© 2005 by the American Diabetes Association, Inc.
Diabetic Retinopathy Is Associated With Mortality and Cardiovascular Disease IncidenceThe EURODIAB Prospective Complications Study
1 Department of Ophthalmology, VU University Medical Center, Amsterdam, the Netherlands Address correspondence and reprint requests to Manon V. van Hecke, MD, Department of Ophthalmology, VU University Medical Center, P.O. Box 7057, 1007 MB Amsterdam, Netherlands. E-mail: m.vanhecke{at}vumc.nl
OBJECTIVETo study the relationship of nonproliferative and proliferative retinopathy with all-cause mortality and cardiovascular disease (CVD) incidence in type 1 diabetic patients and, additionally, the role of cardiovascular risk factors in these associations. RESEARCH DESIGN AND METHODSThis prospective study included 2,237 type 1 diabetic patients from 31 centers in 16 European countries at baseline, aged 1560 years, who were examined for retinopathy by taking two-field 45° fundus photographs, which were centrally graded. Mortality and cardiovascular morbidity follow-up was assessed 68 years after baseline examination according to a standardized protocol. RESULTSAfter 7.9 years of follow-up, 64 patients had died and 128 patients had incident CVD. The age- and sex-adjusted hazard ratios (HRs) of all-cause mortality were 1.45 (95% CI 0.712.96) and 4.16 (1.968.84) in patients with nonproliferative and proliferative retinopathy at baseline, respectively. Adjustments for cardiovascular risk factors completely obliterated the association with nonproliferative retinopathy, whereas the association with proliferative retinopathy remained twofold increased, although nonsignificant. The age- and sex-adjusted HRs of incident CVD were 1.73 (1.152.60) and 2.05 (1.223.45) in patients with nonproliferative and proliferative retinopathy, respectively. After adjustments for cardiovascular risk factors, both associations were attenuated and lost statistical significance. CONCLUSIONSThis study shows that type 1 diabetic patients with nonproliferative or proliferative retinopathy have an increased risk for all-cause mortality and incident CVD. The presence of cardiovascular risk factors explained the associations to a large extent, except for the associations with proliferative retinopathy, which suggests that other shared mechanisms may be involved.
Abbreviations: AER, albumin excretion rate CVD, cardiovascular disease ECG, electrocardiogram
There is increasing evidence that micro- and macrovascular complications of diabetes share certain pathophysiological mechanisms. This may explain why microangiopathy has been associated with macroangiopathy and with mortality (110). For example, microalbuminuria has strongly and independently been associated with the development of cardiovascular disease (CVD) and mortality in type 2 diabetic patients (13,10). Further, retinopathy has also been associated with increased cardiovascular and all-cause mortality risk, particularly in type 2 diabetes (49). In type 1 diabetes, however, this association has been explored only in a limited fashion (4,11). Type 1 diabetes, though, offers a better opportunity to study the relationship between retinopathy and cardiovascular events than type 2 diabetes, because of a younger population with less confounding variables associated with the metabolic syndrome. The pathophysiology underlying the association of retinopathy with CVD and mortality is not well understood. Cardiovascular risk factors, such as hypertension, dyslipidemia, and elevated HbA1c level are also known as risk factors for the development of diabetic retinopathy (12,13), and thus, any association may simply reflect shared risk factors. Previous studies (4,7,14), however, reported that conventional cardiovascular risk factors can partly, but not entirely, account for the association between retinopathy and macrovascular disorders. Consequently, it can be hypothesized that, similar to the unknown mechanism by which microalbuminuria increases the risk of CVD (1), an unknown pathway may be involved in the relationship between retinopathy and macrovascular disorders. Importantly, if mechanistic pathways for any association for microalbuminuria and retinopathy are dissimilar, retinopathy could possibly reflect a new risk marker for CVD. Therefore, the aims of our study were to further explore the relationship of retinopathy with macrovascular disorders in type 1 diabetes and to investigate the role of cardiovascular risk factors in this association. Thus, we described the relationship of nonproliferative and proliferative retinopathy with all-cause mortality and CVD incidence (fatal and nonfatal together) during an 8-year follow-up of a large European sample of type 1 diabetic patients. In addition, we studied the role of known cardiovascular risk factors in these associations.
The EURODIAB Prospective Complications Study is a follow-up of the EURODIAB IDDM Complications Study (15). Full details of the design, methods, and recruitment to the EURODIAB cohort have been published elsewhere (13,15,16). Baseline investigations were performed on 3,250 men and women with type 1 diabetes aged between 15 and 60 years and drawn from 31 centers in 16 European countries. The sampling frame contained all type 1 diabetic patients attending at least once in the last year for each center. Sample selection was stratified by age, sex, and duration of diabetes to ensure sufficient representation in all categories (15). Type 1 diabetes was defined as a diagnosis made before the age of 36 years, with a need for continuous insulin therapy within a year of diagnosis. Of those invited, 85% participated. Patients with a duration of diabetes <1 year and pregnant women were excluded. The local ethics committees approved this study at each center, and all persons provided written informed consent.
Ophthalmologic examination
Follow-up and outcomes Outcome variables were all-cause mortality and fatal and nonfatal CVD incidence. Causes of death were coded according to the ICD-9 classification and assigned to different categories, such as coronary heart disease, stroke, other CVD, non-CVD, and unknown. Two observers (N.C. and J.H.F.) separately allocated cause of death with 100% agreement. CVD was defined as a positive medical history of a cardiovascular event, including myocardial infarction, angina pectoris, coronary artery bypass graft and/or stroke, and/or ischemic changes on a conventional 12-lead electrocardiogram (ECG) (16) and was assessed at baseline and at follow-up examination. Two observers classified the ECG abnormalities according to the Minnesota Code. Any discrepancies between the two observers were adjudicated by a third. ECG abnormalities suggestive of probable ischemia consist of codes 1.1 and 1.2 (major Q/QS waves) and code 7.1 (complete left bundle branch block). Possible ischemia consists of code 1.3 for minor Q waves, codes 4.1, 4.2, and 4.3 for ST segment abnormalities, and codes 5.1, 5.2, and 5.3 for T wave abnormalities.
Additional measurements
Statistical analysis
Of the 3,250 patients recruited at baseline, 2,237 had usable photographs and information on vital status and were included in the present study. We excluded 771 subjects with missing fundus photographs and 463 subjects with missing vital status at follow-up. Vital status was not obtained for 463 persons because of the following reasons: 4 local centers did not participate in the prospective study (n = 437), 8 patients did not fulfill the inclusion criteria, and 18 had an unknown vital status. Subjects with missing fundus photographs and/or missing vital status at follow-up had significantly higher HbA1c levels, larger waist circumferences, higher diastolic blood pressure levels, a higher prevalence of prior CVD, and a less favorable lipid profile. For the analyses with incident CVD as outcome variable, we excluded subjects with missing follow-up morbidity data (n = 134) and/or prevalent CVD (n = 174) and/or unknown cause of death (n = 23), resulting in 1,914 individuals with complete data. Baseline distribution of risk factors did not differ between patients with known and missing causes of death, but the group with missing morbidity data at follow-up had higher systolic blood pressure levels, larger waist circumferences, and higher triglyceride levels. Table 1 shows the baseline characteristics, 8-year mortality, and fatal and nonfatal CVD incidence according to baseline retinopathy status.
Retinopathy and all-cause mortality After a median follow-up of 7.5 years (range 7.07.9), 64 patients died. Among patients with no, nonproliferative, and proliferative retinopathy, the percentages of deaths were 1.5% (n = 18; two cardiovascular, nine noncardiovascular, seven unknown), 3.0% (n = 24; 6, 9, 9, respectively), and 9.5% (n = 22; 8, 7, 7, respectively) (Table 1). The Kaplan-Meier curves in Fig. 1 show that all-cause mortality was significantly higher in patients with proliferative retinopathy than in patients with nonproliferative retinopathy and subjects without retinopathy (P < 0.001). All-cause mortality in patients with nonproliferative retinopathy was also significantly elevated compared with patients without retinopathy (P = 0.034). The age- and sex-adjusted HRs of all-cause mortality were 1.45 (95% CI 0.712.96) and 4.16 (1.968.84) in patients with nonproliferative and proliferative retinopathy, respectively, compared with patients without retinopathy (Table 2). The increased mortality risks associated with nonproliferative and proliferative retinopathy were considerably attenuated by adjustment for lipids and AER and, for nonproliferative retinopathy, was completely obliterated in the multivariate model, while remaining large (2.06), but statistically nonsignificant, in proliferative retinopathy (Table 2).
Retinopathy and incident CVD After a median follow-up of 7.4 years (range 6.97.9), 128 patients had incident CVD (either fatal or nonfatal). Among patients with no, nonproliferative, and proliferative retinopathy, the percentages of cases were 4.1% (n = 43; 2 fatal, 43 nonfatal), 8.9% (n = 60; 6 fatal, 58 nonfatal), and 13.6% (n = 25; 8 fatal, 21 nonfatal). The age- and sex-adjusted HRs of incident CVD were 1.73 (95% CI 1.152.60) and 2.05 (1.223.45) in patients with nonproliferative and proliferative retinopathy compared with patients without retinopathy, respectively (Table 2). The increased CVD risks associated with nonproliferative and proliferative retinopathy were explained to some extent, but not entirely, by other risk factors, notably HbA1c, hypertension, and albumin excretion (Table 2).
Additional analyses
The present study shows that both nonproliferative and proliferative retinopathy were predictive of subsequent all-cause mortality and cardiovascular events in type 1 diabetic patients. After adjustment for cardiovascular risk factors, the increased risk for all-cause mortality disappeared in subjects with nonproliferative retinopathy but remained twofold greater in those with proliferative retinopathy. Lipid levels and AER explained a considerate part of the association between retinopathy and mortality. Surprisingly, HbA1c and diabetes duration, both important risk factors for retinopathy as well as mortality in the EURODIAB study (10,13), did not contribute to the relation of retinopathy and mortality. The increased risk for CVD in people with nonproliferative or proliferative retinopathy was attenuated but not completely explained by adjustment for cardiovascular risk factors, in particular HbA1c, hypertension, and AER. The present findings support results from previous studies (4,69,11,25) in type 1 and type 2 diabetes that demonstrated that retinopathy is positively associated with mortality and cardiovascular events and that these associations can be explained to an important extent, but not entirely, by other cardiovascular risk factors (4,8). The Wisconsin Epidemiologic Study of Diabetic Retinopathy, the only prospective study of retinopathy and mortality in a type 1 diabetic population, showed after 16 and 20 years follow-up a slightly increased risk for CVD and mortality associated with retinopathy (4,11), which is consistent with our results. In the present European study, the association with all-cause mortality was relatively stronger but lost statistical significance in the multivariate model due to small sample sizes. Moreover, the number of fatal CVD cases was too small for separate analyses. In the Atherosclerosis Risk in Communities study (26), diabetic retinopathy was not associated with any clinical manifestation of atherosclerosis, such as stroke, coronary heart disease, and peripheral artery disease. However, these were cross-sectional results, and only one fundus photograph was taken of each participant, which may have influenced the associations. Our finding that proliferative retinopathy was more strongly associated with all-cause mortality than nonproliferative retinopathy is consistent with other studies (4,6,8) in type 2 diabetic patients and suggests that proliferative retinopathy is a stronger risk marker than nonproliferative retinopathy. Risk of fatal and nonfatal CVD, however, was almost similar in both nonproliferative and proliferative retinopathy. The stronger association of proliferative retinopathy with all-cause mortality (fourfold) than with combined fatal and nonfatal cardiovascular events (twofold) is striking, given that we hypothesized that any association should be related to vascular disorders. This discrepancy may be partly due to, first, the considerable proportion of deaths with unknown causes, which are likely to be cardiovascular in origin. Second, the majority of cardiovascular events were nonfatal and based on criteria such as ECG abnormalities, which may result in a degree of misclassification. Life and death status is clearly easier to classify correctly. In contrast to previous reports (4,7,8) in type 2 diabetic patients, the increased risk of all-cause mortality and incident CVD associated with nonproliferative retinopathy was completely obliterated after adjustment for known cardiovascular risk factors. However, proliferative retinopathy was associated with a twofold increased risk for all-cause mortality and CVD, although nonsignificant. Thus, proliferative retinopathy may reflect other pathways than the conventional cardiovascular risk factors leading to an increased risk of mortality or CVD. Advanced glycated end products could play an important role in the relationship between retinopathy and cardiovascular outcomes, as advanced glycated end products are reported to be involved in the pathogenesis of retinopathy and atherosclerosis (27,28). Other unidentified mechanisms could include endothelial dysfunction and inflammatory activity, both reported to be associated with microvascular and macrovascular disorders (2933). These possibilities require further study. The EURODIAB Prospective Complications Study offers a unique possibility to study complications of type 1 diabetes, as it provides a large, multicenter, European cohort of type 1 diabetic patients attending a clinic minimally once a year and where the same standardized methods were used in each center. Furthermore, type 2 diabetes is often associated with numerous other risk factors for CVD in the, often elderly, patients affected, so that disentangling analysis may remain open to imprecision. Type 1 diabetes may offer a "cleaner" model to study such relationships. However, since cardiovascular events are relatively rare among these younger patients, a similar study could be carried out only in such large population as the one studied prospectively by the EURODIAB Prospective Complications Study. Limitations were also present in this study. First, of the baseline population, 31% had missing fundus photographs and/or were lost to follow-up, and 14% had missing morbidity data. These individuals with missing data of mortality and morbidity follow-up and/or fundus photographs at baseline had a more atherogenic profile. This may have lead to an underestimation of the true association of retinopathy with mortality and CVD incidence. Second, the release of confidential cause of death data was difficult due to legal restrictions in a number of countries, which led to a relatively large number of unknown causes of death. Finally, the measurement at baseline of all covariates used in this study was done only once. Therefore, the role of residual confounding related to imprecision of measurements of covariates due to this single measurement, leading to an underestimation of the confounding role of a covariate, cannot be discounted. Nevertheless, the twofold increased risk of total mortality associated with proliferative retinopathy despite multivariate adjustments suggests that residual confounding cannot wholly account for our findings. In conclusion, we have shown that nonproliferative and proliferative retinopathy in type 1 diabetes are associated with an increased risk of all-cause mortality and incident CVD. Adjustments for cardiovascular risk factors completely obliterated the association with nonproliferative retinopathy, whereas the association with proliferative retinopathy remained increased, although nonsignificant. These data suggest that mechanisms other than shared cardiovascular risk factors may be involved in the relationship between retinopathy and CVD and mortality.
This work was generously supported by grants from the Wellcome Trust, European Community, and Diabetes U.K. We thank all the investigators and all the patients who took part. The EURODIAB Prospective Complications Study Group: B. Karamanos, A. Kofinis, K. Petrou, Hippokration Hospital, Athens, Greece. F. Giorgino, G. Picca, A. Angarano, G. De Pergola, L. Laviola, R. Giorgino, Department of Emergency and Organ Transplantation, Internal Medicine, Endocrinology and Metabolic Diseases, University of Bari, Bari, Italy. C. Ionescu-Tirgoviste, A. Coszma, C. Guja, Clinic of Diabetes, Nutrition & Metabolic Diseases, Bucharest, Romania. M. Songini, A. Casu, M. Pedron, S. Pintus, M. Fossarello, Diabetes Unit Ospedale San Michele, Cagliari, Italy. J.B. Ferriss, G. Grealy, D.O. Keefe, Cork University Hospital, Cork, Ireland. M. Toeller, C. Arden, Diabetes Research Institute, Heinrich-Heine University, Dusseldorf, Germany. R. Rottiers, C. Tuyttens, H. Priem, University Hospital of Gent, Gent, Belgium. P. Ebeling, M. Kylliäinen, V.A. Koivisto, University Hospital of Helsinki, Helsinki, Finland. B. Idzior-Walus, J. Sieradzki, K. Cyganek, B. Solnica, Department of Metabolic Diseases, Jagiellonian University, Krakow, Poland. H.H.P.J. Lemkes, J.C. Lemkes-Stuffken, Leiden University Medical Centre, Leiden, The Netherlands. J. Nunes-Correa, M.C. Rogado, L. Gardete-Correia, M.C. Cardoso, A. Silva, J. Boavida, M. Machado Sa Marques, Portuguese Diabetic Association, Lisbon, Portugal. G. Michel, R. Wirion, S. Cardillo, Centre Hospitalier, Luxembourg. G. Pozza, R. Mangili, V. Asnaghi, Ospedale San Raffaele, Milan, Italy. E. Standl, B. Schaffler, H. Brand, A. Harms, City Hospital Schwabing, Munich, Germany. D. Ben Soussan, O. Verier-Mine, P. Fallas, M.C. Fallas, Centre Hospitalier de Valenciennes, Valenciennes, France. J.H. Fuller, J. Holloway, L. Asbury, D.J. Betteridge, University College London, London, U.K. G. Cathelineau, A. Bouallouche, B. Villatte Cathelineau, Hospital Saint-Louis, Paris, France. F. Santeusanio, G. Rosi, V. DAlessandro, C. Cagini, P. Bottini, G.P. Reboldi, Diaprtimento di Medicina Interna, Perugia, Italy R. Navalesi, G. Penno, S. Bandinelli, R. Miccoli, M. Nannipieri, Dipartimento di Endocrinologia e Metabolismo, Pisa, Italy. G. Ghirlanda, C. Saponara, P. Cotroneo, A. Manto, A. Minnella, Universita Cattolica del Sacro Cuore, Rome, Italy. J.D. Ward, S. Tesfaye, S. Eaton, C. Mody, Royal Hallamshire Hospital, Sheffield, U.K. M. Borra, P. Cavallo Perin, S. Giunti, G. Grassi, G.F. Pagano, M. Porta, R. Sivieri, F. Vitelli, M. Veglio Dipartimento di Medicina Interna, Università di Torino and ASO CTO/CRF/Maria Adelaide, Turin, Italy. N. Papazoglou, G. Manes, General Hospital Papageorgiou of Thessaloniki, Thessaloniki, Greece. M. Muggeo, M. Iagulli, V. Cacciatori, V Cattedra di Malattie del Metabolismo, Verona, Italy. K. Irsigler, H. Abrahamian, Hospital Vienna Lainz, Vienna, Austria. S. Walford, J. Sinclair, S. Hughes, V. McLelland, J. Ward, New Cross Hospital, Wolverhampton, U.K. G. Roglic, Z. Metelko, Z.R. Pepeonik, Vuk Vrhovac Institute for Diabetes, Zagreb, Croatia. Steering committee members: J.H. Fuller (London), B. Karamanos, Chairman (Athens), A.-K. Sjolie (Aarhus), N. Chaturvedi (London), M. Toeller (Dusseldorf), G. Pozza, cochairman (Milan), B. Ferriss (Cork), M. Porta (Turin), R. Rottiers (Gent), G Michel (Luxembourg). Coordinating center: J.H. Fuller, N. Chaturvedi, J. Holloway, D. Webb, L. Asbury, University College London, London, U.K. Central laboratories: G.-C. Viberti, R. Swaminathan, P. Lumb, A. Collins, S. Sankaralingham, Guys and St. Thomas Hospital, London, U.K. Retinopathy grading center: S. Aldington, T. Mortemore, H. Lipinski, Royal Postgraduate Medical School of Imperial College London, London, U.K.
A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances. Received for publication December 3, 2004. Accepted for publication March 14, 2005.
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