Accommodation and presbyopia : part 1: physiology of accommodation and development of presbyopia

Accommodation is a dynamic change in the dioptric power of the eye. According to the widely accepted and experimentally confirmed theory of Helmholtz, it is achieved by release of zonular tension with contraction of the ciliary muscle and consecutive modelling of the shape of the crystalline lens by the elastic lens capsule. The ability to accommodate is gradually lost with age (presbyopia). Because of difficulties in examining the accommodative apparatus in vivo, many theories, in part contradictory, about the mechanism of accommodation and the origin of presbyopia have been developed. In recent years experimental studies have greatly increased the knowledge about the acommodative apparatus and suggest a multifactorial aetiology of presbyopia. A better understanding of the physiology of accommodation and presbyopia can contribute to the development of effective treatments.     

Akkommodation und Presbyopie

Accommodation is a dynamic change in the dioptric power of the eye. According to the widely accepted and experimentally confirmed theory of Helmholtz, it is achieved by release of zonular tension with contraction of the ciliary muscle and consecutive modelling of the shape of the crystalline lens by the elastic lens capsule. The ability to accommodate is gradually lost with age (presbyopia). Because of difficulties in examining the accommodative apparatus in vivo, many theories, in part contradictory, about the mechanism of accommodation and the origin of presbyopia have been developed. In recent years experimental studies have greatly increased the knowledge about the acommodative apparatus and suggest a multifactorial aetiology of presbyopia. A better understanding of the physiology of accommodation and presbyopia can contribute to the development of effective treatments.     

The aetiology of presbyopia: a summary of the role of lenticular and extralenticular structures

Presbyopia is a condition of age rather than ageing and, as such, is devolved from the lamentable situation where the normal age-related reduction in amplitude of accommodation reaches a point when the clarity of vision at near cannot be sustained for long enough to satisfy an individual's requirements. Most of our facility to accommodate has been lost by 55 years-of-age and subsequent deterioration in visual performance at near is attributable to characteristics of senescent vision familiar to the optometrist. Our understanding of the cause of presbyopia has then to be derived principally from our understanding of the mechanism of accommodation in young eyes. Hermann von Helmholtz did much to clarify these mechanisms, but despite much research in the 100 years since his death, there is still no consensus on their precise nature. This paper presents a summary of issues, past and present, which have figured in the literature on the physiology of accommodation and presbyopia, and confirms that the pathophysiology of presbyopia is likely to result from deterioration in structure and function of a number of inter-related tissues. Changes in crystalline lens dimensions with age, the associated change in geometry of zonular attachments, and changes in viscoelastic properties of the lens capsule and lens matrix would, however, appear to be the principal correlates for the onset of presbyopia. Recent models of the biomechanics of accommodation have drawn attention to the feasibility of extralenticular contributions to presbyopia and have examined properties of the elasticity and leverage provided by posterior, anterior and tensile fibre systems. The stimulus for innovation in the correction and remediation of presbyopia is strong as at least 98% of individuals from the industrialised nations will survive until 45 years-of-age; it is salutary to note that the survival rate 100 years ago was only 60%.     

Refraction, including prisms

The literature in the past year on refraction is replete with several isolated but very important topics that have been of interest to strabismologists and refractionists for many decades. The refractive changes in scleral buckling procedures include an increase in axial length as well as an increase in myopia, as would be expected. Tinted lenses in dyslexia show little positive effect in the nonasthmatic patients in one study. The use of spectacles or bifocals as a way to control increase in myopia is refuted in another report. It has been shown that in accommodative esotropia not all patients will be able to escape the use of bifocals in the teenage years, even though surgery might be performed. The hope that disposable contact lenses would cut down on the instance of giant papillary conjunctivitis and keratitis has been given some credence, and the conventional theory that sclerosis alone is the cause of presbyopia is attacked. Also, gas permeable bifocal contact lenses are reviewed and the difficulties of correcting presbyopia by this method outlined. The practice of giving an aphakic less bifocal addition instead of a nonaphakic, based on the presumption of increased effective power, is challenged. In the review of prisms, the majority of articles concern prism adaption. The most significant report is that of the Prism Adaptation Study Research Group (Arch Ophthalmol 1990, 108:1248-1256), showing that acquired esotropia in particular has an increased incidence of stable and full corrections surgically in the prism adaptation group versus the control group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Questioning our classical understanding of accommodation and presbyopia

The development of precise instrumentation, electron microscopy, and enhanced analytical capabilities have renewed interest in understanding the mechanisms of accommodation and presbyopia. The properties of the ciliary muscle, the zonule, the lens capsule, and the crystalline lens are being reevaluated, suggesting, for example, that these components' elasticities change significantly with age and that the biochemical properties of the crystalline lens may be altered as the lens ages. The recent findings, mainly during the past decade, are contrasted with or incorporated into our classical understanding of the accommodative mechanism and presbyopia as stated originally by Helmholtz and Fincham.     

老视理论及治疗进展

老视是一种常见的眼部退行性病变,表现为年龄相关性调节力下降,它影响到每一个老年人。随着全新的Schachar调节理论的提出以及传统理论的发展,近年来出现了多种老视治疗方法。目前除了通过各种框架眼镜和接角镜来矫正老视,还可以应用手术方法如前睫状区巩膜切开样(AGS)以及巩膜扩张(SRP)来恢复调节能力。采用人工晶状体植入形成单眼视,多焦点人工晶状体,可调节型人工晶状体,LASIK手术也都是可行的老视治疗措施。本文总结了目前有关老视的各种机制和治疗方法。     

老視理論及治療進展

老視是一種常見的眼部退行性病燮,表現為年齡相關性調節力下降,它影響到每一個老年人.隨着全新的Schachar調節理論的提出以及傳統理論的發展,近年來出現了多種老視治瘵方法.目前除了通過各種框架眼鏡和接觸鏡來矯正老視,還可以應用手術方法如前睫狀區鞏膜切開術(ACS)以及鞏膜擴張(SRP)來恢復調節能力.采用人工晶狀體植入形成單眼視,多焦黠人工晶狀體,可調節型人工晶狀體,LASIK手術也都是可行的老視治瘵措施.本文總結了目前有關老視的各種機制和治瘵方法.     

Conductive keratoplasty

PURPOSE OF REVIEW: Conductive keratoplasty is a noninvasive, in-office procedure for the correction of hyperopia, hyperopic astigmatism, and management of presbyopia. It serves as an alternative to laser-based refractive surgery with essentially no intraoperative or postoperative complications. RECENT FINDINGS: In the past decade, photorefractive keratectomy and laser in-situ keratomileusis have been the most popular refractive surgical procedures to correct myopia, hyperopia and astigmatism. Although relatively safe, flap-related complications often result in undesirable visual acuity. Since US Food and Drugs Administration approval in 2002, conductive keratoplasty has become a promising technique to correct low to moderate hyperopia and astigmatism. The procedure was first used by Mendez and colleagues in 1993. It is a nonlaser, no cutting procedure that delivers radio-frequency energy to corneal stroma in a circular fashion to steepen the cornea. Multiple studies have shown that conductive keratoplasty offers equal or superior efficacy, predictability, stability and safety than currently used refractive procedures to correct hyperopia or hyperopic astigmatism. In addition, monovision conductive keratoplasty has been shown to be successful for the management of presbyopia. SUMMARY: Conductive keratoplasty, an alternative to the laser-based procedure, is effective, predictable, and safe to correct low to moderate hyperopia, astigmatism, and manage presbyopia.     

The mechanism of presbyopia

Accommodation in humans refers to the ability of the lens to change shape in order to bring near objects into focus. Accommodative loss begins during childhood, with symptomatic presbyopia, or presbyopia that affects one's day to day activities, striking during midlife. While symptomatic presbyopia has traditionally been treated with reading glasses or contact lenses, a number of surgical interventions and devices are being actively developed in an attempt to restore at least some level of accommodation. This is occurring at a time when the underlying cause of presbyopia remains unknown, and even the mechanism of accommodation is occasionally debated. While Helmholtz' theory regarding the mechanism of accommodation is generally accepted with regard to broad issues, additional details continue to emerge. Age-related changes in anterior segment structures associated with accommodation have been documented, often through in vitro and/or rhesus monkey studies. A review of these findings suggests that presbyopia develops very differently in humans compared to non-human primates. Focusing on non-invasive in vivo human imaging technologies, including Scheimpflug photography and high-resolution magnetic resonance imaging (MRI), the data suggest that the human uveal tract acts as a unit in response to age-related increasing lens thickness and strongly implicates lifelong lens growth as the causal factor in the development of presbyopia.     

老视矫正手术的临床应用与展望

随着人们对生活质量的需求提高,老视问题越来越受到关注。像屈光不正矫正手术一样,老视的激光矫正治疗无论是设备开发还是临床手术的应用都逐年增多,如角膜的激光、角膜板层间镶嵌、角膜热重塑、巩膜气化等。但是老视又与屈光不正（近视、远视和散光）不同,到了一定年龄屈光度会相对稳定,而老视则与年龄有紧密的相关性。目前老视人群庞大,因此有许多人期待着安全、有效、可预测性强、远中近视力都能兼顾的老视矫治手术。     

Presbyopia in light of accommodation

The "dual, indirect, active" mechanisms of accommodation proposed by Helmholtz is reviewed. New supporting evidence shows that the insertion of the ciliary muscle is onto the span fibrils of the zonule in the interciliary process regions of the ciliary body. Thus, the peripheral zonule is the elastic antagonist of a unified ciliary muscle. A biomechanical model illustrating accommodation is put forward that makes consistent and explicit the force and length changes that the six mechanical component elements undergo. The Hess-Gullstrand lenticular theory of presbyopia is compatible with this model.     

Restoration of accommodation

PURPOSE OF REVIEW: This review examines the current status of accommodation restoration concepts with reference to the recent, published peer-reviewed literature with an emphasis on physiological aspects of accommodation and presbyopia. RECENT FINDINGS: The mechanisms of accommodation and the causes of presbyopia are described. The physiological amenability of the accommodative structures in the presbyopic eye to accommodation restoration is discussed. General theoretical concepts of accommodation restoration are introduced. The methods that have been used to assess accommodation restoration, including the use of animal models, drug stimulated accommodation, subjective near-vision tests and objective measurements, are reviewed. SUMMARY: While physiological and clinical evidence supports the notion that accommodation can be restored to the presbyopic eye, progress in this potentially exciting area is hindered by the scarcity of good, large-scale clinical studies using objective measurement techniques to evaluate the outcomes of accommodation restoration concepts.     

Analysis of the optical performance of presbyopia treatments with the defocus transfer function

PURPOSE: A variety of techniques for treating presbyopia are available. However, these techniques are often disparate in the mechanisms used to cope with the lack of accommodation. Multizone-refractive, diffractive, and apodized intraocular lens technologies are currently being used. This article evaluates the optical performance of these markedly different technologies in a simple and comparable manner. METHODS: The Defocus Transfer Function is a mathematical technique for illustrating the Optical Transfer Function for all levels of defocus. By calculating the Defocus Transfer Function, the simulated performance of presbyopia correction technologies on distance, intermediate, and near vision can be evaluated simultaneously. Simulations of an opaque annular ring, a zonal refractive lens, a full-aperture diffractive lens, and an apodized diffractive lens are analyzed at pupil sizes of 3, 4, and 6 mm with this technique. RESULTS: The opaque annulus begins to act like a Fresnel amplitude zone plate for small pupils. The diffractive lens technologies had sharp optical performance at two distinct foci with a shift in performance from balanced to distance-biased for the apodized diffractive lens. The zonal refractive lens exhibited a multifocal effect. However, the optical performance of this technology fell below that of the diffractive lenses. CONCLUSIONS: The Defocus Transfer Function is a useful tool for analyzing presbyopia treatments. Different strategies for creating simultaneous vision or extended depth of field are easily compared. It may also be possible to reverse this technique to create an ideal pupil function that meets a desired multifocal performance specification.     

The quality of life associated with presbyopia

PURPOSE: To determine the health-related quality of life associated with the presbyopia. DESIGN: Cross-sectional, patient preference-based, time trade-off utility analysis. METHODS: One hundred and ten patients with spectacle-corrected presbyopia and a best-corrected visual acuity of 20/40 or better were interviewed in a cross-sectional fashion using a standardized, validated, time trade-off utility analysis questionnaire. RESULTS: The mean utility associated with the health state of presbyopia in our sample was 0.980 (standard deviation, 0.086; 95% confidence interval, 0.964 to 0.996). Ten percent of the cohort had a presbyopia-associated utility of 0.95 or less. Age, gender, level of education, marital status, and the degree of presbyopia did not significantly affect the time trade-off utilities associated with presbyopia. CONCLUSIONS: Presbyopia corrected with glasses is associated with a nominal decrease in quality of life, similar to that of treated hypertension, for the average person with the condition. Approximately 10% of such patients theoretically may be candidates for an intervention other than spectacles to correct the condition.     

Monovision: a review

In presbyopia, patients can no longer obtain clear vision at distance and near. Monovision is a method of correcting presbyopia where one eye is focussed for distance vision and the other for near. Monovision is a fairly common method of correcting presbyopia with contact lenses and has received renewed interest with the increase in refractive surgery. The present paper is a review of the literature on monovision. The success rate of monovision in adapted contact lens wearers is 59-67%. The main limitations are problems with suppressing the blurred image when driving at night and the need for a third focal length, for example with computer screens at intermediate distances. Stereopsis is impaired in monovision, but most patients do not seem to notice this. These limitations highlight the need to take account of occupational factors. Monovision could cause a binocular vision anomaly to decompensate, so the pre-fitting screening should include an assessment of orthoptic function. Various methods have been used to determine which eye should be given the distance vision contact lens and the literature on tests of ocular dominance is reviewed. It is concluded that tests of blur suppression are most likely to be relevant, but that ocular dominance is not fixed but is rather a fluid, adaptive, phenomenon in most patients. Suitable patients can often be given trial lenses that allow them to experiment with monovision in real world situations and this can be a useful way of revealing the preferred eye for each distance. Of course, no patient should drive or operate machinery until successfully adapted to monovision. Surgically induced monovision is less easily reversed than contact lens-induced monovision, and is only appropriate after a successful trial of monovision with contact lenses.     

Lens Refilling

Cataract surgery has developed into a very safe and highly reproducible procedure but the ultimate goal to also restore physiological accommodation has not yet been achieved. A variety of accommodative intraocular lenses (IOLs) and surgical techniques have been suggested to cure presbyopia but all showed only poor accommodative effects by objective measurements. Complete lens refilling with flexible polymers might be an interesting alternative. Recent investigations on accommodation and presbyopia have given support to the lens refilling method. After development of suitable surgical techniques and filling materials only secondary cataract formation and the unsolved intraoperative control of refraction restrict the clinical use of this technique.     

老视及其矫正

老视是一种生理现象，不论屈光状态如何，每个人均会发生老视。随着人类社会的老龄化，老视的矫正问题成为我们日益严峻的挑战。本文拟就有关老视的调节机制及近年来一些矫正方法的原理、手术方式、临床应用结果作一综述。     

A physiological theory of depth perception from vertical disparity

It has been known since the time of Helmholtz that vertical differences between the two retinal images can generate depth perception. Although many ecologically and geometrically inspired theories have been proposed, the neural mechanisms underlying the phenomenon remain elusive. Here we propose a new theory for depth perception from vertical disparity based on the oriented binocular receptive fields of visual cortical cells and on the radial bias of the preferred-orientation distribution in the cortex. The theory suggests that oriented cells may treat a vertical disparity as a weaker, equivalent horizontal disparity. It explains the induced effect, and the quadrant and size dependence of vertical disparity. It predicts that horizontal and vertical disparities should locally enhance or cancel each other according to their depth signs, and that the effect of vertical disparity should be orientation dependent. These predictions were confirmed through psychophysical experiments.     

Design und optische Prinzipien von Multifokallinsen

Multifocal intraocular lenses (MIOLs) of different designs have been used for more than 20 years in modern cataract surgery. The first IOLs were either refractive 2-3-zone designs or of diffractive concept. All MIOL types, regardless of manufacturer, show reduced contrast sensitivity and increased glare for years. Nevertheless, even first-generation MIOL patients had very good functional results when patient selection, IOL calculation, and surgical techniques were appropriate. Today's gold standard is foldable, aspherical diffractive, or refractive MIOLS. Advancements in technology allow the possibility that MIOLs will be used for aphakia correction as well as presbyopia correction.     

Correlation between presbyopia, age and number of births of mothers in the Kumasi area of Ghana

There is an assumption in Ghana that motherhood has an effect on presbyopia. This study thus attempts to correlate the relationship between presbyopia, age and number of births of mothers in the Kumasi area of Ghana. The data were compiled from records of patients who attended routine eye examinations and refraction tests at the Morny Optical Centre, Kumasi, during the last quarter of 1993. Presbyopia in males and females, and those of mothers and childless female adults for particular age groups were compared. Correlations were found between age and reading additions for both sexes, and presbyopia occurs at a younger age than published. This preliminary study does not indicate any inter-dependence of births on female presbyopia. Further studies with select data may be required to confirm the above findings.