34 Multifocal and monofocal intraocular lenses

Background

Current techniques of cataract surgery and IOL implantation allow accurate prediction of postoperative refraction such that there can now be a reasonable expectation of good uncorrected distance acuity. This has been driven partly by the change from cataract surgery using a large (10 mm) incision to small (3–4 mm) incision phacoemulsification surgery. Among other benefits, this change offers greater predictability of refractive outcomes, which is a necessary pre-requisite for good visual acuity without the need for glasses.1,2

Treatment options

Standard IOLs used have a fixed refractive power, so that the focal length is fixed (monofocal). This means that most patients will require a reading addition to their distance glasses.3 While the majority of people undergoing cataract surgery may be happy to use reading glasses, a proportion are likely to seek good unaided near vision as well as distance vision. The need for reading glasses for near vision is unlikely to be considered an important issue at present in developing countries, where the burden of blindness due to cataract is very high.

One approach to improve near visual acuity is to modify the IOL. An IOL may provide near and distance vision if both powers are present within the optical zone. This has been attempted using diffractive optics or with zones of differing refractive power. Both types of IOL divide light up to focus at two (bifocal) or more (multifocal) points, so that both near and distant objects may be focussed on the retina. Optical evaluation of multifocal IOLs indicates that a twoto threefold increase in the depth of field is achieved at the expense of a 50% reduction in the contrast of the retinal image.4

Question

Does the use of multifocal intraocular lenses offer benefits over the current standard treatment of monofocal intraocular lens implantation?

The evidence

A systematic review of randomised controlled trials of multifocal intraocular lenses was undertaken to determine whether their use offers benefits over the current standard treatment of monofocal intraocular lens implantation.5 Eight RCTs were identified for inclusion in the review,5–12 and the characteristics of these trials are summarised in Table 34.1.

Question

Does the use of multifocal intraocular lenses offer benefits over monofocal intraocular lenses for visual acuity (unaided and corrected)?

The evidence

Distance acuity was described in five trials as the proportion of participants achieving specified acuity levels (Figure 34.1). The proportion of participants achieving less than 6/6 unaided was not significantly different between multifocal and monofocal groups (Peto odds ratio 1·21, 95% CI 0·75−1·96). Three trials describe mean acuity rather than proportions (Figure 34.2). There was no evidence of any difference in acuity between multifocal and monofocal groups (standardised mean difference −0·03, 95% CI −0·24 −0·18).

Similarly, there was no difference between IOL types with respect to the proportion of participants achieving less than 6/6 best corrected visual acuity in seven studies (Peto odds ratio 1·43, 95% CI 0·99–2·09, Figure 34.3). There was no difference in mean best-corrected acuity in the three refractive IOL trials that reported this outcome

(standardised mean difference 0·19, 95% CI −0·03–0·40, Figure 34.4).

Because of significant heterogeneity no meta-analysis was conducted on the data for near visual acuity. The six trials that reported this outcome found that near vision tended to improve with the use of a multifocal IOL (Figure 34.5).

(Continued)

Outcomes

Intervention

Method quality* Participants

USA, Germany and Austria Foldable 3-piece silicone optic, Distance acuity

Jadad score 5

Randomised

.,al etJavitt

(Progress 3) or monofocal IOL Near acuity

Allocation concealment Gender not given

Single centre

.,al et

Double-masked trial of Allocation concealment Over 50 years of age or monofocal IOL implantation Near acuity

.,al et

Comment

There was no difference between multifocal and monofocal IOL groups with respect to distance visual acuity. Unaided near vision is critical to the assessment of multifocal efficacy, but was reported in a manner that makes comparison between trials difficult. It is not made

clear in most trials whether the reported print size read has been corrected for reading distance so as to allow a near acuity to be calculated. Only two trials explicitly report near acuity.7,13 A further problem is the use of Jaeger cards. These are not standardised between manufacturers, so that J3 from one trial cannot be

assumed to equal J3 from another. Despite these caveats, it is clear that unaided near acuity is improved by the use of multifocal IOLs. It is important to remember, however, that monofocal IOL near acuity can be restored by the use of reading glasses.

Question

Does the use of multifocal intraocular lenses offer benefits over monofocal intraocular lenses in terms of spectacle dependence?

The evidence

In all RCTs the majority of multifocal IOL participants still used spectacles for some tasks – usually for reading small print. Freedom from spectacles was, however, more frequent with multifocal than monofocal IOLs in seven trials

(Peto odds ratio for spectacle dependence 0·15, 95% CI 0·11–0·22, Figure 34.6).

Comment

Spectacle independence is more likely to be achieved with use of the multifocal IOL than monofocal IOLs.

However, in no trial did more than half of the participants achieve spectacle independence.

Question

Does the use of multifocal intraocular lenses offer benefits over monofocal intraocular lenses for in terms of depth of field?

The evidence

Four trials measured depth of field (the amount of defocus consistent with retention of useful acuity). Depth of field was improved with the multifocal IOL compared to the monofocal. One trial tested acuity with defocus from emmetropia, and reported the proportion achieving better than or equal to 6/12 acuity at each level of defocus. In the trial, 76% and 57% of multifocal and monofocal patients respectively achieved 6/12 with minus 1·25 D defocus, compared with 96% and 4% at minus 2·5 D defocus. Three trials measured the number of dioptres of defocus through which a 6/12 acuity was achieved, across a range of 3–6 dioptres plus or minus from emmetropia. The data were not presented in such a way as to be combined in a metaanalysis. However, each study reported increased depth of field with the multifocal IOL.

Question

Does the use of multifocal intraocular lenses offer benefits over monofocal intraocular lenses in terms of contrast sensitivity and glare?

The evidence

Contrast is the difference between the brightness of an image and its background divided by the total brightness of image plus background. Contrast sensitivity is the inverse of target contrast threshold. Six trials assessed contrast sensitivity (Table 34.3). All reported lower contrast sensitivity with the multifocal IOL, which is consistent with the expected optical effect of the lens.

Three trials objectively assessed glare using the Brightness Acuity Tester. The differences between lenses was not statistically significant. Seven trials reported subjective results and all describe an increased incidence of adverse visual phenomena with the multifocal IOL (Figure 34.7). In those trials that separated glare from haloes, the latter is the more frequently observed. The TyPE questionnaire quantifies the degree of bother from glare, haloes, and rings around lights on a scale of 0–4, where “not at all” scores 0, “a little bit” scores 1, “moderately” scores 2, “quite a bit” scores 3 and “extremely” scores 4. The mean scores (without glasses on) were 1·57 for the multifocal IOL and 0·43 for the monofocal.7 Median

scores of 1 (multifocal) and 0 (monofocal) were reported in the other study using the TyPE instrument.12

Comment

Contrast sensitivity was lower in participants with the multifocal IOL. The differences were smaller than would be expected, given the division of light between distance and near focus, which may result from post-receptoral visual processing. Whether the reduction in contrast sensitivity induced by the IOL would be clinically significant would depend on the contrast presented by the visual target and the contrast sensitivity of the patient’s retina. No significant differences between IOLs with respect to objective glare were reported. Subjective experience of adverse visual phenomena (glare/halo) was more likely with multifocal IOLS.

Question

Does the use of multifocal intraocular lenses offer benefits over monofocal intraocular lenses in terms of quality of life?

The evidence

The overall visual satisfaction results from the monofocal IOL control groups illustrate the high level of satisfaction with these

lenses. The non-validated multifocal subjective data are inconclusive. Using the TyPE instrument, one study found a small but statistically significant increase in overall visual satisfaction with the multifocal IOL7 (8·4/10 with the multifocal compared to 7·9/10 with the monofocal) and, as expected, a larger beneficial effect with respect to near vision (7·4/10 and 5·3/10). Another study found unaided overall visual satisfaction to be the same in the two IOL groups (median 8/10).12

Only El-Maghraby et al. (1992)6 did not report any subjective assessment. Of the other seven trials, only Javitt and Steinert (2000)7 and Leyland et al. (2002)12 used a validated instrument: the TyPE cataract questionnaire. The data could not be combined for meta-analysis, and are instead presented in Table 34.2.

Comment

Subjective outcomes are fundamental to the evaluation of multifocal IOLs but, like near vision, measurements were flawed in most of the trials. There was no consistent effect on visual satisfaction evident.

Adverse subjective visual phenomena, particularly haloes or rings around lights, were more prevalent and more troublesome in participants with the multifocal IOL. The lack of a consistent drop in patient satisfaction despite the prevalence of these phenomena could be interpreted as evidence that patients do not perceive them as severe.

Discussion

Six of the eight included trials involved participants who had surgery on only one eye.5,6,8–12 Unilateral trials allow measurement of uniocular outcomes such as visual acuity, but are of limited use when attempting to measure the effect of the multifocal intraocular lenses on quality of life, especially where the fellow eye has good vision.

Implications for practice

There is good evidence that use of multifocal intraocular lenses improves near vision without any adverse effect on distance acuity. Spectacle independence is considerably more likely with use of these intraocular lenses when compared to the standard practice of monofocal implantation.

Whether the improvement in unaided near vision and increased incidence of spectacle independence are sufficiently high to outweigh the loss of contrast sensitivity and the experience of haloes is a matter for an individual patient to decide. The final choice for a patient is likely to depend on his or her motivation to be free of spectacles, guided by realistic expectations as to the likelihood of achieving this aim and understanding of the compromises involved.

Implications for research

The optical and visual effects of multifocal intraocular lenses are now well known. Future research on these and similar intraocular lenses should use validated subjective outcome criteria and strive for clarity in reporting of objective outcomes, particularly with regard to near vision. The search for alternative strategies to achieve spectacle independence such as monovision and accommodating intraocular lenses should continue.

References

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10.Steinert RF, Post CT Jr, Brint SF et al. A prospective, randomized, double-masked comparison of a zonal-progressive multifocal

intraocular lens and a monofocal intraocular lens. Ophthalmology 1992;99:853–60.

11.Kamlesh M, Dadeya S, Kaushik S. Contrast sensitivity and depth of focus with aspheric multifocal versus conventional monofocal intraocular lens. Can J Ophthalmol 2001;36:197–201.

12.Leyland M, Langan L, Goolfee F, Lee N, Bloom P. Prospective randomised double-masked trial of bilateral multifocal, bifocal or monofocal intraocular lenses. Eye 2002;16:481–90.

13.Jadad AR, Moore RA, Carroll D. Assessing the quality of reports of randomized clinical trials: is blinding necessary? Control Clin Trials 1996;17:1–12.