ACTA OPHTHALMOLOGICA SCANDINAVICA 1996 -

The incidence of registered blindness caused by age-related macular degeneration Thomas Rosenberg and Fleming Klie

National Eye Clinic for the Visually Impaired, Hellerup, Denmark

ABSTRACT. Membership applications to the Danish Society of the Blind were used as a register source of legal blindness (visual acuity 56\60) . Based on application forms completed by specialists in oph- thalmology 1585 subjects were recorded as blind in 1993.1132 subjects (71.4%) had age-related macular degeneration. Only 5 % of the registered subjects with age-related macular degeneration were below 70 years of age. The median age was 82, equal for both gender. A female overrep- resentation of 2.8 :1 was found. Five-year age-specific incidence rates demonstrated an exponential rise of registered blindness due to age-re- lated macular degeneration from age 60 to 90. A decline in incidence after the age of 90 is assumed to reflect underregistration of very old per- sons. This tendency of non-registration was particularly pronounced in elderly males. Estimated prevalence rates of registered blindness due to age-related macular degeneration increased 100-fold from the age group 60-64 to the age group 80-84. The age specific incidence rate for the age group 60-99 years was 140:100,000 for females and only 66:100,000 for males. The corresponding incidence rate for both sexes was 108:100,000. It is still a matter of dispute whether ‘true’ prevalence rates of blindness are higher in females than in males. The sex difference seems not to be explained by differences in ‘visual impairment threshold of registration’ among the registered persons.

Key words: age-related macular degeneration - AMD - age-specific incidence rates - blind- ness - register - visual impairment.

Acta Ophthalmol. Scand. 1996: 74: 399-402

ge-related macular degeneration A (AMD) is a collective term for various age-related degenerative changes in the posterior pole of the eye primarily affecting the retinal pigment epithelium and Bruchs membrane (Bresler et al. 1988). AMD has become the leading cause of severe visual loss in the western world (Sorsby 1972;

Goldstein 1980; Ferris 1983; Bruce et al. 1991). Outstanding population sur- veys have delineated the prevalence and the clinical spectrum of AMD (Leibowitz et as. 1980; Vinding 1989; Klein et al. 1991; Vingerling et al. 19 9 5). Nevertheless, prevalence figures on blindness due to AMD from population surveys are based on rela-

tively small numbers. Register studies, on the other hand, are hampered by a virtually unknown extent of underesti- mation (Editorial 1994).

The present study making no excep- tion, was strictly population based on newly registered cases of blindness in consequence of AMD from a single year. In another paper we have com- pared the results from 1993 with data from three corresponding annual counts from 1977,1984, and 1988, re- spectively (Rosenberg & Klie 1996).

Material and Methods The study material was selected by au- diting all new membership applica- tions to the Danish Society of the Blind during the year 1993. The inclusion criterion was a best corrected visual acuity (VA) 56 /60 in subjects with AMD as the principal cause of visual impairment. Further details on data collection are mentioned by Rosen- berg & Klie (1996).

Age and sex specific incidence rates were calculated in S-year groups by dividing the observed number in each group by the number of years lived through within the corresponding age and sex group, i.e. the number of per- sons at risk during 1993 alive at January 1, 1994 in the Danish popula- tion.

The estimated prevalences, i.e. the risk for a person to be blind by AMD, was calculated as the sum of the in- cidence rates for the preceding ages

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Table 1. Yearly incidences from four studies of registered blindness (VA 5 6/60) caused by AMD in the Danish male and female population. ~ ~ ~ ~~

1977 1984 1988 1993 Age Males Females Males Females Males Females Males Females (years) N ASIR N ASIR N ASIR N ASIR N ASIR N ASIR N ASIR N ASIR

45- 49 0 0 0 0 0 1 1 1 0.5 1 0.5 50- 54 0 2 1 0 1 1 1 1 5 5 - 59 0 4 3 1 1 6 4 1 1 2 2 1 0.8 1 0.7 60 - 64 2 2 5 4 5 4 10 7 3 2 12 9 1 0.9 11 9 65 - 69 12 10 25 19 15 14 20 16 18 16 29 22 5 5 31 25 70 - 74 31 37 53 49 43 46 96 81 40 45 79 70 35 38 76 65 75- 79 49 88 102 123 74 120 163 175 61 90 170 170 75 114 179 188 8 0 - 84 68 213 126 242 92 271 198 321 74 197 271 389 90 214 243 326 85 - 89 26 197 77 326 54 353 123 391 59 357 207 528 70 390 190 457 90- 94 6 159 30 423 18 372 43 401 30 587 60 457 18 338 81 519 95 - 99 1 179 2 174 1 143 3 158 2 215 11 394 2 223 18 570

1 1 1 0.6 0

100 - 104 0 0 0 0 0 0 2 2299 0

45-104 195 426 303 663 289 843 30 1 831

N =number, ASIR = age specific incidence per 100,000.

(cumulated incidence rates). The ex- responding population counts. Statis- Results pected number of prevalent cases was calculated for each 5-year group by multiplying cumulated incidences for

tics on cumulated incidence and pre- valence were carried out by the Danish Cancer Society, Division for Cancer

Table 1 presents the age distribution and the age specific incidence rates of

ages 62.5,-67.5, . . . 92.5 by the cor- Epidemiology.-

Percentage

100

90

80

70

60

50

40

30

20

10

0 50 60 70 80 90 100

Age

Fig. 1. Registered blindness (VA < 6/60) caused by age-related macular degeneration in 1993. Cumulated numbers (in percentage) as a function of age. Both sexes.

registered blindness of AMD per 100,000 in four one-year studies from a 17-year period for females and males, respectively.

Fig. 1 shows the cumulated age dis- tribution curve of registered blindness on account of AMD. From the age of 60 a slow rise in frequency is noted, and only five per cent of the registered cases are under 70 years of age. But from age 75 a sharp rise occurs, level- ing off over age 90. The median age of all AMD cases proved to be 82 years, equal for men and women.

Table 2 enumerates the estimated prevalences of registration per 100,000 and the estimated number of prevalent AMD cases in Denmark January 1,1994.

Among the 1132 persons with AMD as the principal cause of legal blindness, the 829 (73.2%) were women corresponding to a female to male proportion of 2.8:l. The age spe- cific incidence rate for the age group 60-99 years was 140:100,000 for fe- males and only 66:100,000 for males. The corresponding incidence rate for both sexes was 108:100,000.

A comparison of WHO visual ca- tegories among 5-year age groups is presented in Table 3. It should be noted that the proportion between im- pairment categories was largely inde-

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ACTA OPHTHALMOLOGICA SCANDINAVICA 1996 -

Table 2. Cumulated incidencedestimated prevalences of blindness caused by age-related macular degeneration per 100,000, and the estimated number of prevalent AMD cases in Denmark January 1,1994.

Cumulated incidence/ Estimated prevalent cases Age estimated prevalence (years) Females Males Females Males

60 - 64 22 2 27 2 65 - 69 107 16 131 17 70 - 74 332 123 389 112 75 - 79 963 504 915 329 80 - 84 2245 1326 1676 559 85 - 89 420 1 2837 1765 512 90 + 6656 4601 1270 290

Total 6173 1821

pendant of age. In addition, no obvious difference was observed between males and females with respect to the distribution among visual impairment categories.

Discussion During the last 25 years the annual number of newly registered blind per- sons as a consequence of AMD has in- creased 4-fold (Rosenberg & Klie 1996).

A similar tendency to a rise in blind- ness due to AMD has been reported in several studies (Lovie-Kitchin & Bow- man 1985). This increase is generally explained by the age displacements within the populations. Nevertheless, due to the unknown fraction of non registration an increasing incidence of registrable AMD as proposed by Evans &Wormald (1994) cannot be ruled out (Rosenberg & Klie 1996).

Fig. 1 demonstrates the influence from age on the registered incidence of blindness from AMD. Only 5% of the registered cases were below 70-years of age. From age 75 to 90 a rapid in- crease is seen. The median age was 82 years, equal for both gender.

The pronounced impact of age on the incidence of blindness from AMD as demonstrated in Fig. 1 and Table 1 is essential to the choice of age limits in population surveys, as well as to the way of presenting the data. In our opi- nion incidence figures calculated on 5-year age groups are most suitable for comparison. This is of special import- ance in the very old, who often are lumped into a common age group, e.g. 75 and more or SO+. In our study from 1993 the highest yearly incidence rate of blindness in females was 570 per 100,000 in the age group 95-99. A maximal incidence rate for males of 2299 per 100,000 in the age group

Table 3. Blindness caused by age-related macular degeneration. Age distribution cross tabu- lated with WHO/ICIDH visual impairment categories (World Health Organization 1980). The number (N) of females and males registered in 1993 are listed separately.

Visual impairment category

~

40 - 49 1 1 (0.1) 1 1 (0.3) 50 - 59 1 1 (0.1) 1 1 2 (0.7)

70 - 79 97 90 57 11 255 (31) 25 47 36 2 l lO(37) 80 - 89 159 136 111 27 433 (52) 48 58 45 9 160 (53) 90 - 99 28 32 28 11 99 (12) 4 6 8 2 20 ( 7) 2 100 1 1 2 (0.7)

60 - 69 19 16 5 2 4 2 ( 5 ) 4 1 1 6 ( 2)

Total 303 275 202 51 831 83 114 91 13 301 Percentage C/O) 36.5 33.1 24.3 6.1 100 27.6 37.9 30.2 4.3 100

100-104 is questionable because it is based on only two persons (Table 1).

AMD may be considered as a late stage of age-related maculopathy (ARM). Vinding (1990) in a cross sec- tional survey on 1000 individuals aged 60-80 described the occurrence of blindness (i.e. visual acuity 56/60) as ‘the top of the iceberg’, comprising 13% of the total number of AMD- eyes. According to a proposal from the international ARM epidemiological study group, both clinical designa- tions, ARM and AMD, are defined without making use of a visual acuity criterion (Bird et al. 1995). According to a.0. Ferris I11 (1993), Vinding (1989), and Sperduto & Siege1 (1980), the prevalence of ARM is increasing steadily with age, reaching levels of about 50% in people aged 75 or more. It may be argued that a ‘disease’ affect- ing half of an elderly population should be conceived as a normal phenomenon of aging, or with the words of Young (1987) ‘a continuum of senescence and disease’. If AMD and blindness from AMD are later stages of ARM, it follows that the incidence of blindness from AMD is increasing throughout life, making the relative decline of the age specific incidences in the oldest age groups (Table 1) an artefact due to increasing underregistration among the oldest people. Another hypotheti- cal explanation for the declining rise in age specific rates of blindness due to AMD in the very old might be corre- lated with the presence of at least two types of AMD; the ‘dry’, geographic type and the neovascular or disciform type. A relative dominance of neovas- cular types among the cases, leading to severe visual impairment was reported by Hyman et al. (1983). If, in addition, the incidence of neovascular AMD peaked somewhere in the 70’s or go’s, a declining increase in the incidence of blindness during the subsequent pen- tades would be the result.

Incidence rates on AMD-blindness (< 6/60) from Avon in the U.K. (Grey et al. 1989) are very close to our data. Other reports on the incidence of blindness due to AMD are scarce and comparisons are furthermore invali- dated by differences in the definition of blindness and the age groups chosen for calculation of age specific in- cidence rates.

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A significant female dominance among the registered AMD cases was observed. The fema1e:male ratio among registered subjects with AMD was 2.8:l. This distribution is partly explained by a majority of elderly women in the general population: In the age group 8 5 - 89 the number of fe- males is more than twice the number of males, and among the 90-94 year olds there are nearly three females for every male. Nevertheless, looking at the esti- mated prevalences (Table 2), female gender still is overrepresented. This tendency has been noted in other regis- ter-studies as well (Bruce et al. 1991). According to our observations the ex- planation should not be sought in a higher visual impairment registration- threshold among registered males (Table 3). The Rotterdam study showed females to be more seriously affected by ARM than males (Vinger- ling et al. 1995). This is in keeping with the findings of the Beaver Dam study of a higher prevalence of neovascular AMD in women (Klein et al. 1993). If this is a general feature, one would ex- pect relatively more women within ca- tegories of the lowest visual acuity (Table 3). Yet, this seems not to be the case.

We believe that the skewed sex ratio at least partly might be explained by socio-cultural differences between fe- males and males, rather than a sex de- pendant difference in the natural his- tory of AMD. Indeed, a number of population surveys were unable to demonstrate any sex difference in the prevalence of ARM. (Ferris 111 1983; Lovie-Kitchin &Bowman 1985; Vind- ing 1989).

The calculation of estimated pre- valence rates of registered blindness from AMD equating with cumulated incidence rates is based on the as- sumption that the incidence of blind- ness from AMD is constant and that survival is unaffected by the presence of AMD. Our estimated prevalence rates were only calculated for 5-year age groups up to age 90 due to the small number of persons registered above this age (Table 2). The increas- ing prevalence with age is evident with a maximum of 67 per 1000 in females aged 90 years or more. Vinding (1990) calculated a legal blindness rate of 4.3 per 1000 among subjects aged 60-80,

and Leibowitz et al. (1980) obtained 5.6 per 1000 in the age group 65-85. Our prevalence estimates on the corre- sponding age groups (both sexes) was 2.6 per 1000 and 7.0 per 1000, respec- tively. It is notable that the prevalence rates are in the same order of magni- tude in the aforementioned population surveys and in the present register study. It may indirectly be taken as an indication of a rather high sensitivity in the last mentioned investigation.

Aknowledgments The Danish Society of the Blind (DBS) gave this study support by copying all application forms. We thank Kirsten Marie Klie for her help with data processing. Director Hans Storm and statistician Gerda Engholm, The Danish Cancer Society, generously helped with statistical calculations.

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Received on August 24th, 1995.

Corresponding author: Thomas Rosenberg National Eye Clinic for the Visually Impaired 1 Rymarksvej DK-2900 Hellerup, Denmark.

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