Nine hundred and ninety-five participants contributed 3,719 participant-visits for the analysis of the prevalence of VI. Characteristics of the remaining participants at the start of each examination are shown in . Persons participating at later examinations were older, had lower glycosylated hemoglobin A1 levels, higher systolic blood pressure, lower diastolic blood pressure, higher frequency of PDR and macular edema, and were more likely to have hypertension, to be college graduates, and have photocoagulation for PDR or macular edema than those seen at earlier examinations. There did not appear to be a difference in socioeconomic status, proteinuria or visual impairment status. Because the prevalence of severe VI was low (< 4%), further analyses are restricted to the prevalence of any VI.
Characteristics of Cohort at Beginning of Each Wisconsin Epidemiologic Study of Diabetic Retinopathy Examination.
For most duration of diabetes groups, the prevalence of VI was lower in those persons diagnosed more recently than in those diagnosed earlier (). For example, the prevalence of VI in people with 15–19 years of duration of T1DM at the time of examination was 13% among those diagnosed with T1DM in 1960–69, 2% among those diagnosed in 1970–74, and 4% among those diagnosed in 1975–79. The prevalence of VI in people with 30–34 years of duration of T1DM at the time of examination was 16% among those diagnosed with T1DM in 1922–59, 15% among those diagnosed in 1960–69, and 9% among those diagnosed in 1970–74.
Differences in prevalence of visual impairment by duration and period of diagnosis of type 1 diabetes mellitus in the Wisconsin Epidemiologic Study of Diabetic Retinopathy. *CNE=cannot estimate (defined by < 50 persons at risk)
Models incorporating duration and period of T1DM cohort as categorical and, in addition, ordered factor variables are presented in . The prevalence of VI increased with longer duration of T1DM (Odds Ratio (OR) per duration group 1.15, 95% Confidence Interval (CI) 1.05, 1.27, P =.002). After controlling for age, sex, glycosylated hemoglobin A1, systolic and diastolic blood pressure, proteinuria, and photocoagulation status, the association was no longer evident (OR 0.93, 95% CI 0.79, 1.10, P =.38). There was a lower prevalence of VI with more recent diagnosis (OR per more recent period of diagnosis 0.91, 95% CI 0.88, 0.93, P <.0001). After controlling for, age, sex, glycosylated hemoglobin A1, systolic and diastolic blood pressure, proteinuria, and photocoagulation status, the association remained unchanged (OR 0.91, 95% CI 0.88, 0.94, P <.0001). By controlling further for Duncan’s Socioeconomic Index, the association remained unchanged (data not shown). There were no significant interactions between period of diagnosis of T1DM cohort and age, sex, glycosylated hemoglobin A1, and Duncan’s Socioeconomic Index and the prevalence of VI.
Diabetes Duration and Period of Diagnosis of Type 1 Diabetes Models for Visual Impairment.
Changes in incidence of visual loss over the 25 year study period are presented in . To adjust for the variable lengths of the intervals between examinations, the rates are presented on an annual basis. For any VI, the annualized incidence rates decreased from 1.19% in the first follow-up interval to 0.30% in the fourth follow-up interval. Differences in annualized incidence rates based on intervals of greatly differing lengths may be exaggerated due to the greater cumulative impact of the competing risk of death in longer intervals. However, if we consider the interval between visits 2 and 4 as a single 11 year interval comparable in length to the last 10 year interval, the annualized incidence of VI for the earlier interval (0.52%) was still higher than that of the last interval (0.30%). This relationship persisted within strata based on duration of T1DM (data not shown).
Annualized Incidence of Visual Impairment in the Wisconsin Epidemiologic Study of Diabetic Retinopathy.