Incidence of Blindness in Southern Germany Due to Glaucoma and Degenerative Conditions

Abstract

purpose. To estimate population-based incidence rates of registered blindness separately, to determine its main causes.

methods. The files of all newly registered blindness-allowance recipients in Württemberg-Hohenzollern, Germany (population: approximately 5 million), between 1994 and 1998 were reviewed. From ophthalmological reports on file the fulfillment of the German criteria for blindness (visual acuity of 1/50 or less or equivalent reduction of visual function) was ascertained, and the causes of blindness were obtained. Incidence rates of blindness due to macular degeneration, glaucoma, cataract, optic atrophy, and diabetic retinopathy were estimated.

results. There were 3531 newly registered blindness-allowance recipients (67.1% female; mean age, 72.8 ± 21.0 years). Standardized incidence rates in the general population (per 100,000 person-years; 95% confidence interval): All causes 12.27 (11.87–12.68), macular degeneration 5.29 (5.02–5.55), cataract 3.32 (3.11–3.52), optic atrophy 2.86 (2.66–3.05), glaucoma 2.43 (2.25–2.61), diabetic retinopathy 2.13 (1.96–2.30), other or unknown causes 5.17 (4.91–5.43). In many cases, blindness was attributable to more than one cause. Assuming that incidence rates are the same in other parts of the country, 9,939 (9,608–10,270) new cases of blindness were estimated to occur in Germany per year.

conclusions. The most common single cause of blindness was macular degeneration. Incidence rates of blindness due to such treatable conditions as glaucoma were also high. This finding suggests that the taking of measures for secondary prevention (e.g., early detection and optimal treatment of patients with glaucoma and diabetic retinopathy) should be intensified.

In the literature, there is little reliable population-based information about incidence rates of blindness due to its different causes. Most of the rare studies showing incidence rates are based on incomplete registries, based on voluntary registration. ^{1 2 3} A number of studies show relative frequencies (percentages) of incident or prevalent cases of blindness due to different causes. ⁴ However, relative frequencies are interdependent, and little information about absolute risks or incidence rates can be drawn from such studies. 

We used data collected in a register of blindness-allowance recipients to study incident cases of blindness in a large district of southern Germany (Württemberg-Hohenzollern; population approximately 5 million). German law entitles nearly all blind persons to receive a substantial blindness allowance. Because it is independent of other sources of income, it can be assumed that almost all blind persons apply for this allowance and are therefore registered by the welfare administration. ⁵ Previous publications on blindness in this district included the incidence of blindness due to diabetes and a possible time trend in the incidence of blindness. ^{5 6} In this article, we report incidence rates of each of the main causes of blindness—age-related macular degeneration, glaucoma, cataract, optic atrophy, and diabetic retinopathy—extracted from the registry of blindness-allowance recipients. In addition, we estimated the total number of new cases of blindness that occur in Germany each year due to these causes. 

Subjects and Methods

In Württemberg-Hohenzollern, Germany (one of the two districts of the federal state of Baden-Württemberg), a list of all individuals newly registered as blind between January 1, 1994, and December 31, 1998, was established (blindness-allowance recipients). The data were based on administrative files of the welfare administration. A detailed description of the database and the methods used, an in-depth analysis of incidence rates and risks due to diabetes, and an analysis of a potential time trend have been published. ^{5 6} Briefly, all applicants for the blindness allowance were included who met the German criteria for blindness (visual acuity of 1/50 or less based on the best corrected acuity in the better eye, visual field reduced to a radius of 5° or less, or equivalent reduction of vision—e.g., due to central scotoma, making the person unable to find his or her way). Written medical and ophthalmological statements were required in each case. Each file was reviewed for date of registration, basic demographic data, and ophthalmological findings. In each case, the reviewers determined the main cause of blindness, as well as up to five additional contributory causes, on the basis of the ophthalmological statement on file. The state’s office of statistics provided population data. The total population of the study area as of December 31, 1995, was 5,587,895. The study was performed in accordance with the Declaration of Helsinki for research involving human subjects. 

Statistical Analysis

Age-specific incidence rates were estimated. They were expressed per 100,000 person years of observation. The rates were calculated separately for age-related macular degeneration, glaucoma, cataract, and optic atrophy. For completeness, incidence rates due to diabetic retinopathy, other or unknown causes, and all cases combined were also included. 

To obtain an overall incidence rate and to make the results comparable with other studies, the rates were also standardized to the West German population of 1991. A method of direct standardization was used. ⁷ These methods, as well as the year used as the standard, were chosen to facilitate comparison with previously published data. 

Two separate analyses were performed: The first was based on all causes mentioned in the ophthalmological reports, so that one subject could have more than one cause of blindness. The second was based only on the causes considered to be the main causes of blindness in each case. 

Approximate 95% confidence intervals of incidence rates in the age-specific strata were computed on the basis of the empiric variance of the Poisson distribution. Confidence intervals of standardized incidence rates were calculated by using the weighted sum of the variances of the strata. ⁷  

In addition, absolute numbers of incident cases of blindness in Germany per year were estimated, assuming that the incidence rates found in the region studied are the same all over Germany. The number of incident cases (c _i ) in an age-specific stratum i was estimated as c _i = IR_i · n _i · 100,000⁻¹, where IR_i is the incidence rate obtained from the main causes of blindness in Württemberg-Hohenzollern, and n _i is the number of the German population in stratum i as of December 31, 1998. The estimated total of incident cases resulted from summing up c _i over all strata. All calculations were performed electronically (SAS statistical software, ver. 6.12; SAS, Cary, NC). 

Results

Patients’ Characteristics

In the region studied 3531 new blindness-allowance recipients were registered between 1994 and 1998. The characteristics of these individuals (sex and age) are shown in Table 1 . Mean age was 72.8 ± 21.0 years (SD; range, 0–102; 67.1% female). The cause of blindness could not be determined in 63 (1.8%) of the subjects. Of the subjects, 42.4% had one diagnosis, 36.9% had two diagnoses, 15.0% had three diagnoses, and 3.9% had between four and six diagnoses. 

Epidemiologic Measures

Age-standardized incidence rates are shown separately for each cause of blindness in Table 2 . Age-specific incidence rates are shown in Tables 3 and 4 . The pattern of incidence rates according to age was very similar for macular degeneration, glaucoma, cataract, optic atrophy, and all causes combined: Up to the age of 59 years, incidence rates remained very low. After age 59, the rates rose dramatically. The data are presented together for both sexes, because there was little difference between rates in men and women, and there were small numbers in some age- and sex-specific subgroups. These results indicate that all the major causes of blindness were typical phenomena of the older age groups. Slightly greater incidence rates at younger ages were observed in the relatively rare and unknown causes, which were combined in one category because of the small number of cases. This finding can be explained by the occurrence of congenital and traumatic causes of blindness in younger age groups. Discussion of the different pattern that was found in diabetic retinopathy, with incidence rates increasing at younger ages, has been published. ^{5 6}  

When only the main cause of blindness in each individual was considered, the ranking of causes changed, compared with the table of all contributory causes (Table 2) . This was mainly because cataract was, in most cases, only one of several contributory causes of blindness. In both analyses, macular degeneration was by far the most frequent cause of blindness. 

Estimated absolute numbers for the main causes of blindness in Germany are shown in Table 5 . Assuming that incidence rates are the same in other parts of the country, approximately 10,000 new cases of blindness were estimated to occur in Germany per year. 

Discussion

Completeness

This study is based on all newly registered blind people in a defined geographic area during a specified period. Because of the substantial and unique financial incentive for registration, the sample of incident cases of blindness may be considered to be almost complete. Although it must be assumed that a few subjects who meet the German criteria of blindness did not apply for the blindness allowance, the database used herein can be regarded as including a much greater proportion of blind individuals than registries elsewhere. ⁵ However, there is no information about people who may have been eligible but did not register. As we have shown previously, during the past 9 years, a very slight tendency toward a reduction of incident blindness has been demonstrated, with some variability between subgroups according to sex and diabetic status. However, no substantial change in the incidence of blindness occurred between 1990 and 1998. ⁶ It seems unlikely that a major change in incidence rates will occur in the short term. Therefore, it seemed reasonable to combine cases that were reported over 5 years for this analysis. 

Limitations

The study was limited by the fact that we had to rely on information from the administrative files. Therefore, the number of variables that we could study was restricted, and it was not possible to check the causes of blindness beyond the information available in the records. This led to some diagnostic uncertainty. In elderly individuals with multiple morbidities it may be difficult to establish clearly which of several conditions was the cause of blindness. Other limitations and possible sources of bias have been discussed in depth in our previous publications. ^{5 6}  

The estimation of absolute numbers of incident cases of blindness in the whole of Germany rests on the assumption that incidence rates do not substantially differ across the country. In most parts of the country similar studies are not available. Therefore, these numbers have to be interpreted with caution. However, similar incidence rates were found in Hessia and North Rhine. ^{8 9} No substantial change in incidence rates between 1990 and 1998 was demonstrated. ⁶ The healthcare system is the same in all parts of the country. On this basis, it may be reasonable to assume that incidence rates are similar in other parts of Germany. Differences may be present, however, in the eastern part of the country, because of socioeconomic differences and the changes in the healthcare system after the reunification of Germany in 1990. 

Comparison with Other Studies

In Hessia, a German state adjacent to Württemberg-Hohenzollern, a study found an overall incidence of blindness of 14 per 100,000 person-years, which is of the same order of magnitude as our findings. ⁸ This study was based on only one year (1996) with 767 cases. In Upper Bavaria an incidence rate of 17.5 per 100,000 person-years was found in 1984 (645 cases). ¹⁰ There is one study that analyzed incidence rates on the basis of our database, using the data of only one calendar year (1994). ¹¹ The overall incidence was similar to our findings (11.6 per 100,000 person-years). However, the ascertainment of cases in the previous study is likely to be incomplete. The small sample size of all these studies did not allow the reliable estimation of age-, sex- and cause-specific rates and no standardization by age was performed. 

A study of incident blindness in Denmark found an incidence of seven cases of blindness due to glaucoma per 100,000 person-years in people aged 65 years or more. ¹ This figure is smaller than the incidence rate in our study. However, the Danish registry is likely to be incomplete mostly because it is based on voluntary admissions to a private organization of blind people. 

Several British studies have also found higher incidence rates of blindness than the Danish study and higher also than in our study, but the definition of legal blindness is less strict and more vaguely defined than in Germany (“so blind as to be unable to perform any work for which eyesight is essential,” where, in practice, a threshold of 1/20 is recommended). As a consequence, the rates are hardly comparable. ^{2 3 12} The same problem arises when our results are compared with data from Italy. Although the definition of blindness is less strict in Italy than in Germany (1/20 instead of 1/50), the overall incidence rate of blindness that we have found is similar to that reported from Italy between 1987 and 1991. ¹³ This finding may be explained either by underreporting of blindness in Italy, which we consider to be the most plausible explanation, or by higher actual incidence rates in Germany. Reports from developing countries indicate that cataract is a major cause of blindness in those countries, whereas it was of less importance in our study. ¹⁴ This difference can be explained by widespread use of highly developed cataract surgery in Germany. 

Several cross-sectional surveys on the prevalence of blindness have been published in recent years. All these studies show a considerable increase of blindness with increasing age. For example, the Rotterdam study examined the prevalence of blindness in a sample (n = 6775) of elderly population in The Netherlands. ¹⁵ Because the prevalence of blindness is low, only 32 blind subjects were identified, despite the considerable sample size of the study. Age-related macular degeneration was the commonest cause of blindness, which is in accordance with the results of our study. Cataract was the most frequent cause of impaired vision, but much less frequent as a cause of blindness. This corresponds to our finding that cataract was rarely the main cause of blindness but contributed to blindness together with other causes. Diabetic retinopathy was not a common cause of prevalent blindness in the Rotterdam study. The most plausible explanation for this finding is that cases of blindness due to diabetic retinopathy are found in an incidence study, such as ours, but are less common in a prevalence study, because of the poor survival of blind people with diabetes. ¹⁶ The Beaver Dam Eye Study ¹⁷ and the Salisbury Eye Evaluation Study, ¹⁸ both conducted in the United States, and the Copenhagen City Eye Study ¹⁹ are similar to the Rotterdam study in design and results. They also show age-related macular degeneration as the commonest cause of prevalent blindness. The small number of blind people who were identified in these studies must be taken into consideration. For example, the Salisbury and the Copenhagen studies found 21 and 10 blind persons, respectively. ^{18 19}  

Perspectives

To monitor a potential time trend in the future, the observation of incidence rates on the basis of the database used herein should be continued. A database, as well as a feasible and reliable methodology, are now available to evaluate the development of incidence rates of blindness over time. However, the collection of these data is cumbersome and time consuming. With a relatively small additional effort, diagnoses could be included in computer files used for administrative purposes. Together with the administration, and on the basis of the study presented in this article, we are developing feasible ways to make these data more easily available for regular monitoring of the incidence of blindness in the future. This way, it will be possible to evaluate the long-term effects of improved therapy and prevention on the incidence of blindness much more easily and with sufficient statistical power. ⁵ With some additional effort, such data could be collected in larger areas and should be made available for monitoring the complications of diabetes in the framework of routine quality control. To reach this goal, decisions of relevant organizations, such as the welfare administration, public health bodies, and boards of physicians are needed. 

The leading causes of blindness are degenerative conditions for which, to date, there is no effective treatment. However, incidence rates of blindness due to glaucoma, a treatable condition, are still high. The same is true of diabetic retinopathy. This demonstrates the need for increased effort and more effective ways to prevent blindness. Early detection and consequent treatment of glaucoma may help in reaching this goal. In Germany, the statutory health insurance does not provide screening for glaucoma at this time. Specific, well-designed interventions are needed in the future to reduce the loss of vision. Those interventions should be thoroughly evaluated for feasibility, medical effectiveness, and economic impact. 

⁴  Deceased, August 18, 2002.

 Supported by Gesundheitsforschung e.V., Berlin; the Public Health Research Network North Rhine-Westphalia (Forschungsverbund Public Health Nordrhein-Westfalen), and Chibret Pharmacia and Pharm-Allergan (CT).

 Submitted for publication March 28, 2002; revised July 8, 2002; accepted August 9, 2002.

 Commercial relationships policy: F (CT); N (all others).

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Table 1.

 

 

Patients’ Characteristics

Table 1.

 

Patients’ Characteristics

Age (y)	Men	Women	All
0–39	180	133	313
40–59	151	140	291
60–79	388	692	1080
≥80	441	1406	1847
Total	1160	2371	3531

Table 2.

 

 

Age-Standardized Cause-Specific Incidence Rates of Blindness

Table 2.

 

Age-Standardized Cause-Specific Incidence Rates of Blindness

	Incidence Rates*	CI	Incidence Rates, †	CI
All causes	12.27	11.87–12.68	12.27	11.87–12.68
Macular degeneration	4.18	3.95–4.42	5.29	5.02–5.55
Cataract	0.58	0.49–0.66	3.32	3.11–3.52
Optic atrophy	0.91	0.80–1.02	2.86	2.66–3.05
Glaucoma	1.40	1.26–1.54	2.43	2.25–2.61
Diabetic retinopathy	1.71	1.56–1.87	2.13	1.96–2.30
Other or unknown causes	3.50	3.28–3.71	5.17	4.91–5.43

Table 3.

 

 

Age-Specific Incidence Rates of All Contributory Causes of Blindness

Table 3.

 

Age-Specific Incidence Rates of All Contributory Causes of Blindness

	Age (y)
		0–39	40–59	60–79	≥80
All causes	2.12 [1.89–2.36]	3.97 [3.51–4.43]	23.02 [21.65–24.39]	159.02 [151.77–166.27]
Macular degeneration	0.13 [0.07–0.19]	0.26 [0.14–0.38]	7.91 [7.10–8.71]	97.46 [91.78–103.14]
Cataract	0.13 [0.07–0.19]	0.55 [0.38–0.72]	5.65 [4.97–6.33]	54.93 [50.67–59.19]
Optic atrophy	0.63 [0.50–0.76]	0.82 [0.61–1.03]	5.07 [4.43–5.72]	37.11 [33.60–40.61]
Glaucoma	0.10 [0.05–0.15]	0.35 [0.22–0.49]	4.62 [4.01–5.24]	38.49 [34.92–42.05]
Diabetic retinopathy	0.13 [0.07–0.19]	0.83 [0.62–1.04]	7.80 [7.00–8.60]	13.69 [11.56–15.82]
Other or unknown causes	1.63 [1.43–1.84]	2.80 [2.41–3.18]	8.61 [7.77–9.45]	54.07 [49.84–58.30]

Table 4.

 

 

Age-Specific Incidence Rates of Main Causes of Blindness

Table 4.

 

Age-Specific Incidence Rates of Main Causes of Blindness

	Age (y)
		0–39	40–59	60–79	≥80
All causes	2.12 [1.89–2.36]	3.97 [3.51–4.43]	23.02 [21.65–24.39]	159.02 [151.77–166.27]
Macular degeneration	0.12 [0.06–0.17]	0.18 [0.08–0.27]	6.10 [5.39–6.80]	77.75 [72.68–82.82]
Cataract	0.04 [0.01–0.07]	0.11 [0.03–0.18]	0.98 [0.70–1.26]	9.21 [7.47–10.96]
Optic atrophy	0.47 [0.36–0.59]	0.42 [0.27–0.57]	1.36 [1.03–1.70]	8.01 [6.38–9.63]
Glaucoma	0.02 [0.00–0.04]	0.15 [0.06–0.24]	2.37 [1.93–2.81]	24.28 [21.45–27.11]
Diabetic retinopathy	0.12 [0.07–0.18]	0.72 [0.53–0.92]	6.59 [5.85–7.32]	8.95 [7.23–10.68]
Other or unknown causes	1.35 [1.16–1.54]	2.39 [2.03–2.74]	5.63 [4.95–6.31]	30.82 [27.63–34.02]

Table 5.

 

 

Estimated Absolute Number of Incident Cases of Blindness in Germany per Year

Table 5.

 

Estimated Absolute Number of Incident Cases of Blindness in Germany per Year

	Absolute Number per Year	CI
All causes	9939	[9608–10270]
Macular degeneration	3290	[3103–3477]
Cataract	460	[389–531]
Optic atrophy	734	[643–825]
Glaucoma	1113	[1003–1223]
Diabetic retinopathy	1488	[1354–1622]

 

The authors thank the welfare administration (Landeswohlfahrtsverband Württemberg-Hohenzollern) for their continued support of the study by providing access to their data; Jean-Cyriaque Barry (Department of Ophthalmology II, University of Tübingen) for valuable advice and comments on the draft of this article; Markus Werner, Tatjana Heemcke, and Christa Bendiek for collecting the raw data; and Ulli Monigatti for building the database.