Diabetic retinopathy is not an independent marker of Parkinson disease (PD) among individuals aged 50 years and older, according to study findings published in Brain Communications.
A previous association between diabetic retinopathy and PD has been proposed due to the fact that both conditions may be related to an abnormal dopaminergic system. The objective of the current study aimed to evaluate and explore the prevalence and incidence of PD among patients screened for diabetic retinopathy.
The matched cohort study is based on data from several Danish registries. The first part of the study evaluated PD prevalence among patients with diabetes and diabetic retinopathy at baseline, and the second part assessed the incidence of PD in a 5-year prospective cohort.
Patients aged 50 years and older with diabetes were screened for diabetic retinopathy between 2013 and 2018, and data were extracted from the Danish Registry of Diabetic Retinopathy. The patients were randomly matched 1:5 based on sex and birth year with control individuals without diabetes from the Danish Civil Registration System.
The study population included 1,017,349 individuals—173,568 had diabetes (mean age, 68.1 years; 56.7% male), and 843,781 did not.
At the index date, 481 patients (0.28%) and 3733 control individuals (0.44%) were diagnosed with PD. The cross-sectional analysis found that patients had a lower risk of PD (odds ratio [OR] 0.79; 95% CI, 0.71-0.87) at the index date vs the control group. This finding was also observed in patients without diabetic retinopathy (odds ratio 0.76; 95% CI, 0.68-0.85), but not for patients with diabetic retinopathy levels 1 to 4 (OR 0.93; 95% CI, 0.72-1.21).
A total of 2305 PD events occurred in the 5-year follow-up, of which 365 were in the case group and 1940 occurred among the control individuals, indicating a trend toward a reduced risk of PD in patients with diabetes (adjusted hazard ratio [HR] 0.88; 95% CI, 0.78-1.00) compared with individuals without diabetes.
In patients with diabetic retinopathy level 0 or 1 to 4, no association was found between diabetic retinopathy and PD (adjusted HR 0.91; 95% CI, 0.79-1.03 and 0.77; 95% CI, 0.56-1.05, respectively).
Among individuals without diabetic retinopathy, 310 developed PD in 45,229 person-years vs 55 events in 94,798 person-years for patients with diabetic retinopathy (multivariable adjusted HR 0.74; 95% CI, 0.54-1.02).
The 5-year risk of incident diabetic retinopathy was not different between patients with and without PD at the index date (26 events in 811 person-years vs 10,399 events in 359,302 person-years, respectively; multivariable adjusted HR 0.99; 95% CI, 0.67-1.45).
Study limitations include the lack of data from the primary sector, and patients with a less severe degree of PD who have not visited the hospital are not included in the results. Also, data were unavailable regarding current smoking status and HbA1c level, and use of statins may have affected the results.
“While patients with diabetes were less likely to develop Parkinson disease, diabetic retinopathy did not affect the risk of prevalent or incident Parkinson disease,” the researchers concluded. “This study, therefore, does not support the hypothesis of diabetic retinopathy being an independent risk factor for Parkinson’s disease.”
Larsen MEC, Thykjaer AS, Pedersen FN, et al. Diabetic retinopathy as a potential marker of Parkinson’s disease: a register-based cohort study. Brain Commun. November 8, 2021. doi: 10.1093/braincomms/fcab262