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Contrast sensitivity as a function of focus in patients with the diffractive multifocal intraocular lens
Contrast sensitivity as a function of focus in patients with the diffractive multifocal intraocular lens Thomas Olsen, M.D., Leif Corydon, M.D.
ABSTRACT Contrast sensitivity as a function of focus and visual acuity as a function of contrast were investigated in 19 patients with a diffractive multifocal intraocular lens and compared with 19 control patients with a conventional monofocal implant. The contrast sensitivity of the multifocal patients followed a bimodal curve with a maximum sensitivity at the far focus and a second peak at the near focus, corresponding to about + 3 diopters in the spectacle plane. The maximum sensitivity of the multifocal group was 0.14 log units lower than the control group (P< .05). In the near region, the contrast sensitivity of the multifocal patients exceeded that of the control group from + 2 diopters and inward. No difference in distance visual acuity was found with high contrast letters. With intermediate contrast letters, the visual acuity of the multifocal patients was lower than that of the control group (P< .05).
Key Words: cataract urgery, contrast sensitivity, diffraction , focu , multifocal intraocular len, visual acuity
A multifocal intraocular lens (IOL) is a lens with more than one focal length. In contrast to the monofocallens, which has just one image plane, the multifocallens produces multiple images along the optic axis. When correctly powered and placed in the eye, it is capable of giving a simultaneous imaging of near and distant objects on the retina. This arrangement has important implications for visual perception. While viewing a distant object through the distant focus, some of the light will be focused via the near focus in front of the retina and, conversely, while viewing a near object through the near focus, some of the light will be focused via the far focus behind the retina. Which image is perceived as a meaningful image will depend on which image is focused on the retina. However, because of
the broader distribution of the "focus" in the vicinity of the retina, the formation of the perceived image will occur at some loss of contrast. Clinical studies 1 ,2,3 have shown that the diffractive multifocal IOL from 3M has the potential to give the pseudophakic patient a normal distance vision as well as a normal reading vision without any spectacle correction. Published studies on the contrast sensitivity function in these patients are few in number, however. In a preliminary investigation, 4 we found a normal contrast sensitivity for distance vision and a slightly reduced contrast sensitivity for near vision without near addition, indicating the near focus of the diffractive multifocal IOL to be somewhat less efficient than the far focus. In this study, we report on contrast sensitivity as a function of focus to
From the Department of Ophthalmology, Vejle Sygehus, Vejle, Denmark. Presented at the Symposium on Cataract, IOL and Refractive Surgery, Los Angeles, March 1990. Reprint requests to Thomas Olsen, M.D., dr. med., University Eye Clinic, Aarhus Kommunehospital, DK-8000 Aarhus C, Denmark.
J CATARACT
REFRACT SURG-VOL 16, NOVEMBER 1990
703
investigate the sensory representation of the broadened focus of the multifocallens design.
SUBJECTS AND METHODS Nineteen patients with a multifocal IOL and 19 patients with a conventional IOL were studied. Both series were selected from a computerized file of operated patients using the following inclusion criteria: below 80 years of age, a postoperative time interval of four to eight months, no pre-existing ocular pathology, and a visual acuity better than 20/40 with correction, if any. In the multifocal group, 25 patients fulfilled the first three criteria. Four of them had some agerelated macular degeneration at the follow-up visit and two had a capsular opacification, leaving 19 patients with a visual acuity of more than 20/40 in the age range from 47 to 79 years, mean 71 years. In the control group, 21 patients fulfilled the first three criteria; two of them had a visual acuity worse than 20/30 because of capsular opacification, leaving 19 patients in the age range 48 to 79 years, mean age 72 years. The patients in these two groups were different from those previously reported. 4 All patients had extracapsular cataract extraction (ECCE) or phacoemulsification, usually with a continuous tear capsulotomy technique and in-the-bag placement of the IOL. The multifocal IOL was the 815le (3M/Vision Care) with a diffractive near add of + 3.5 diopters (D). The control group comprised several monofocal designs: the Pharmacia 700b, the
3M/Vision Care style 17le and 17xe, and other all poly(methyl methacrylate) lenses. The contrast sensitivity was measured using the Pelli-Robson test charts 5 consisting of fixed-sized letters of varying contrast. The size of the letters subtend 0.5 degrees at 3 meters. The contrast sensitivity was measured as a function of focus in the following way: The optimal spectacle correction for vision at 3 meters was recorded. To this correction were added spheres of +7.0 D, +6.5 D, +6.0 D, + 5.5 D, etc., while recording the contrast sensitivity at each correction. The procedure was repeated with the addition of -7.0 D, - 6.5 D, - 6.0 D, - 5.5 D, etc., thus covering a series of contrast sensitivity measurements from -7 D to + 7 D around the far focus. Regan visual acuity6 with high and intermediate contrast was measured with the optimal spectacle correction at 3 meters.
RESULTS Figure 1 shows the contrast sensitivity as a function of focus. The contrast sensitivity of the multifocal group followed a bimodal curve with a maximum of 1.48 log units (±O.ll S.D.) at the optimal distance correction (zero refraction) and an additional top of 1.37 log units (± 0.07) at a near focus of about + 3 D. The contrast sensitivity at the near focus was significantly lower than at the far focus (P< .05 by paired t-test).
LOG CONTRAST SENSITIVITY
2.0.---------------------------------------.
1.5
_
Multifocal
o
Monofocal Fig. 1.
1.0
0.5
2
345
678
DEFOCUS (Dpt) 704
J CATARACT REFRACT SURG-VOL
16, NOVEMBER 1990
(Olsen) The contrast sensitivity as a function of focus in 19 patients with a dift'ractive multifocal IOL and in 19 control patients with a monofocal IOL. The abscissa shows the spectacle equivalent of the focus of the IOL. A plus value indicates the near region and a minus value the hyperopic region.
The contrast sensitivity of the monofocal group reached a maximum of 1. 62 log units ( ± 0.15) with a symmetrical decline around the far focus. The difference of 0.14 log units between the maxima of multifocal and the control group was significantly different from zero (P< .05 by unpaired t-test). In the near region, the contrast sensitivity of the multifocal group was found to exceed that of the control group at + 2 D and inward. At the near focus of the multifocal group (spectacle equivalent about + 3 D, corresponding to a reading distance of 33 cm), the difference was 0.50 log units between the contrast sensitivity of the monofocal and the multifocal group. No difference in Regan visual acuity was found under high contrast conditions. The mean scores were 7.57 (± 0.82) and 7.61 (± 1.02) in the multifocal and the monofocal groups, respectively. With letters of intermediate contrast, however, a significant lower visual acuity was found in the multifocal group, the mean values being 3.38 (± 1.25) and 4.78 (±0.91), respectively (P<.05) (Figure 2). DISCUSSION When the focal point of an optical system is broken up into several focal pOints along the axis, some blurring will occur at anyone image formation because of the formation of several blurred images on top of the focused image. The result is a loss of contrast. It is therefore not surprising that a certain loss of contrast sensitivity could be detected in the patients with a multifocal lens implant . We have previously found a statistically nonsignificant decrease of 0.1 log units in the contrast sensitivity measured through the far focus of the multifocal IOL and a significant decrease of 0.19 log units measured through the near focus of the multifocal IOL compared with the normal focus of the monofocal IOL. 4 This was found in a limited number of patients using the Vistech test charts of sinusoidal gratings. According to Rubin, 7 the PelliRobson method is more reliable than the Vistech method in test-retest performance, making the Pelli-Robson the method of choice when repeated measurements are made on the same patient. The greater precision of the Pelli-Robson method may be the reason it was possible in the present study to find a statistically significant difference of only 0.14 log units between the far focus of the multifocal and the monofocal group. At the near focus, a further reduction of 0.10 log units was found, making a total reduction of 0.24 log units from the far focus of the monofocal group. This should be compared with our previous reported result of 0.19 log units as the average decrease of the near focus across a range of spatial frequencies. Considering the differences in J CATARACT REFRACT
REGAN VISUAL ACUITY 10.-----------------------------------~
00 Multifocal
8
D
6
Monofocal
P < .05
4
2
High contrast Fig. 2.
Medium contrast
(Olsen) Regan visual acuity as a function of contrast in patients with a diffractive multifocal IOL and in control patients with a monofocal IOL.
methodology, we believe the present results agree well with our previous results. According to the manufacturer, 41 % of the light is directed to the far focus after passing through the diffractive multifocal IOL (zero order of diffraction), 41 % is directed to the near focus (one order of diffraction), and the remaining 18% is directed to higher orders. Compared with the monofocallens in which 100% of the light is directed to one focus only, and assuming a uniform scattering of the out of focus light, the contrast of the near and distance image formed by the multifocallens will be expected to be reduced 59% or 0.4 units on a log scale. This is, however, significantly different from the observed loss of only 0.14 log units. This discrepancy is not readily explained. It may be speculated that the image processing system of the retina may have compensatory mechanisms by which it can respond to reduced contrast levels. Further investigations are needed, however, to study the visual mechanisms underlying this phenomenon. The actual benefit of the multifocal IOL in the near region is readily illustrated by the relation between contrast sensitivity and focus (Figure 1). Without a near add, a patient with a multifocal IOL will have better vision than a patient with a monofocal IOL for object distances nearer than 50 cm. The + 3.5 D near add of the diffractive IOL was found to be equivalent to about + 3 D in the spectacle plane, which accords with theoretical predictions. 8 In the present study, no difference between the visual acuity of the multifocal group and the monofo-
SURG- VOL 16, NOVEMBER 1990
705
cal group using high contrast letters was found. This finding agrees with other studies showing a favorable distance visual acuity with the multifocal IOL.l,2,3 With intermediate contrast letters, however, a significant reduction in acuity was found in the multifocal group. We therefore conclude that the eye with a multifocal implant may maintain a normal acuity with a high contrast object despite the physically reduced contrast at the retina. With a low contrast object, however, the acuity may be reduced. REFERENCES 1. Hansen TE, Corydon L, Krag S, Thim K. New multifocal intraocular lens design. J Cataract Refract Surg 1990; 16: 38-41 2. Percival P. Early experience with the diffractive bifocal'lens. Eur J Implant Refract Surg 1989; 1:3-4
706
J CATARACT REFRACT
3. Jacobi KW, Nowak MR, Strobel J. Bifokale Intraokularlinsen nach Kataraktextraktion - eigene Erfahrungen, Europaische Multizentrische Studie und FDA-Studie. In: Freyler H, Skorpik C, Grasl M, eds. 3. Kongress der Deutschen Gesellschaft fur Intraokularlinsen Implantation, Wien, 1989. Springer-Verlag, Wien, New York 1990; 353-355 4. Olsen T, Corydon L. Contrast sensitivity in patients with a new type of multifocal intraocular lens. J Cataract Refract Surg 1990; 16:42-46 5. Pelli DG, Robson JG, Wilkins AJ. The design of a new letter chart for measuring contrast sensitivity. Clin Vis Sci 1988; 2:187-199 6. Regan D, Neima D. Low-contrast letter charts as a test of visual function. Ophthalmology 1983; 90:1192-1200 7. Rubin GS. Reliability and sensitivity of clinical contrast sensitivity tests. Clin Vis Sci 1988; 2: 169-177 8. Olsen T. Refraction factor in IOL power calculation. J Cataract Refract Surg 1990; 16:129-130
SURG-VOL 16, NOVEMBER 1990
ABSTRACT Contrast sensitivity as a function of focus and visual acuity as a function of contrast were investigated in 19 patients with a diffractive multifocal intraocular lens and compared with 19 control patients with a conventional monofocal implant. The contrast sensitivity of the multifocal patients followed a bimodal curve with a maximum sensitivity at the far focus and a second peak at the near focus, corresponding to about + 3 diopters in the spectacle plane. The maximum sensitivity of the multifocal group was 0.14 log units lower than the control group (P< .05). In the near region, the contrast sensitivity of the multifocal patients exceeded that of the control group from + 2 diopters and inward. No difference in distance visual acuity was found with high contrast letters. With intermediate contrast letters, the visual acuity of the multifocal patients was lower than that of the control group (P< .05).
Key Words: cataract urgery, contrast sensitivity, diffraction , focu , multifocal intraocular len, visual acuity
A multifocal intraocular lens (IOL) is a lens with more than one focal length. In contrast to the monofocallens, which has just one image plane, the multifocallens produces multiple images along the optic axis. When correctly powered and placed in the eye, it is capable of giving a simultaneous imaging of near and distant objects on the retina. This arrangement has important implications for visual perception. While viewing a distant object through the distant focus, some of the light will be focused via the near focus in front of the retina and, conversely, while viewing a near object through the near focus, some of the light will be focused via the far focus behind the retina. Which image is perceived as a meaningful image will depend on which image is focused on the retina. However, because of
the broader distribution of the "focus" in the vicinity of the retina, the formation of the perceived image will occur at some loss of contrast. Clinical studies 1 ,2,3 have shown that the diffractive multifocal IOL from 3M has the potential to give the pseudophakic patient a normal distance vision as well as a normal reading vision without any spectacle correction. Published studies on the contrast sensitivity function in these patients are few in number, however. In a preliminary investigation, 4 we found a normal contrast sensitivity for distance vision and a slightly reduced contrast sensitivity for near vision without near addition, indicating the near focus of the diffractive multifocal IOL to be somewhat less efficient than the far focus. In this study, we report on contrast sensitivity as a function of focus to
From the Department of Ophthalmology, Vejle Sygehus, Vejle, Denmark. Presented at the Symposium on Cataract, IOL and Refractive Surgery, Los Angeles, March 1990. Reprint requests to Thomas Olsen, M.D., dr. med., University Eye Clinic, Aarhus Kommunehospital, DK-8000 Aarhus C, Denmark.
J CATARACT
REFRACT SURG-VOL 16, NOVEMBER 1990
703
investigate the sensory representation of the broadened focus of the multifocallens design.
SUBJECTS AND METHODS Nineteen patients with a multifocal IOL and 19 patients with a conventional IOL were studied. Both series were selected from a computerized file of operated patients using the following inclusion criteria: below 80 years of age, a postoperative time interval of four to eight months, no pre-existing ocular pathology, and a visual acuity better than 20/40 with correction, if any. In the multifocal group, 25 patients fulfilled the first three criteria. Four of them had some agerelated macular degeneration at the follow-up visit and two had a capsular opacification, leaving 19 patients with a visual acuity of more than 20/40 in the age range from 47 to 79 years, mean 71 years. In the control group, 21 patients fulfilled the first three criteria; two of them had a visual acuity worse than 20/30 because of capsular opacification, leaving 19 patients in the age range 48 to 79 years, mean age 72 years. The patients in these two groups were different from those previously reported. 4 All patients had extracapsular cataract extraction (ECCE) or phacoemulsification, usually with a continuous tear capsulotomy technique and in-the-bag placement of the IOL. The multifocal IOL was the 815le (3M/Vision Care) with a diffractive near add of + 3.5 diopters (D). The control group comprised several monofocal designs: the Pharmacia 700b, the
3M/Vision Care style 17le and 17xe, and other all poly(methyl methacrylate) lenses. The contrast sensitivity was measured using the Pelli-Robson test charts 5 consisting of fixed-sized letters of varying contrast. The size of the letters subtend 0.5 degrees at 3 meters. The contrast sensitivity was measured as a function of focus in the following way: The optimal spectacle correction for vision at 3 meters was recorded. To this correction were added spheres of +7.0 D, +6.5 D, +6.0 D, + 5.5 D, etc., while recording the contrast sensitivity at each correction. The procedure was repeated with the addition of -7.0 D, - 6.5 D, - 6.0 D, - 5.5 D, etc., thus covering a series of contrast sensitivity measurements from -7 D to + 7 D around the far focus. Regan visual acuity6 with high and intermediate contrast was measured with the optimal spectacle correction at 3 meters.
RESULTS Figure 1 shows the contrast sensitivity as a function of focus. The contrast sensitivity of the multifocal group followed a bimodal curve with a maximum of 1.48 log units (±O.ll S.D.) at the optimal distance correction (zero refraction) and an additional top of 1.37 log units (± 0.07) at a near focus of about + 3 D. The contrast sensitivity at the near focus was significantly lower than at the far focus (P< .05 by paired t-test).
LOG CONTRAST SENSITIVITY
2.0.---------------------------------------.
1.5
_
Multifocal
o
Monofocal Fig. 1.
1.0
0.5
2
345
678
DEFOCUS (Dpt) 704
J CATARACT REFRACT SURG-VOL
16, NOVEMBER 1990
(Olsen) The contrast sensitivity as a function of focus in 19 patients with a dift'ractive multifocal IOL and in 19 control patients with a monofocal IOL. The abscissa shows the spectacle equivalent of the focus of the IOL. A plus value indicates the near region and a minus value the hyperopic region.
The contrast sensitivity of the monofocal group reached a maximum of 1. 62 log units ( ± 0.15) with a symmetrical decline around the far focus. The difference of 0.14 log units between the maxima of multifocal and the control group was significantly different from zero (P< .05 by unpaired t-test). In the near region, the contrast sensitivity of the multifocal group was found to exceed that of the control group at + 2 D and inward. At the near focus of the multifocal group (spectacle equivalent about + 3 D, corresponding to a reading distance of 33 cm), the difference was 0.50 log units between the contrast sensitivity of the monofocal and the multifocal group. No difference in Regan visual acuity was found under high contrast conditions. The mean scores were 7.57 (± 0.82) and 7.61 (± 1.02) in the multifocal and the monofocal groups, respectively. With letters of intermediate contrast, however, a significant lower visual acuity was found in the multifocal group, the mean values being 3.38 (± 1.25) and 4.78 (±0.91), respectively (P<.05) (Figure 2). DISCUSSION When the focal point of an optical system is broken up into several focal pOints along the axis, some blurring will occur at anyone image formation because of the formation of several blurred images on top of the focused image. The result is a loss of contrast. It is therefore not surprising that a certain loss of contrast sensitivity could be detected in the patients with a multifocal lens implant . We have previously found a statistically nonsignificant decrease of 0.1 log units in the contrast sensitivity measured through the far focus of the multifocal IOL and a significant decrease of 0.19 log units measured through the near focus of the multifocal IOL compared with the normal focus of the monofocal IOL. 4 This was found in a limited number of patients using the Vistech test charts of sinusoidal gratings. According to Rubin, 7 the PelliRobson method is more reliable than the Vistech method in test-retest performance, making the Pelli-Robson the method of choice when repeated measurements are made on the same patient. The greater precision of the Pelli-Robson method may be the reason it was possible in the present study to find a statistically significant difference of only 0.14 log units between the far focus of the multifocal and the monofocal group. At the near focus, a further reduction of 0.10 log units was found, making a total reduction of 0.24 log units from the far focus of the monofocal group. This should be compared with our previous reported result of 0.19 log units as the average decrease of the near focus across a range of spatial frequencies. Considering the differences in J CATARACT REFRACT
REGAN VISUAL ACUITY 10.-----------------------------------~
00 Multifocal
8
D
6
Monofocal
P < .05
4
2
High contrast Fig. 2.
Medium contrast
(Olsen) Regan visual acuity as a function of contrast in patients with a diffractive multifocal IOL and in control patients with a monofocal IOL.
methodology, we believe the present results agree well with our previous results. According to the manufacturer, 41 % of the light is directed to the far focus after passing through the diffractive multifocal IOL (zero order of diffraction), 41 % is directed to the near focus (one order of diffraction), and the remaining 18% is directed to higher orders. Compared with the monofocallens in which 100% of the light is directed to one focus only, and assuming a uniform scattering of the out of focus light, the contrast of the near and distance image formed by the multifocallens will be expected to be reduced 59% or 0.4 units on a log scale. This is, however, significantly different from the observed loss of only 0.14 log units. This discrepancy is not readily explained. It may be speculated that the image processing system of the retina may have compensatory mechanisms by which it can respond to reduced contrast levels. Further investigations are needed, however, to study the visual mechanisms underlying this phenomenon. The actual benefit of the multifocal IOL in the near region is readily illustrated by the relation between contrast sensitivity and focus (Figure 1). Without a near add, a patient with a multifocal IOL will have better vision than a patient with a monofocal IOL for object distances nearer than 50 cm. The + 3.5 D near add of the diffractive IOL was found to be equivalent to about + 3 D in the spectacle plane, which accords with theoretical predictions. 8 In the present study, no difference between the visual acuity of the multifocal group and the monofo-
SURG- VOL 16, NOVEMBER 1990
705
cal group using high contrast letters was found. This finding agrees with other studies showing a favorable distance visual acuity with the multifocal IOL.l,2,3 With intermediate contrast letters, however, a significant reduction in acuity was found in the multifocal group. We therefore conclude that the eye with a multifocal implant may maintain a normal acuity with a high contrast object despite the physically reduced contrast at the retina. With a low contrast object, however, the acuity may be reduced. REFERENCES 1. Hansen TE, Corydon L, Krag S, Thim K. New multifocal intraocular lens design. J Cataract Refract Surg 1990; 16: 38-41 2. Percival P. Early experience with the diffractive bifocal'lens. Eur J Implant Refract Surg 1989; 1:3-4
706
J CATARACT REFRACT
3. Jacobi KW, Nowak MR, Strobel J. Bifokale Intraokularlinsen nach Kataraktextraktion - eigene Erfahrungen, Europaische Multizentrische Studie und FDA-Studie. In: Freyler H, Skorpik C, Grasl M, eds. 3. Kongress der Deutschen Gesellschaft fur Intraokularlinsen Implantation, Wien, 1989. Springer-Verlag, Wien, New York 1990; 353-355 4. Olsen T, Corydon L. Contrast sensitivity in patients with a new type of multifocal intraocular lens. J Cataract Refract Surg 1990; 16:42-46 5. Pelli DG, Robson JG, Wilkins AJ. The design of a new letter chart for measuring contrast sensitivity. Clin Vis Sci 1988; 2:187-199 6. Regan D, Neima D. Low-contrast letter charts as a test of visual function. Ophthalmology 1983; 90:1192-1200 7. Rubin GS. Reliability and sensitivity of clinical contrast sensitivity tests. Clin Vis Sci 1988; 2: 169-177 8. Olsen T. Refraction factor in IOL power calculation. J Cataract Refract Surg 1990; 16:129-130
SURG-VOL 16, NOVEMBER 1990