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The Aging Eye: Problems That Affect Acuity and Contrast Sensitivity

Theodore Grosvenor, OD, PhD, FAAO
5673 Correll Dr.
Ferndale WA 98248

 

Contents

 

Introduction

 

When I was asked to prepare courses on the aging eye, I was reminded of the book by that name written some forty years ago by R. A. Weale, Head of the Department of Physiological Optics at the Institute of Ophthalmology in London (1). In this book, Weale took a broad approach to the age-related changes that occur in the eye, including ocular morphology; refraction; visual thresholds; spatial and temporal resolution; spectral sensitivity; and color vision.

In my course on this subject, I will follow Weale's lead and also take a broad approach to the effects of aging on vision and the eye. Although optometrists currently tend to be concerned mainly with disease-related conditions that can cause severe vision impairment or blindness, there are many relatively subtle - but nevertheless important - age-related changes that affect almost everyone beyond the age of 45 or 50-years.

This course will focus on changes in refractive error, retinal illumination, visual acuity, contrast sensitivity, problems with the lacrimal system, and age-related cataracts. Although cataracts have traditionally been considered as a major cause of age-related vision loss, it is fortunate that in countries where the current 'no-stitch' cataract surgery with phaco-emulsification, followed by the implantation of an IOL is available -- including the United States, Canada, England, Australia, and New Zealand -- cataracts may be considered as a 'temporary' loss of visual acuity.

 

Presbyopia

The most obvious of the "less-threatening" age-related changes in vision is presbyopia, which usually becomes evident by 40 to 45 years of age. As shown in Figure 1, the near-point of accommodation gradually moves outward from about 10 inches (20 cm) at age 40, to 12 inches (30 cm) at age 45, and 16 inches (40 cm) at age 50 years.

 

Figure 1. Near-point of accommodation as related to age.

 

Derived from the Greek presby and ops meaning old-age sight, presbyopia occurs because the eye’s lens gradually loses its ability to change shape when acted upon by the ciliary muscle - the smooth muscle within the ciliary body that surrounds the lens.

 

Figure 2. Two strategies for dealing with presbyopia. This is one of the many ‘Eye Doctor’ cartoons collected by Prof. Henry Hofstetter, Indiana University School of Optometry. It appeared in the Aug.18, 1968 issue of The Optical Journal and Review of Optometry.

 

Determining the Power for a Presbyope's Reading Addition

Either (or both) of two methods can be used to determine the power of the add power to be given in the form of reading glasses, bifocals, trifocals, or progressive addition lenses.

1. On the basis of the near-point of accommodation.

With the patient wearing his or her correction for distance vision, the near-point card is placed on the reading rod at a distance of 40 to 50 cm, and it is gradually moved inward until the patient reports that the 20/20 (6/6) letters begin to blur. The near-point of accommodation, expressed in cm, is converted to the amplitude of accommodation by means of the formula:

Amplitude of accommodation = 40/near point of accommodation

For a near-point of accommodation of 10 cm, the amplitude of accommodation would be: 40/10, which equals 4.00 D. When the amplitude of accommodation has been measured, the reading addition is determined as the amount of plus lens power that will require the patient to use only one half of his or her amplitude.

2. On the basis of the binocular crossed-cylinder finding.

The Jackson crossed-cylinders (typically with plus and minus 0.50 D powers) are placed in the phoropter with the minus axes in the 90-degree position, and the patient views a crossed-cylinder grid (Figure 3) under very low illumination at a distance of 40 cm. The test is begun with +1.00 D lens power (combined with the subjective lens power) more than the patient has currently worn for near vision. For example, if the patient has worn no near-point addition, the starting added power is +1.00 D; and if the patient has been wearing a bifocal addition, the additional power would be +1.00 D more than the currently worn addition.

 

Figure 3. The crossed-cylinder grid.

 

The patient is asked to report which lines are darker or more distinct: those going up-and-down (vertical) or those going across (horizontal). The expected answer is “up-and-down,” in which case the plus power in the phoropter is binocularly reduced, 0.25 D at a time, until the patient reports that the two sets of lines are equally distinct, or the “across” lines are more distinct than the “up-and-down” lines.

Modification of the Tentative Add Power

Once the tentative addition power has been determined, it may be modified by means of: (a) the results of the negative and positive relative accommodation (plus and minus to blur) tests; and/or (b) the patient’s desired working distance.

1. The relative accommodation tests.

With the near-point card at the 40 cm distance and the tentative addition in place, the examiner asks the patient to keep the bottom line of letters in sharp focus, and to report when the bottom line of letters begins to blur, as plus lens power is added, 0.25 D at a time. Similarly, the patient is asked to keep the letters in sharp focus as minus lenses are added, 0.25 D at a time. The plus-to-blur and minus-to-blur findings are recorded as the amount of relative plus lens power and minus lens power from the desired add power, respectively, required to blur the bottom line.

The expected result is for the two findings to be equal, or for the plus-to-blur to be slightly higher than the minus to blur; for example, +1.00 D and -0.75 D.

2. The patient’s desired working distance.

gain, with the near-point card at 40 cm and the tentative addition in place, the patient is asked to report when the bottom line of letters begin to blur, as the card is moved farther away on the reading rod. The finding is recorded as the position of the card in cm or inches.

The card is again placed at the 40 cm distance, and the patient is asked to report when the bottom line of letters begin to blur as the card is moved toward the patient. The position of the card when this happens is recorded. The card is then moved to a position halfway between the near and far positions, and the patient is asked whether this is his or her desired reading distance. This test may also be performed out-of-phoropter in a trial frame.

The typical patient will require a reading add of +0.75 D or +1.00 D at age 40 or 42, but possibly at an earlier age for a hyperopic patient, or at a later age for a myopic patient, depending upon the amount of myopia or hyperopia. The power of the addition will typically increase to between +2.25 D and +2.50 D by the age of 54 or 55 years.

The Effects of Pupil Size and Anisometropia

Occasionally an optometrist encounters a patient over age 50 who has never worn glasses or contact lenses, but has no difficulty reading small print at the 40 cm distance. This mystery can often be solved by checking the patient’s pupil size. If the pupil diameter is no more than 2 or 3 mm, the patient’s depth of field is sufficiently large to provide clear vision at a much closer distance than with a larger pupil (the smaller pupils create a ‘pseudo-pinhole’ effect).

Another mystery sometimes occurs when an ‘over 50’ person who has never worn glasses or contacts is able to read the small letters on both the distance and near acuity charts. In this case, the patient might have uncorrected anisometropia and is using one eye for near and the other for distance. I once examined a patient whose refraction was right eye (OD) +0.50 D and left eye (OS) -1.50 D. He wondered why other people his age all were wearing glasses for reading when he could read easily without glasses!

Reading Glasses, Bifocals, Trifocals, or Progressive-Addition Lenses?

Many emmetropes are understandably reluctant to wear glasses in any form because they believe that glasses are a sign of ‘old age.’ These ‘reluctant presbyopes’ will often insist on having reading glasses only so they won’t have to appear in public wearing glasses. But, once they have encountered the problem of having to remove their glasses for distance vision, many patients are willing to accept the idea of wearing bifocals or progressive-addition lenses for reading and other near work. This is especially true if they are reminded that they can always remove their glasses for distance vision. A possible alternative, of course, is the use of ‘half-eye’ reading glasses.

As for my own situation - having moderately high myopia, astigmatism, and anisometropia - I wore bifocals for several years, then I switched to trifocals, and later I switched to progressive-addition lenses. When computers began to play an important role in my life, I found that progressive-addition lenses were far superior to trifocals for computer work, as well as for general wear.

Optical Properties of Conventional Multifocal Lenses

Since the original innovation of Benjamin Franklin (who combined the top halves of his distance spectacles with the bottom halves of his reading spectacles into one frame), multifocal lenses have become a regular staple of the prescribing doctor. Several segmented designs are available, including:

Figure 4 shows several types of segmented multifocal spectacle lens designs.

 

Figure 4: Multifocal spectacle designs. Upper images left to right: Panoptic, Round Top, Flat Top, Curved Top. Lower images left to right): B-Seg (Ribbon), R-Seg (Ribbon), Executive®, Altax®. Images from http://www.eynak.com/ed/00108001.asp

 

Optical Properties of Progressive-Addition Lenses

Progressive-addition lenses differ from segmented lenses in that progressives have aspheric front surfaces that provide a gradual increase in plus power as the line of sight sweeps downward through a progressive corridor (also referred to as a channel). These lenses typically provide blurred vision on either side of the corridor due to the presence of astigmatism.

 

Figure 5. Schematic representation of a progressive-addition lens. The shaded areas represent the presence of varying amounts of astigmatism.

 

‘Hard’ and ‘soft’ design lenses.

Progressive-addition lenses can be considered as hard or soft designs, on the basis of the length of the progressive corridor and its effect on the induced astigmatism.

A lens having a hard design has a short progressive corridor with relatively large distance and reading areas that are free of unwanted astigmatism, and with the induced astigmatism concentrated into relatively small regions of the lens. With this design, the larger distance and reading areas are provided at the cost of relatively large amounts of astigmatism, which can cause blurred vision and spatial distortion. The Ultravue® lens is an example of a hard design.

A lens having a soft design has a long progressive corridor with relatively small distance and reading areas that are free of astigmatism. The astigmatism is spread over larger areas of the lens at the expense of smaller astigmatism-free distance and near areas. The Varilux 2® is an example of a soft design.

Multidesign lenses.

Borish, Hitzeman and Brookman (2) found that younger presbyopes tend to prefer lenses having a soft design; whereas older presbyopes show about equal preference for lenses having hard and soft designs. Because soft progressive-addition lenses tend to be preferred by presbyopes who require relatively low addition powers, and hard designs tend to be preferred presbyopes who require higher addition powers, multidesign lenses having soft designs in the lower add powers and hard designs in the higher add powers were introduced.

 

The first multidesign lens to be introduced was the Varilux Infinity®, in 1988, which has individual designs for each add power from +0.75 D to +3.00 D. This was followed in 1993 by the Varilux Comfort®, described by the manufacturer as a multidesign lens having a high and wide reading area and a soft, usable periphery. Other multidesign lenses include the American Optical Pro-15® and Pro-16® lenses, the Silor Adaptor No-Line®, and the Kodak progressive lens. Newer designs that incorporate front and back surface asphericity to decrease induced astigmatism include the Divinity® and Visio® (Essilor) progressive lenses.

 

VDU progressive-addition lenses.

Horgen, et al., (3) conducted an investigation of the use of progressive-addition lenses that were especially designed for use with computer screens (visual display units - VDUs). Subjects for the study were four groups of VDU users, each consisting of approximately 40 VDU workers.

 

Subjects filled in a questionnaire concerning visual conditions, working conditions, discomfort, the status of the their optical corrections, etc., before the investigation began and at periods of 6 months and one year. VDU lenses used in the study were the Interview® (Essilor), Gradal RD® (Zeiss) and Technica® (American Optical).

The subjective evaluation of the area of clear vision and overall satisfaction were significantly improved for wearers of Interview® and Gradal RD® lenses. Horgen, et al., concluded:

"Lens designs that cover viewing distances from near out to approximately 2 meters work well compared to lens designs trying to cover a greater range of clear vision. When task analysis shows that single-vision correction may be used, this is still an acceptable solution."

Occupational progressive lenses.

Sheedy and Hardy (4) used a Rotlex Lens Analyzer to determine the optical properties of seven different designs of occupational progressive lenses (OPLs) intended for a patient requiring a +2.50 D add. Lenses used were: AO Technica®, Cosmolit Office®, Essilor Interview®, Hoya Tract®, Shamir Office®, Sola Access®, and Zeiss Gradal RD®.

Sheedy and Hardy also described occupational progressive lenses designed to provide near vision in the lower portion of the lens having a wide field of intermediate vision in the lower portion of the lens and farther intermediate vision at the top of the lens. They emphasized that the lenses were not intended to meet typical distance viewing needs. The results of their investigation showed large optical differences between the various designs in terms of add powers, their locations, and the zone widths. They concluded:

"The literature and clinical experience support that OPLs are successful at meeting the computer, general office, and other intermediate viewing-distance needs of many patients. However because of the large differences in several OPL designs, patient success can likely be enhanced by selecting the design that best suits his or her viewing needs."

Contact Lens and Intraocular Lens Corrections for Presbyopia

An alternative for presbyopia remediation can be multifocal contact lenses (both rigid and soft designs). However, selection of the right candidate for these lenses is of great importance (e.g., good anterior segment ocular health, larger pupil size to allow both the distant and near components of the contact lens to be utilized, and the ability to insert and remove the lenses safely and effectively). (Figure 6)

 

Figure 6. Some multifocal contact lens designs. Left – Aspheric design with near power center, distance power peripheral. Middle – Concentric design with near power center, distance power peripheral. Right – Segment design with near power in lower segment (+). Images from http://www.allaboutvision.com/contacts/bifocals.htm, and http://www.dankerlabs.com/austil.htm.

 

Newer intraocular lenses that provide multifocal capability through refractive and diffractive means are also available for elderly patients who require cataract surgery. (Figure 7)

 

Figure 7. Some multifocal intraocular lens designs. Left – Advanced Medical Optics Array IOL with different annular refractive zones. Right – Alcon ReStor multifocal IOL with refractive and diffractive annular zones. Images from http://www.eyesurgerycenterla.com/emerging_tech.htm, and http://www.revoptom.com/index.asp?page=2_1533.htm.

 

There are other investigational intraocular lenses that incorporate elements such as flexible haptics that allow the ciliary muscle to change the position of the artificial lens. Corneal surgery creating an add power out of the patient’s own corneal tissue, or from inserting an intracorneal lens for near viewing are also being explored. However, all of these options are considered experimental at this time.

With the advent of LASIK and other refractive surgeries, it is important to explain to patients who have a distance vision correction that the need for near correction will still occur as they get older. Refractive surgery does not mean that near vision will be clear for a lifetime. True surgical correction of presbyopia has not yet been perfected.