In this study, we used a free-viewing HFP to measure the optical
density of MP in 46 healthy subjects and demonstrated an age-related
decline in the optical density of the pigment in a group from a
northern European population. Furthermore, we observed significantly
less MP in nine healthy eyes known to be at high risk for AMD compared
with nine matched eyes at no such risk.
The mean optical density of MP in this study was 0.289 ± 0.156,
higher than measurements taken in 217 subjects from Arizona (0.22 ± 0.13) in a recent study,
54 but comparable with most
previous reports.
55 56 There are two possible explanations
for the discrepancy between our results and those of the Arizona study.
First, Hammond and Caruso–Avery used a reference for MP measurement at
only 4° retinal eccentricity, where MP is still optically detectable,
and slightly lower values are therefore unsurprising. And second, all
subjects reported in the current study have been living in the
northwest of England since childhood, where ambient light levels are
much lower than in Arizona, and it is possible that MP is depleted in
response to the oxidant load arising from greater cumulative light
exposure. The interindividual variability of MP measurement among our
subjects, represented by a range of 0.0124 to 0.646 ± 0.156,
is entirely consistent with previous reports.
54 57
The current results showing an age-related decline in MP optical
density are contrary to some of the early studies
57 58 but
consistent with the most recent.
54 It is important to
note, however, that the early reports took no account of recently
identified variables that are believed to be related to the optical
density of MP, such as iris color,
59 tobacco
use,
60 gender,
55 and lens
density.
61 In 2000, Hammond and
Caruso–Avery
54 reported a statistically significant
inverse relationship between the optical density of MP and age
(
r = −0.14;
P < 0.02) among subjects
living in Arizona. However, it was unclear whether the ages of the
study group were related to the variables that are associated with MP
density. This is of particular importance, because those associations
were confirmed in that study.
54 In the present study,
there was no relationship between any of these variables and age. It is
of interest that age and years spent in Arizona were positively
correlated in Hammond and Caruso–Avery, allowing for the possibility
that the age-related decline in MP optical density was attributable to
cumulative exposure to very high ambient levels of light. That we have
reproduced this finding in subjects from a northern European population
indicates that other factors play a role.
The age-related decline in MP optical density must be attributable to
either inadequate uptake or excessive depletion of the retinal
carotenoids. The decline of MP optical density with increasing age may
simply reflect the age-related loss of photoreceptors and their axons
in which L and Z are found,
62 especially in view of the
demonstration by Elsner et al.
63 of the close spatial
relationship between cone photopigment and MP distribution. However,
because cones are relatively spared in age-related loss
64 and because our technique of HFP excluded rod contributions, this is
unlikely. Alternatively, depletion of MP may result from utilization of
L and Z in response to the age-related increase in oxidant
load.
65 66 67 68
Given that micronutrient deficiencies are seen in 18% to 40% of the
elderly population,
69 70 71 the age-related decline in MP
may be nutritional in origin. However, because dietary carotenoid
intake is difficult to measure, reflected in the wide variability of
reported values for L and Z consumption (0.8–4
mg/day),
72 73 this is difficult to investigate. Of note, a
recent study has confirmed a significant and positive correlation
between dietary intake of carotenoids and serum levels in the elderly,
with the exception of L and Z, indicating that the macular carotenoids
may be inadequately absorbed in this age group.
74 Further,
because carotenoids act synergistically with α-tocopherol and
ascorbate, deficiency of either of these vitamins results in excessive
depletion of its carotenoid coantioxidant.
75 76 Beyond
dietary and absorptive factors, it is also possible that age-related
changes in carotenoid transport in blood and accumulation of L and Z in
the retina may be important.
Of nine eyes at high-risk of AMD and nine eyes at no such risk, when
paired eyes were matched on the basis of variables associated with MP
optical density, less MP was seen in the predisposed eye in eight
cases. The bilaterality of AMD has recently been investigated in the
Blue Mountains Eye Study, in which it was reported that both eyes were
affected in 80% of patients with early or late AMD.
77 Of
30 fellow eyes in subjects with unilateral neovascular AMD, early AMD
was seen in 20 (66%), atrophic AMD in 7 (23%), and a healthy macula
in only 3 (10%). The second eye of patients with unilateral
neovascular AMD is at high risk of development of the condition because
of the significant age-related increase in the bilaterality of
neovascular AMD, even after adjusting for variables such as age,
smoking, and family history.
39 77 The incidence of
choroidal neovascularization in the contralateral eye in cases of
unilateral neovascular AMD has been estimated to lie between 28% and
35% at 4 years,
39 40 78 79 with a 12% risk per
annum.
41 Of the nine high-risk eyes reported here, three
exhibited soft drusen with pigmentary changes within 18 months of
testing, and a further three showed these changes with choroidal
neovascularization.
The relative absence of MP in eyes predisposed to AMD should be
interpreted in the context of the excellent interocular agreement of MP
measurements demonstrated in this study and in previous
studies.
80 81 In other words, low MP optical density in
the fellow eye of a patient with neovascular AMD indicates that MP was
probably absent in the diseased eye, although the latter cannot be
measured because of fibrovascular scarring and loss of central vision.
Further, because we matched eyes in terms of putative risk factors for
AMD, which are reportedly associated with MP optical density, the
observed absence of MP in the predisposed eyes appears to be an
independent association with high risk for AMD. However, whether this
deficiency of MP resulted in neovascular AMD in the diseased eye and
will do the same in the healthy eye or is the result of subclinical
disease warrants discussion.
Because MP is located within some part of the photoreceptor
cell
82 or its membrane,
83 and because cone
and rod systems appear to be functionally impaired in early
AMD,
84 85 it is possible that photoceptor loss in
preclinical AMD may result in depletion of the pigment. However, this
mechanism is unlikely to have played a role in our study, because
sensitivity was similar for AMD-predisposed eyes and age-matched
control eyes at the parafovea, where AMD typically begins. Further,
Curcio et al.
62 have shown a differential loss of rods and
cones in AMD, with sparing of foveal cones and relative sparing of
parafoveal cones in early disease. In brief, therefore, because the
healthy predisposed eyes exhibited no clinical signs of disease and
rods did not contribute to HFP measurements because we used a frequency
of 25 Hz, we do not believe that photoreceptor dropout accounts for the
absence of MP we observed in these eyes. This conclusion is consistent
with the findings of Bone et al.,
86 who found that L and Z
concentrations, as measured by high-performance liquid chromatography
(HPLC), were significantly reduced in the central and peripheral retina
of eyes with AMD, suggesting that the loss of retinal carotenoids is
not the result of the disease process.
It is interesting, however, that some predisposed eyes had greater
quantities of MP than some of the standard-risk eyes. In other words,
our finding does not support the view that there is a critical value
below which AMD is likely to develop. Rather, the results suggest
depletion of preexisting MP and are therefore consistent with the view
that the retinal carotenoids are used in response to an age-related
process, possibly oxidative stress.
66 Clearly, the main
limitation of the present study rests on the small sample size, which
reflects the rarity of healthy fellow eyes in patients with unilateral
neovascular AMD.
The evidence in support of the view that MP protects against AMD has
been reviewed elsewhere.
87 The Eye Disease Case–Control
Study (EDCC) reported that a high dietary intake and high serum levels
of L and Z were associated with a reduced chance of development of
AMD.
36 88 Parallels between several putative risk factors
for AMD and an absence of MP have been observed by Hammond et
al.,
59 including light iris color, cigarette
smoking,
60 female gender,
55 loss of visual
sensitivity,
89 and increasing lens density.
61 Furthermore, reduced concentrations of L and Z have been demonstrated
in the macula and whole retina of human donor eyes with early AMD
compared with control subjects.
90 Weiter et
al.
91 noted that the area of central sparing seen in cases
of annular macular degeneration, including cases of atrophic AMD,
correlated strongly with the lateral extent of MP, which may be due to
the absorptive properties of the pigment, in that the lipofuscin
fluorophore A2E is known to mediate blue-light–induced apoptosis of
RPE cells.
92 93 Indeed, it has even been suggested that
the focal reduction in RPE lipofuscin concentration at the fovea is
attributable to the protection afforded to the photoreceptor outer
segments, the phagocytosed elements of which contribute to
lipofuscinogenesis
94 by the MP.
95 96 Although
these findings are consistent with the plausible rationale that MP
protects the central retina from blue light damage and oxidative
stress, they should be interpreted in the context of our current and
incomplete understanding of the disease and with full appreciation of
the limitations of the observational nature of the studies involved.
The hypothesis that MP reduces the risk of development of AMD is
particularly enticing because MP is entirely of dietary origin, thus
suggesting that the most common cause of blind registration in the
western World could be delayed, or even averted, with appropriate
dietary modification. Hammond et al.
97 have shown that
dietary supplements of spinach and corn, representing approximately
four times as much L and two to three times as much Z as a typical
diet, result in a significant increase in the optical density of MP and
the serum concentration of L in most subjects. Of note, after
discontinuation of the modified diet, serum levels of the carotenoids
returned to normal but MP optical density remained augmented,
reflecting the low turnover of these compounds in the retina. However,
the subjects involved varied in age from 30 to 65 years and are
therefore not representative of the population at risk for AMD.
It remains uncertain whether the age-related decline in MP optical
density or the relative absence of MP in predisposed eyes is the result
of inadequate dietary intake of L and Z or some other mechanism, and
this is an area that requires further investigation. To our knowledge,
there are no World Health Organization (WHO) guidelines for optimal
nutritional intake of specific carotenoids, and the recommendation of
preparations containing these micronutrients cannot be justified on the
basis of current evidence. Nevertheless, because MP is entirely of
dietary origin, it seems prudent to encourage our patients to eat a
balanced diet rich in fruits and vegetables, especially those that are
yellow, orange, or dark green.
72
In conclusion, we have shown that the two most important risk factors
for AMD, age and advanced disease in the fellow eye, are associated
with reduced optical density of MP. Ultimately, longitudinal studies
involving serial measurements of MP and serum levels of L and Z in a
large cohort of subjects are needed to establish whether supplemental L
and Z augments MP in those subjects at risk of development of AMD and
whether such MP augmentation can delay, avert, or modify the course of
the disease.