Abstract
purpose. To assess prospectively whether development of age-related macular
degeneration is influenced by experimentally induced chronic
high-pressure glaucoma, and whether age-related macular degeneration
influences the appearance of the optic nerve head in experimental
chronic high-pressure glaucoma in older rhesus monkeys.
methods. The longitudinal study included 102 eyes of 52 rhesus monkeys. The
total study group was divided into a group with experimentally induced
unilateral chronic high-pressure glaucoma (n = 40 eyes)
and a normal control group (n = 62 eyes). Additionally,
arterial hypertension and atherosclerosis were experimentally induced
in both study groups in a similar percentage of monkeys. Mean monkey
age at the end of the study was 19.6 ± 3.1 years (range, 13–24
years). The macular region, optic disc, and retinal nerve fiber layer
were morphometrically evaluated by color wide-angle fundus photographs
taken at baseline and at the end of the study.
results. The degree of age-related macular degeneration, measured as number and
area of drusen in the foveal and extrafoveal region of the macula, did
not differ significantly between the two study groups. In the
glaucomatous group, the degree of macular degeneration was
statistically independent of the development of parapapillary
atrophy, loss of neuroretinal rim, and decrease in the visibility of
the retinal nerve fiber layer.
conclusions. Development of age-related macular degeneration in rhesus monkeys is
independent of concomitant chronic high-pressure glaucoma, including
the development of glaucomatous parapapillary chorioretinal atrophy.
Conversely, age-related macular degeneration does not markedly
influence the course of experimental chronic high-pressure glaucoma or
the development of parapapillary atrophy in
monkeys.
Age-related macular degeneration and glaucomatous optic
neuropathy are among the leading causes of marked visual impairment in
the elderly population in Western countries.
1 2 3 4 The
purpose of the present longitudinal experimental study in rhesus
monkeys was to evaluate whether experimentally induced chronic
high-pressure glaucoma influences the development of age-related
macular degeneration, and conversely, whether age-related macular
degeneration influences the degree of glaucomatous optic neuropathy.
The study comprised 102 eyes of 52 rhesus monkeys (
Macaca
mulatta;
Table 1 . Selection criterion of inclusion of the eyes in the study was
the availability of wide-angle color fundus photographs with the
posterior pole of the fundus fully illuminated. The total study group
was divided into an arterial hypertensive–atherosclerotic subgroup
consisting of 33 monkeys in which systemic arterial hypertension and/or
diet-induced atherosclerosis had been induced experimentally, and into
a second subgroup including 19 monkeys without chronic arterial
hypertension or atherosclerosis
(Table 1) . The reason for inducing
arterial hypertension and atherosclerosis in a subgroup of monkeys was
that in humans, age-related macular degeneration and glaucoma are
diseases of the elderly population, who often have systemic arterial
hypertension and atherosclerosis. Atherosclerosis was produced
experimentally by feeding the animals a special atherogenic diet
(consisting of 1 mg cholesterol per calorie [0.8% by weight]) and
43% of total calories as fat)
5 continuously for many
years. Chronic arterial hypertension was produced by modified
Goldblatt’s procedure
6 and maintained for a period of
years, as confirmed by serial blood pressure measurements.
To examine the relationship between age-related macular degeneration
and glaucoma, experimental high-pressure glaucoma was unilaterally
produced in 40 eyes of 40 monkeys by multiple applications of argon
laser to the trabecular meshwork.
7 Mean age (±SD) was
19.6 ± 3.3 years, mean duration of elevation of intraocular
pressure was 24.0 ± 13.0 months (median, 21 months; range, 4–55
months). Before the application of laser photocoagulation, intraocular
pressure was measured three times on 3 days to establish a baseline for
each eye. During the follow-up period, the frequency of intraocular
pressure measurements depended on the level of intraocular pressure in
each eye. The higher the intraocular pressure, the more frequently it
was measured: two to three times a week when the intraocular pressure
was more than 60 mm Hg, weekly for intraocular pressure of 40 to 50 mm
Hg, and monthly for intraocular pressures of less than 40 mm Hg. Our
objective was to maintain an intraocular pressure between 30 and 40 mm
Hg to mimic the clinical situation of moderate ocular hypertension.
Because, with the argon laser trabecular application, it is impossible
to achieve a desired level of intraocular pressure on a long-term
basis, the intraocular pressure often tended to go higher than the
desired levels if left alone. Therefore, to maintain our desired level
of intraocular pressure (i.e., 30–40 mm Hg), we had to use ocular
hypotensive drops, such as topical β-blockers and miotics, in 90% of
the glaucomatous eyes. Because age was significantly different between
the glaucomatous group and the nonglaucomatous control group, a second
nonglaucomatous control group was formed, consisting of 43 eyes with a
mean age of 19.53 ± 2.31 years
(Table 2) . The percentage of monkeys with arterial hypertension,
atherosclerosis, and combined arterial hypertension with
atherosclerosis, respectively, in each study group did not differ
significantly (
P > 0.50; χ
2 test) between the nonglaucomatous control group and the group with
experimental glaucoma.
The study design complied with the National Institutes of Health’s and
the University of Iowa’s Institutional Guidelines for the Care and Use
of Laboratory Animals, and the ARVO Statement for the Use of Animals in
Ophthalmic and Vision Research. All animals were examined at baseline
of the study and serially thereafter under ketamine anesthesia (8–10
mg/kg body weight) and before and during follow-up of elevation of
intraocular pressure in the glaucomatous group. These examinations
included intraocular pressure measurement by Goldmann applanation
tonometry, ophthalmoscopic examination, and stereoscopic color fundus
photography. All experimental procedures were performed, and the fundus
photographs were taken at the University of Iowa, Iowa City, and
morphometrically evaluated at Friedrich-Alexander University, Erlangen,
Germany, as has previously been described in detail.
8 We
evaluated the areas of the neuroretinal rim and of the alpha zone and
beta zone of the parapapillary atrophy
(Fig. 1) and the visibility of the retinal nerve fiber layer. The degree
of age-related macular degeneration was evaluated by counting the
number of drusen, separated in the foveal region and in the extrafoveal
region within the temporal vascular arcade. The mean size of the drusen
was classified according to three grades scaled from 1 for very small
to 3 for very large. The possibility of grading the severity of
age-related macular degeneration on fundus photographs has been
described in detail.
9 10 To determine the reproducibility
of the assessment of age-related macular degeneration, photographs of
10 randomly selected eyes were re-evaluated five times.
For evaluation of the statistical significance of differences in
measurements between the study groups, the Mann–Whitney test was
applied. For the comparison of frequencies, theχ 2 test was used. For assessment of differences
in the follow-up examinations, the Wilcoxon test was applied. The
coefficient of variation was calculated as the ratio of the mean of the
SD divided by the mean of the mean.
All measurements considered, the number of drusen increased
significantly (
P = 0.007) with increasing age.
Comparing the glaucomatous group (40 eyes) with the age-matched
nonglaucomatous control group (43 eyes), no significant
(
P ≥ 0.30) differences were detected in total drusen
count, drusen count in the foveal and extrafoveal regions, total drusen
area, drusen area in the foveal region, or drusen area in the
extrafoveal region
(Table 2) . In the follow-up examination, when the
photographs taken at baseline and the end of the study were compared,
the increase in total drusen count, foveal drusen count, and
extrafoveal drusen count in the total drusen, foveal drusen, and
extrafoveal drusen areas did not vary statistically (
P > 0.40) between the glaucomatous group and the nonglaucomatous
age-matched control group
(Table 3) .
In the whole study group, at baseline and the end of the study,
respectively, total drusen count (P = 0.75 and P = 0.28, respectively), foveal drusen count
(P = 0.26 and P = 0.95, respectively),
extrafoveal drusen count (P = 0.26 and P = 0.08, respectively), total drusen area
(P = 0.28 and P = 0.35, respectively),
foveal drusen area (P = 0.42 and P =
0.77, respectively), and extrafoveal drusen area (P =
0.22 and P = 0.28, respectively) were statistically
independent of neuroretinal rim area. The same held true if only the
eyes of the glaucomatous group were taken into account
(P > 0.25). Neuroretinal rim area decreased
significantly (P < 0.001) during the period of
elevation of intraocular pressure within the glaucomatous group. Within
the glaucomatous group, number and area of drusen at end of the study,
and the change in number and area of drusen during the period of
elevation of intraocular pressure were statistically (P > 0.30) independent of the decrease in neuroretinal rim area. When the
glaucomatous group was compared with the nonglaucomatous age-matched
control group, neuroretinal rim area was significantly
(P < 0.001) smaller in the glaucomatous group. In the
normal eyes, neuroretinal rim area was independent (P =
0.96) of age.
In the entire study group, size of beta and alpha zones of
parapapillary atrophy were statistically independent of total drusen
count (P = 0.28 and P = 0.94,
respectively), foveal drusen count (P = 0.98 and P = 0.33, respectively), extrafoveal drusen count
(P = 0.76 and P = 0.33, respectively),
total drusen area (P = 0.70 and P =
0.63, respectively), foveal drusen area (P = 0.95 and P = 0.23, respectively), and extrafoveal drusen area
(P = 0.61 and P = 0.95, respectively).
The same result was found when the glaucomatous group
(P > 0.25) and the nonglaucomatous age-matched control
group (P > 0.20) were analyzed separately. Within the
glaucomatous group, number and area of drusen at the end of the study,
and the change in number and area of drusen during the period of
elevation of intraocular pressure were statistically (P > 0.30) independent of the development and enlargement of the beta
zone and alpha zones of parapapillary atrophy.
In the entire study group and in each of the subgroups, total drusen
count (P = 0.40), foveal drusen count
(P = 0.65), extrafoveal drusen count (P = 0.35), total drusen area (P = 0.13), foveal drusen
area (P = 0.15), and extrafoveal drusen area
(P = 0.19) were statistically (P >
0.30) independent of the visibility of the retinal nerve fiber layer,
with squared correlation coefficients lower than 0.04. Visibility of
the retinal nerve fiber layer decreased significantly
(P < 0.001) during the period of elevation of
intraocular pressure within the glaucomatous group. When the
glaucomatous group and the nonglaucomatous age-matched control group
were compared, visibility of the retinal nerve fiber layer was
significantly (P < 0.001) lower in the glaucomatous
group.
If in the statistical analysis the total study group was divided into
monkeys with and without arterial hypertension–atherosclerosis, count
and size of macular drusen were not significantly (P >
0.10) correlated with any of the parameters: neuroretinal rim area,
size of alpha and beta zones of parapapillary atrophy, and retinal
nerve fiber layer visibility. Correspondingly, the differences in size
and count of macular drusen between the glaucomatous eyes and the
nonglaucomatous eyes were not statistically significant
(P > 0.20).
The coefficient of variation for the reassessment of the number of
drusen in the foveal region was 0.189, and for the count of the drusen
in the extrafoveal region, it was 0.174.
The results of the present study suggest that the courses of both
diseases, age-related macular degeneration and experimental chronic
high-pressure glaucoma, are independent of each other. Monkey eyes with
pronounced age-related macular degeneration and those without did not
vary significantly in degree of neuroretinal rim loss and size and
increase of beta zone of parapapillary atrophy. As a corollary, monkeys
with glaucoma and monkeys without glaucoma did not vary significantly
in the number and size of macular drusen. In the glaucomatous group,
the severity of age-related macular degeneration was statistically
independent of neuroretinal rim area and visibility of the retinal
nerve fiber layer, which decreased significantly during the time of
elevated intraocular pressure. However, the median and mean number and
size of macular drusen tended to be smaller, although not statistically
lower, in the glaucomatous eyes compared with the nonglaucomatous
age-matched control eyes
(Tables 2 and 3) . There were monkeys in which
the number of drusen decreased after elevation of intraocular pressure
(Figs. 1A 1B) , whereas in the contralateral nonglaucomatous eye the
number of drusen increased.
Although these differences can be explained by the physiologic
fluctuation in the appearance of macular drusen,
11 they at
least suggest that the induction of glaucoma in the present study did
not favor the development of age-related degeneration. This may be
astonishing, because vascular insufficiency has been thought
responsible for the development of both diseases.
12 13 14 15 16 17 18 19 20 21 22 Studies have suggested that patients with age-related macular
degeneration have a reduced choroidal blood flow that may lead to the
development of the disease.
12 13 14 15 16 17 18 Other studies have
suggested that, besides elevated intraocular pressure, vascular
insufficiency, including vasospasm with vascular dysregulation, is
among the main risk factors of glaucoma.
19 20 21 Correspondingly, calcium channel blockers as vasospasmolytics have been
reported to be helpful in the treatment of patients with
glaucoma.
22
Interestingly, age-related macular degeneration and parapapillary
chorioretinal atrophy in monkey eyes with glaucoma were statistically
independent of each other, although both are associated with
degenerative changes of the retinal pigment epithelium, and although
for both of them, again a vascular pathogenesis has been
discussed.
8 12 13 14 15 16 17 18 19 20 Previous histomorphometric and
perimetric studies have shown that parapapillary atrophy in
glaucomatous eyes is the clinical–histopathologic equivalent of
structural and pigmentary irregularities and of loss of retinal pigment
epithelium in the parapapillary region.
8 23 It corresponds
psychophysically to relative and absolute scotomata in the visual
field. Vascular insufficiency in the choroid has been thought
pathogenetically responsible for the development of
parapapillary atrophy in glaucomatous eyes.
8 20 21 In a
parallel manner, age-related macular degeneration shows morphologic
changes in the macular retinal pigment epithelial layer, leading to
structural and pigmentary irregularities and finally to a loss of
retinal pigment epithelial cells.
24 Microperimetric
studies have revealed that the lesions of age-related macular
degeneration represent relative and absolute visual field
defects.
25 Pathogenetically, an impairment of the
choroidal circulation has been reported.
12 13 14 15 16 17 18
Despite these similarities between age-related macular degeneration and
parapapillary atrophy, no connection between them has been discovered,
either in the present experimental study or in clinical investigations
in patients.
26 It can be concluded that parapapillary
atrophy is neither a risk factor nor a protective factor for
age-related macular degeneration and that they have a different
pathogenesis.
In conclusion, development of age-related macular degeneration in
rhesus monkeys may be independent of concomitant chronic high-pressure
glaucoma, including the development of glaucomatous parapapillary
chorioretinal atrophy. Conversely, age-related macular degeneration may
not markedly influence the course of experimental chronic high-pressure
glaucoma or the development of parapapillary atrophy in monkeys.
Although the two diseases may not influence each other in their
development, the coexistence of both diseases in patients has a
cumulative negative effect on visual function from a clinical point of
view. The central visual field, which may be relatively spared by the
glaucomatous process, is markedly affected by age-related macular
degeneration.