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EW Interview:
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EyeWorld: How would you define presbyopia?
Adrian Glasser, PhD: First, it is necessary to accurately define accommodation. Accommodation is a dioptric change in power in the eye. That means the eye’s optical power increases when the eye accommodates. Accommodation is not simply the ability to see clearly at near. Presbyopia is the gradual age-related loss of accommodation — a progressive decline in accommodative amplitude from childhood. EW: What causes presbyopia? Glasser: Many aspects of the eye and accommodative apparatus undergo age changes. Potentially, these factors contribute to the progressive loss of accommodation. Presbyopia is fundamentally an inability of the eye and hence the crystalline lens to undergo accommodative optical changes. If nothing else in the eye changed, but the lens lost the ability to accommodate, presbyopia would occur. That is not to suggest that this proves that the age changes in the lens cause presbyopia, but certainly age changes in the lens must be considered as important factors in the progression of presbyopia. It could be that presbyopia is simply a manifestation of a gradual progressive change that occurs in the lens toward the progression of cataract. It may also be that presbyopia is truly a comprehensive system failure, where many aspects of the accommodative system fail together. What is clear about presbyopia is that if accommodation is to be restored, a fundamental requirement is a lens that is capable of undergoing accommodative optical changes. EW: How does accommodation occur? Glasser: In 1855, Helmholtz published his paper on the accommodative mechanism of the human eye. He described that when the eye is focused for distance vision there is resting tension on the zonular fibers at the lens equator. When accommodation occurs, the ciliary muscle contracts and the apex moves forward in the eye to release the resting zonular tension and allow the lens to accommodate. Fincham identified that the lens capsule molds the lens into an accommodated form, and more recent evidence has demonstrated that the posterior of the lens surface moves backward during accommodation. The general mechanism first described by Helmholtz has been demonstrated to be correct and is generally accepted. EW: What is the best method to measure accommodation? Glasser: An instrument that can objectively measure the optical power of the eye and how that changes with accommodation should be used. Objective measurement of accommodation requires no subjective assessment from the patient or from the clinician. Objective infrared refractometers, such as autorefractors, when used correctly, are good instruments to use. Accommodation is a dynamic process and instruments capable of measuring accommodation dynamically can also provide important information, such as how fast accommodation occurs. It is also necessary that accommodation be stimulated effectively. If a near reading chart is presented to the patient, it should represent a compelling accommodative stimulus. Pharmacological means such as topical application of pilocarpine can also be used to stimulate accommodation. It is also necessary that convergent eye movements that occur with accommodation be minimized to avoid artifactual results. Clinically, accommodation is routinely measured with the push-up technique. While this is easy to use in a clinical setting, it is a poor way of measuring accommodation. EW: Which approach to presbyopia correction do you think has the best chance of success? Glasser: Given the frequency and success of surgical interventions and laser treatments for cataract and refractive errors, surgical interventions are likely to prove most beneficial. Prospects are being explored for laser modifications of the crystalline lens. This approach is highly investigational and will need significant testing to demonstrate safety, viability, and efficacy. The prospects for the development of successful accommodative intraocular lenses are looking promising. Several such lenses are either being used in Europe or undergoing Food and Drug Administration clinical trials in the United States. Several other such accommodative IOLs are undergoing early stages of development and testing. In addition, there has been much interest in the prospects for developing an accommodative IOL by injecting a soft polymer into the capsular bag after cataract extraction. This work is also investigational and the outcomes of this testing and development will have to be followed closely. The massive potential patient base, the market-driven forces, the huge interest in the field, and new technological developments mean that we will converge on not just one, but many viable solutions. Within 5 years, I expect there to be many device lenses available for patients to choose from that will afford some relief from presbyopia. EW: Why is there so much debate about the cause and treatment of presbyopia? Glasser: The accommodative apparatus within the eye is a complex and multifaceted system that is difficult to observe and study. New instruments are only now being developed that allow the actions of the accommodative structures to be studied in detail. Many aspects of the accommodative apparatus are very fine anatomical structures and are not easily studied without disrupting their function. The causes of presbyopia are not well understood and characterizing this system requires a comprehensive understanding of anatomy, optics, physiology, mechanics, and pharmacology. Few unanimously agree about anything to do with accommodation and presbyopia. While there are just a few fundamentally different theories of accommodation, there are many different theories of presbyopia. Age changes in just about every anatomical component of the accommodative apparatus have been implicated as a primary causative factor in the progression of presbyopia. EW: Are scleral expansion procedures, such as scleral band expansion surgery, effective? Glasser: No. They are based on unfounded theories of how accommodation occurs. Our experiments do not support Schachar’s theories of accommodation and presbyopia and no independent evidence exists to support them.1 Our research shows that with increasing age, the human lens loses the ability to accommodate and undergoes an exponential increase in hardness.2,3 Surgically expanding the sclera cannot restore the accommodative capacity to the lens. In addition, objective attempts to measure accommodation in scleral expansion patients showed that no accommodation occurs.4 Furthermore, even subjective assessments show only infrequent and at best transient benefits.5 Finally, with the high prevalence of cataract in the elderly, any serendipitous benefits of scleral expansion surgery will be lost with cataract surgery.6 EW: How should ophthalmologists go about deciphering the new research regarding presbyopia? Glasser: Critically. Important and relevant scientific information is disseminated. Anyone interested in this area should be conservative, cautious, and critical. Contact Information References
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Adrian Glasser, PhD, discusses the massive potential presbyopia
patient base, market-driven forces, huge interest in the field, and
technological developments. |
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