Evidence weak for blue light-filtering IOLs

Only weak evidence supports the use of IOLs that filter visible blue light, researchers say.

“On the basis of currently available evidence, one cannot advocate for the use of blue-light-filtering IOLs over UV-only filtering IOLs,” wroite X. Li, Waterford Institute of Technology, Waterford, Ireland, and colleagues.

They published their review of the research on the lenses in the journal Eye.

Manufacturers and distributers have been claiming benefits for IOLs that filter visible short-wavelength light.

Blue light is scattered more than light of longer wavelengths, Li and colleagues wrote, and blue light scatter is the predominant cause of veiling luminance and glare disability.

In a healthy eye, lutein, zeaxanthin, and mesozeaxanthin-collectively referred to as macular pigment-absorb blue light peaking at 460 nm. An average amount of macular pigment filters out about 40% of blue light incident on the macula.

Hypothetically, increasing macular pigment would improve contrast between a background consisting of blue haze and a target, thereby increasing visual range and improving discernibiltiy of a target’s low-contrast internal details.

The crystalline lens blocks ultraviolet (UV) radiation between about 300 and 400 nm. Over time, the damage caused by radiation, oxidation, and post-translational modification increases light scatter, fluorescence, and spectral absorption, especially at the short-wavelength end of the visible spectrum.

As a consequence, a 53-year-old lens transmits about 70% of visible blue light, while a 75-year-old lens transmits about 25% of blue light.

Early IOLs did not include chromophores to block UV radiation, but by 1978 researchers realized that UV radiation was damaging retinas in eyes implanted with these lenses, and by 1980 most IOLs contained UV-blocking chromophores.

A standard IOL now absorbs wavelengths up to 420 nm. Blue light-filtering IOLs block wavelengths between 400 and 500 nm. They are subdivided into blue-blockers, which absorb visible light in the 450-500 nm range and violet-blockers, that absorb visible light in the 410-440 nm range.

Li and colleagues could not find any studies on violet-blocking IOLs. They found 21 studies reporting on outcomes following implantation of blue-light-filtering IOLs. The studies involved  a total of 8,914 patients and 12,919 study eyes undergoing cataract surgery.

The researchers classified 7 of these as individual cohort studies or low-quality randomized controlled trials. There were no systematic reviews of cohort studies or individual randomized-controlled trials with narrow confidence intervals.

Of these 7 best studies, only one found better vision with blue-light-filtering IOLs than UV-only filtering IOLs. In this study, researchers randomly selected 30 eyes for implantation with UV-only light filtering IOLs and 30 for implantation with blue-light filtering IOLs. There were no differences in visual acuity or colour vision up to 6 months after surgery, but the blue-light filtered eyes scored better in contrast sensitivity at select frequencies.

This study did not measure or report contrast sensitivity preoperatively in either group, so Li et. al. reasoned the finding may simply reflect better preoperative contrast sensitivity in the eyes scheduled to be implanted with the blue-light-filtering IOL.

In another study, patients implanted with standard IOLs that block UV light only wore either clip-on blue light-filtering spectacles of clip-on UV-only filtering spectacles. The researchers found that the blue-light filtering spectacles increased the patient’s ability to tolerate glare and enhanced their recovery following photostress under conditions of intense light. It is hard to determine how well the results of this experiment applies to actual blue light-filtering IOLs, Li and colleagues noted.

On the other hand, a comparison of another IOL (AcrySof Natural IOL, Alcon), which filters blue light, to single-piece IOLs (single-piece AcrySof IOL, Alcon), which filter only UV light, showed no difference between them in visual acuity, contrast sensitivity, or colour perception among the 93 patients implanted with one or the other.

Likewise, researchers implanted each of 30 patients with UV-only filtering lenses (AcrySof SA60AT lenses, Alcon) in one eye and blue light-filtering lenses (AcrySof SN60WF, Alcon) in the other eye. Two years later they found no differences in visual acuity, colour vision, contrast sensitivity, macular thickness, or macular volume.

Some researchers have tested the influence of blue-light filtering IOLs on night vision, which depends on blue light. In one study, 22 patients with bilateral pseudophakia and early age-related macular degeneration were less able to sort blue socks from navy socks while wearing blue light-filtering spectacles than without the spectacles in dim conditions.

Other studies found no such differences. These studies, however, all used luminance values of at least 1 cd/m², which means the subjects’ vision was at least partly mediated by cones rather than rods, Li and colleagues wrote.

Blue light can suppress melatonin, so some researchers have speculated that blue-filtering IOLs might affect sleep patterns. Studies on UV-only filtering IOLs show improvements in sleep patterns, perhaps because the procedure replaced yellow lenses. One study comparing patients implanted with UV-only filtering IOLs and blue-light filtering IOLs found improvements in sleep only for those with the UV-only filtering IOLs. Other similar studies have also found no differences.

Could blue light filtering lenses affect macular degeneration? It is difficult to design a study answering that question, Li et. al. wrote, because “it would be impossible to control for the cumulative exposure to such visible wavelengths before surgery.”

However, one small, observational study found increased fundus autofluorescence, a marker for geographic atrophy and neovascular age-related macular degeneration, in eyes implanted with UV-only filtering IOLs and not in eyes implanted with blue light-filtering IOLs. Li and colleagues noted measures of autofluorescence are influenced by the nature and density for a cataract before surgery and the absorbance properties of the IOL.

Another study found geographic atrophy progressed more slowly in eyes implanted with blue light-filtering IOLs. However, the study did not control for age, genetic background, and other confounding factors.

“In general, the quality of evidence informing the surgeon’s selection of IOLs on the basis of light transmittance properties is deficient,” Li and colleagues concluded.