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Keywords:

  • bifocal spectacle lens;
  • children;
  • myopia;
  • near work;
  • vergence

Abstract

  1. Top of page
  2. Abstract
  3. EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA
  4. RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS
  5. PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS
  6. ACCOMMODATION VERGENCE INTERACTIONS
  7. DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER
  8. VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS
  9. PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS
  10. CONCLUSION
  11. GRANTS AND FINANCIAL ASSISTANCE
  12. REFERENCES

Bifocal spectacle lenses have been used as a strategy to slow myopic progression in children since the 1950s and perhaps earlier. The reported success of this strategy varies greatly, as does the design of studies reporting the outcomes of their use—from earlier retrospective analysis of records to later prospective clinical trials. Collectively, published data support the suggestion that bifocal lenses inhibit myopic development in children but only by a small amount and only in a subset of children. Possible reasons for the greatly varying outcomes include a lack of individualism of the treatment and failure to take the vergence system into account. This review summarises the results of bifocal and multifocal studies, describes how accommodation, convergence and their interaction are linked to myopic development and details how a bifocal treatment that takes this into account may be devised. Also discussed is whether alterations to peripheral retinal blur contribute to bifocal lens effects.

Since the 1950s and perhaps earlier by Wick1 in 1947, bifocal spectacle lenses have been prescribed to children with the aim of inhibiting myopic progression. Collective analysis of published data shows that bifocal lenses inhibit myopic development but only by a small amount and not in all children. Standard bifocal lenses reduce the accommodative demand at near and increase the fusional vergence demand. Even though there is the suggestion that the act of convergence at near is also related to myopic development,2,3 this is not routinely taken into account when these lenses are prescribed. The lack of accounting for the state of the vergence system is a possible reason for bifocal spectacles’ lack of success.4

This review summarises the results of bifocal and multifocal studies in children, describes how accommodation, convergence and their interaction are linked to myopic development and details how a bifocal treatment that takes this into account may be devised. Whether alterations to peripheral retinal blur contribute to bifocal lens effects is also discussed. Excessive near work is a risk factor for myopic development,5 though it is a complex variable to examine and especially to quantify. Although its impact has recently been questioned,6 many studies show an association between near work and myopia, particularly for near work induced transient myopia,7,8 close working distances and prolonged durations of continuous reading.9 While there are many other theories on how close work could cause myopia, including increased negative spherical aberration,10–12 altered Stiles-Crawford functions,13,14 contrast adaptation,15 visual deprivation due to the unchanging nature of text16 and lack of outdoor activity,6 these are outside the scope of this particular review.

EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA

  1. Top of page
  2. Abstract
  3. EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA
  4. RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS
  5. PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS
  6. ACCOMMODATION VERGENCE INTERACTIONS
  7. DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER
  8. VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS
  9. PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS
  10. CONCLUSION
  11. GRANTS AND FINANCIAL ASSISTANCE
  12. REFERENCES

Existing literature contains numerous conflicting reports on the benefits of bifocals and multifocal spectacle lenses to control myopia. A statistically significant reduction in myopic progression has been reported by Miles,17 Roberts and Banford,18 Oakley and Young,19 Neetens and Evens,20 Goss,21 Leung and Brown22 (multifocal), Fulk, Cyert and Parker,23 Gwiazda and colleagues24 (multifocal), Hasebe and co-workers25 (multifocal), Yang and associates26 (multifocal), and Cheng and collaborators.27 In contrast, bifocals were shown to have no statistically significant effect in the studies by Mandell,28 Shotwell,29 Grosvenor and co-workers,30 Parssinen, Hemminki and Klemetti,31 Jensen,32 Fulk and Cyert,33 Shih and colleagues34 (multifocal) and Edwards and associates35 (multifocal). Given that several well-designed studies document a beneficial effect, the negative results of others may have arisen from procedural differences that masked or weakened a real but small positive effect.36 The outcomes of the retrospective and then prospective studies are evaluated separately in this review.

RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS

  1. Top of page
  2. Abstract
  3. EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA
  4. RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS
  5. PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS
  6. ACCOMMODATION VERGENCE INTERACTIONS
  7. DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER
  8. VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS
  9. PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS
  10. CONCLUSION
  11. GRANTS AND FINANCIAL ASSISTANCE
  12. REFERENCES

The earliest clinical studies analysed the records of practitioners who routinely prescribed bifocals for myopic children (Table 1). Due primarily to their retrospective nature, reliable, robust conclusions from these studies are not possible. These publications include confusing wearing times and assessment periods,18,21,28 lack of bifocal design details,18,20,21,28 lack of near addition information,17,18,20,28 poor patient selection criteria28 and potential measurement bias,18,28 yet they provide an argument that bifocals might control myopia in some children and point to the importance of the vergence system. For example, the study of Oakley and Young19 showed that bifocals had a strong control effect in myopic Caucasian children (reduced progression of 0.51 D per year). The authors attributed the success of the treatment to the high placement of the reading portion of the lens and also a high proportion of children with near esophoria. The greater treatment effect of bifocals in children with near esophoria was a finding later supported by Goss21 in a similarly retrospective study (control 0.22 D per year).

Table 1. Results of retrospective bifocal spectacle lens wear studies in myopic children
StudyAge (years) and locationNumberDuration (years) and outcome measureType and power of bifocalRate of myopic progression (D/year)
  1. SV = Single vision lenses, BF = Bifocal lenses, NSR = Non-cycloplegic subjective refraction, CSR = Cycloglegic subjective refraction, UM = Unmasked. a: Mean change in refraction was not reported but it was concluded that bifocals did not reduce progression of myopia more than would have occurred by chance alone. b: A cross-over study in which subjects wore single vision lenses for two years and then bifocal lenses for the following two years.

Mandell28SV = 17.1 BF = 14.3 CaliforniaSV = 116 BF = 59Checked at least twice before 30; NSR, UMNot knownNot calculateda
Miles17SV = 6–14b BF = 8–16b St LouisSV = 103 BF = 482; NSR and CSR, UM28 mm flat top, decentred for slight base-in effectSV = −0.75 BF = −0.35
Roberts and Banford18Examined at least twice before 17; New York StateSV = 396 BF = 85Checked at least twice before 17; NSR, UMType unknown, most adds +0.75 to +2.00 DSV = −0.41 BF = −0.31 Significant p < 0.02
Oakley and Young19SV = 6–17 BF = 6–17 OregonSV = 275 BF = 2693–4; NSR and CSR, UMFlat top with top at pupil, +1.50 to +2.00 D addCaucasian SV = −0.53 BF = −0.02 Significant p < 0.001 Native American SV = −0.38 BF = −0.10 Significant p < 0.05
Neetens and Evans20SV = 8–9 BF = 8–9 HollandSV = 733 BF = 5439–10; NSR, UMMyopia up to 3 D, total near point power equal to 0; Myopia ≥ 3 D, +2.50D addSV = −0.45 BF = −0.30 Significant p < 0.001
Goss21SV = 6–15 BF = 6–15 Illinois, Iowa and OklahomaSV = 52 BF = 60Checked four times or more between 6–15; NSR, UMType unknown, most adds +0.75 and +1.00 DOrtho or Exo SV = −0.44 BF = −0.45 Not significant Eso SV = −0.54 BF = −0.32 Significant p < 0.05

PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS

  1. Top of page
  2. Abstract
  3. EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA
  4. RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS
  5. PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS
  6. ACCOMMODATION VERGENCE INTERACTIONS
  7. DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER
  8. VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS
  9. PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS
  10. CONCLUSION
  11. GRANTS AND FINANCIAL ASSISTANCE
  12. REFERENCES

Published prospective clinical studies have shown that bifocal and multifocal lenses are not particularly effective at controlling myopic progression in children (Figure 1 and Table 2). In Figure 1, it is apparent that only the studies of Leung and Brown22 and Cheng and collaborators27 show an overall treatment effect of greater than 0.25 D per year. In the work of Leung and Brown,22 the primary outcome measure was non-cycloplegic subjective refraction by an unmasked examiner—this has the potential to inadvertently bias the study outcomes. Unfortunately, there are also reports (Grosvenor and co-authors30 and Parssinen, Hemminki and Klemetti31) that bifocals accelerate the rate of myopic progression, though this detrimental effect is not statistically significant.

image

Figure 1. The magnitude of myopic reduction (D per year) with bifocals and multifocals of different addition-lens power. The studies marked with * show a significant effect (p < 0.05).

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Table 2. Results of prospective bifocal and multifocal spectacle lens wear studies in myopic children
BIFOCAL LENSES
StudyAge (years) and locationNumberDuration (years) and outcome measureType and power of bifocalRate of myopic progression (D/year)
Shotwell2917–21 United States Naval Academy, MarylandSV = 21 PP = 27 BF = 134; CSR, SM+1.25 D single vision with 2Δ base-in; 25 mm flat top, 3 mm above lower lid, +1.50 D addSV = -0.06 PP = -0.07 BF = -0.04 Not significant
Grosvenor et al306–15 HoustonSV = 39 +1.00 DBF = 41 +2.00 D BF = 443; NSR, SMExecutive, 2 mm below pupil centre, +1.00 D and +2.00D addSV = -0.34 +1.00D BF = -0.36 +2.00D BF = -0.34 Not significant
Parssinen, Hemminki and Klemetti319–13 FinlandFull-time SV = 79 Distance SV = 79 BF = 792–5; CSR, ISM28 mm flat top, 2–3 mm below pupil centre, +1.75 D addFulltime SV = -0.49 Distance SV = -0.63 BF = -0.53 Not significant
Fulk and Cyert33Esophoric children Male = 6–14 Female = 6–13 OklahomaSV = 14 BF = 141.5; CAR and AL, SM28 mm flat top, 1 mm above limbus, +1.25 D addSV = -0.57 BF = -0.39 Not significant
Fulk, Cyert and Parker23Esophoric children Male = 6–13 Female = 6–12 OklahomaSV = 40 BF = 422.5; CAR and AL, SM28 mm flat top, 1 mm above limbus, +1.50 D addSV = -0.50 BF = -0.40 Adjusted for age, significant p = 0.046
Cheng et al278–13 CanadaSV = 41 BF = 48 PBF = 482; CAR and AL, UMExecutive, 2 mm above lower limbus, +1.50 D add, +1.50 D and 3Δ base-inSV = -0.78 BF = -0.48 PBF = -0.35 Significant p < 0.001
BIFOCAL /MULTIFOCAL LENSES AND DRUG TREATMENT/ LENS COMBINATION
StudyAge (years) and locationNumberDuration (years) and outcome measureType and power of bifocal, drug treatmentRate of myopic progression (D/year)
Schwartz3825 monozygotic twin pairs, one in each group; 7–13 Washington, DCSV = 25 tBF = 253.5; CSR, SMType not known, +1.25D add, tropicamideSV = -0.27 tBF = -0.24 Not significant
Jensen32Children in 2nd through 5th grades DenmarkSV = 49 BF = 51 TBF = 592; CAR and AL, UM35 mm flat top, lower pupil margin, +2.00D add, timololSV = -0.57 BF = -0.48 TBF = -0.59 Not significant
Shih et al346–13 Taipei, TaiwanSV = 61 MF = 61 AMF = 661.5; CAR and AL, DMProgressive (multifocal), power unknown, atropineSV = -0.93 MF = -0.79 Not significant AMF = -0.27 Significant p < 0.001
MULTIFOCAL LENSES
StudyAge (years) and locationNumberDuration (years) and outcome measureType and power of bifocalRate of myopia progression (D/year)
  1. SV = Single vision lenses, BF = Bifocal lenses, MF = Multifocal lenses, PP = +1.25 D single vision spectacles with 2Δ base-in each eye, PBF = Prismatic bifocal lenses, tBF = Tropicamide and bifocal lenses, TBF = Timolol and bifocal lenses, AMF = Atropine and multifocal lenses, CSR = Cycloglegic subjective refraction, NSR = Non-cycloplegic subjective refraction, CAR = Cycloglegic automated refraction, AL = Axial length, UM = Unmasked, SM = Single masked, ISM = Incomplete single masked, DM = Double masked.

Leung and Brown229–12 Hong KongSV = 32 +1.50D MF = 22 +2.00D MF = 142; NSR and AL, UMProgressive, 1 mm above pupil centre, +1.50 and +2.00 D addSV = -0.62 +1.50D MF = -0.38 Significant p < 0.001 +2.00D MF = -0.33 Significant p < 0.001
Edwards et al357–10.5 Hong KongSV = 133 MF = 1212; CAR and AL, DMProgressive, on pupil centre, +1.50 D addSV = -0.63 MF = -0.56 Not significant
Gwiazda et al246–11 Boston, Philadelphia, Birmingham, HoustonSV = 234 MF = 2353; CAR and AL, DMProgressive, top of channel 4 mm above pupil centre, +2.00 D addSV = -0.49 MF = -0.43 Significant p = 0.004
Hasebe et al256–12 Two-stage cross-over trial, JapanSV = 46 MF = 461.5/1.5; CAR and AL, DMProgressive, on pupil centre and 3 mm above pupil centre, +1.50 D addFirst 1.5 yr SV = -0.80 MF = -0.59 Second 1.5 year SV = -0.61 MF = -0.63 3 year analysis, significant p < 0.001
Yang et al267–13 ChinaSV = 75 MF = 742; CAR and AL, DMProgressive, on pupil centre, +1.50 D addSV = -0.75 MF = -0.62 Significant p = 0.01

Collectively, the studies reporting a benefit show either a small overall inhibitory effect22–27 or that bifocal lenses are only effective in subsets of children. This includes myopic children with a high lag of accommodation24,25,27,37 (Table 3) and those with a high rate of myopic progression.22,27 The greater treatment effect of bifocals in esophoric children, reported in a number of retrospective studies,19,21 has been confirmed in very few prospective trials23,25,26 (Tables 2 and 3). The greater beneficial effect observed in children with fast myopic progression versus those with lower annual progression rates could reflect the fact that children with low annual progression are close to the end of the myopic developmental process. This means that myopic progression would be slow in all groups, even those prescribed single vision lenses and thus, any treatment effect would be impossible to observe. Why bifocal spectacles are more effective in children with high lags of accommodation is unknown. It is possible that the near addition has a greater effect on retinal defocus when the lag of accommodation is high (and thus the growth promoting effect of the hyperopic focus error greater). Alternatively, in children with low lags of accommodation, the near addition could create a lead of accommodation instead of a reduced lag, especially if the power of the addition is too high. The resultant defocus would vary in type and amount, making the refractive response unpredictable.

Table 3. Results of prospective studies showing bifocals/multifocals have stronger treatment effect for children with greater lag of accommodation and/or near esophoria
StudyAge (years) and locationNumberDuration (yr) and outcome measureType and power of bifocalRate of myopia progression (D/year)
Gwiazda et al376–11 Reanalysis of Gwiazda et al37 data from Boston, Philadelphia, Birmingham, HoustonSV = 234 MF = 2353; CAR and AL, DMProgressive, top of channel 4 mm above pupil centre, +2.00 D addLag of Acc. (≥0.43 D) (n = 119) SV = -0.53 (n = 115) MF = -0.42 Significant p < 0.05 Lag of Acc. (≥0.43 D) with eso (≥2Δ) (n = 34) SV = -0.57 (n = 42) MF = -0.36 Significant p < 0.05
Hasebe et al256–12 Two-stage cross-over trial JapanSV = 46 MF = 461.5/1.5; CAR and AL, DMProgressive, on pupil centre and 3 mm above pupil centre, +1.50 D addOnly in first 1.5 year Lag of Acc. (≥1.8 D) (n = 18) SV = -0.99 (n = 18) MF = -0.58 Significant p < 0.05 Esophoria (≥4Δ) (n = 24) SV = -0.88 (n = 20) MF = -0.51 Significant p < 0.05
Yang et al267–13 ChinaSV = 75 MF = 742; CAR and AL, DMProgressive, on pupil centre, +1.50 D addEsophoria (≥2Δ) (n = 22) SV = -0.83 (n = 13) MF = -0.44 Significant p < 0.01
Cheng et al278–13 CanadaSV = 41 BF = 48 PBF = 482; CAR and AL, UMExecutive, 2 mm above lower limbus, +1.50 D add, +1.50 D and 3Δ base-inLag of Acc. (≥1.01 D) (n = 20) SV = -0.88 (n = 23) BF = -0.44 (n = 24) PBF = -0.42 Significant p < 0.001 Lag of Acc. (≤1.01 D) (n = 21) SV = -0.68 (n = 22) PBF = -0.29 Significant p < 0.001

Ocular drugs, such as tropicamide,38 timolol32 and atropine,34 have been prescribed, theoretically, to enhance the bifocal/multifocal lens treatment effect. Of the drugs studied, only atropine combined with multifocal lenses show a significant myopic control effect (control 0.66 D per year). Whether the control of myopia was due solely to the atropine or to atropine's interaction with the multifocal lenses is uncertain. When prescribed without multifocal lenses, atropine has also been shown to control myopic progression (by 0.70 D per year).39 Due to the ocular side-effects experienced when using atropine, atropine has not become mainstream treatment for myopic progression in children.40

ACCOMMODATION VERGENCE INTERACTIONS

  1. Top of page
  2. Abstract
  3. EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA
  4. RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS
  5. PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS
  6. ACCOMMODATION VERGENCE INTERACTIONS
  7. DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER
  8. VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS
  9. PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS
  10. CONCLUSION
  11. GRANTS AND FINANCIAL ASSISTANCE
  12. REFERENCES

Accommodation and convergence are elements of the oculomotor near response mechanism (with the near triad being accommodation, convergence and pupillary constriction). These responses contribute to the production of a clear and single image at near under normal binocular viewing conditions. The closer the target, the greater will be the accommodative and convergence demand. For that reason, it has been postulated that the increased amounts of accommodation and convergence that occur at near are linked to the development of myopia.2 Further to this, myopia is suggested as an adaptation to a mismatch of the accommodative and vergence systems on the near visual plane.36,41 The importance of establishing a balance between the accommodative and vergence systems for an effective bifocal lens treatment for myopia has been advocated by Hung and Ciuffreda.4

Why measure the lag of accommodation with a near addition?

In the majority of published prospective studies for the control of myopia using bifocals and multifocals, the power of the prescribed near addition is usually the same for all children. The power chosen was based on past experience and beliefs of the researchers conducting the study or for practical reasons. For example, all children may receive a +1.00 D,30+1.25 D,33+1.50 D,22,23,25–27,29,35+1.75 D31 or +2.00 D addition,22,24,30,32 with the same addition power prescribed to all children in a particular study. This approach was often carried out in the belief that the addition would always help to improve the near focus and that the data analysis would be simplified, if only one addition power was used.

In fact, not only does the magnitude of the lag of accommodation show large variability among individuals, the effect of the near addition on the accommodative error may be quite different, even for individuals in whom the magnitude of the accommodative lag is the same. Some individuals even over-accommodate at near, that is, they have a lead of accommodation and the near addition makes the lead even greater.42–44 Individual variations in measured lag of accommodation for a near target at 33 cm range from 0.10 D (young adults)45 to more than 1.00 D (young adults,46 children24,25,46–48 and young adults and children49). In addition, the optimal lens power to minimise these errors varies across subject groups. For example, for a 33 cm target, optimal lens powers of +1.00 D43 and +1.28 D50 for myopic adults and +2.57 D51 for myopic children have been reported. In emmetropic adults, +2.00 D lenses were reported in one study as creating the least amount of accommodative error in young emmetropic adults,46 whereas later studies reported significant leads of accommodation with +2.00 D lenses.43,50,52 Collectively, the results of these studies clearly show that the same near-addition power would not be optimal for all individuals. Therefore, if positive lenses are to be prescribed to minimise the accommodative lag in myopic children, it is important to monitor the actual accommodation of the children wearing the lenses.

Why measure the phoria with a near addition?

In addition to reducing the accommodative demand and thus the accommodative response at near, positive lenses will decrease accommodative convergence due to the accommodation-convergence linkage. This would result in the phoria shifting to a more divergent position (that is, relatively more exophoria would be measured).44,51 Therefore, the near addition not only reduces the accommodative lag, it also creates a higher demand on positive fusional vergence, especially for individuals with near exophoria.44,51 In contrast, positive lenses would decrease both the lag of accommodation and the measured esophoria in esophoric individuals, reducing the demand on negative fusional vergence and enhancing the equilibrium between accommodation and vergence. This suggests that in terms of the vergence system, positive lenses at near would benefit esophoric myopic children more.44

Retrospective21 and prospective23,25,26 studies have shown that incorporating a near addition is more effective in reducing the rate of myopic progression in individuals demonstrating baseline esophoria at near, through the distance correction, compared with those that are exophoric at near under the same circumstances. In other studies, the baseline near phoria alone was not a strong determinant of the success of bifocal treatment.24,27,35,53 Though children with esophoria are expected to benefit more from bifocal wear, it is important to point out that a large proportion of the esophoric children will have lens-induced exophoria with the bifocals in place, because the AC/A ratio is usually high in this subset of children.54 This could explain why esophoric children wearing bifocals do not reliably have a significantly reduced rate of myopic progression. Therefore, in prescribing bifocals for the control of myopia, it is the state of lens-induced near phoria instead of baseline near phoria that plays a role in determining success because it is this uncontrolled phoria that causes oculomotor imbalance.

DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER

  1. Top of page
  2. Abstract
  3. EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA
  4. RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS
  5. PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS
  6. ACCOMMODATION VERGENCE INTERACTIONS
  7. DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER
  8. VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS
  9. PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS
  10. CONCLUSION
  11. GRANTS AND FINANCIAL ASSISTANCE
  12. REFERENCES

To determine the optimal near-addition lens power, a direct method is to measure and plot the accommodative error as a function of near-addition lens power. The power that produces zero accommodative error is the optimal lens power.42,50 Using this method, +1.28 D was found to be optimal for a group of young adult subjects for a viewing distance of 33 cm.50 Another method employed is to correlate the initial accommodative error of the subjects with the optimal lens power determined from the aforementioned analysis. By fitting a linear regression line on the correlation plot, a regression equation is used to predict the optimal lens power from the accommodative error of a given individual. For an accommodative error of 0.50 D, the predicted optimal lens power is about +1.50 D,42,50 however the robustness of this method is questionable as the r-value of the correlation is only 0.7.42,50 Furthermore, this type of correlation has shown to be particularly weak for myopic children with an r-value of only 0.3.51

Published data suggest (although this has not been directly tested) that the subgroup of children who achieve the best oculomotor equilibrium, as advocated by the oculomotor interactive theory,4 would benefit the most from the near addition. It has been established that when the positive-lens addition is strong enough to produce zero accommodative error, a large exophoria is induced in the majority of children.44,51 Jiang and associates50 adopted Morgan's norm of 3Δ exophoria at near as the criterion to determine the optimal addition power for oculomotor balance. The authors plotted the near phoria data of the subjects as a function of lens power and evaluated the lens power to produce 3Δ exophoria; they considered this the optimal oculomotor lens power. The mean optimal lens power based on this approach is +0.20 D for a viewing distance of 33 cm, greatly different from the power determined from zero accommodative error. In fact, +0.20 D would have only a very marginal beneficial effect on the lag.

It is apparent that the antagonistic relationship of the lens power on accommodative error and lens-induced exophoria would complicate the optimal lens power selection. To avoid such uncertainty, Cheng, Schmid and Woo51 suggested coupling the accommodative errors and near phoria using the accommodation and vergence interaction (that is, the AC/A ratio) and plotted them as functions of lens power (Figure 2). The significance of this plot is that the point where the accommodation and phoria lines intersect is the lens power at which the lag and phoria are simultaneously closest to zero.51 From the intersection point, the optimal lens power is found to be +1.13 D for a group of children viewing a near target at 33 cm. Myopic children often have greater lags which require higher lens powers to minimise the accommodative error. For these children, Cheng, Schmid and Woo51 suggest the use of base-in prism to reduce the lens-induced exophoria. In Figure 2, the incorporation of 3Δ base-in prism in the near segment (total 6Δ base-in) will allow the use of higher addition powers, up to +2.25 D, to reduce the accommodative error. The benefit of incorporating base-in prism in the near segment has been demonstrated in a recent clinical trial.27

image

Figure 2. Lag of accommodation (D) and phoria (Δ) interaction. The points of significance are where the accommodation lines and phoria lines intersected (marked with arrow), indicating the optimal positive-addition and prism power. (Adapted from Cheng, Schmid and Woo51)

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VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS

  1. Top of page
  2. Abstract
  3. EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA
  4. RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS
  5. PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS
  6. ACCOMMODATION VERGENCE INTERACTIONS
  7. DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER
  8. VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS
  9. PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS
  10. CONCLUSION
  11. GRANTS AND FINANCIAL ASSISTANCE
  12. REFERENCES

Unlike presbyopes, children have a full accommodative range that allows them to see clearly through both the distance and near portions of bifocal and multifocal lenses. If myopic children in the clinical trial used the distance instead of the near portion of the lens for reading, the treatment effect of the positive lenses would be reduced. This treatment shortcoming is more likely with multifocal lenses, as children receive no feedback on which part of the multifocal lenses they are using and therefore may inadvertently use the distance portion. Even when given a specific instruction such as ‘look down’ or ‘use the lower part of the glasses’ for reading, children may still use other portions of the multifocal lenses if they experience no difference in their vision. This problem is worsened if the spectacles slip downward as frequently occurs in children wearing glasses. The study of Hasebe and colleagues55 confirmed that children who wear multifocal lenses do not consistently use the near-addition portion of the spectacles during reading, which means that their lag of accommodation is not controlled for a large amount of the time. In contrast to multifocal lenses, the segment line of bifocals provides feedback to the children, enabling them to deliberately choose the reading portion for near work. Two factors against their use are the poor cosmetic appearance and the hesitant attitude of some parents and children towards a lens that is usually prescribed for presbyopic adults. These concerns should be carefully considered in a decision of whether multifocal or bifocal spectacles are to be prescribed for the purpose of controlling myopia.

PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS

  1. Top of page
  2. Abstract
  3. EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA
  4. RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS
  5. PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS
  6. ACCOMMODATION VERGENCE INTERACTIONS
  7. DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER
  8. VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS
  9. PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS
  10. CONCLUSION
  11. GRANTS AND FINANCIAL ASSISTANCE
  12. REFERENCES

There is mounting evidence for the importance of the peripheral retina for the control of eye growth and the development of myopia. This evidence includes the finding that children with hyperopic defocus on the retinal peripheries are more likely to become myopic56 and that monkeys fitted with lenses that blur only the retinal periphery develop myopia.57 In addition, this has been postulated as the potential reason concentric bifocal contact lenses58 and orthokeratology59 inhibit myopic progression. In these cases, the myopic defocus incident on the retinal periphery may limit the myopic ocular elongation. Nevertheless, there have been studies showing that under-correction of the distance refractive error of myopic children with spectacles, which produces myopic defocus for both central and peripheral retina, does not lower the rate of myopic progression.60,61 The lack of beneficial effect of under-correction may be a consequence of the central peripheral blur or the relatively low degree of myopic defocus in the periphery (under-correction was limited to 0.75 D).

Thus, another possible reason for a potentially superior myopic inhibitory effect of bifocals (particularly for the executive design, where the entire lower half of the lens has the near addition) over multifocal lenses relates to the lens effects on the retinal periphery. Relatively speaking, a greater portion of the periphery (typically superior retina) will experience myopic defocus in the case of an executive design bifocal versus a D-segment bifocal versus a multifocal lens design. This peripheral myopic defocus may be inconsequential or fundamental to the beneficial effects observed. In terms of bifocal spectacle lens effects, this theory remains untested. A qualitative assessment of this effect for previous bifocal and multifocal spectacle studies was considered but could not proceed due to variations in near-addition power and the vastly differing rates of myopic progression in single vision lens groups across studies (Tables 1, 2 and 3).

CONCLUSION

  1. Top of page
  2. Abstract
  3. EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA
  4. RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS
  5. PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS
  6. ACCOMMODATION VERGENCE INTERACTIONS
  7. DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER
  8. VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS
  9. PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS
  10. CONCLUSION
  11. GRANTS AND FINANCIAL ASSISTANCE
  12. REFERENCES

Bifocal and multifocal lenses can limit myopic progression in children with fast progression, near esophoria and/or high lags of accommodation. By considering the binocular vision characteristics of myopic children and adopting a systematic approach to determine the lens/prism combination that produces optimal focus at near, a bifocal/multifocal lens design that has the best chance of limiting progression can be selected. Although such lenses are not yet commercially available to achieve this aim, technical advances mean they could be in the near future.

REFERENCES

  1. Top of page
  2. Abstract
  3. EFFECT OF BIFOCAL AND MULTIFOCAL SPECTACLE LENSES ON MYOPIA
  4. RETROSPECTIVE ANALYSIS OF PRIVATE PRACTICE RECORDS
  5. PROSPECTIVE CLINICAL TRIALS OF BIFOCALS/MULTIFOCALS
  6. ACCOMMODATION VERGENCE INTERACTIONS
  7. DETERMINATION OF OPTIMAL NEAR ADDITION AND PRISMATIC POWER
  8. VALUE OF BIFOCAL OVER MULTIFOCAL LENS DESIGNS
  9. PERIPHERAL RETINAL BLUR AND BIFOCAL LENS EFFECTS
  10. CONCLUSION
  11. GRANTS AND FINANCIAL ASSISTANCE
  12. REFERENCES
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