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An overview of bifocal contact lenses
Journal of the British Contact Lens Association, Vol. 14, No. 2, pp 71-74, 1991 Printed in Great Britain
01991 British Contact Lens Association
AN OVERVIEW OF BIFOCAL CONTACT LENSES B r u c e J. W. E v a n s *
and Dorothy A. Thompson?
KEY WORDS:Bifocal contact lenses, presbyopia, review.
Introduction HE MEAN age of the population is increasing at a surprising rate. Between 1987 and 2001 the UK population is predicted to increase by 4%. 1 However, there will actually be a reduction in the number of pre-presbyopes, so that a major increase is anticipated in the 40-55 year-old age-group. Many of these people currently wear single-vision contact lenses, and many more are emmetropic at present. Neither of these groups are likely to be willing spectacle wearers, so that the demand for bifocal contact lenses should increase.
T
Modes of Correcting Presbyopia with Contact Lenses Monovision Monovision is the simplest method of correcting presbyopia with contact lenses, and is probably the most frequently used. One eye is given the distance correction and the other the near correction. It is assumed that for a given visual task the appropriate; clearer image will be selected by the patient's higher levels of visual processing, and the irrelevant blurred image will be suppressed. Any type of single-vision contact lens can be used, and these are fitted in the usual way.
Figure 1. Schematic illustration of the action of a translating bifocal contact lens, shown in bold outline (shaded area represents the pupil). In (a), the eye is in the primary position and the pupil views through the upper portion of the bifocal. On downward gaze, shown in (b), the contact lens is held in place by the lower lid and the pupil now views through the lower, near portion of the lens. * BSc, MBCO, DCLP t BSc, PhD, MBCO
Alternating Vision (Translating) Bifocals These lenses (Figure 1) in some respects resemble bifocal spectacle lenses; when the patient looks down the lower lid pushes the lens up so that the near segment, which is normally below the pupil, comes into effect. The lenses are prism ballasted to achieve orientation, and a variety of segment sizes and shapes are available. This design can work well with rigid materials, but it is very difficult to achieve enough movement for this correction mode to work with soft lenses. Simultaneous Vision Bifocals With this mode of correction each eye is simultaneously presented with images from the distance and near segments, and lens movement is not necessary to preferentially select one image. For a given visual task, these lenses rely on the brain's ability to automatically select the appropriate (clear) image and ignore the other. This principle can therefore be applied to rigid or soft materials, although lens movement can decrease the performance with rigid lenses. Simultaneous-vision lenses first became available in a concentric design with a distance powered centre surrounded by an annulus powered for near vision (Figure 2); the so-called centre-distance or CD lenses. These lenses are sensitive to pupil size, and near
Figure
2.
Schematic
representation
of a
concentric
simultaneous-vision bifocal: its 'construction is shown in bold outline (shaded area represents the pupil). Under dim illumination, the pupil dilates (a) and the image from the surrounding annulus is favoured. During near vision or under bright illumination, pupillary constriction (b) favours the central portion.
71
BRUCE J. W. EVANS AND DOROTHY A. THOMPSON
vision miosis can bias vision towards the inappropriinated 57% of them by careful screening. This is an ate distance centre, while mydriasis causes the important point, since careful screening can greatly anthithesis during night vision. To counter this disad- improve the success rate. They found that monovision vantage, some manufacturers made lenses with the was the most successful system, with 67% success, reverse construction; centre-near or CN lenses. This whilst the two simultaneous-vision systems each had design exploits near-vision miosis and is effective success rates of 40%. It should be noted that this under certain lighting conditions, but when reading study used a fixed and very limited range of parain poor light or driving in bright conditions, difficult- meters for their simultaneous-vision options, so this ies can still arise. A significant development of these 40% success rate is probably an underestimate. generic types of simultaneous vision-correction has One of Back et al. 's 4 most surprising findings was been the manufacture of aspheric forms, some of that disrupted stereopsis was not a significant reason which have a progressive power change between for failure with monovision. This point' was explored distance and maximum near power. : b y McGill and Erickson 5, who compared stereopsis Simultaneous-vision lenses that utilise diffractive during monovision with that when the same presbyoptics are now available in rigid or soft designs. The opes wore each of four different makes of distan.ce portion of such lenses extends over virtually simultaneous-vision lenses. They found that stereopthe whole lens diameter, including the central area. sis was no worse with monovision than with the The near focus is achieved by a diffractive grating bifocals. However, they suggested that this may be etched centrally on t h e b a c k surface of the lens. due to insufficient effective adds with the bifocals, a Although this etched area occupies a diameter of problem that some modern designs may have overcome. only 4-hmm, the manufacturers claim that visual performance with these lenses is essentially 'pupil There are two often-cited disadvantages of monoviindependent'. sion: stereopsis, discussed above, and monocular blur. The distance blur that occurs in the eye corrected Review of Comparative Studies for reading has attracted particular interest because There are many specific manufacturers' designs of its possible medico-legal implications for driving. within each of the generic categories described However, Collins et al. 6 found that monovision did above 2, and the practitioner has to decide which of not have a significant effect on peripheral visual these is most likely to be successful in a given case. acuity at eccentricities of 10 ° or more, and that there Information to help the practitioner to make this were no significant differences in peripheral vision decision is available in several forms. Obviously, manwith monovision, simultaneous vision, or hard bifocal ufacturers' literature is biased and case studies, • contact lenses. This is not surprising, since the resolvalthough interesting, can only really be described as ing ability of the retina drops off very rapidly as we anecdotal evidence. The best sources of information move away from the fovea onto the peripheral retina. are controlled, comparative clinical trials, preferably For example, 1.hD of blur would only be expected to ones that have been published in journals requiring effect the central 3-4 ° of the visual field, and even approval by expert referees. In the following review 3.00D of blur would not have a significant effect on of some of these studies, the lenses discussed are vision at eccentricities greater than about 10°.7 So hydrophilic unless otherwise stated. monovision does not mean monocularity, but merely Most workers have found monovision to be a blurs the central few degrees of one eye. surprising success. For example, Collins et al. 3 asked Since monovision still seems to be one of the most subjects to compare the visual performance of dissuccessful modes of correction, perhaps we should tance contact lenses and half-eye spectacles with briefly look at ways of optimising success with this simultaneous-vision spherical and aspherical bifocals, system. It is generally recommended that the dominmonovision and modified monovision. This last alterant eye is given the lens for distance correction. native is a single-vision distance lens in one eye and However, recent research on ocular dominance s-ll a simultaneous-vision bifocal biased for near vision in shows that the idea of one eye being dominant for the other. They found few significant differences, all tasks is a gross over-simplification, and sighting but monovision, modified monovision, and spherical tests of ocular dominance seem to be of limited practsimultaneous-vision bifocals were the preferred ical use.* A more relevant test for monovision might options. However, their 11 subjects only wore each be to fog each eye in turn when the patient is pair of lenses for two days, and no cross-over compari- binocularly viewing polarised letters, to see which son was made. eye's blurred image is most readily suppressed. Back et al. 4 compared monovision with two spheriStein 12 suggested the pragmatic technique of permitcal simultaneous-vision modes of wear. One of these ting the patient to wander around the room with a was a pair of conventional CN lenses, while the other ÷ 1.50D lens over each eye in turn. The eye that is option was a CN lens in one eye and a CD lens in the other eye (CNCD). This last mode of wear has * Since this paper was presented, an experimental study has shown that tests of sighting dominance correlated poorly with the binocular effect of being pupil independent. These an illumination-controlled blur suppression test, and neither researchers started off with 470 presbyopes, but elimwere useful for selecting the distance monovision eye.
72
OVERVIEW OF BIFOCAL CONTACT LENSES
most comfortable with this lens is then fitted with the near vision lens. This same principle could also be used to select which eye should wear each lens in a simultaneousvision CNCD lens combination. We felt this binocularly pupil-independent mode of correction needs more investigation. Although monovision seems to be surprisingly successful, the suspension of central binocular vision inevitably prevents it from being the ideal mode of correction. However, one problem with simultaneousVision lenses is that the brain is expected to suppress a blurred image which occurs in the same eye as the clear one; this problem is said to be minimised with diffractive bifocals. 13 Papas e t al. 14 compared a soft diffractive bifocal lens (Echelon) with monovision. This was a cross-over study involving 21 subjects and they found that the Echelon lens gave better stereopsis, although monovision was subjectively preferred for reading. They concluded that similar patient acceptance can be expected for both modalities. It is worth noting that they found relatively poor low-contrast acuity with the bifocal lens. This loss of contrast is an inevitable problem with simultaneous-vision lenses, which may produce difficulties for patients in low lighting levels, with low-contrast targets, or when prolonged critical vision is required. An alternative approach is used in lenses such as the PS45, CALS, and Unilens, which are designed to reduce the spherical aberration of the eye, hence allowing an increase in its depth of focus. There have been several anecdotal reports of success with these designs, but Young et al. 15 showed that the PS45 was, in fact, pupil-dependent and Charman and Saunders 16 showed that, with the PS45 and CALS designs, the effective addition varied with distance prescription. Saunders 17 compared the PS45 with monovision, Diffrax, and Alges. The PS45 was preferred by 40% and monovision by 60%, although the comparison of a rigid lens with soft lenses may not be fair on the Dfffrax. Spherical aberration lenses may be an important development, but the ability of these lenses to correct la{ge reading additions still needs to be demonstrated. So, if monovision suspends central binocular vision, and simultaneous vision reduces contrast, what can be done for the presbyope who wants sustainable good binocular vision at distance and near with good contrast sensitivity? Van Meter et al. 18 found that approximately 90% of patients fitted with one of two types of alternating rigid bifocals ha.d normal contrast sensitivity at distance and near. Their success rate after one year was 67% with a concentric design, and 93% with the tangent streak design. As always, patient selection was an important factor in optimising lens success; but it does seem that a translating lens can work very well for patients who are preadapted .to rigid lenses and whose lid shape and tension is appropriate. The tangent streak bifocal is
a recent design using a fluoropolymer material, and success rates of 82% and 70% have been reported with this lensl ~'2° Ames et al. 21 have looked at the factors that influence vision with rigid gas permeable alternating bifocals. They found that prism, segment height, and the fitting relationship can influence distance and near vision. The most useful predictors of visual performance were lens movement, post-blink segment positioning, and return time. Their findings illustrate the inescapable fact that this type of lens does demand a certain level of practitioner expertise. By comparison, most modern simultaneous lenses are easy to fit and come with a limited range of parameters. It has also been suggested that alternating vision lenses are better for patients who have small pupils. 22
Conclusions An unequivocal conclusion cannot be drawn from this review of the literature; however, general recommendations do emerge. For patients who do not demand critical or low-contrast vision and do not spend a lot of time in dim environments, one of the modern simultaneous-vision lenses (available in rigid or soft materials) may be the lens of first choice. Alternatively, and particularly if the patient is visually more demanding and can tolerate rigid lenses, the first option may be a rigid alternating-vision lens. If neither of these is satisfactory, then monovision is still a viable option, and it might be possible to test the success of this modality by a blur suppression test. Practitioner success can be improved by very careful patient selection (with particular attention to visual needs), the judicious use of trial lenses, and by involving patients in the decision-making process. Address for Correspondence Bruce J.W. Evans, Vision Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, United Kingdom.
REFERENCES
i Optical Services Audit Committee Report, Chapter 5: Future developments of optical services, p. 25 (1990). 2 Kerr, C. The Contact Lens Yearbook. 2nd edn, Medical & Scientific Publishing Ltd., Hythe (1990). 3 Collins, M.J., Brown, B., and Bowman, K.J. Short term responses to soft contact lens corrections for presbyopia. Clin. Experim. Optom., 72, 40-45 (1989). 4 Back, A.P., Holden, B.A., and Hine, N.A. Correction of presbyopia with contact lenses: comparative success rates with three systems. Optom. Vis. Sci., 66, 518-525 (1989). 5 McGill, E. and Erickson, P. Stereopsis in presbyopes wearing monovision and simultaneous vision bifocal contact lenses. Am. J. Optom. Physiol. Optics., 65, 619-626 (1988). 6 Collins, M.J., Brown, B., Verney, S.J., Makras, M., and Bowman, K.J. Peripheral visual acuity with monovision and other contact lens corrections for presbyopia. Optom. Vis. Sci., 66, 370-374 (1989). 7 Borish, I.M. Clinical Refraction, 3rd edn, The Professional Press, Inc., Chicago, pp 399-418 (1975). 8 Collinge, A.J. A possible physiological basis for ocular dominance. Br. J. Physiol. Optics, 33, 21-22 (1979).
73
BRUCE J.W. EVANS AND DOROTHY A. THOMPSON Moseley, D. Dominance, reading and spelling. Bull. Audiophonol. Ann. Sc. Univ. Franche-Comte., 4, 443-464 (1988). J0 Schor, C., Carson, M., Peterson, G., Suzuki, J., and Erickson, P. Effects of interocular blur suppression ability on monovision task performance. J. Am. Optom. Assoc., 60, 188-192 (1989). 1i Evans, B.J.W., and Drasdo, N. In preparation. 12 Stein, H.A. The management of presbyopia with contact lenses: a review. Contact Lens Assoc. Ophthalmol. J., 16, 33-38 (1990). 13 Allergan Optical. New directions in managing presbyopia with contact lenses. Continuing Education Forum, 11-12 (1989). 14 Papas, E., Young, G., and Hearn, K. Monovision vs. soft diffractive bifocal contact lenses: a crossover study. Inter: Contact Lens Clin., 17, 181-186 (1990). 1~ Young, G., Grey, C.P., and Papas, E.BI Simultaneous vision bifocal contact lenses: a comparative assessment of the in vitro • optical performance. Optom. Vis. Sci., 67, 339-345 (1990). 1~ Charman, W.N. and Saunders, B. Theoretical and practical factors influencing the optical performance of contact lenses for the presbyope. J. Br. Contact Lens Assoc., 13, 67-75 (1990).
17 Saunders, B.D. Optical performance of bifocal contact lenses in vivo. Trans. Br. Contact Lens Assoc., 6, 71-74 (1989). 18 Van Meter, W.S, Gussler, J.R., and Litteral, G. Clinical evaluation of 3 bifocal contact lenses. Contact Lens Assoc. Ophthalmol. J., 16, 203-207 (1990). 19 Remba, M.J. The tangent streak rigid gas permeable bifocal contact lens. J. Am. Optom. Assoc., 59, 212-216 (1988). :0 Josephson, J.E., Wong, M., and Caffery, B.E. Clinical experience with the tangent streak RGP bifocal contact lens. J. Am. Optom. Assoc., 60, 166-170 (1989). 2l Ames, K.S., Erickson, P., Godio, L., and Medici, L. Factors influencing vision with rigid gas permeable bifocals. Optom. Vis. Sc/., 66, 92-97 (1989). 2: Erickson, P., Robboy, M., Apollonio, A., and Jones, W.F. Optical design considerations for contact lens bifocals. J. Am. Optom. Assoc., 59, 198-205 (1988).
01991 British Contact Lens Association
AN OVERVIEW OF BIFOCAL CONTACT LENSES B r u c e J. W. E v a n s *
and Dorothy A. Thompson?
KEY WORDS:Bifocal contact lenses, presbyopia, review.
Introduction HE MEAN age of the population is increasing at a surprising rate. Between 1987 and 2001 the UK population is predicted to increase by 4%. 1 However, there will actually be a reduction in the number of pre-presbyopes, so that a major increase is anticipated in the 40-55 year-old age-group. Many of these people currently wear single-vision contact lenses, and many more are emmetropic at present. Neither of these groups are likely to be willing spectacle wearers, so that the demand for bifocal contact lenses should increase.
T
Modes of Correcting Presbyopia with Contact Lenses Monovision Monovision is the simplest method of correcting presbyopia with contact lenses, and is probably the most frequently used. One eye is given the distance correction and the other the near correction. It is assumed that for a given visual task the appropriate; clearer image will be selected by the patient's higher levels of visual processing, and the irrelevant blurred image will be suppressed. Any type of single-vision contact lens can be used, and these are fitted in the usual way.
Figure 1. Schematic illustration of the action of a translating bifocal contact lens, shown in bold outline (shaded area represents the pupil). In (a), the eye is in the primary position and the pupil views through the upper portion of the bifocal. On downward gaze, shown in (b), the contact lens is held in place by the lower lid and the pupil now views through the lower, near portion of the lens. * BSc, MBCO, DCLP t BSc, PhD, MBCO
Alternating Vision (Translating) Bifocals These lenses (Figure 1) in some respects resemble bifocal spectacle lenses; when the patient looks down the lower lid pushes the lens up so that the near segment, which is normally below the pupil, comes into effect. The lenses are prism ballasted to achieve orientation, and a variety of segment sizes and shapes are available. This design can work well with rigid materials, but it is very difficult to achieve enough movement for this correction mode to work with soft lenses. Simultaneous Vision Bifocals With this mode of correction each eye is simultaneously presented with images from the distance and near segments, and lens movement is not necessary to preferentially select one image. For a given visual task, these lenses rely on the brain's ability to automatically select the appropriate (clear) image and ignore the other. This principle can therefore be applied to rigid or soft materials, although lens movement can decrease the performance with rigid lenses. Simultaneous-vision lenses first became available in a concentric design with a distance powered centre surrounded by an annulus powered for near vision (Figure 2); the so-called centre-distance or CD lenses. These lenses are sensitive to pupil size, and near
Figure
2.
Schematic
representation
of a
concentric
simultaneous-vision bifocal: its 'construction is shown in bold outline (shaded area represents the pupil). Under dim illumination, the pupil dilates (a) and the image from the surrounding annulus is favoured. During near vision or under bright illumination, pupillary constriction (b) favours the central portion.
71
BRUCE J. W. EVANS AND DOROTHY A. THOMPSON
vision miosis can bias vision towards the inappropriinated 57% of them by careful screening. This is an ate distance centre, while mydriasis causes the important point, since careful screening can greatly anthithesis during night vision. To counter this disad- improve the success rate. They found that monovision vantage, some manufacturers made lenses with the was the most successful system, with 67% success, reverse construction; centre-near or CN lenses. This whilst the two simultaneous-vision systems each had design exploits near-vision miosis and is effective success rates of 40%. It should be noted that this under certain lighting conditions, but when reading study used a fixed and very limited range of parain poor light or driving in bright conditions, difficult- meters for their simultaneous-vision options, so this ies can still arise. A significant development of these 40% success rate is probably an underestimate. generic types of simultaneous vision-correction has One of Back et al. 's 4 most surprising findings was been the manufacture of aspheric forms, some of that disrupted stereopsis was not a significant reason which have a progressive power change between for failure with monovision. This point' was explored distance and maximum near power. : b y McGill and Erickson 5, who compared stereopsis Simultaneous-vision lenses that utilise diffractive during monovision with that when the same presbyoptics are now available in rigid or soft designs. The opes wore each of four different makes of distan.ce portion of such lenses extends over virtually simultaneous-vision lenses. They found that stereopthe whole lens diameter, including the central area. sis was no worse with monovision than with the The near focus is achieved by a diffractive grating bifocals. However, they suggested that this may be etched centrally on t h e b a c k surface of the lens. due to insufficient effective adds with the bifocals, a Although this etched area occupies a diameter of problem that some modern designs may have overcome. only 4-hmm, the manufacturers claim that visual performance with these lenses is essentially 'pupil There are two often-cited disadvantages of monoviindependent'. sion: stereopsis, discussed above, and monocular blur. The distance blur that occurs in the eye corrected Review of Comparative Studies for reading has attracted particular interest because There are many specific manufacturers' designs of its possible medico-legal implications for driving. within each of the generic categories described However, Collins et al. 6 found that monovision did above 2, and the practitioner has to decide which of not have a significant effect on peripheral visual these is most likely to be successful in a given case. acuity at eccentricities of 10 ° or more, and that there Information to help the practitioner to make this were no significant differences in peripheral vision decision is available in several forms. Obviously, manwith monovision, simultaneous vision, or hard bifocal ufacturers' literature is biased and case studies, • contact lenses. This is not surprising, since the resolvalthough interesting, can only really be described as ing ability of the retina drops off very rapidly as we anecdotal evidence. The best sources of information move away from the fovea onto the peripheral retina. are controlled, comparative clinical trials, preferably For example, 1.hD of blur would only be expected to ones that have been published in journals requiring effect the central 3-4 ° of the visual field, and even approval by expert referees. In the following review 3.00D of blur would not have a significant effect on of some of these studies, the lenses discussed are vision at eccentricities greater than about 10°.7 So hydrophilic unless otherwise stated. monovision does not mean monocularity, but merely Most workers have found monovision to be a blurs the central few degrees of one eye. surprising success. For example, Collins et al. 3 asked Since monovision still seems to be one of the most subjects to compare the visual performance of dissuccessful modes of correction, perhaps we should tance contact lenses and half-eye spectacles with briefly look at ways of optimising success with this simultaneous-vision spherical and aspherical bifocals, system. It is generally recommended that the dominmonovision and modified monovision. This last alterant eye is given the lens for distance correction. native is a single-vision distance lens in one eye and However, recent research on ocular dominance s-ll a simultaneous-vision bifocal biased for near vision in shows that the idea of one eye being dominant for the other. They found few significant differences, all tasks is a gross over-simplification, and sighting but monovision, modified monovision, and spherical tests of ocular dominance seem to be of limited practsimultaneous-vision bifocals were the preferred ical use.* A more relevant test for monovision might options. However, their 11 subjects only wore each be to fog each eye in turn when the patient is pair of lenses for two days, and no cross-over compari- binocularly viewing polarised letters, to see which son was made. eye's blurred image is most readily suppressed. Back et al. 4 compared monovision with two spheriStein 12 suggested the pragmatic technique of permitcal simultaneous-vision modes of wear. One of these ting the patient to wander around the room with a was a pair of conventional CN lenses, while the other ÷ 1.50D lens over each eye in turn. The eye that is option was a CN lens in one eye and a CD lens in the other eye (CNCD). This last mode of wear has * Since this paper was presented, an experimental study has shown that tests of sighting dominance correlated poorly with the binocular effect of being pupil independent. These an illumination-controlled blur suppression test, and neither researchers started off with 470 presbyopes, but elimwere useful for selecting the distance monovision eye.
72
OVERVIEW OF BIFOCAL CONTACT LENSES
most comfortable with this lens is then fitted with the near vision lens. This same principle could also be used to select which eye should wear each lens in a simultaneousvision CNCD lens combination. We felt this binocularly pupil-independent mode of correction needs more investigation. Although monovision seems to be surprisingly successful, the suspension of central binocular vision inevitably prevents it from being the ideal mode of correction. However, one problem with simultaneousVision lenses is that the brain is expected to suppress a blurred image which occurs in the same eye as the clear one; this problem is said to be minimised with diffractive bifocals. 13 Papas e t al. 14 compared a soft diffractive bifocal lens (Echelon) with monovision. This was a cross-over study involving 21 subjects and they found that the Echelon lens gave better stereopsis, although monovision was subjectively preferred for reading. They concluded that similar patient acceptance can be expected for both modalities. It is worth noting that they found relatively poor low-contrast acuity with the bifocal lens. This loss of contrast is an inevitable problem with simultaneous-vision lenses, which may produce difficulties for patients in low lighting levels, with low-contrast targets, or when prolonged critical vision is required. An alternative approach is used in lenses such as the PS45, CALS, and Unilens, which are designed to reduce the spherical aberration of the eye, hence allowing an increase in its depth of focus. There have been several anecdotal reports of success with these designs, but Young et al. 15 showed that the PS45 was, in fact, pupil-dependent and Charman and Saunders 16 showed that, with the PS45 and CALS designs, the effective addition varied with distance prescription. Saunders 17 compared the PS45 with monovision, Diffrax, and Alges. The PS45 was preferred by 40% and monovision by 60%, although the comparison of a rigid lens with soft lenses may not be fair on the Dfffrax. Spherical aberration lenses may be an important development, but the ability of these lenses to correct la{ge reading additions still needs to be demonstrated. So, if monovision suspends central binocular vision, and simultaneous vision reduces contrast, what can be done for the presbyope who wants sustainable good binocular vision at distance and near with good contrast sensitivity? Van Meter et al. 18 found that approximately 90% of patients fitted with one of two types of alternating rigid bifocals ha.d normal contrast sensitivity at distance and near. Their success rate after one year was 67% with a concentric design, and 93% with the tangent streak design. As always, patient selection was an important factor in optimising lens success; but it does seem that a translating lens can work very well for patients who are preadapted .to rigid lenses and whose lid shape and tension is appropriate. The tangent streak bifocal is
a recent design using a fluoropolymer material, and success rates of 82% and 70% have been reported with this lensl ~'2° Ames et al. 21 have looked at the factors that influence vision with rigid gas permeable alternating bifocals. They found that prism, segment height, and the fitting relationship can influence distance and near vision. The most useful predictors of visual performance were lens movement, post-blink segment positioning, and return time. Their findings illustrate the inescapable fact that this type of lens does demand a certain level of practitioner expertise. By comparison, most modern simultaneous lenses are easy to fit and come with a limited range of parameters. It has also been suggested that alternating vision lenses are better for patients who have small pupils. 22
Conclusions An unequivocal conclusion cannot be drawn from this review of the literature; however, general recommendations do emerge. For patients who do not demand critical or low-contrast vision and do not spend a lot of time in dim environments, one of the modern simultaneous-vision lenses (available in rigid or soft materials) may be the lens of first choice. Alternatively, and particularly if the patient is visually more demanding and can tolerate rigid lenses, the first option may be a rigid alternating-vision lens. If neither of these is satisfactory, then monovision is still a viable option, and it might be possible to test the success of this modality by a blur suppression test. Practitioner success can be improved by very careful patient selection (with particular attention to visual needs), the judicious use of trial lenses, and by involving patients in the decision-making process. Address for Correspondence Bruce J.W. Evans, Vision Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, United Kingdom.
REFERENCES
i Optical Services Audit Committee Report, Chapter 5: Future developments of optical services, p. 25 (1990). 2 Kerr, C. The Contact Lens Yearbook. 2nd edn, Medical & Scientific Publishing Ltd., Hythe (1990). 3 Collins, M.J., Brown, B., and Bowman, K.J. Short term responses to soft contact lens corrections for presbyopia. Clin. Experim. Optom., 72, 40-45 (1989). 4 Back, A.P., Holden, B.A., and Hine, N.A. Correction of presbyopia with contact lenses: comparative success rates with three systems. Optom. Vis. Sci., 66, 518-525 (1989). 5 McGill, E. and Erickson, P. Stereopsis in presbyopes wearing monovision and simultaneous vision bifocal contact lenses. Am. J. Optom. Physiol. Optics., 65, 619-626 (1988). 6 Collins, M.J., Brown, B., Verney, S.J., Makras, M., and Bowman, K.J. Peripheral visual acuity with monovision and other contact lens corrections for presbyopia. Optom. Vis. Sci., 66, 370-374 (1989). 7 Borish, I.M. Clinical Refraction, 3rd edn, The Professional Press, Inc., Chicago, pp 399-418 (1975). 8 Collinge, A.J. A possible physiological basis for ocular dominance. Br. J. Physiol. Optics, 33, 21-22 (1979).
73
BRUCE J.W. EVANS AND DOROTHY A. THOMPSON Moseley, D. Dominance, reading and spelling. Bull. Audiophonol. Ann. Sc. Univ. Franche-Comte., 4, 443-464 (1988). J0 Schor, C., Carson, M., Peterson, G., Suzuki, J., and Erickson, P. Effects of interocular blur suppression ability on monovision task performance. J. Am. Optom. Assoc., 60, 188-192 (1989). 1i Evans, B.J.W., and Drasdo, N. In preparation. 12 Stein, H.A. The management of presbyopia with contact lenses: a review. Contact Lens Assoc. Ophthalmol. J., 16, 33-38 (1990). 13 Allergan Optical. New directions in managing presbyopia with contact lenses. Continuing Education Forum, 11-12 (1989). 14 Papas, E., Young, G., and Hearn, K. Monovision vs. soft diffractive bifocal contact lenses: a crossover study. Inter: Contact Lens Clin., 17, 181-186 (1990). 1~ Young, G., Grey, C.P., and Papas, E.BI Simultaneous vision bifocal contact lenses: a comparative assessment of the in vitro • optical performance. Optom. Vis. Sci., 67, 339-345 (1990). 1~ Charman, W.N. and Saunders, B. Theoretical and practical factors influencing the optical performance of contact lenses for the presbyope. J. Br. Contact Lens Assoc., 13, 67-75 (1990).
17 Saunders, B.D. Optical performance of bifocal contact lenses in vivo. Trans. Br. Contact Lens Assoc., 6, 71-74 (1989). 18 Van Meter, W.S, Gussler, J.R., and Litteral, G. Clinical evaluation of 3 bifocal contact lenses. Contact Lens Assoc. Ophthalmol. J., 16, 203-207 (1990). 19 Remba, M.J. The tangent streak rigid gas permeable bifocal contact lens. J. Am. Optom. Assoc., 59, 212-216 (1988). :0 Josephson, J.E., Wong, M., and Caffery, B.E. Clinical experience with the tangent streak RGP bifocal contact lens. J. Am. Optom. Assoc., 60, 166-170 (1989). 2l Ames, K.S., Erickson, P., Godio, L., and Medici, L. Factors influencing vision with rigid gas permeable bifocals. Optom. Vis. Sc/., 66, 92-97 (1989). 2: Erickson, P., Robboy, M., Apollonio, A., and Jones, W.F. Optical design considerations for contact lens bifocals. J. Am. Optom. Assoc., 59, 198-205 (1988).