Presbyopia and

If we believe demographic studies, in 2002 there were almost 20 million people over the age of 50, approximately a third of the UK population.1
The over-50s have more disposable income due to mortgages that have been paid off, and children that have left home. The over-50s, unlike previous generations, lead a very active life, both in business and in leisure and look to their eye care practitioner for help with the frustration of their failing near sight.
When patients become presbyopic, they aspire to the freedom of not having to wear spectacles but, at the same time, being able to read clearly. This opens the opportunity for contact lenses, but unfortunately very few multifocal contact lenses are fitted. There has been a variety of studies looking at the usage of contact lenses in the over-50s, with a figure of 3 per cent of the whole contact lens market being given.2 A third of the population represents just 3 per cent of the contact lens market.
There are a multitude of reasons why so few over-50s elect to wear contact lenses, but without doubt one of the main reasons is a reluctance to fit multifocal contact lenses. Practitioners perceive RGP multifocals as complicated, difficult to fit, and expensive. In fact, the reverse applies to many of today's multifocal RGPs. They are no more difficult to fit than single-vision lenses and cost similar to a pair of varifocal spectacle lenses.

LENS DESIGN
One of the key factors when fitting the presbyope is to have some form of clinical strategy. As patients become more presbyopic, monovision should be considered first.
Most of our contact lens patients are myopic, undercorrection will improve reading. Unfortunately, there is a limit to how far you can undercorrect a myope, as reducing the prescription by more than 0.50D will mean they will lose half a line with the distance acuity. By their mid-40s, many patients' amplitude of accommodation is down to 3.00D or less. They complain of difficulty reading in poor lighting, and with very small text. It is at this point that many practitioners would consider monovision. This technique has been described extensively in the literature, and in the previous two articles accompanying this series, but monovision with RGP patients works better than with soft contact lenses, because RGP lenses correct the astigmatism thus giving enhanced visual acuity.
Monovision works well for most of our emerging presbyopes, provided their near vision tasks are not too small, but for those with greater demands at near, more than 0.50D may be required in the non-dominant eye. As the power before the non-dominant eye is increased, the distance vision is reduced and stereopsis becomes compromised. This may lead to difficulties (such as with driving3 or for commercial airline pilots4), and possible legal implications. Many patients are happy using three lenses, a pair of
lenses corrected for the distance, and
a third lens for use at work.
Once all the monovision possibilities have been exhausted, the practitioner and patient should consider multifocal RGPs. This may mean a multifocal lens is introduced to the non-dominant eye. With modern RGP multifocals the posterior surface is aspheric in design and fits the same as a single-vision lens. The patient therefore cannot physically tell the difference between their single-vision lens in the dominant eye, and multifocal lens in the non-dominant eye.
Once the patient moves beyond 50, their amplitude of accommodation is reduced further. This means that their symptoms of blurred vision at near become worse, and their ability to do near vision tasks uncorrected becomes almost impossible. At this point, multifocal RGPs are considered for both eyes.
Over the years there have been many different multifocal designs and, broadly speaking, they can be put into two groups; aspheric/concentric RGP multifocals and alternating RGP multifocals.

Alternating RGP bifocals
Alternating RGP bifocals are also called translating bifocals because, similar to spectacle lenses, there are two distinct areas, one for distance and one for reading.
So, as the patient is in downgaze, the lens rides up and the near segment covers part of the pupil, allowing the patient to read. When the patient looks straight ahead the lens re-centres, and they look through the distance portion. For these lenses to be effective they have prism ballast incorporated within them, and are often truncated.
There have been many alternating designs, each with its own advantages and disadvantages, but the two lenses that were most commonly fitted were the Tangent Streak (Fused Kontacts, Kansas City, Missouri, US), and the ST Fluoroperm bifocal (Paragon Vision Sciences).
Unfortunately, the latter is no longer commercially available. The Tangent Streak (Figure 1) is an executive bifocal with add powers from +0.75D to +3.50D. The position of the segment height is 1.3 mm from the lower lid to the visual axis. An important element of the tangent streak is its truncation component, as the position of the lower lid is critical in its assessment. If the lower lid rests below the limbus, (Figure 2) then use a 0.2mm truncation. When the lower lid is at a tangent or above the limbus then a 0.4mm should be used. Movement and centration with this lens is critical, as excessive rotation with the blink can cause the lens to misalign. Excessive rotation is often associated with a base curve radius that is too steep, while a lens that sits superiorly may require increased prism ballast to ensure it centres correctly.
Another innovative alternating multifocal is the Presbylite II RGP bifocal (Pro Cornea, Netherlands). This unique multifocal design has a triangular near segment, a spherical zone that encompasses 70 per cent of the front optics for distance needs, and a small aspheric area at the top of the triangle for intermediate tasks. The Presbylite II is a comfortable, thin lens design, that does not use truncation. The lens has monocentric optics, with all optical cuts on the front surface. The normal base curve is spherical, single cut, but can be made with a base toric. Any distance or near add power is available and the Boston XO material is recommended.

Aspheric/concentric RGP multifocals
Aspheric/concentric RGP multifocals have become far more popular over the past 10 years and the introduction of CNC (computer numeric controlled) lathes has enabled laboratories to manufacture lenses very accurately, in many cases with tolerances of just a few microns.
This has meant the discomfort often associated with alternating designs can be avoided. This group of lenses are fitted like a single-vision lens and, more importantly, feel to the patient like single-vision lenses, hence their popular use in modified monovision. The designs of the aspheric/concentric RGP multifocals are quite varied and can be broadly put into three groups; back-surface aspherics, front-surface aspherics, and concentric bifocals. Each design has its own merits and three commercially available lenses are described later in this article.
Back-surface aspherics rely on the lens to translate. The back surface of the lens is steeper than the corneal surface, and the the add power increases with eccentricity. The rate of flattening induces a progressive increase in plus power as the cornea/tear/contact lens relationship changes in downward gaze. As the near portion of the lens positions itself, the pressure of the fitting relationship flattens the central corneal configuration and creates a steep configuration inferiorly, where there is a clearance between the cornea and contact lens. Generally speaking, back-surface aspheric designs work very well but do result in some corneal moulding. This phenomenon has no effect on contact lens vision, but does cause spectacle blur. This is more prevalent when using low Dk materials, and this author advises the use of high-Dk materials, such as Boston XO, to minimise this effect.
Front-surface aspherics work in a different way to back-surface aspherics, but still rely on a degree of translation. The theory behind front surface aspherics is very contentious, with many believing they work on the principle of simultaneous vision. Simultaneous vision is where two images, a distance and near image, are placed on the retina and transmitted to the visual cortex. The brain then has to decipher which image to accept. Generally speaking, this works well, but is dependent upon many factors, such as pupil size, lens centration, and lens movement. The front-surface aspherics are designed to produce a power curve, which increases from the centre out towards the periphery. This means the majority of lenses are centre distance with the reading in the periphery. By controlling the spherical aberration within the power gradient, there is a smooth transition from distance to near, which is why laboratories tell us they have a 'varifocal effect'.
Concentric bifocals have now come of age due to the use of high Dk materials. These lenses are available in both centre-distance and centre-near designs, but the centre distance design is the more popular. A good example of a concentric bifocal is the SAM multifocal lens, described as an aplanatic concentric multifocal. This new technology gives precise control over spherical aberration, thus allowing greater optical clarity. Front-surface optics allow practitioner control over fitting profile, allowing for better intermediate correction making the lens truly multifocal. As with virtually all the RGP multifocals, the lens relies on translation on downward gaze to be totally effective at near.

PATIENT SELECTION
The literature is full of papers quoting different success rates with RGP multifocal lenses5-8 but careful and accurate patient selection is the key to successful RGP fitting, whether it be single vision or multifocal lenses.
Both patient and practitioner need to be aware of all possibilities to correct the patient's presbyopia, thus allowing the patient to make an informed choice. There are many considerations to be made when fitting somebody with RGP lenses.
The first and foremost is the 'physical fit'. Before the practitioner puts any lens on the eye he must assess the corneal shape. This can be done with either a keratometer, or some form of corneal mapping system - a big advantage of the latter is that it will indicate profile beyond the central 3mm (Figure 3). Indications for an RGP multifocal include:

Corneal astigmatism
Early presbyopes benefit from simply having their astigmatism corrected, and perhaps a small monovision correction in the non-dominant eye allows them to continue working at near.
Due to the rotational effects of toric soft lenses, their vision can be impaired and not as defined as an RGP. Soft toric lenses have become very popular over the past few years with estimates of 15 per cent of the total contact lens market. As these patients go through their 40s they become increasingly presbyopic with the associated symptoms. Monovision is often successful until they reach a reading addition of more than +1.5D, at which point monovision has been exhausted, and the patient requires a multifocal correction.
At the time of writing, there is only one soft multifocal toric on the market. This is a low water content, made-to-order lens, and gives, like all soft multifocals, compromised vision. This is an ideal opportunity for the practitioner to introduce the patient to RGP multifocals. In general, corneal astigmats, especially those with 1.00D or more, are suitable for RGP multifocal lenses.
Keratoconus
All keratoconics eventually become presbyopic. When the disease becomes more advanced the only way a keratoconic can achieve good acuity is with an RGP lens.

Post refractive surgery
As patients become presbyopic, they will benefit from an RGP the multifocal lens, as soft lenses will be of little use to them.

Corneal exhaustion syndrome
There are still a number of soft lens patients wearing low-Dk lenses that cause chronic hypoxic problems such as polymegathism, neovascularisation, and reduced wearing times. For many of these patients the only way they will be successful is by being refitted in a RGP lens, and eventually an RGB multifocal.

Visual demands
Visual demands often dictate the type of presbyopic correction the patient requires. The patient in a near vision occupation, such as a bookkeeper or accountant, will not tolerate compromised near vision. For these patients it is best to start with RGPs, and resort back to soft lenses if RGPs fail. Other patients, such as the retired or those requiring lenses just for leisure, can accept more freely the visual compromise that comes with soft multifocals.

Wearing modality
It is essential for a practitioner to recommend a suitable material and wearing schedule for a patient's particular demands. Some not only require long wearing times, but wish to sleep in the lenses. The Menicon Z lens has FDA approval for 30-day wear in the US. The lens is made in an RGP bifocal design, called the Menifocal Z, and was reported by Lakkis et al 9 who found that 46 per cent of patients could successfully wear their lenses continuously for 22 to 30 days throughout the trial.

Clinical data gathering
The following information is important before fitting RGP multifocal lenses:

<25C6> Refraction. Corneal astigmats, especially those with 1.00D or more, are successfully corrected with an RGP multifocal lens
<25C6> Assessment of the visual acuity is important. If the patient has an amblyopic eye, then obviously monovision cannot be considered. These patients are very difficult to fit successfully
<25C6> Full binocularity needs to be assessed. Patients without full binocularity (such as an alternating strabismus) but with good vision in each eye, do very well with monovision. In contrast to this, if the patient has a high phoria, with low fusional reserves, then monovision may not be readily accepted. Poor convergence at near can be another barrier to success with monovision
<25C6> The position of the lids should also be noted, particularly if the intention is to fit the patient with a prism ballasted alternating bifocal
<25C6> Measurement of pupil diameter in both scotopic and photopic conditions is of vital importance, especially when fitting the aspheric/concentric multifocals. If the pupil is too small then this distance vision will be fine, but the patient will have great difficulty reading, contrary, if the pupil is large then difficulty will be found with night driving. Patients will complain of a 'ghosting' with night driving, and may have to wear an old single-vision RGP in the dominant eye.

FITTING LENSES
It is important to be familiar with the manufacturer's fitting advice for any particular lens design. There now follow three examples to illustrate various fitting strategies.

Quasar Plus
This lens was one of the original RGP multifocals, or more accurately termed varifocals, and was developed by Tony Hough and it is now made exclusively by No 7 in a variety of different RGP materials.
Technically, this is a distance centre multifocal with a graduated annulus of near vision, where the progressive power is incorporated into the optics of the back surface of the lens. The aspheric design of the back optical zone is based upon the patient's degree of ametropia, required reading addition, and corneal topography. By incorporating the asphericity into the back surface, the profile of the tear film will be altered when the patient is in downward gaze (Figure 4). Without this the patient will not be able to see at near and this puts the Quasar Plus into the translating category of lenses. Once the lens had been developed it became apparent that the back-surface optics alone was not sufficient to create sufficient visual effect at near, so a front surface modification was made to boost the reading vision.
Clinically, the lens is fitted in a very similar manner to any single-vision RGP, though the fluorescein pattern seems quite different to conventional fitting RGPs (Figure 5 shows a correctly fitted Quasar Plus which looks like a steeply fitting lens). Unlike a single-vision lens, the movement of the lens is critical if the vision is to be stable. Excessive movement destabilises the vision, while a lens that has restricted movement and cannot properly translate will provide poor near vision. The secret is to get this lens moving enough to create a translating effect that allows the patients to read. It is the author's experience that adjustment should be made to the fit first, before attempting to change the power. When the practitioner is satisfied that the fit and movement of the lenses are correct, then power alterations can be made, but caution should be given to making the reading add too strong as this can result in transient distance blurring.
Finally, because of the back surface design of this lens, it can alter the corneal topography and induce spectacle blur. This problem is more common when using low-Dk RGP materials, and can be alleviated by the use of materials such as Boston XO. All the same, it is wise to caution the patient that the spectacle vision may be a little blurred immediately after taking out their contact lenses which may last for up to 30 to 40 minutes. For those patients who interchange between RGP multifocals and spectacles, Hough developed the Quasar Plus XV version, a very complex front surface geometry lens, thus alleviating the problem with the back surface.
Menifocal Z
Menifocal Z is a concentric bifocal. The optics that create the bifocal effect are found on the front surface, as shown in Figure 6.
The Menifocal Z has three distinct areas for the patient to see through and is very dependent on the lens translating fully. When the patient is looking in the distance it is imperative that the lens centres over the pupil, as any decentration will cause flare and visual disturbances. As the patient looks down to read, the lens translates approximately 2mm, thus allowing the patient to see through the near portion. In between the distance and near zones lies a transitional zone, which is a smooth transition from distance to near. As the power of the reading addition is increased the zone diameters alter. Using the fact that as we get older our pupils get smaller, Menicon made the distance zone smaller and the reading zone larger as the reading addition increases.
Accurate centration is the key to success with this lens, as once the lens decentres its visual performance is compromised. Broadly speaking, the way a lens centres is dependent on the back surface design. With the Menicon Z there is a large spherical back zone, with an aspheric periphery, so it fits similarly to a single-vision lens. A great deal of attention needs to be given to the position in primary gaze, and free, unimpeded, movement of the lens in downward gaze. It is almost impossible to assess this while the patient is on the slit lamp, so it is the author's opinion that the use of a Burton lamp is essential to get a true impression of how the lens positions and moves.
One of the key advantages of the Menifocal Z is the material, which is a polymer composed of siloxanylstylene and fluoromethacrylate. This innovative RGP material has a very high Dk value at 163, and at the present time is the only RGP lens licensed by the FDA for 30-day continuous wear. This gives the practitioner a great deal of flexibility, both with wearing modality and power (particularly high plus powers). For those astigmatic presbyopes (or those with a compromised cornea) seeking a continuous wear solution, then the Menifocal Z is the answer.

Profile Additions
This RGP multifocal is manufactured by David Thomas, and utilises the company's successful polynomial aspheric design for the back surface (as with the 'Profile' lens), and uses a sophisticated design that incorporates SAM (spherical aberration management) technology on the front.
This is the ideal introduction lens when the patient has exhausted the monovision option. It can be used in enhanced monovision, with the patient wearing their single vision lens in the dominant eye only, and the Profile Additions in the non-dominant eye. The lens is fitted in the normal way, with a little more attention to centration, as is common with all the RGP multifocals. Decentred lenses can cause flare and visual discomfort. The front surface is designed with increasing eccentricity from the centre to the periphery, so controlling spherical aberration and enhancing the vision. With CNC lathes, accurate surfaces can be produced with tolerances of just a few microns, allowing the laboratory the flexibility to manufacture a range of reading additions.
The limitation with these lenses is that, as the reading addition is increased, the central area for distance becomes smaller. This can create problems with night driving. As the pupils dilate in scotopic conditions, more light passes through the reading area than the distance and this can create a ghosting effect which can be troublesome (Figure 7). This can easily be resolved by advising patients to wear a single-vision lens in their dominant eye when driving at night, leaving their Profile Additions in their non-dominant eye for near tasks. The other advantage of this lens, and all other RGP multifocals, is the practitioner has a choice of materials. There is also the possibility of fitting toric multifocal RGP lenses. The back surface is worked like any other lens with a toroidal surface and the aspheric surface is on the front.

FOLLOW-UP REGIME
The lens care and follow-up regime is no different to any other RGP lens. Patients should be instructed of the importance of cleaning their lenses as surface deposits could cause the lens to dry, creating discomfort, redness and reduced wearing times. It should be remembered that the tear film in the over-50s differs from those who are younger.
As our patients age, their tear secretions decline due to a steady increase in the collagen content of the lacrimal gland, with a 50 per cent reduction in the tear secretions from the late teens to the early 50s. The required centering and movement of an RGP multifocal has been stressed throughout this paper, so any surface deposits that can alter this should be avoided. Patients should be advised from the very outset that their lenses should last 12 months, but no more than 24 months. So, just like they have become accustomed to replacing their spectacles every two years, similarly, they should accept the need to replace their RGP multifocals every two years.
When patients return for an aftercare visit it is inevitable that the reading addition may require increasing. With our spectacle-wearing patients the solution is quite straight forward; we just reorder the same lenses with a stronger reading addition. With RGP multifocals the solution is not so easy, and quite often the cautious practitioner will apply a modified monovision approach and increase the reading addition in the non-dominant eye. With some of the aspheric/concentric RGP multifocals the advantage of increasing the reading addition is lost by the visual compromise it creates. In this scenario, thought should be given to trying a translating lens.

DISCUSSION
The reason RGP multifocals are not used more commonly in everyday practice is multifactorial. As contact lens practice has become more commercial, patients are lured into the belief that soft disposable contact lenses, is all the profession has to offer. There are many instances when a patient wants to supplement spectacle wear with contact lenses in their leisure time, for these patients soft multifocal lenses work well. But it is part of our professional duty to advise patients on all aspects of contact lenses, including the possibility that they could wear RGP multifocals, thus allowing them to make an 'informed' choice. By denying them the opportunity to try RGP multifocals, denies them the chance to see crisply at both distance and near.
One of the main reasons soft multifocal lenses have not penetrated the contact lens market is due to the success of varifocal spectacle lenses. Many companies boast adaptation rates of 95 per cent with their varifocal spectacles, a major improvement in the last few years. These same patients understandably expect the same level of vision from their contact lenses. If they are not forewarned about the compromised vision that soft multifocal contact lenses give, then they will be disappointed.
The reason for the compromised vision with soft multifocal lenses is that they work on the principle of simultaneous vision, when both the distance and near images are focused on the retina. It is inevitable that there will be some degree of degradation of the retinal image, resulting in the patient complaining of compromised vision. In clinical practice, quite often, soft multifocal patients struggle to achieve 6/6 and N6 at near, but for many this is sufficient for leisure activities.
If a patient requires crisp vision, at both distance and near, then the lens needs to translate to allow the patient to see through a dedicated reading area. Currently, there are only a handful of translating soft bifocals, the best example being the Triton lens (Gelflex Lab). When you analyse the RGP multifocal lenses, they all translate to some degree, and because of this, give superior vision to their soft lens counterparts.

Acknowledgement
The author thanks Tony Hough for his contribution to this article.

References
1 www.Statistics.gov.uk
2 Morgan P. Taking the pulse of the UK contact lens market optician, June 2004 No 5954 Vol 227 26-27.
3 Wood J M et al. The effect of monovision contact lens wear on driving performance Clin Exp Optom, 1998; 81:3: 100-103.
4 www.airlinesafety.com/editorials/Delta554. htm The crash of Delta flight 554: Was monovision contact lenses use the cause?
5 Young G, Grey C, Papas E. Simultaneous vision bifocal contact lenses: a comparative assessment. Optom Vis Sci, 1990 ; 67, 339-345.
6 Barr JA. Bifocals, multifocals, monovision - what works today? Contact Lens Spectrum, 2003; 18(5); 41-45.
7 Hanson DW. Advanced multifocal fitting and management. Contact Lens Spectrum, 1999; 14(8)
8 Hanson DW. Multifocal contact lenses - the next generation. Contact Lens Spectrum, 2002; 17(11): 42-48.
9 Lakkis C. An investigation into the performance of the Menifocal Z during continuous wear. Poster American Academy of Optometry, Tampa 2004.

<25C6> Jonathan Walker works in private practice in Walsall and is conference scientific programme officer for the BCLA