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.     

眼调节装置与老视的研究

近几年来关于老视机制和老视治疗的研究在眼科领域中成为关注的热点，许多学者在眼调节装置(晶状体、悬韧带、睫状体)的年龄相关性变化及相关于老视进展机制的研究工作基础上，提出许多令人鼓舞和全新的观点和理论。着重对一些有代表性的研究结果进行了综述，并且简要描述了近几年出现的老视治疗方法和对未来发展的展望。     

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.     

Comparison of the behavior of natural and refilled porcine lenses in a robotic lens stretcher

The mechanism by which the eye dynamically changes focal distance (accommodation), and the mechanism by which this ability is lost with age (presbyopia), are still contested. Due to inherent confounding factors in vivo, in vitro measurements have been undertaken using a robotic lens stretcher to examine these mechanisms as well as the efficacy of lens refilling - a proposed treatment for presbyopia. Dynamic forces, anterior and posterior curvatures, and lens thickness are all correlated for young natural and refilled porcine lenses. Comparisons are made to lenses refilled with a homogeneous polymer system. The amplitude of accommodation of the young porcine lens is very small such that it may be a suitable model for presbyopia. The behavior of refilled lenses was highly dependent on the refill volume. The volume could be tuned to maximize accommodative amplitude in the refilled lens.     

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%.     

老视的角膜矫正手术

老视是随着年龄增长、调节能力丧失出现视近物困难的一种生理现象,发病机制尚不明确。人口老龄化使老视人口及老视矫正的需求逐渐增多,老视的手术矫正方法正成为眼科医师关注的热点。随着技术和材料的不断发展,出现了多样化的老视矫正手术,包括经角膜老视矫正手术、晶状体老视矫正手术和巩膜老视矫正手术。尽管还没有一种完美的老视矫正手术能真正恢复眼的调节功能,现有的手术已取得一些临床效果。现对老视的角膜手术矫正方法做一综述。     

激光老视术后调节的双因素理论

目的 :对于“激光老视治疗”的临床结果及基于弹性改变假说及晶状体前移及晶状体松弛模式的眼调节机制进行分析。方法 :根据光在模型眼中的传导原理 ,导出了总屈光力因晶状体与角膜屈光力、前房深度、眼轴长度及折射率而改变的函数。使用晶状体伸张及调节实验值导出与总屈光力变化相关的眼参数的实验方程。激光术后调节幅度使用林氏动态模型进行描述 ,此模型含晶状体的前移及松弛两个分量。同时提出林氏 卡达假说 ,认为术后调节改变及低回退率来自于巩膜 睫状肌弹性的增加。结果 :调节 (AA)幅度大约为晶状体屈光力改变的 73%～ 76 % ,并且与睫状体的收缩及晶状体厚度增加呈非线性变化。我们提出对于 2 .0D的调节可由晶状体松弛 (LR)或前移 (AS)或LR及AS两者组合而达到。对于硬化的老年晶状体 ,AS是主要因素。结论 :激光老视治疗 (LAPP)的临床结果可由光在眼球的传导方式计算及由弹性改变假说和包含晶状体前移和松弛的动态模型的调节机制来分析 ,其中动态模式包含晶状体前移及松弛两个因素。     

Involutional changes in the human eye accommodative apparatus as evidenced by ultrasound biometry and biomicroscopy

The paper presents the results of studying involutional changes in the accommodative apparatus of the eye in presbyopia by ultrasound biometry and biomicroscopy. Persons aged 40-70 years who had presbyopia and emmetropic refraction and without concomitant eye disease were examined. In addition to routine studies, all the examinees underwent ultrasound biomicroscopy and ultrasound biometry. The age-related lenticular enlargement was established to be accompanied by a considerable reduction in the orbicular space of the posterior chamber of the eye to the extent of its transformation to a slotted space. The presbyopic patients were found to have a significantly diminished tone in the middle and posterior portions of the lenticular ligamentous apparatus to the extent of its sag, with the altered direction of their passage between the crystalline lenticular equator and the ciliary body crown, i. e. from meridional to radial. The revealed significant age-related topographic and anatomic changes in the orbicular portion of the posterior chamber of eye and in ligamentous tone, which are associated with the involutional increase in the size of the crystalline lens point to the great importance of decreasing the working accommodation distance in the development of presbyopia.     

Accommodation and presbyopia

The mechanism of accommodation has been studied for at least four hundred years. The most interesting aspect of accommodation is that its time course is well in advance of other physiological functions – it begins to decline by adolescence and is lost about two-thirds of the way through the normal line span. The state of presbyopia is reached when accommodation has declined sufficiently to interfere with close tasks requiring acute vision. Presbyopia is generally considered to originate with the 'plant' of the accommodative system, either within the lens and its capsule or within their support structures. One of the lenticular theories, the Hess–Gullstrand theory, is distinguished from other theories by its claim that as age increases there is an increasing excess amount of ciliary muscle contraction beyond the ability of the lens and capsule to respond to it. For all other theories, the maximum possible amount of ciliary muscle contraction is always necessary to produce maximum accommodation, at least beyond the age at which it reaches its peak. From my review of the present understanding of the mechanisms of accommodation and the theories of the development of presbyopia, I conclude that there is overwhelming evidence against the Hess–Gullstrand theory and that it is unlikely that changes in the ciliary muscle contractility contribute significantly to the development of presbyopia.     

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.     

老视矫正手术方法的现状和发展趋势

老视是因年龄相关性调节幅度的下降使得近阅读渐渐困难的一种生理老化现象。现阶段老视矫正的主流方式是配戴单焦框架眼镜、双焦框架眼镜和渐变多焦点眼镜。目前所有针对老视的手术方法都未能带来持久的真正生理意义上的调节改善,本文就目前各种老视矫正手术方法（包括单眼视方法的、施于角膜的、施于巩膜的与施于晶状体的）的现状与发展趋势做一综述。     

Presbyopia toward the end of the 20th century

Recent advances in our understanding of the anatomy and physiology of accommodation have contributed to current concepts of the possible constituent factors in presbyopia, largely supplanting the long-held belief that presbyopia is due to sclerosis of the crystalline lens. In this review, the author examines epidemiologic, basic scientific, and clinical evidence for a multifactorial decrease in accommodative amplitude with age. Methods of measurement, oculomotor effects of presbyopia, and approaches to correcting it are also considered.     

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.     

晶状体手术矫正老视的治疗进展

老视是与眼睛调节能力下降密切相关的生理现象。不论年轻时屈光状态如何,老视都是人一生中必须经历的阶段。随着老龄化人口数量的增多,老视问题得到越来越多眼科工作者的关注,各种治疗老视的方法与技术也应运而生。现就晶状体手术用于矫正老视进行综述。     

Presbyopietherapie mit Femtosekundenlaser

Presbyopia is by far the most common refractive error worldwide, with no permanent therapeutic option available. All efforts to restore accommodation by the use of surgery have not led to a generally accepted therapy. However, there is evidence from an animal model that the use of a femtosecond (fs) laser might influence the modulus of elasticity in the lens. Fs-laser impulses can create intralenticular disruption in animal eyes as well as human cadaver lenses and improve elasticity. The concept of treating presbyopia with fs-laser requires a new, complex theory combining the optical and the mechanical aspects of accommodation in the eye. Diagnostic tools for measuring optical change in power and geometrical modification as the eye views from far to near are needed to obtain objective clinical data. A non-invasive treatment of presbyopia to restore accommodation might be possible in the future.     

Physiopathology in accommodation and presbyopia, clinical and surgical approach

Accommodation is a complex of phenomena by which the refracting power of the eye changes for focusing on near vision. Accommodative amplitude declines progressively with increasing age. Until today, this phenomenon called presbyopia was explained by Helmholtz's hypothezis which involves only ciliary muscle. It seems that in accommodation are involved, besides the ciliary muscle, the lens and extraventicular elastic components, biophysical dynamics of the lens capsule, vitreous, iris. According with the role of these factors and theirs pathophysiological implications, new surgical technique has developed to reverse presbyopia in the human eye (Surgical reversal presbyopia, Anterior ciliary sclerotomy) and offers a potential for understanding ocular hypertension and treatment of primary open-angle glaucoma.     

Artificial crystalline lens

If presbyopia is caused by hardening of the crystalline lens, replacing it with a material with mechanical properties similar to the young crystalline lens should restore accommodative ability. Such a silicone material has been developed. Refilling the capsular bag with this material results in 3 to 5 D of accommodation in primates in response to pilocarpine.     

Surgical treatment of presbyopia: scleral, corneal, and lenticular

PURPOSE OF REVIEW: Having solved most of the problems concerning myopia, hyperopia, and astigmatism, it is perfectly understandable that the surgical treatment of presbyopia should be next on the agenda. It is a new challenge where every possible aspect is explored. RECENT FINDINGS: The past five years have been troubled by the debate over von Helmholtz theory on accommodation. Numerous investigations have been carried out on the primate and humans using various procedures. The more we learn about this mechanism, the nearer we will be to finding a solution to presbyopia. It appears essential to refer to recent works confirming von Helmholtz theory. Therefore, understanding presbyopia requires a great deal of optical ingenuity such as monovision, scleral modifications, which still remain controversial, or clear lens exchange or refilling. SUMMARY: In 2003, with all the different techniques available, the surgeon has a wide choice to offer patients that are satisfactory from a practical, theoretical, and ethical point of view. However, these techniques must only be proposed once the patients have been carefully informed.     

Accommodation- presbyopia: mechanism etiology

The purpose of the presentation is to elucidate the mechanism of accommodation and etiology of the age‐related decline in accommodative amplitude that results in presbyopia in the fifth decade of life. Multiple physical, mathematical, clinical, in vitro, and in vivo experiments demonstrate that the human crystalline lens develops an unusual shape during accommodation. Central lens thickness increases; the curvatures of the central surfaces of the lens steepen; while the peripheral surfaces of the lens flatten. This ‘steep profile’ also occurs in other biconvex objects that have an aspect ratio ≤ 0.6 (minor axis to major axis ratio) in response to a small equatorial displacement when the volume of the object only minimally changes. The ‘steep profile’ even occurs when the major axis of an ellipse with an aspect ratio ≤ 0.6 is increased and the area enclosed by the ellipse is held constant or permitted to decrease by 2%. The universality of the occurrence of a ‘steep profile’ implies that the lens is under increased tension during accommodation. This hypothesis was confirmed by using high‐resolution anterior segment optical coherence tomography (OCT) to measure the change of stress on the anterior lens capsule in patients who had undergone a phakic refractive intraocular lens (PRL) at least 1 year prior to the study. The PRL served as an internal control. It was found that during a mean of 8‐diopters of in vivo accommodation, the stress on the lens was increased during accommodation, p < 0.001. The increased stress on the lens capsule during accommodation occurs as a consequence of increased equatorial zonular tension. Normal equatorial lens growth predicts both the age‐related decline in accommodative amplitude and the age‐related increase in intraocular pressure (IOP). The rapid decline in accommodative amplitude and rapid increase in IOP that occur during childhood and their slower changes thereafter, both match the logarithmic pattern of equatorial lens growth, R² = 0.90. In summary, the lens is under increased stress during accommodation as a consequence of increased equatorial zonular tension. Normal equatorial lens growth is the etiology of both the age‐related decline in accommodative amplitude that results in presbyopia in the fifth decade of life and the increase in IOP that occurs with age.     

Accommodation: mechanism and measurement

A clear understanding of the anatomy of accommodative structures, the mechanism of accommodation, and the causes of presbyopia is necessary to understand whether accommodation may be restored to the presbyopic eye. Not withstanding the significant challenges that are faced owing to age-related changes in the eye, theoretically, it may be possible to restore accommodation to a presbyopic eye with an artificial accommodative intraocular lens.