, a power profile of a refractive annular multifocal IOL is considered with a base power of 0.0 D, and approximately +2.50 D of addition. The total optical zone is 6 mm in diameter. An elliptical pupil shape is used for the analysis, together with a circular one for comparison purposes. Again, metrics based on the PSF and the OTF are obtained, such as the D25 or the VSOTF. The through-focus analysis of the D25 for the centred position yields similar results for both pupils at the near focus, whereas the distance focus gives greater values for the circular pupil. The through-decentring analysis reveals that the near focus is more affected with decentring. As for the VSOTF ratio, it is shown that the near focus is reduced with increasing decentring for both pupils, whereas the far focus varies more discreetly. The robustness to decentring seems to be similar for both pupils for the near focus. However, the tolerance seems to be slightly better for the elliptical aperture in the vertical direction. As a conclusion, the pupil shape together with decentring has an impact on the physical metrics that were analysed. This means that the shape of the pupil might affect the effectiveness of this kind of optical solutions for presbyopia. Nevertheless, the clinical implications of these variations are not directly transposable due to the effect of other factors such as the contribution of the rest of the ocular media and the neural processes. Chapter 6 is titled “Clinical evaluation of light distortion with multifocal optical solutions for presbyopia”. In section 6.1, which is titled “Light distortion of soft multifocal contact lenses with different pupil size and shape”, a clinical evaluation of light disturbances is presented. As it has been previously stated, the number of users of MCLs has been rising due to the increasing popularity of these solutions as a modality to correct presbyopia. Most of the current MCLs designs are based on simultaneous image formation. However, this principle can imply visual side effects, such as an augmented sensitivity to disability glare or the presence of haloes, especially under low light conditions due to the increased pupil size. Earlier studies have reported visual such side effects with MCLs. Increased disturbing photic phenomena can be an obstacle to perform everyday tasks, such as night driving or driving with a low sun. The vast majority of the simultaneous image formation designs are based on concentric annular areas with a central circular optical zone surrounded by one or more annular zones, which yield rotationally symmetric power profiles. These designs are well related to the shape of a normal circular pupil, but the effect on non-circular shaped pupils and their clinical implications have not been previously investigated. This topic is discussed in this part. A total of 14 eyes of 7 healthy contact lens wearer patients (3 females and 4 males) aged from 25 to 40 years (mean 28.57 ± 8.46 years) were analysed. The LDA device has been used for light characterization. The selected lenses for the study were the monthly disposable Biofinity Multifocal (CooperVision, CA, USA), with both “D” and “N” designs, and an addition power of +2.50 D. The D design consists of a centre for distance vision with progressive positive shift towards the periphery, whereas the N design has a centre for near vision with progressive negative shift. The dominant eye (DE) of the patients wore the corresponding “D” design and the fellow non-dominant eye (NDE) the “N” design. The monofocal version of these lenses was included for comparison purposes. Two circular pupils, one of 3 mm (P1) and one of 5 mm (P2) were used. Also, one elliptical shape of 3 mm in the horizontal and 5 mm in the vertical direction was included (P3). In this study, optical spherical aberrations and metrics such as the light disturbance index (LDI), best fit circle radius (BFCr) and its corresponding irregularity standard deviation (BFCirregSD) are analysed. Meridional difference of the light distortion between the vertical and horizontal directions has also been calculated. As a first approach, a comparison between the monofocal and the corresponding multifocal versions of commercially available lenses is performed. In light of our results, the MCLs induce a generalized increasing of the LDI (and thus, the distortion size) with all kind of pupils. The highest values of LDI are obtained for the DE, with more pronounced differences than the NDE. More specifically, the greatest statistically significant difference in LDI is obtained for the DE with P2. The value of the LDI for the DE with P2 and a MCL was 6.09 ± 3.28 (%), the highest one. Indeed, in terms of total area, P2 is the one that lets the biggest amount of light in. The analysis of the pupil factor reveals some significant differences. The greatest differences are obtained for the pair comparison including P1 and P2 for the multifocal “D” design, which yielded differences in the LDI, BFCr and the spherical aberration, with greater disturbance size for P2. In average, the disturbance is greater with P3 than with P1 for both the DE and NDE. An additional analysis is performed in order to compare the differences in size of the light disturbance between the vertical (M90) and horizontal directions (M0), for all pupils with MCLs. Although the size of the disturbance changes with the pupil size or its total area, the shape of the disturbance can be different among pupils. A first step is considering the BFCirregSD. This parameter, however, does not show statistically significant differences for any pair comparison. In spite of this, the greatest BFCirregSD is obtained for P3 with the multifocal D design. For confirmation, a more thorough approach has been adopted taking into account the shape of the non-circular pupil, so the difference between M90 and M0 are of special interest due to their association with the maximum and minimum aperture sizes of P3. This analysis shows that the shape of the pupil might have an impact on the shape of the light distortion, since the differences between M90 and M0 only turns out to be significant with P3 for the D multifocal design, which yields the greatest difference value (0.23 degrees), in accordance to the previous greatest BFCirregSD. With regard to the type of design of the multifocal lenses (D and N), the differences are only found to be statistically significant for the spherical aberration, but not for the light disturbance metrics. The coefficient becomes more negative and greater in modulus for the NDE with greater aperture size. Likewise, it is more positive and greater in modulus for the DE with greater pupil. As a conclusion, it has been shown that MCLs increase light disturbance effects under low light conditions. Also, the size of the distortion is increased with pupil size. As well as the size of the distortion is associated with the size of the pupil, it seems that the shape of the distortion might also be related with the shape of the pupil. In summary, the performance of new progressive centre-distance MCL designs in terms of the total amount of total addition achieved depends on the diameter of the pupil size and the total distance-vision area. For this reason, the pupil size of the patients, as well as their visual needs, are of crucial importance. Besides, the pupil shape affects the physical parameters derived from the analysed lens power profiles of different MCLs, and also of IOLs under decentration. This means that the effectiveness of these optical solutions for the correction of presbyopia might be altered with irregular pupil shapes. Nevertheless, the clinical implications of these phenomena might be different from the real physical measurements, due to the influence of additional factors such as ocular wavefront aberrations or the neural adaptation process. Also, MCLs increase light disturbance effects under low light conditions. Besides, the size of the distortion was increased with pupil size. As well as the size of the distortion is associated with the size of the pupil, it seems that the shape of the distortion might also be related with the shape of the pupil. Lastly, it is important to highlight the importance of the computing software tools for the development of this thesis, since specific custom-made software was made for optical modelling including different pupil sizes and shapes. Future lines of research should aim to solve the current limitation of this methods. New developments of optical software would provide a powerful tool not only for characterization of already exiting designs, but also for predicting the optical performance of new experimental ones. This thesis provides a deeper knowledge on existing methods. Looking into the future, investigation should aim to achieve new solutions for the correction or prevention of presbyopia in order to preserve the most optimal quality of vision, and thus, of life.