The results of the Rayleigh match in this study showed no
difference between the diabetic patients and the control group, a
finding in agreement with Elsner et al.,
46 who found that
at low illuminance levels the Rayleigh match was normal in diabetes and
that the optical density of the M and L cone photopigments was normal.
At higher levels of illumination, the diabetic patients exhibited
bleaching abnormalities, a finding that was not investigated in this
study.
The experiments designed to measure the lens and macular pigment
densities relied on the underlying assumption that the cone absorption
spectra are the same in both the diabetic and control groups. If the
optical density of a cone photopigment changes, there is a
corresponding change in the absorption spectrum—the bandwidth
broadening for increasing optical density and narrowing for decreasing
optical density.
47 In a disease state leading to cone
dysfunction, it is possible that the photopigment density could be
reduced, because of absence of pigment production or of disc
replacement in the diseased receptor outer segment. This could lead to
a change in the color-match ratios that in turn would give an apparent
change in the optical density of the prereceptoral filters in the
diabetic patients. Although M-cone photopigment density is probably
normal in diabetes, there is evidence of S cone pathway
dysfunction.
48 49 Later studies suggest that this
localizes to the postreceptoral pathway
50 51 and the
dysfunction thus may not reflect an S-cone change. However, a model was
developed (see Appendix) to investigate the effect of cone dysfunction
on the color-match ratios and the derived media optical densities,
based on a consideration of the cone excitations and the effect of
reduced photopigment concentration and decreased outer segment length.
The results are shown in
Figure 4 which shows that, overall, the effect of changes in the cones on the
Rayleigh match, apparent lens density, and apparent macular pigment
density is small, even for a 2 log unit (100-fold) decrease in the
product of cone photopigment concentration and outer segment length.
The one-tailed 95% confidence intervals for Rayleigh match in the
control subjects and the diabetic patients are plotted in
Figure 4 (top), indicating that the spread of data in this experiment is well
within the bounds of the change predicted by the model (i.e., M and L
cone optical densities are not significantly affected by diabetes).
The estimated lens optical density would be decreased in the case of S
cone photopigment loss (
Fig. 4 , middle), which is opposite to the
effect seen in the results of the 420- and 515-nm color match.
The model was also used to investigate apparent changes in macular
pigment optical density that may occur due to cone dysfunction in
foveal and extrafoveal locations. A 2-log-unit reduction in S cone
photopigment and outer segment length product in the fovea, maintaining
normal S cones in the extrafoveal location, would result in an
underestimation of the macular pigment density by 0.05 log units (
Fig. 4 , bottom). This does not explain the reduction measured in the
diabetic patients. An apparent reduction in macular pigment density
caused by S-cone change would also be accompanied by an underestimation
of the optical density of the crystalline lens, as all the patients who
performed the macular pigment color matches had also performed the lens
density match. Our results for the optical density of the lens agree
closely with those obtained by Lutze and Bresnick.
11 The
two methods used in the present study to derive a diabetes
duration-dependence relationship gave values of 0.019 log units/year
and 0.021 log units/year, which agree with the result of 0.018 log
units/year derived by Lutze and Bresnick. Estimating the age of a
lens-matched normal subject using
equation 3 agrees well with the
equations from Moreland,
12 who combined the Lutze and
Bresnick data with the two-component model of Pokorny et
al.
5
The similarity of the results in the two studies is particularly
important, considering the different experimental methods used. Lutze
and Bresnick
11 measured dark-adapted absolute thresholds
to two wavelengths equally absorbed by rhodopsin. They conducted
further experiments to verify that rods were the receptors involved in
detection rather than cones. Our method uses a color threshold at low
photopic luminance, which relies on cones. The similarity of the
results obtained in the present work in comparison with those obtained
previously suggests that they represent a true ocular media change
rather than an apparent change caused by receptoral abnormality in the
foveal cones of the diabetic patients.
The results of autofluorescence studies have shown an increase in the
lens absorption in diabetes.
6 7 8 9 10 The origin of the
increased light loss in the short-wavelength end of the spectrum is not
entirely clear, but it has been suggested that it is due to an
accumulation of advanced glycosylated end products
(AGEs).
8 52 Autofluorescence studies have shown that
glycosylated collagen absorbs at 370 nm and emits at 440
nm,
53 and that autofluorescence occurs at these
wavelengths in “browned” lenses.
54 55 Browned lenses
also autofluoresce at other wavelengths,
56 with a
significant emission at 520 nm.
9 10 In diabetes, the
long-term exposure of the delicate lenticular environment to
hyperglycemia is likely to lead to an increased accumulation of AGEs,
with accompanying optical effects. AGEs have been implicated in the
pathogenesis of complications of diabetes
57 58 and
specifically in the formation of cataract.
59 60 61 62 Glycosylation of lens α-crystallin has been measured in excised
diabetic and normal lenses.
63 The diabetic lenses had a
threefold increase in α-crystallin glycosylation in comparison with
normal lenses, although there was no significant difference in the
degree of lens browning between the two groups in this study. The
authors suggest that differences in lens browning between diabetic
patients and control subjects may be due to glycosylation of proteins
other than crystallins. Studies of bovine lens have shown that
nonenzymatic glycosylation occurs both in the
crystallins
64 and the membrane proteins.
65 Further study may be able to identify the relative contribution to
overall lens browning resulting from the glycosylation of different
proteins.
Lutein and zeaxanthin are the only carotenoids present in the
lens
66 and are concentrated in the lens epithelium and
cortex.
67 A high dietary intake of carotenoids has been
linked with a reduced incidence of nuclear cataract
68 (although this study provided only weak support for the association),
and a reduced need for cataract extraction is seen in
women
69 and men
70 in the United States with
high carotenoid intake. The use of vitamin supplements containing
vitamin C and E for longer than 10 years may also lower the risk of
cataract,
71 suggesting a protective role of these
antioxidants. In our study there was no association between lens
optical density and macular pigment optical density for either the
control group or the diabetic patients
(Fig. 3) . The wavelengths used
to measure lens density are equally absorbed by lutein and zeaxanthin,
and consequently the result of this color match would be unaffected by
both macular and lenticular carotenoid concentrations.
Macular pigment optical density showed no dependence on age in our
study, a finding that is in agreement with some previous studies in
normal subjects,
21 22 23 although a small age-dependent
effect has also been reported.
25 26 The possibility of an
age-related decline in macular pigment density is not resolved at
present.
There are several mechanisms by which macular pigment levels could be
reduced in diabetes. First, there may be a genetic influence. There is
a wide variation in macular pigment density in the patients with no
maculopathy
(Fig. 2) , which makes a strong genetic influence on macular
pigment density in diabetic patients unlikely. Macular pigment density
has been measured in monozygotic twins,
72 with results
suggesting that pigment levels are not entirely genetically determined.
Second, the diabetic diet could be deficient in lutein and zeaxanthin
or absorption from the gut could be reduced. Granado et
al.
73 studied the serum levels of antioxidants in a group
of European insulin-dependent diabetic patients, first-degree
relatives, and control subjects. They found no significant difference
in serum levels of lutein and zeaxanthin between groups, although the
diabetic group had lower levels of retinol and higher levels ofβ
-carotene, α-carotene, and β-cryptoxanthin than did first-degree
relatives without diabetes. However, Ford et al.
74 found a
significant reduction in the serum levels of macular carotenoids in
patients with newly diagnosed and established diabetes patients in the
United States in comparison with normal subjects. These findings may
relate to different diets in the two study populations. Study of the
absorption of carotenoids in diabetic persons would help to resolve
this issue.
Finally, the pigment density could become low as a result of a reduced
rate of incorporation into retinal tissue or an increased rate of
removal from the retina. Thickening of basement membranes of retinal
capillaries in diabetes,
75 76 77 78 the increased affinity of
oxygen for glycosylated hemoglobin,
79 the existence of a
redox shift due to the effects of hyperglycemia on glycolysis and
sorbitol metabolism,
80 and the presence of abnormal
vasculature in the parafovea of diabetic persons
81 imply
that diabetic retinas are under continuous oxidative stress. An
analysis of retinal tissues from primate and human eyes for oxidation
products of lutein and zeaxanthin showed that, indeed, these pigments
appear to play a role as antioxidants.
28
Analysis of the data in this study has shown that the only factor with
a statistically significant correlation with lower levels of macular
pigment among the diabetic patients was grade of maculopathy. Because
the grade of maculopathy provides an indication of the severity of
microvascular disease in the macula, this may imply that lower pigment
levels are found in diabetic maculae that are under greater oxidative
stress. The macular pigment density in an individual is likely to
represent an equilibrium value of rate of incorporation into retinal
tissue combined with rate of removal from tissue, including conversion
to other compounds. A low macular pigment level could result from
either reduced incorporation into retinal tissue or from increased rate
of removal. It is possible that increasing antioxidant protection to
the diabetic retina may reduce the probability of development of
microvascular complications. In the San Luis Valley Diabetes study of
antioxidants in diabetes, the effect of dietary and supplement intakes
of vitamin C, vitamin E, and β-carotene on progression of diabetic
retinopathy was examined in patients with type II
diabetes.
82 There was no observed protective effect of
these nutrients against diabetic retinopathy, and indeed among those
patients not taking insulin, increased vitamin E intake was associated
with an increased risk for severity of retinopathy, as was increased
intake of β-carotene in patients taking insulin. This study
highlights the complexity of the relationship between antioxidants and
diabetic retinopathy, but the effect of dietary intake of lutein and
zeaxanthin on grade of maculopathy was not specifically assessed nor
were serum levels of any antioxidant measured.
Hammond et al.,
83 showed that macular pigment levels could
be modulated by diet in some but not all subjects. Of 11 participants,
8 showed increased serum levels and macular pigment density, 2 showed
an increase in serum level but not in macular pigment density, and 1
showed no response in either parameter. The increase in macular pigment
optical density persisted to the posttest time point (range, 1–6
months after cessation of the diet).
Our study showed that diabetic patients have increased differential
lens optical density and reduced levels of macular pigment. The
lenticular changes probably result from an accumulation of AGEs. Lens
density may also be affected by oxidative stress in diabetic persons,
although the color match used in the study cannot provide any
information regarding the concentration of carotenoids in the lens. The
results of a study of antioxidant levels in the diabetic lens,
including lutein and zeaxanthin, would be very interesting indeed.
Reduced macular pigment density may result from increased oxidative
stress in the diabetic macula. Further study is indicated to pinpoint
the cause more precisely. A controlled trial of dietary modification
with serial measurement of serum lutein and zeaxanthin and macular
pigment optical density would be of interest in patients with diabetes.
If pigment density does not increase after a dietary supplement, the
incorporation of the carotenoids into retinal tissues would be at
fault. Conversely, demonstration of an increase in pigment optical
density would indicate that an increased rate of elimination of pigment
from the tissues is the likely reason for the observed low value.