SPECIAL ARTICLE
On Seeing Yellow
The Case for, and Against, Short-Wavelength Light–Absorbing Intraocular Lenses
Matthew P. Simunovic, MB, BChir, PhD
T
he normal human crystalline lens absorbs UV and short-wavelength visible electromagnetic radiation. Early intraocular lenses (IOLs) permitted the transmission of such
radiation to the retina following cataract extraction. Experimental studies of the absorption profile of the crystalline lens and animal studies demonstrating the deleterious effects of short-wavelength radiation on the retina led to the development of UV-absorbing,
and later, short-wavelength light–absorbing (SLA) IOLs. Short-wavelength light–absorbing IOLs
were designed to mimic the absorption properties of the normal crystalline lens by absorbing some
short-wavelength light in addition to UV radiation; however, debate continues regarding the relative merits of such lenses over UV-absorbing IOLs. Advocates of SLA IOLs suggest that they may
theoretically offer increased photoprotection and decreased glare sensitivity and draw on in vitro,
animal, and limited clinical studies that infer possible benefits. Detractors suggest that there is no
direct evidence supporting a role for SLA IOLs in preventing retinal dysfunction in humans and
suggest that they may have negative effects on color perception, scotopic vision, and circadian rhythms.
This article examines the theoretical and empirical evidence for, and against, such lenses.
Arch Ophthalmol. 2012;130(7):919-926
The human crystalline lens absorbs UV and
short-wavelength visible electromagnetic radiation (Figure 1); this property
appears to be derived from cellular components present at birth (such as amino
acids) and from so-called lens pigments,
which are believed to be derived largely
from the amino acid tryptophan and which
accumulate with age.2,3 Several roles for UV
and short-wavelength visible radiation absorption by the crystalline lens have been
mooted, including a reduction in the effects of chromatic aberration by filtering
out highly refracted short-wavelength light,
glare reduction,4 and photoprotection.2
Early cataract surgery permitted the
postoperative transmission of UV and
short-wavelength visible radiation to the
retina. Studies on the spectral absorption
profile of the crystalline lens2,5 and animal experiments demonstrating the deleterious effects of short-wavelength radiation on the retina6-8 led to the development
of UV-absorbing intraocular lenses [IOLs]
ARCH OPHTHALMOL / VOL 130 (NO. 7), JULY 2012
919
in the early 1980s (conventional UVabsorbing IOLs). This development was
followed in the late 1980s by IOLs that also
absorbed short-wavelength visible radiation9 (short-wavelength light–absorbing
[SLA] IOLs).
Advocates of SLA IOLs suggest that they
afford useful photoprotection by absorbing potentially harmful short-wavelength
visible light, reduce glare sensitivity, reduce postoperative cyanopsia, and improve contrast sensitivity.10 Opponents of
SLA IOLs suggest that they diminish photoreception, interfere with circadian
rhythms, and do not provide additional
useful photoprotection when compared
with conventional UV-absorbing IOLs.11
There is a clear divergence of opinion regarding the benefits and drawbacks of SLA
IOLs. This article examines the theoretical advantages and disadvantages of SLA
IOLs and also discusses the empirical studies of such lenses.
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3.0
Aged 20 y
Aged 70 y
SLA IOL
UV-absorbing IOL
Optical Density, AU
2.5
2.0
1.5
1.0
0.5
0.0
400
450
500
550
600
650
Wavelength, nm
Figure 1. Optical densities of the crystalline lens of the average patient aged 20 years and 70 years
together with those of a short-wavelength light–absorbing intraocular lens (SLA IOL) (AcrySof Natural
[Alcon]) and a conventional UV-absorbing IOL (AcrySof [Alcon]) plotted against wavelength.1 AU
indicates absorbance units.
0
Normalized Efficacy
–1
–2
–3
–4
–5
–6
300
350
400
450
500
550
600
650
700
750
Wavelength, nm
Figure 2. The so-called aphakic blue-light hazard function. Normalized log efficacy is plotted against
wavelength.
PHOTOPROTECTION AND
RETINAL DYSFUNCTION
The so-called sunlight age-related
macular degeneration (ARMD) hypothesis suggests that age-related
maculopathy (ARM)/ARMD results from sustained or repeated
exposure to high-energy shortwavelength electromagnetic radiation.12 As a consequence, some epidemiological studies have examined
the relationship between lifetime
exposure to sunlight and the development of ARM/ARMD. Results have
been mixed12,13; some studies find a
positive association, or such an association in patient subgroups,14-19
whereas others find no association20-22 or a negative association.23
However, the crystalline lens may be
an important confounding factor in
any epidemiological study investi-
gating the role of light in ARM/
ARMD. Specifically, variations in the
optical density of the lens pigments
modulate the dose of short-wavelength electromagnetic radiation
incident on the retina; estimates of
environmental light exposure are at
best estimates at the corneal surface rather than at the retinal surface. The effect of removing the crystalline lens is in itself a controversy.
Meta-analysis of the cumulative evidence (from 24 studies involving
113 780 subjects) suggests, however, that cataract surgery may be associated with an increased risk of late
ARMD,24 although the contribution of light to this increased risk remains uncertain. In addition, the
mechanism of ARMD is generally
agreed to be multifactorial; even if
we accept that short-wavelength
electromagnetic radiation expo-
ARCH OPHTHALMOL / VOL 130 (NO. 7), JULY 2012
920
sure may have a role in the etiology
of ARM/ARMD, many variables
could modulate the tissue response
to such radiation, including previous surgical intervention, diet and
other lifestyle factors, systemic disease, macular pigment density, and
genetic factors.13,15
Animal studies have investigated the effects of light exposure on
the retina, although such studies are
inherently limited to studying its
acute effects. Two types of light-induced damage are recognized. The
first, known as type I, was originally described by Noell and colleagues 8 and results from prolonged exposure to light within the
short- to medium-wavelength range
of the visible spectrum. The peak of
the action spectrum roughly coincides with that of the normal human scotopic sensitivity function,
which lead to the suggestion that the
effect may be mediated by the absorption of light by rhodopsin or one
of its intermediates.25 The second
type of induced damage, which occurs after exposure to shorter durations of more intense (by approximately 2 log U) light is known as type
II phototoxicity and was first described by Ham and colleagues.7 In
phakic animals, this so-called bluelight hazard peaks in the shortwavelength range of the visible spectrum and falls with increasing
wavelength. Such damage is not only
mediated by receptor photopigments, but also by lipofuscin absorption within the retinal pigment
epithelium.25 Ham et al26 demonstrated that aphakic rhesus monkeys—in whom the ocular media
transmit a significant proportion of
UV radiation to the retina—show an
increased susceptibility to retinal
phototoxicity (Figure 2).
In a thorough theoretical analysis, van de Kraats and van Norren27
measured the transmission spectra
and calculated the effects on visual
function and photoprotection of a variety of SLA IOLs (in order of increasing short-wavelength visible absorption: YellowFlex [PhysIOL],
AcrySof Natural [Alcon], Hoya AF-1
UY [Hoya Surgical Optics GmbH],
Optiblue [Abbott Medical Optics
Inc], and PC-440 Orange [Ophtec])
and conventional UV-absorbing
IOLs (AT45 [Eyeonics], AcrySof [Al-
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con], and Clariflex [Abbott Medical Optics Inc]). Their data clearly
show that all SLA IOLs are not equal,
and this finding raises the important issue of standardization. At present, no universally accepted means
exists to quickly and conveniently
convey the photoreceptive and photoprotective effects of IOLs.27,28 Furthermore, the data accumulated by
van de Kraats and van Norren27 show
that SLA IOLs that do not have sharp
cutoff characteristics (ie, those in
which absorption tapers off gradually with increasing wavelength) vary
significantly in their transmission
with varying lens power. They suggest that sharp cutoff filters may limit
this problem,27,28 although changes
to the manufacturing process could
also reduce the effect. Their calculations also suggest that the addition of an SLA filter does not necessarily render an IOL superior to all
conventional UV-absorbing IOLs in
terms of photoprotection. Indeed,
PhysIOL’s YellowFlex SLA IOL offered the worst of both worlds: inferior photoprotection with impaired photoreception when
compared with the best-performing conventional UV-absorbing IOL.
Such a lens had little to recommend
itself in terms of spectral transmission and is no longer commercially
available. Calculations by van de
Kraats and van Norren27 suggest that
all the remaining SLA IOLs should
increase the threshold for type II
phototoxicity by 0.26 to 0.52 log U
when compared with the bestperforming conventional UVabsorbing IOL (for northern skylight as the light source and using the
blue-light hazard spectrum of Ham
and colleagues7). Although they did
not directly calculate the theoretical
protective effect for type I damage, if
we assume its action spectrum resembles that of rhodopsin, the protective effect is more modest, ranging from 0.02 to 0.20 log U. In
addition, all conventional IOLs and
1 SLA IOL (YellowFlex) afforded less
protection than the average crystalline lens in a 20-year-old subject.
The theoretical protective effects of SLA IOLs are supported by
in vitro and animals models. Sparrow and colleagues29 irradiated lipofuscin A2E fluorophore–laden
retinal pigment epithelial cells with
light filtered by conventional UVabsorbing IOLs (AcrySof, Sensar
[Abbott Medical Optics Inc], ClariFlex, and CeeOn Edge [Pharmacia
and Upjohn]) or SLA IOLs (AcrySof Natural). They were able to
demonstrate that the photoprotective effect of the SLA IOL resulted
in a reduction of cell death by about
80%.29 Rezai and colleagues30 performed similar experiments using
retinal pigment epithelial cells irradiated with short-wavelength light
(430-450 nm) filtered through a conventional UV-absorbing IOL (Alcon AcrySof ) or an SLA IOL (AcrySof Natural). They found a 50%
reduction in cell death when this
light was filtered by the SLA IOL.
Marshall and colleagues31 demonstrated that proliferation of human
uveal melanoma cell lines stimulated by short-wavelength light (475
nm) was suppressed when the light
was first filtered through an SLA IOL
(Acrysof Natural); this suppression
was not observed with a conventional UV-absorbing IOL (AcrySof). Kurihara and colleagues32 exposed pseudophakic mice implanted
with bespoke IOLs created from an
SLA IOL (AcrySof Natural) or a conventional UV-absorbing IOL (AcrySof) to white light of an appropriate intensity and duration (5000 lux
for 24 hours) to induce retinal phototoxicity. They demonstrated that
animals undergoing implantation
with an SLA IOL showed significantly less retinal dysfunction—as
assessed by electroretinography—
and fewer apoptotic retinal cells on
histopathological examination than
animals implanted with a conventional UV-absorbing IOL.32
Although modeling and in vitro
studies using retinal pigment epithelial cells may apply to the bluelight hazard, this itself is an acute
condition. Extrapolating such data
to model chronic retinal conditions, such as ARM/ARMD, may not
be appropriate.11 Similar reservations should be held in regard to in
vitro studies using uveal melanoma cell lines33 and to murine models of acute retinal phototoxicity.
Nevertheless, short-term photic
maculopathy may be underrecognized; for example, as many as onethird of ophthalmologists who view
operating microscope lights and/or
ARCH OPHTHALMOL / VOL 130 (NO. 7), JULY 2012
921
perform retinal laser procedures
without appropriate barrier protection have been shown to have an
asymptomatic acquired tritan color
vision deficiency.34 The magnitude
of this deficiency appears to correlate to the dose of light exposure,34
although the action spectrum of this
effect is unknown. Evidence also
suggests that individuals with yellower crystalline lenses maintain better short-wavelength cone function
in later life,35 suggesting a photoprotective or an adaptive effect of filtering short-wavelength visible light.
In a small retrospective study of
pseudophakic patients by Miyake
and colleagues,36 the incidence of
postoperative blood–retinal barrier
breakdown and fundus autofluorescence was higher in those with conventional UV-absorbing IOL implants (Hoya MC-5 [Hoya Surgical
Optics GmbH]) when compared
with those with SLA IOL implants
(Hoya UVCY [Hoya Surgical Optics GmbH] or the Yellow-Colored
IOL [Menicon Co, Ltd]). More recently, Nolan and colleagues37 used
a psychophysical paradigm to investigate the effects of cataract surgery
on macular pigment optical density in a group of 42 patients undergoing cataract extraction and implantation of an SLA IOL (AcrySof
Natural) or a conventional UVabsorbing IOL (AcrySof). These authors suggest that implantation of an
SLA IOL is associated with a progressive increase in macular pigment optical density from 3 months
and as long as 1 year after surgery
whereas implantation of a conventional UV-absorbing IOL is not associated with a significant change.
In a subsequently published nonrandomized study, Obana and colleagues 38 used Raman spectroscopy to directly assess macular
pigment optical density in 259 subjects with an SLA IOL implant (AcrySof Natural) or a conventional UVabsorbing IOL implant (Acrysof ).
Although they found that both types
of lens were associated with a decline in macular pigment optical
density, those receiving SLA IOL implants had significantly higher densities from 1 year postoperatively until their final follow-up point of 2
years. The increased macular pigment optical density in those with
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Rods
S cones
M cones
L cones
1.0
0.9
Normalized Sensitivity
0.8
Melanopsin
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
400
450
500
550
600
650
Figure 3. Normalized spectral sensitivity curves for the photoreceptors of the retina and melanopsin.
Data for the photoreceptors are the estimates of Dartnall and colleagues42 for human photoreceptors.
Melanopsin sensitivity is estimated for a photopigment with a peak at 480 nm using the photopigment
template of Lamb.43 Normalized log sensitivity is plotted against wavelength.
SLA IOLs—when compared with
conventional UV-absorbing IOLs—
may reflect a decreased consumption of macular pigment by retinal
biochemical processes triggered by
short-wavelength light absorption.
The findings of Nolan and colleagues37 would suggest that such a
mechanism may not act alone—at
least in the year after surgery—
because macular pigment optical
density was found to be lower in the
phakic state (ie, when shortwavelength light absorption would
have been even higher than with an
SLA IOL). Whatever the mechanism of increased macular pigment
optical density, this finding warrants further investigation because
it may in turn afford improved photoprotection,39 protection from oxidative stress,39 and optimization of
red-green color discrimination via its
notch filter absorption profile40 (although possibly at the expense of
tritan discrimination40).
Despite the theoretical considerations and empirical results discussed thus far, no studies have been
published that directly support the
hypothesis that the photoprotection afforded by SLA IOLs results in
a decreased incidence of macular
dysfunction. In fact, limited evidence to the contrary exists: a recent study by Kara-Junior and colleagues 41 of 30 patients with no
preoperative retinal pathology undergoing cataract extraction and implantation of discordant IOLs (AcrySof Natural/AcrySof) suggested no
interocular difference in retinal structure (as assessed by optical coherence tomography), visual acuity, or
contrast sensitivity during a 5-year
period after surgery. The limitations of this study include a small
sample size and the fact that the results may not be generalizable to
patients who could be especially vulnerable to increased short-wavelength light exposure, such as those
with preexisting retinal disease or
young patients, who would potentially face several decades of increased exposure to such radiation.
PHOTORECEPTION AND
VISUAL FUNCTION
A Note on Light Sources,
Reflection Spectra, Transmission
Curves, and Photon Detection
All natural and most artificial light
sources emit light across a wide range
of wavelengths. Similarly, the reflectance spectrum of almost all objects
is broad. Furthermore, the visual pigments absorb light across a wide range
of wavelengths (Figure 3). For these
reasons, the effects of filters such as
SLA IOLs under real circumstances
may be minimal and restricted to special circumstances, such as visual
comparisons in which a target differs from a background in its emission or reflectance within the shortwavelength region of the visible
spectrum. The simplest way to demonstrate the inferiority of SLA IOLs
is to exploit this prediction by test-
ARCH OPHTHALMOL / VOL 130 (NO. 7), JULY 2012
922
ing sensitivity to near-monochromatic short-wavelength stimuli under conditions in which Weber’s law
breaks down, to present such stimuli
against a background of a longer
(dominant) wavelength, or to perform tests that assess relative sensitivity to short- and long-wavelength
stimuli. The cynic would suggest that
such studies merely recapitulate laboratory absorbance measures, albeit
with a noisy and imprecise photodetector (ie, the human visual system). However, such studies are essential in order to place the effects of
SLA IOLs into context given the interindividual and intraindividual variances in sensitivity known to affect
psychophysical estimates and the possibility of medium- to long-term adaptive mechanisms.35 In addition, the
anticipated effects of SLA IOLs are not
equal and depend on their absorption profile27; these differences may
account for some of the discrepancies in findings between studies.
Color Vision
Because SLA IOLs influence the
spectral quality of light incident on
the retina, one of the anticipated deleterious effects of such lenses is on
color vision. Compared with conventional UV-absorbing IOLs, SLA
IOLs would be anticipated to effectively decrease the chromaticity difference between warm and cool colors (ie, they should induce a tritan
color vision deficiency).44 In theory,
this could be compounded by the
recently reported increases in macular pigment optical density found to
be associated with SLA IOL insertion. 37,38,40 Conversely, one predicted benefit would be a reduction in postoperative cyanopsia.
Experimental studies show that
although a theoretical loss of tritan
color discrimination should result,
the effect is generally too small to be
detected by clinical tests of color vision, such as the FarnsworthMunsell D-15 panel test (and its
derivatives),45-48 the FarnsworthMunsell 100-Hue test,41,47,49-58 and the
Moreland equation,46,59 under photopic conditions. Experiments using
central color contrast detection tasks
in patients with discordant IOL implants in either eye (Hoya AF-1 UV/
Hoya AF-1 UY) also suggest that
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color discrimination is not adversely affected by SLA IOLs.60 One
study that used external filters to
mimic the effects of SLA IOLs in
pseudophakic AMD patients implanted with conventional UVabsorbing IOLs suggests a decreased performance in a practical
task of color discrimination: the sorting of black from navy socks.61 However, in comparison with the SLA
IOL the authors sought to replicate, the external filter attenuated
more short wavelength light.61,62 Several studies suggest that tritan discrimination is slightly impaired in
those with SLA IOLs under mesopic conditions,48,49,56,63 and 1 study
found that photopic tritan discrimination is slightly impaired in the
early (6 months) but not the late (12
months) postoperative period. 63
However, the measured impairment of mesopic tritan discrimination is small and certainly far less
than what should be produced by the
average crystalline lens of agematched normal eyes (Figure 1).
A few studies have investigated
the effects of SLA IOLs on shortwavelength automated perimetric64
thresholds in patients with discordant IOL implants (AcrySof/
AcrySof Natural65,66 and Hoya AF-1
UV/Hoya AF-1 UY).46 The reduction in average sensitivity ranged
from 1.6 dB 66 (AcrySof/AcrySof
Natural) to 2.7 dB46 (Hoya AF-1 UV/
Hoya AF-1 UY). The difference was
found to achieve statistical significance in two46,66 of the 3 studies.
Short-wavelength light–absorbing IOLs appear to reduce the incidence of cyanopsia significantly in
the early postoperative period,55,67 although patients generally adapt to
such chromatopsias within a few
months of cataract surgery regardless.67
Scotopic Vision
The rod photoreceptor’s peak wavelength of sensitivity (␭max) lies at
about 495 nm,42,68 which is approximately 60 nm below the visual system’s peak sensitivity in photopic
conditions (see Figure 3). 69 Because of this fact and because under extremely low luminance levels sensitivity is independent of
background illumination, it has been
proposed that scotopic sensitivity
should be reduced by SLA IOLs. Calculations suggest that the effect is
modest.27,70,71 Data for a variety of
SLA IOLs from van de Kraats and
van Norren27 suggest a theoretical reduction in sensitivity in rods ranging from 0.04 to 0.22 log U (compared with the best-performing
conventional UV-absorbing lens).
However, their study used daytime
skylight as the assumed light source.
Because the natural scotopic light
source—the night sky—is often rich
in long-wavelength light,72,73 these
calculations may represent an overestimation in the negative effects of
such lenses. Our visual system is well
adapted to extract information about
relative differences in brightness/
lightness, thus negating the effect of
fixed filters under many circumstances; if the spectral quality of the
target and background are identical, then one would predict no reduction in scotopic sensitivity under lighting conditions where
Weber’s law holds. Reductions in
sensitivity should only be evident
under very low lighting conditions
(ie, at absolute threshold), and such
conditions are seldom encountered
in the modern world.74 Furthermore, these calculations overlook
the possibility of long-term compensatory mechanisms.
Despite the fact that absolute
threshold should theoretically be
minimally increased by SLA IOLs,
empirical studies have failed to demonstrate such differences in practice. Greenstein and colleagues51 examined scotopic sensitivity in 9
patients with discordant IOL implants (Acrysof/Acrysof Natural) to
narrowband stimuli (440, 500, and
650 nm) as well as to a white stimulus. Although the authors found a
trend toward slightly lowered sensitivity in eyes with Acrysof Natural IOL implants, such differences
were minimal (⬍1 dB) and did not
achieve statistical significance in
their small sample. Muftuoglu and
colleagues59 examined the effects of
SLA IOLs on scotopic vision using
a contrast detection paradigm with
and without a glare source in the
field of vision. They found no significant difference in scotopic contrast sensitivity between 38 eyes with
SLA IOL implants (AcrySof Natu-
ARCH OPHTHALMOL / VOL 130 (NO. 7), JULY 2012
923
ral) and 38 eyes with conventional
UV-absorbing IOL implants (AcrySof ). Kiser and colleagues61 attempted to assess the effects of SLA
IOLs on scotopic visual function in
22 elderly pseudophakic patients
with AMD. All their subjects had bilateral conventional UV-absorbing
IOLs, and their performance at a scotopic threshold task was assessed
with and without an external filter
supplied by an IOL manufacturer
and designed to mimic an SLA IOL
(AcrySof Natural). Performance was
not found to be significantly affected by the filter.61
Contrast Detection and Glare
Yellow spectacle lenses are anecdotally believed to improve vision under certain conditions, and some empirical studies support the assertion
that they may slightly improve contrast sensitivity.75 Part of the potential benefit may be from eliminating those wavelengths that are
especially vulnerable to scatter and
(in phakic subjects) that induce crystalline lens fluorescence.76 An additional hypothesis is that such lenses
may improve the contrast of objects presented against backgrounds that are comparatively
richer in short-wavelength light.76
Most studies find no difference in
contrast sensitivity between eyes undergoing implantation with conventional UV-absorbing IOLs and SLA
IOLs.* Niwa and colleagues77 assessed contrast sensitivity functions
in patients undergoing implantation with conventional UV-absorbing IOLs (Hoya UV) or SLA IOLs
(Hoya UVCY). They found a statistically significant superiority for SLA
IOLs at medium to low spatial frequencies under photopic and mesopic conditions. Yuan and colleagues55 similarly found that contrast
sensitivity at middle to low spatial frequencies was slightly, although significantly, superior in patients undergoing implantation with SLA IOLs
(lenses not specified). A recent study
by Gray and colleagues78 investigating subjects’ performance at a simulated driving task in the presence of
a glare source suggests that those im*References 41, 45, 47, 48, 53, 54, 58, 59,
63, 67.
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planted with SLA IOLs (AcrySof
Natural) perform significantly better than those implanted with conventional IOLs (AcrySof ). Hammond and colleagues4 investigated
the effects of glare and photobleaching on patients with discordant IOL
implants (AcrySof/AcrySof Natural). Their results suggest SLA IOLs
provide a significant reduction in susceptibility to glare and improved photostress recovery. This study supported their previous findings in
patients with the same type of IOL
implants bilaterally.79 Wang and colleagues56 found evidence to suggest that low-contrast acuity was reduced by SLA IOLs under mesopic
lighting conditions (Hoya AF-1 vs
MC 611 MI [HumanOptics]).
Wirtitsch and colleagues 46 similarly found a small but significant decrease in contrast acuity associated
with SLA IOLs that appears to arise
from differences under mesopic conditions (discordant Hoya AF-1 UV/
Hoya AF-1 UY IOLs). Pierre and colleagues80 used a minimum-motion
paradigm of a red-blue grating to assess changes in contrast perception. This study effectively measured the effect of SLA IOLs on the
spectral sensitivity function of their
patients, and perhaps unsurprisingly found a small but significant
reduction in relative sensitivity to the
blue portion of their stimulus.
Circadian Rhythm
One of the most intriguing findings
in visual science during the past decade or so is that of the intrinsic photopigment of the retinal ganglion cells,
melanopsin, which accounts for the
partial maintenance of circadian
rhythm in animals deficient of rods
and cones.81 Melanopsin’s ␭max lies at
about 480 nm (see Figure 3),82 and
thus SLA IOLs would be anticipated
to reduce light absorption by this
photopigment. The theoretical reduction in light absorption by melanopsin caused by SLA IOLs, when
compared with the best-performing
conventional UV-absorbing IOLs, is
small83-85 and has been estimated to
range from 0.08 to 0.27 log U (assuming northern skylight as the
source).27 Such a small reduction is
unlikely to be of consequence for several reasons. First, such consider-
ations overlook possible compensatory adaptation mechanisms 8 6 ;
second, rods and cones have known
input into the circadian mechanism87; and third, complete abolition of light absorption by melanopsin has a surprisingly modest negative
effect on circadian cycles.87
The only published empirical evidence comes from a small retrospective study. Landers and colleagues88
administered a standardized questionnaire on sleep patterns to 49
patients with conventional UVabsorbing IOL implants (31 patients with S140NB [Abbott Medical Optics]) or SLA IOL implants (18
patients with AcrySof Natural) during the year preceding their study.
They found no significant difference in test scores between the 2
groups, although it should be noted
that their study was conducted in a
geographical region remarkable for
its sunny climate. It has been argued that reduced photoreception by
the circadian mechanism may have
greater implications for those exposed to chronically low light levels, such as those living at high latitudes and/or those who spend
extended periods indoors.9
COMMENT
Although SLA IOLs should theoretically affect color discrimination and
absolute thresholds, rigorous analysis suggests that these effects would
be minimal27 and reserved for special testing conditions. Furthermore, empirical studies have failed
to demonstrate any functional difference in scotopic vision51,59,61 or
color discrimination at clinical color
vision tests under photopic conditions.45-47,49-54 Sensitivity at certain
specialized visual tasks (eg, shortwavelength automated perimetry, 4 6 , 6 6 mesopic tritan color
discrimination)48,49,56 has been found
to be reduced in some studies; however, such differences are small and
of questionable functional significance. Although it has been argued
that light absorption by melanopsin should be reduced by SLA IOLs,
the calculated effect is modest and
likely to be of little consequence to
circadian rhythms.27,83-85 This conclusion is supported by the currently available empirical data.88 A
ARCH OPHTHALMOL / VOL 130 (NO. 7), JULY 2012
924
few studies4,55,77,78 support the assertion that SLA IOLs may improve contrast sensitivity and susceptibility to
glare compared with conventional
UV-absorbing IOLs, although several studies suggest no significant difference† and two suggest a slight decrement in contrast sensitivity.46,56
In terms of photoprotection, UV
radiation and short-wavelength visible light have clearly been demonstrated to be capable of producing
retinal phototoxicity (with the efficacy of such radiation falling with
increasing wavelength).26 Consequently, experts generally concur
that UV absorption by IOLs is recommended; however, opinions differ on the absorption of shortwavelength visible light. Although
exposure to high levels of shortwavelength visible radiation can certainly produce acute retinal phototoxicity in animals and although
occupational exposure to intense
light sources may produce subclinical photic maculopathy,34 epidemiological studies investigating the role
of light in ARM/ARMD have produced mixed results.12,13 Studies have
demonstrated that SLA IOLs provide a protective effect in vitro29,30
and in vivo in a murine model.32 Furthermore, they have been shown to
be associated with a decreased
blood–retinal barrier breakdown in
a small retrospective clinical trial.36
Recent clinical trials also suggest that
SLA IOLs are associated with increased macular pigment optical
densities postoperatively.37,38 No differences in retinal structure were
found, however, in a small clinical
trial performed on middle-aged patients with no preoperative retinal
pathology who underwent implantation with discordant IOLs.41
The normal crystalline lens absorbs UV and short-wavelength visible radiation from an early age. At
this time, no convincing theoretical arguments or empirical data suggest that the filtering of shortwavelength visible radiation should
be eliminated at the time of cataract
surgery. The choice of IOL clearly
remains at the discretion of the surgeon, who can weigh the perceived
benefits and drawbacks of each type
†References 41, 45, 47, 48, 53, 54, 58, 59,
63, 67.
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of lens for the individual patient; this
choice could be aided in the future
by a standardized means of quantifying the photoprotective and photoreceptive effects of IOLs.27,28 In the
absence of conclusive evidence, the
logical default should be to replace
the crystalline lens with an SLA IOL
that mimics the absorption properties of the normal adult lens within
the short-wavelength range of the
visible spectrum until empirical evidence clearly supports a case for doing otherwise.
Author Affiliations: Sydney Eye Hospital and Save Sight Institute, University of Sydney, Sydney, Australia.
Correspondence: Matthew P.
Simunovic, MB, BChir, PhD, Sydney Eye Hospital, 8 Macquarie St,
Sydney, New South Wales, Australia 2000 ([email protected]).
Financial Disclosure: None reported.
Additional Contributions: Jim
Schwiegerling, PhD, provided the
transmission data for AcrySof IOLs.
12.
13.
14.
15.
16.
17.
18.
19.
20.
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