With the aging of the population, patients and ophthalmologists have recently shown increasing interest in surgical approaches to the correction of presbyopia. Presbyopia is the age-related loss of the ability to accommodate that occurs in all people by approximately 55 years of age. Presbyopia is primarily due to an increase in the stiffness of the crystalline lens.1-4
Accommodation is not simply the ability of an eye corrected for distance vision to see clearly at near; it is an optical change in the power of the eye that occurs due to an increase in the dioptric power of the lens.
Strategies to surgically correct presbyopia fall into two general groups: (1) attempts to restore the eye's ability to perform active and dynamic accommodation, and (2) use of optical devices to alleviate presbyopic symptoms. The latter group, which includes multifocal IOLs, presby-LASIK, and corneal inlays, among other strategies, are aimed at providing monovision or pseudoaccommodation in the presbyopic eye. Some of these modalities are currently being used to provide functional distance and near vision in presbyopic patients.
For the former group, endeavoring to restore accommodation, a number of methods are under investigation, including scleral expansion surgery; softening of the lens with a laser or chemical; and replacing the hardened presbyopic lens with an artificial, accommodating IOL.
To evaluate the ability of these methods to restore accommodation, it is important to understand the mechanism of accommodation, the ways accommodation is measured, and the causes of presbyopia. This article reviews these points and ends with a look at some developing technologies for correction of presbyopia through the restoration of accommodation.
MECHANISM OF ACCOMMODATION
An emmetropic eye is focused for distance when the ciliary muscle is relaxed. Resting tension on the zonular fibers extending between the ciliary body and the lens edge holds the lens in a flattened and unaccommodative state. Contraction of the ciliary muscle occurs with an effort to focus at near. When the ciliary muscle contracts, the ciliary body moves inward, toward the axis of the eye, resulting in a release of resting zonular tension. When the zonular tension is released, the elastic capsule surrounding the lens of a young eye molds the malleable lens into a more spherical and accommodative form.5 This results in a decrease in lens equatorial diameter, an increase in lens axial thickness, and, most important for the accommodative optical change, a steepening of the lens anterior and posterior surface curvatures.6-9
Accommodation is a true optical change in the power of the eye. Therefore, to evaluate whether the eye undergoes accommodation, accommodation should be measured objectively. However, clinically the subjective push-up test is often used. In this test, the distance-corrected patient moves a near-reading chart toward the eyes until first sustained blur is reported. The reciprocal of the distance in meters from the chart to the eyes is then used as a measure of the accommodative amplitude in diopters. This test does not actually measure accommodation, but rather it is a measure of near reading distance.
There are limitations with this type of subjective measurement, in that it includes the eye's depth of field, which is not the same as accommodation. In fact, in patients who cannot accommodate, such as presbyopic patients or those with a monofocal IOL in place, this method measures only depth of field. Further, this test relies on the patient's ability to perceive blur. The perception of blur varies with each patient, as does the patient's interpretation of what blur means. Measurements can vary depending on the instructions given to the patient.
Because of these limitations, this type of subjective measurement is inadequate to accurately assess restoration of accommodation. Additionally, other subjective measures, such as defocus curve testing, are also inadequate because they too do not differentiate between an optical change in power and the depth of field of the eye.
Objective measurement of accommodation can be accomplished with refractometers, autorefractors, and aberrometers. The Hartinger coincidence refractometer (HCR), for example, is a laboratory instrument widely used to objectively measure accommodation. The patient focuses on a distant letter chart, and minus-powered trial lenses are placed in front of the eye to defocus the image and stimulate accommodation. When the patient accommodates to overcome the induced defocus, the response is measured with the HCR.
A commercially available autorefractor (Grand Seiko WR-5100K; Grand Seiko Co., Hiroshima, Japan) was compared with HCR to evaluate the autorefractor's potential to measure accommodation reliably.10 In 22 patients 21 to 30 years of age, accommodation was stimulated and measured using three methods: (1) the subjective push-up test, (2) a near target pushed toward the patient with response measured by the autorefractor and HCR, and (3) a distant target viewed with increasing blur through trial lenses with response measured by the autorefractor and HCR. The subjective test overestimated accommodative amplitude in comparison with the objective methods. The measurements with the autorefractor showed good agreement with those of the HCR. The results of the study showed that the Grand Seiko WR-5100K is well suited for objective clinical measurement of accommodation in young, normal adults.
In another study, the autorefractor and another commercially available instrument, the iTrace wavefront aberrometer (Tracey Technologies, Houston), were compared for their ability to measure accommodation.11 In 30 young adults (age range, 21 to 31 years) and 15 prepresbyopic individuals (age range, 38 to 49 years), accommodation was stimulated with near charts presented at various distances, and repeat measurements were taken with both instruments. The accommodative responses measured by the two instruments were not statistically significantly different. The results of the study showed that both instruments are suitable for objective measurement of accommodation in phakic prepresbyopic patients with low accommodative amplitudes.
SCLERAL EXPANSION BANDS
One approach that has been investigated for restoration of accommodation is scleral expansion, based on the Schachar theory of accommodation.12 Schachar's theory, stating that there is an increase in zonular tension during accommodation, is opposite to the widely accepted theory proposed by Helmholtz in the 19th century, stating that relaxation of the zonules allows the lens to become more spherical, increasing its optical power.
In scleral expansion band (SEB) surgery,7 the bands are suggested to increase the circumlental space to allow the equatorial expansion of the lens that supposedly takes place during accommodation, according to Schachar's theory.
Subjective and objective test methods were used to examine a satisfied patient who had undergone SEB implantation.13
This 50-year-old man was examined 19 months after SEB surgery. No preoperative data on the patient were available. Nine age-matched normal presbyopic control patients, ages 48 to 52 years, and one normal nonpresbyopic control patient, age 27 years, were also examined. Accommodation was measured subjectively using the push-up method and objectively using HCR and a dynamic infrared optometer.
The satisfied SEB patient achieved 20/20 distance acuity with correction of 0.50 D in the right eye and 1.00 D in the left. Distance-corrected near visual acuity at 40 cm was 20/100 in the right eye and 20/70 in the left. Corneal topography and aberrometry showed no suggestion of optical multifocality.
Accommodative amplitude as measured subjectively with the push-up technique was 3.30 D in the right eye and 2.50 D in the left, whereas objective measures showed amplitudes of 0.25 D to 1.33 D. Accommodative amplitude as measured objectively in this patient was no greater than in age-matched presbyopic controls.
The results of this study showed that SEBs do not restore accommodation. The satisfaction expressed by this patient may have been due to his high expectations for a positive outcome from the surgery.
OTHER APPROACHES TO ACCOMMODATION RESTORATION
A number of accommodating IOLs are currently in use or under investigation. They fall into three general categories: (1) those whose mechanism depends on forward translation of a single optic, (2) those whose mechanism depends on the movement of dual optics, and (3) those whose mechanism depends on a change in curvature of the lens surface.
The first category includes the Eyeonics Crystalens (Bausch & Lomb; Rochester, New York), the Tetraflex (LensTec; St. Petersburg, Florida), and the C-Well dynamically accommodating IOL (Acuity, Ltd.; Or-Yehuda, Israel). The second category includes the Synchrony Dual-Optic IOL (Visiogen; Irvine, California). The third category includes the FluidVision lens (Power Vision; Belmont, California) and the NuLens (NuLens, Ltd.; Herzeliya, Israel).
The first of these devices to achieve US Food and Drug Administration (FDA) approval was the Crystalens, and this is the only accommodating IOL with US regulatory approval. The investigational and regulatory status of the other devices ranges from preclinical to clinical trial stage to commercial availability in some markets outside the United States.
Another avenue that has been investigated for restoration of accommodation is polymer refilling of the capsular bag.14 In a rhesus monkey model, changes in optical refraction were measured after refilling the capsular bag with a new polymer material. Care was taken to minimize postoperative inflammation to delay the occurrence of posterior capsular opacification. Thirty-seven weeks postoperatively, the greatest accommodative amplitude measured in five adolescent monkeys was 6.30 D, although this subsequently decreased to zero in three of the monkeys. Two monkeys maintained accommodative amplitude of 4.00 D throughout follow-up, and all five monkeys maintained clear enough capsules to allow measurement of refraction to 37 weeks. Although some accommodation occurred in the polymer refilled eyes, this amounts to a relatively small percentage of the 10.00 to 14.00 D of accommodation available in the normal eyes of monkeys of this age.
With growing interest in the correction of presbyopia and restoration of accommodation, it is clear that proposed solutions will continue to proliferate. For them to succeed, it is necessary that they be based on sound principles and an understanding of the mechanism of accommodation and causes of presbyopia.
Additionally, for clinicians and researchers to evaluate the efficacy of these proposed solutions, it is important that they use appropriate instruments for objective measurement. These instruments must be able to distinguish true restoration of accommodation from artifacts, such as the eye's depth of focus. It is clear from our objective investigations, for instance, that SEB surgery cannot and does not restore accommodative ability to the presbyopic crystalline lens.
Although efforts to restore accommodation have been disappointing to date, other techniques are under investigation, and a number of styles of accommodating IOLs may hold promise for the future.
Adrian Glasser, PhD, is a Professor in the College of Optometry at the University of Houston. Of the companies or products mentioned in this article, the author has, during the past year, served as a paid consultant to Advanced Medical Optics, Inc., Eyeonics, Bausch & Lomb, and Visiogen. The author serves as a paid consultant to other companies not mentioned in the article, with interests in accommodation, presbyopia, and accommodation restoration concepts. Dr. Glasser may be reached at tel: +1 713 743 1876; e-mail: email@example.com.
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