Visual outcomes with the yellow intraocular lens

Authors


Dr Dinesh Selva
Department of Ophthalmology
and Visual Sciences
Royal Adelaide Hospital
North Terrace
Adelaide 5000
South Australia
Australia
Tel: + 61 8 8222 5222
Fax: + 61 8 8222 5221
Email: leiboigal5@yahoo.com.au

Abstract.

Purpose: To compare postoperative best distance visual acuity (VA), contrast sensitivity and colour perception with the blue light-filtering AcrySof® Natural (SN60AT) and AcrySof® single-piece (SA60AT) intraocular lenses (IOLs).

Methods: This was a prospective, randomized, comparative, interventional study comparing postoperative performance between the SN60 and SA60 IOLs. There were nine patients (nine eyes) in the SN60 group and 10 patients (10 eyes) in the SA60 group. All patients were operated using phacoemulsification. Postoperative VA (Snellen chart), contrast sensitivity (Pelli−Robson contrast sensitivity chart) and colour perception (Farnsworth−Munsell D-15 panel test) were measured at 1, 3 and 6 months postoperatively.

Results: Postoperative best corrected VA after 6 months was 20/20 or better in 89% of SN60 eyes and 100% of SA60 eyes. Postoperative contrast sensitivity scores improved significantly in both groups under both photopic and mesopic conditions. There were no statistically significant differences in contrast sensitivity scores between the SN60 and SA60 groups at any of the postoperative evaluation time-points. Postoperative colour perception improved significantly in both the SN60 and SA60 groups, and there were no statistically significant differences in colour perception performance between the two groups.

Conclusion: The blue light-filtering AcrySof® Natural (SN60) IOL has postoperative visual performance comparable with that of the AcrySof® single-piece (SA60) IOL.

Introduction

Blue light is a high-energy light in the 400–500-nanometre (nm) wavelength region. It is a component of solar radiation, as well as artificial light (office lighting and computers). Several studies have indicated that high levels of exposure to ultraviolet (UV) and blue light may result in damage to retinal cells (Ham et al. 1984; Sparrow et al. 2000) and contribute to the development of age-related macular degeneration (AMD) (Mainster 1987; Taylor et al. 1992; Cruickshanks et al. 1993, 2001; Darzins et al. 1997).

The healthy human lens gradually becomes yellow as part of the normal ageing process. This yellowing reduces the transmission of blue light, thereby reducing the amount of blue light that reaches the retina (Weale 1988; Bron et al. 2000). As cataract surgery entails replacing a yellow lens with a clear lens, retinal exposure to blue light will increase after surgery.

The possibility that blue light may accelerate AMD after cataract extraction has led to the development of intraocular lenses (IOLs) that absorb blue light wavelengths. One of the most recently designed is the blue light-absorbing AcrySof® Natural IOL (SN60AT) (Alcon, Inc., Fort Worth, Texas, USA). It is identical to the AcrySof® single-piece IOL (SA60), except for the addition of a 0.04% covalently bonded yellow chromophore. The light transmission design is intended to mimic that of the natural human crystalline lens with the ability to filter UV light as well as visible blue light of wavelengths up to 550 nm.

This pilot study was designed to compare the postoperative visual outcomes (best corrected visual acuity, contrast sensitivity and colour perception) of the SN60 and SA60 IOLs.

Materials and Methods

This was a prospective, randomized, comparative, interventional study. All patients were operated by a single surgeon (GP) and were unaware of the type of IOL implanted. The study protocol and patient informed consent form were approved by the Institutional Review Board. Informed consent was obtained from all study participants.

Patients included in the study had age-related cataracts requiring extraction, but an otherwise normal ocular examination. Patients with other ocular pathologies, high hyperopia or myopia (6.0 D), or neurological diseases and patients using medications with a possible influence on contrast sensitivity or colour vision were excluded from the study.

Preoperative evaluation included a complete ophthalmic examination (refraction, intraocular pressure, anterior and fundoscopic evaluation), as well as biometry and keratometry readings, obtained by the same trained personnel for both groups. Contrast sensitivity and colour perceptions were also tested as part of the preoperative evaluation. The patients were randomized to receive one of the two lenses by drawing a blank envelope for each patient out of a box of 20 envelopes. A note in each envelope stated whether an SA60 or SN60 lens would be implanted.

The SN60 group (nine patients, nine eyes) included three men and six women with a mean age of 74 ± 6 years. The SA60 group (10 patients, 10 eyes) included six men and four women with a mean age of 74 ± 6 years. One of the SN60 patients was excluded early in the study as he refused to be operated on his other eye.

The patients were operated using a standard phacoemulsification technique and treated postoperatively with topical prednisolone and tobramycin drops. There were no intraoperative or postoperative complications and all patients had an uneventful recovery. None of the patients complained of any ocular disturbances or unusual colour perception.

Postoperative evaluation included a complete clinical examination at 1, 3 and 6 months after the operation, including contrast sensitivity and colour perception. Best corrected visual acuity (BCVA) measurements were taken using a Snellen chart. Contrast sensitivity was measured with a Pelli− Robson contrast sensitivity chart in photopic (85 candelas/m2) and mesopic (3 candelas/m2) conditions. The patient sat directly in front of the chart at a distance of about 1 metre, wearing his or her best distance vision correction. The patient's sensitivity was indicated by the faintest triplet in which two of the three letters were named correctly. The log contrast sensitivity for this triplet was given by the number on the scoring pad nearest to the triplet. The operated eye was tested separately. Colour vision was measured in the operated eye using the Farnsworth− Munsell D15 panel test, and the number of errors was recorded.

Statistical analysis

The mean and standard deviation of the log contrast sensitivity values and colour perception errors in the two study groups were compared using a simple two-tailed t-test. A statistically significant difference was considered to exist when p < 0.05.

Results

Visual acuity

Preoperative BCVA in both groups ranged between 20/30 and 20/200. Postoperative BCVA after 6 months in the SN60 group was ≥ 20/20 in all eyes, except one (11%), which had a BCVA of 20/30. Postoperative BCVA in the SA60 group was ≥20/20 in all eyes.

Contrast sensitivity

There was an overall improvement in log contrast sensitivity scores under both photopic and mesopic conditions at 1, 3 and 6 months after the operation in both groups (Figs 1 and 2). The postoperative improvement was statistically significant at all evaluation time-points, in both photopic and mesopic conditions (Tables 1 and 2), except the 1-month postoperative mesopic measurement in the SN60 group (p = 0.08). There were no statistically significant differences in contrast sensitivity scores between the SN60 and SA60 groups at any of the postoperative time-points, in either mesopic or photopic conditions (Tables 1 and 2).

Figure 1.

Single photopic log contrast sensitivity scores in the operated eyes of SN60 (n = 9 eyes) and SA60 (n = 10 eyes) patients at 1, 3 and 6 months postoperatively, showing a marked improvement after the operation compared with preoperatively.

Figure 2.

Single mesopic log contrast sensitivity scores in the operated eyes of SN60 (n = 9 eyes) and SA60 (n = 10 eyes) patients at 1, 3 and 6 months postoperatively, showing a marked improvement after the operation compared with preoperatively.

Table 1.  Mean (± SD) photopic contrast sensitivity log values in the SA60 and SN60 groups.
 Preoperative1 month
postoperative
3 months
postoperative
6 months
postoperative
  • *

    p < 0.001,

  • p = 0.01 (for differences between pre- and postoperative mean log values).

SA60 (n = 10 eyes)1.02 ± 0.211.33 ± 0.32*1.49 ± 0.28*1.52 ± 0.21*
SN60 (n = 9 eyes)0.98 ± 0.321.32 ± 0.371.43 ± 0.26*1.55 ± 0.24*
p-value0.60.90.20.5
Table 2.  Mean (± SD) mesopic contrast sensitivity log values in the SA60 and SN60 groups.
 Preoperative1 month
postoperative
3 months
postoperative
6 months
postoperative
  • *

    = 0.03,

  • p < 0.001,

  • = 0.08 (for differences between pre- and postoperative mean log values).

SA60 (n = 10 eyes)0.68 ± 0.110.94 ± 0.41*1.33 ± 0.211.37 ± 0.19
SN60 (n = 9 eyes)0.69 ± 0.510.85 ± 0.441.20 ± 0.211.32 ± 0.18
p-value0.80.80.30.5

Colour perception

There was an overall improvement in postoperative colour perception in both the SN60 and SA60 groups. The postoperative improvement was statistically significant at all evaluation time-points (Table 3) except the 1-month postoperative evaluation in the SN60 group (p = 0.07). There were no statistically significant differences in postoperative colour perception between the SN60 and SA60 groups (Table 3).

Table 3.  Mean (± SD) number of errors on the D15 colour chart in the SA60 and SN60 groups.
 Preoperative1 month
postoperative
3 months
postoperative
6 months
postoperative
  • *

    p < 0.001,

  • p = 0.07,

  • p = 0.04,

  • §

    p = 0.004 (for differences between pre- and postoperative mean number of errors on the D15 colour chart).

SA60 (n = 10 eyes)6.9 ± 3.63.0 ± 1.9*3.0 ± 2.3*2.4 ± 2.1*
SN60 (n = 9 eyes)6.6 ± 3.14.2 ± 3.04.0 ± 2.73.5 ± 3.8§
p-value0.80.30.30.15

Discussion

Recent clinical and experimental studies suggest that blue light may be associated with AMD. The Beaver Dam Eye Study concluded that the amount of time spent outdoors between the ages of 13 and 19 years and between 30 and 39 years was significantly associated with the development of both early and late AMD (Cruickshanks et al. 1993, 2001). Another study (Taylor et al. 1992) found that cumulative UV exposure was not related to the development of visual loss from AMD, but that there was a significant association between blue light exposure during the 20-year period that preceded the study and the progression to visual loss from AMD. Experimental studies have shown that retinal injury from wavelengths that peak at around 425 nm is initiated by photochemical processes in the retinal pigment epithelium (RPE) (Ham et al. 1984), possibly mediated by lipofuscin fluorophore A2E (Sparrow et al. 2000), and results in RPE atrophy (Sparrow et al. 2002, 2003).

Large epidemiological studies have suggested that cataract extraction modifies the outcome and incidence of AMD. The Beaver Dam Study found a positive association between cataract extraction and clinical signs of early AMD (Klein et al. 1994). This was confirmed in the 5- and 10-year follow-up studies that showed a significant association between cataract surgery before baseline evaluation and higher rates of progression of AMD, incidence of late AMD and geographic atrophy (Klein et al. 1998, 2002). Pooled data from the Beaver Dam and Blue Mountain Eye Studies showed that the risk of late AMD was almost six times higher for pseudophakic than for phakic eyes (Wang et al. 2003).

As lipofuscin fluorophores, including A2E, accumulate with age, the natural yellowing of the ageing human lens may reduce the damaging potential of short wavelength blue light. Removing the yellow crystalline lens in cataract surgery and replacing it with a clear IOL increases retinal exposure to blue light after surgery. This may be a possible explanation for the increased incidence of advanced AMD after cataract surgery. This is the thinking behind the development of a yellow-tinted IOL that simulates the adult crystalline lens, thereby possibly protecting RPE cells from blue light damage and reducing the risk of AMD progression. In a recent study, Sparrow et al. (2004) evaluated several IOLs for their in vitro ability to protect RPE cells from light damage mediated by the lipofuscin fluorophore A2E. They found that by absorbing blue light, the AcrySof® Natural IOL shields RPE cells that have accumulated the ageing lipofuscin fluorophore A2E from the damaging effects of light.

Our study showed excellent postoperative visual outcomes for both groups at 1, 3 and 6 months follow-up, and there were no significant differences in BCVA, contrast sensitivity or colour perception between the SN60 and SA60 IOL groups. Although these findings are encouraging, the relatively small groups in our study may mean that the study was not sensitive enough to identify differences in the parameters that were evaluated, and this precludes a definite conclusion. Nevertheless, our findings are supported by other reports with similar results. Niwa et al. (1996) demonstrated that patients with blue light-filtering lenses had improved contrast sensitivity in both photopic and mesopic conditions, compared to those without blue light-filtering lenses. A recent study by Yuan et al. (2004) evaluated contrast sensitivity and colour vision with a yellow coloured ultra-violet (UV) intraocular lens (IOLs; Hoya, Tokyo, Japan) and found that this IOL was preferable to an ordinary UV IOL in preserving spatial contrast sensitivity and caused less photophobia and cyanopsia in the early postoperative period. The AcrySof® Natural IOL (model SB30AL) is currently involved in a larger clinical trial including 300 patients at six sites. Preliminary results have shown comparable visual outcomes (in terms of BCVA, contrast sensitivity and colour perception) to the control group of patients with non-tinted IOLs (Cionni 2003).

All these studies evaluated short-term visual outcomes with the blue light-filtering lens. However, none of them followed the patients for a period long enough to allow for analysis of longterm outcomes, or for conclusions as to whether this type of IOL provides any protection against AMD. Large-scale, randomized studies with long follow-up periods will be required for that.

In conclusion, our pilot study, as well as other clinical and laboratory reports, suggests that the blue light-filtering AcrySof® Natural (SN60) IOL has comparable postoperative visual performance with the AcrySof® single-piece (SA60) IOL. Given the increasing evidence of accelerated AMD after cataract extraction, possibly as a consequence of blue light exposure, the blue light-filtering IOL may offer a safe option by which to reduce this risk, especially in patients with pre-existing AMD.

Acknowledgements

The authors thank Siew-Kim Teoh, Elizabeth Fenech, Tony Amato and Adrian Cooke for their work in co-ordinating this study.

Ancillary