July 2019

ASCRS NEWS

Review of “The impact of glistenings on the optical quality of a hydrophobic acrylic intraocular lens”


by Megan Law, MD, and Benyam Kinde, MD


Saras Ramanathan, MD

Residency program director University of California,
San Francisco (UCSF)


Benyam Kinde, MD, and Megan Law, MD
Source: UCSF

Glistenings are fluid-filled microvacuoles that develop within IOLs postoperatively. They are appreciated on slit lamp as shiny speckles, and various grading techniques have been employed to better objectively categorize severity. Glistenings are generally thought to develop secondary to water vapor detachment and accumulation within polymer voids of IOLs. As light passes through the IOL, it is refracted and scattered at the water-polymer interface.1 The impact of glistenings on visual performance has been a matter of controversy. While Waite et al. and Hayahi et al. did not demonstrate a significant difference in visual acuity, other groups have reported effects on visual acuity and/or high contrast sensitivity. 2, 3, 4, 5, 6, 7.
In this in vitro study by Weindler et al., the authors attempt to objectively assess the effect of various clinical grades of glistenings on image quality by calculation of the modulation transfer function (MTF) and Strehl ratio, two metrics of IOL image quality. Weindler et al. obtained 38 monofocal hydrophobic acrylic AcrySof SA60AT 21.0 D IOLs (Alcon) for this study. Glistening formation was induced by exposing lenses to a temperature change over time. Of the 38 IOLs used in this study, 20 lenses were initially used as controls, prior to heat exposure. A subset of the total lenses used in this study underwent multiple treatments in order to increase the range in the number of glistenings.
IOLs were then photographed and the number and size of induced glistenings were calculated. Subsequently, lenses were graded: grade 0 (no glistenings), grade 1 (1–100 MV/mm2), grade 2 (101–200 MV/mm2), grade 3 (201–500 MV/mm2), and grade 4 (more than 500 MV/mm2). The MTF and the Strehl ratio were evaluated at aperture diameters of 3 mm and 4.5 mm. The MTF, which is a measure of contrast transfer from the object to image, was determined using three discrete spatial frequencies (20, 50, and 100 lp/mm). The Strehl ratio is another measure of optical performance and is calculated as the peak focal intensity in aberration versus an ideal system . Both MTF and Strehl ratio range from 0 to 1 with higher values representing better optical systems.
Glistenings were observed in all 38 IOLs. The mean glistening number +/– standard deviation (MV/mm2) was 74±12.7 (grade 1), 142±22.2 (grade 2), 297±76.2 (grade 3), and 1509±311.9 (grade 4). The mean glistening diameter ±SD was 15.31±3.13 µm (range: 7.33–24.74 µm), comparable to previous studies.2, 9 There were no significant differences between the MTF values of the clear IOLs and those with grade 1–3 glistenings (Wilcoxon test, p>0.05). Grade 4 IOLs had lower MTF levels compared to clear IOLs (Mann-Whitney U test, p<0.01). The differences were 0.011 (25 lp/mm), 0.024 (50 lp/mm), and 0.048 (100 lp/mm) for the 3-mm aperture, and 0.056 (25 lp/mm), 0.071 (50 lp/mm), and 0.059 (100 lp/mm) for the 4.5 mm aperture. In IOLs with grade 4 glistenings, the Strehl was lower compared to controls by 0.068 (3 mm) and 0.096 (4.5 mm; Mann-Whitney U test, p<0.01). The decreased Strehl ratio for grades 1–3 was not statistically significant (Wilcoxon test, p>0.05).
The authors concluded that image quality is not compromised in IOLs with low–moderate glistenings (grade 1–3). In IOLs with severe glistenings (>500 MV/mm2), the impact on the MTF and Strehl ratio is limited and likely not visually significant. Previous in vivo studies assessing the impact of glistenings on visual acuity and contrast sensitivity have reported mixed results, with some showing reduced contrast sensitivity at higher special frequencies.6,7 This study showed a similar trend: MTF decline was greater in frequencies higher than 25 lp/mm.
Interestingly, the investigators found that monochromatic high order aberrations have a greater impact than glistenings on imaging quality. In assessing the Strehl ratio in the 3.0 and 4.5 mm aperture size for clear IOLs, the ratio was 0.386 lower in the larger aperture group. Because the SA60AT is a spherical lens, an increased aperture size may induce significant spherical aberrations affecting optical performance. By comparison, grade 4 glistening in the 3 mm and 4.5 mm aperture IOLs only caused a decreased Strehl ratio of 0.068 and 0.096, respectively.
This in vitro analysis assessed the impact of glistenings on a single type of hydrophobic IOL of a single power. It is possible that the visual impact of glistenings could depend on IOL power, as suggested previously.10 Furthermore, while a majority of the IOLs with glistenings had a “typical” configuration with highest concentration centrally, some had a peak density of glistenings in an eccentric location. It is not known how this configuration could affect the metrices measured. Lastly, it is unclear how repeated heating of the IOL to reach a desired glistening grade could affect the integrity of the IOL and image quality. Straylight, or light scattering, can be induced by glistenings and is another factor that can affect image quality, causing glare symptoms. While not studied in this paper, straylight may be significantly higher in IOLs with increased glistenings causing visual disturbances even in the presence of a high MTF value or Strehl ratio.
Weindler et al.’s results are consistent with previous in vivo studies assessing the impact of glistenings on visual function. The AcrySof SA60AT lens with a limited degree of glistenings has a comparable optical quality to a clear IOL. Even the development of severe glistenings may not translate to a meaningful reduction in visual acuity or contrast sensitivity. As clinicians examining glistenings in our pseudophakic patients, we should recognize that this seemingly impressive examination finding plays only a marginal role in optical performance.

The question of whether and by how much moderate to severe glistenings affect visual quality is difficult to determine. I asked the UCSF residents to review this study and literature review that appears in the July issue of JCRS.

—David F. Chang, MD,
EyeWorld Journal
Club editor

Contact information

Ramanathan: Saras.Ramanathan@ucsf.edu

References

1. Werner L. Glistenings and surface light scattering in intraocular lenses. J Cataract Refract Surg. 2010;36:1398–420.
2. Waite A, et al. Glistenings in the single-piece, hydrophobic, acrylic intraocular lenses. Am J Ophthalmol. 2007;144:143–4.
3. Hayashi K, et al. Long-term effect of surface light scattering and glistenings of intraocular lenses on visual function. Am J Ophthalmol. 2012;154:240–251.
4. Dhaliwal DK, et al. Visual significance of glistenings seen in the AcrySof intraocular lens. J Cataract Refract Surg. 1996;22:452–7.
5. Christiansen G, et al. Glistenings in the AcrySof intraocular lens: pilot study. J Cataract Refract Surg. 2001;27:728–33.
6. Gunenc U, et al. Effects on visual function of glistenings and folding marks in AcrySof intraocular lenses. J Cataract Refract Surg. 2001;27:1611–4.
7. Xi L, et al. Analysis of glistenings in hydrophobic acrylic intraocular lenses on visual performance. Int J Ophthalmol. 2014;7:446–51.
8. Pieh S, et al. In vitro Strehl ratios with spherical, aberration-free, average, and customized spherical aberration-correcting intraocular lenses. Invest Ophthalmol Vis Sci. 2009;50:1264–70.
9. Henriksen BS, et al. Effect of intraocular lens glistening size on visual quality. J Cataract Refract Surg. 2015;41:1190–8.
10. Colin J, et al. Glistenings in a large series of hydrophobic acrylic intraocular lenses. J Cataract Refract Surg. 2009;35:2121–6.

Review of “The impact of glistenings on the optical quality of a hydrophobic acrylic intraocular lens” Review of “The impact of glistenings on the optical quality of a hydrophobic acrylic intraocular lens”
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