Presbyopia and

It is beyond the scope of this article to discuss all of the various soft multifocal contact lenses on the market. As 95 per cent of new fits in 2004 were with daily or monthly replacement lenses, it seems appropriate to describe some of the monthly disposable soft multifocal lenses available.1
According to Government Actuary Department figures, the UK population is almost 60 million. Of this number, some 30 per cent are between the ages of 45 and 64 (Figure 1). This amounts to some 14.5 million presbyopes who potentially might consider the option of contact lenses as a visual correction.2 At present only 7 per cent of new fits and 19 per cent of refits are with multifocal lenses.1
Presbyopes are a progressively increasing proportion of the population, who are living healthier, more active lifestyles. Having a higher disposable income, they are keen to find ways to maintain a younger looking appearance. Those contact lens wearers fitted 30 years ago are now presbyopic, and demand an alternative to spectacles for near vision correction.
So why does this sector of the market remain almost untouched? Perhaps because practitioners have no confidence in the presently available range of lenses, believing them to be complex and time consuming to fit, or that they simply do not work.
Practitioners who are successful in this area insist that their preferred lens of choice is the only practical solution to the needs of the wearer. Intriguingly, even when condemned by some as unworkable, all the lenses will produce satisfactory results for many wearers, given a little patience and the right fitting approach.
The aim of this article is to describe some of the innovative options available today, and the procedures required to achieve a successful outcome when fitting soft multifocal contact lenses.

Historical influences
Traditionally, there were two specific types of contact lenses designed to correct presbyopia. The translating or alternating forms require the eye to rotate behind the lens to view through an inferiorly positioned segment and achieve the near correction. These lenses suffered from an inherent need for rotational stability to be maintained.
Alternatively, simultaneous or concentric lenses created two superimposed overlapping images, by virtue of two specific zones. The central zone could be powered for distance, while the periphery is corrected for near vision or vice versa. These lenses were sometimes given the name centre distance or centre near, depending on whether the central zone was biased towards distance 'D' or near 'N' vision (Figure 2).
All present-day presbyopic monthly disposable lenses are derived from the simultaneous or concentric designs. However, many of the lenses now incorporate some means of inducing an intermediate range visual effect, giving them the benefit of being genuine multifocals in action and more acceptable to the needs of the modern-day wearer. These lenses are easy to fit, as they are not gaze dependent, having no off centre segment.
Furthermore, due to the much smoother progressive transitions between each zone, they are far less sensitive to the negative effects of pupil size such as flare and/or ghost images, than were the original bi-concentric forms.

Opportunities
The ACLM Contact Lens Year Book 2004 lists six monthly disposable multifocal/bifocal lenses (Table 1).
As can be seen, these lenses are divided into two general designs. Most are classified as 'fully aspheric' while others are described as 'multi-zonal'.

Differentiation
The Acuvue Bifocal is an example of a centre distance multi-zonal lens. Introduced by Johnson & Johnson in September 1998, it was one of the first frequent replacement contact lenses to correct presbyopia.3
Available on either a one- or two-weekly disposable replacement cycle, this lens revolutionised the presbyopic contact lens market by offering the practitioner the opportunity to fit and supply the patient directly from a consulting room diagnostic bank. This has the advantage of being able to immediately demonstrate, to the prospective wearer, the action of the lenses in both distance and near vision.
Sometimes described as 'pupil intelligent' this simultaneous vision design incorporates five alternating distance and near portions over an 8mm optic zone (Figure 3). Based on research of different pupil sizes within the presbyopic population, the size and spacing of each zone is designed to optimise vision for various lighting and viewing conditions.
The lenses have a base curve of 8.5mm and a diameter of 14.2mm, with a distance prescription range +6.00D to -9.00D in 0.25D steps. There are four add powers between +1.00D and +2.50D in 0.50D steps, which gives the practitioner the possibility to modify the prescription to the exact individual requirements of the wearer. Although these lenses are described as bifocals, many practitioners reduce the add power in the dominant eye by 0.50D to 1.00D, which reduces distance blur. Also, an additional effect of this modification is to enhance the clarity of the intermediate visual range, rendering the lens system more multifocal in action.3,4

Alternatives
The Bausch & Lomb SofLens Multifocal is one of a number of monthly disposable aspheric lenses.
Launched in April 2003, this front surface centre near aspheric design is a true multifocal, giving effective vision throughout the visual range. As with all multifocals of this type, the lens front surface progressively changes from the centre towards the lens edge. This creates a gradual power gradient, generating a smooth visual transition from near to far. Good centration is vital for simultaneous vision lenses to perform well and prevent oblique aberrations such as coma.
With this in mind, these lenses are available in two base curves, 8.8mm and 8.5mm with a diameter of 14.5mm. In a recent study by Bausch & Lomb, 94 per cent of the lenses dispensed were of the 8.8mm base curve as per their fitting recommendations. Only when an unsatisfactory fitting result was obtained did the investigators consider the 8.5mm base curve.5
The distance power range is +6.00D to -10.00D in 0.25D steps. Aspheric multifocal lenses do not have specific add powers, but a span of power from a pre-set near point out to distance, created by the aspheric optics of the lenses. Studies have reported inferior visual performance with some aspheric multifocal lenses, when the pre-set near power is high, potentially offering too much plus to the system and wearer.6
Both the SofLens Multifocal and the Biomedic 73 UV have reduced this problem by offering a low and high add range. The Bausch & Lomb multifocal has a low near power range up to +1.50 for the early presbyope. A high near power range from +1.75 to +2.50 is available for those requiring a higher add. Although these lenses are usually supplied incorporating the same amount of reading power in each lens, they may be fitted as modified monovision using the low add for the dominant eye and the high add for the non-dominant eye. This can reduce distance blur without interference to near vision.

Amalgamation
The new Proclear Multifocal lenses from CooperVision are a combination of the various optical principles used in contact lens practice to obtain functional multifocal vision.
'Balanced Progressive Technology' is the registered term that describes the combination of spherical and aspheric optics with unique zone sizes to produce two complementary, but inverse geometry lenses (Figure 4) - a centre distance 'D' lens for the dominant eye and a centre near 'N' lens for the non-dominant eye.
It can be seen that these lenses are multi-zonal with a spherical central area of differing sizes for the two designs, 2.3mm for the distance and 1.7mm for the near. Surrounding the central zone is a 5mm-diameter aspheric annulus and surrounding this a final spherical band, which completes the total optic zone size of 8.5mm. The principle of using both a centre distance and centre near lens at the same time is not new and experience suggests it may be a highly successful formula when fitting presbyopic contact lens wearers. This sounds similar to monovision, but as each lens is a true multifocal in its own right, binocular summation is maintained.
Table 2 shows the full parameters of the CooperVision Proclear Multifocal. The Proclear material aids dehydration resistance for an age group that exhibits a higher than average degree of marginal ocular dryness.7 Four specific adds allow the practitioner to independently prescribe the distance and near powers for the individual wearer's needs, maintaining control of the resultant visual effect.

Patient Selection
Managing patient expectations is probably the one major aspect that governs success. These lenses may not produce perfect vision for all, but will certainly deliver functional near, intermediate and distance vision for most wearers. Choosing suitable patients can significantly reduce the risk of failure.
Certain groups, such as emmetropes and those requiring correction of astigmatism, are notoriously well known for being difficult to fit with presbyopic contact lenses. The initial questioning, ocular examination and measurements will identify whether contact lenses are an appropriate means of optical correction. Careful interpretation of the patient's lifestyle and occupation can determine their priority towards near or distance vision and can identify where to place the emphasis of the correction.
The dilemma that confronts the practitioner is how best to achieve multifocal vision for the contact lens wearer. Much of the public is unaware of the existence of multifocal contact lenses. Once this has been recognised as a possible means of vision correction, it is important to give a simple description of how the lenses work, explaining that both eyes need to view objects simultaneously to obtain the best results and that each lens has an individual part to play in the total experience.
Although these lenses soften the images obtained, one should be positive; it is vital to avoid negative descriptions of potential visual outcome. The availability of consulting room diagnostic lenses has made it so much easier to immediately demonstrate the kind of vision possible. This will help gauge patient expectation against reality. Practitioner expectations also need to be tempered against what can reasonably be attained.
If the wearer obtains acceptable vision at all distances, within legal constraints, the aim has been achieved. The ultimate goal is patient satisfaction with the visual results. A motivated wearer is far more likely to succeed with a longer-term trial in the real world. A trial in their environment, where the lenses can be tested in a variety of conditions is the acid test of genuine acceptance.

Pupil Size
Pupil size varies with light intensity, age and visual task. Due to pupil size variation, it is not possible to accurately predict, in advance of fitting, the likely visual results.
Even with careful measurements of pupil size in the consulting room, there is no guarantee that the lenses will be worn in similar conditions of illumination.
As with most modern-day soft lenses, simultaneous vision multifocals move very little on eye and their action is mainly dependent on the relative contribution of power from the different zones over the pupil. In photopic conditions when the pupil contracts, centre near designed lenses will produce a proportionately greater near vision response. This would be desirable when reading in a brightly lit room, but unacceptable while driving into bright sunlight.
Centre distance designed lenses would theoretically be better suited to the driving conditions described, but might be inadequate for near vision. Clearly, the relationship between pupil size and the overall size of the individual power zones within the lens optics will dictate the degree of effectivity induced. For this reason, manufacturers of monthly disposable multifocals have tried to design lenses with optical aperture zone sizes which best suit average pupil sizes for the various illumination and range tasks.
The Proclear Multifocal system offers an inverse geometry designed lens for each eye, reducing pupil size dependency. Irrespective of light intensity the proportion of power required for a given task at a specific range has been balanced between the two lenses worn. Although it is generally believed that pupil size has a greater effect on simultaneous vision lenses as opposed to translating types, this is not strictly correct. Consider the situation where a segment height of a translating bifocal is measured in a brightly lit room. If the top of the segment were to be placed at the lower edge of the pupil margin, any subsequent reduction in light intensity would cause the pupil to dilate, with an increase in segment coverage for distance and consequent blurring of vision. Setting the segment top slightly within the pupil area in primary gaze, as is commonly the case, would make matters even worse. The only accurate means of measuring the effects of pupil size on vision is to trial wear the chosen option.

Ocular Dominance
One of the most important prerequisites when fitting simultaneous vision lenses is to determine ocular dominance. Even if this is not initially considered a necessary step to fitting success, subsequent visual improvements may be achieved by adjustment to the degree of distance or near component within the lens system.
Practitioners are familiar with the measurement of ocular dominance when prescribing monovision. Although the receptive field for each eye in the primary position of gaze is approximately identical, the input from one eye is often dominant over the other.8
In clinical practice it is usual to bias the dominant eye towards distance vision and the non-dominant towards near. This is based on the assumption that any out of focus blurred image viewed by the non-dominant eye, when viewing a distant object, can be suppressed. Many practitioners enlist sighting dominance tests (hole in card, pointing finger and so on) to confirm the dominant eye. However, there is no evidence in the scientific literature relating sighting dominance with blur suppression. In other words, sighting dominance is an inadequate measure of ocular dominance.9
Sensory dominance tests (fogging techniques) are far more likely to be predictive of success or failure with simultaneous vision lenses, because they are a direct measure of the patient's ability to suppress an out-of-focus image. To assess sensory dominance using the fogging technique, first ensure that the patient is fully corrected for distance. Then, while viewing a distant object binocularly, hold the patient's add power first in front of the right eye and then in front of the left. Simply ask the patient to state which eye notices the greatest degree of blur. The eye that experiences the greatest effect would be corrected for distance on the assumption that the opposing eye was able to suppress the out-of-focus image created by the add power.
This simple test will quickly identify those people unable to tolerate the slightly defocused image obtained through a simultaneous vision multifocal lens.

Fitting Considerations
The majority of frequently replaced soft contact lenses are manufactured in a limited range of fitting parameters.
This does not mitigate the necessity for accurate fitting characteristics. Vision, comfort and ocular health are all degraded by poor apposition of a lens to the eye. Soft lenses have to be large enough to cover the limbus in all directions of gaze. Centration is especially important with multifocal lenses and the correct amount of movement is required to ensure ocular health. Generally, these requirements are accomplished by the draping effect of modern-day monthly disposable lenses, primarily due to the designs having improved thickness profiles.

Optimising Vision
The CooperVision fitting guide for the Proclear Multifocal recommends that having first determined dominance, convert any distance sphero-cylindrical spectacle prescription to the best vision sphere and allow for vertex effect, if necessary.
Choose the lowest equivalent add power to that of the spectacle correction. Place the 'D' lens onto the dominant eye and the 'N' lens onto the non-dominant eye and allow the lenses to settle. Under normal illumination conditions, test visual acuity binocularly. Keep these checks as near 'real world' as possible. Look through a window to check distance and use a coloured magazine for near vision. Using Snellen acuity charts is only necessary to measure and record the level of vision achieved, but research has shown that these charts are a poor means of accurately assessing the true performance of simultaneous vision lenses.10
Once a satisfactory visual response has been obtained, dispense the pair of lenses for a one-week trial period. This will allow the wearer to use the lenses in a variety of environments, and confirm that they are comfortable with their vision at all distances.

Modifications
If at the first aftercare it is decided that the vision is less than optimum, any over-refraction should be done binocularly without the aid of trial frames or phoropter heads. Again, this is to maintain a 'real world' environment and to ensure normality of pupil size without the effects of mydriasis due to shadowing.
The over-refraction should be performed using hand-held lenses or a twirl. As with all these lenses, small increments of power 0.25D can have a significant effect on improvements to vision. Simultaneous vision lenses form multiple overlapping images on the retina. These are created by the pupil being intentionally covered partly by distance, intermediate and near portions of the lens optics at the same time. It is the relative contribution of power from any one zone dominating that stimulates the required effect on visual range. Looking at it another way, multifocal vision is accomplished for all viewing distances by a tenuous balance between the effective distance and near powers of each lens.
This must be kept in mind when performing an over-refraction to attain a successful result.

Distance Enhancement
Aim to maximise vision in the dominant eye. Commonly an increase in minus power is required only in the dominant eye
If a 0.25D change is required for distance in either eye, make the change without any alterations to the add power.

Near Enhancement
Aim to maximise vision in the non-dominant eye. A +0.25D acceptance of power to the distance component will usually improve reading
If 0.50D is accepted in the distance prescription, it may be necessary to consider changing the add powers for both lenses
Keep the add powers as low as possible, consistent with acceptable near vision as this will reduce potential interference to distance sight
Attempt to increase plus/decrease minus in the distance prescription before attempting to change the add powers.

If an alteration appears to be needed for either the distance or near components, always check the effect on the other end of the visual range before adjustments are made to the lens.

Creative Thinking
Many practitioners believe that the only way to succeed with simultaneous vision multifocal lenses is to fit them with odd adds, using a lower add in the dominant eye than in the non-dominant eye. This modified monovision technique places a bias on the dominant eye for distance, while still maintaining comfortable near sight. Centre distance designed lenses tend to favour clearer distance vision, while centre near performs better at near. By combining a variety of these alternatives it is possible to devise a lens system that best suits the individual wearer's lifestyle.

Discussion
Successful fitting of simultaneous vision multifocal/bifocal lenses necessitates some changes in approach on the part of the practitioner. Listening and interpreting accurately the patient's requirements for near and distance vision will identify whether a bias towards either range would be appropriate.
Careful assessment of sensory ocular dominance can pinpoint, prior to fitting, those that have a strong sighting preference or that are able to alternate blur suppression between eyes. Always testing vision binocularly and avoiding trial frames or phoropter heads will ensure a more natural environment and enhances the action of the lens systems. Any over-refraction should be done monocularly, without occlusion to the contra-lateral eye, with hand-held lenses or a twirl.
By employing these techniques the practitioner will discover that many more wearers will achieve the desired results simply and without excessive time in the chair.

Conclusion
Many of the procedures described in this article can be used to fit any of the array of presbyopic lenses available today.
New designs allow the practitioner greater freedom and control of the final lenses supplied. Confidence will be gained with experience in fitting these products, bringing personal satisfaction and financial reward for those prepared to accept the challenge.
The presbyopic population is a growing sector of the marketplace and for those practitioners unprepared for the future demands there will always be others to whom the public can turn.

References
1 Morgan P and Efron N. Trends in UK contact lens prescribing 2004. optician, 2004; 227: 5950 16-17.
2 Population projections for the United Kingdom by the Government Actuary 2004.
3 Meyler J and Veys J. A new 'pupil-intelligent' design for presbyopic correction. optician, 1999; 217: 5687 18-23.
4 Bergenske. First Impressions of a New Disposable Bifocal. CL Spectrum, September 1998; 39-41.
5 Edwards K and Orsborn G. Correcting presbyopes with soft contact lenses - can we do better? optician, 2003; 226:5926 22-26.
6 Guillon M, Maissa C, Cooper P. Visual Performance of a multi-zone bifocal and a progressive multifocal contact lens. CLAO, 2002; 88-93.
7 Quesnel NM, Giasson CJ. On-eye dehydration of Proclear, Resolution 55G and Acuvue contact lenses. Contact lens and anterior eye, 2001; 24 : 88-93.
8 Jain S, Arora I, Azar DT. Success of monovision in presbyopes: review of the literature and potential application to refractive surgery. Survey of Ophthalmology, 1996; 40: 491-499.
9 Run Johannsdottir K and Stelmach L B. Monovision: a Review of the Scientific Literature. Optometry and Visual Science, September 2001; 78: 9 646-651
10 McGill E, Ames K, Erickson P and Robboy M. Quality of Vision With Hydrogel Simultaneous Vision Bifocal Contact Lenses. ICLC, December 1987; 14: 12 476-482.

<25C6> John Rogers is UK Professional Services Manager for CooperVision with particular responsibility for northern Europe. He is also a Practical Contact Lens Examiner for ABDO