Impact of socioeconomic deprivation on the development of diabetic retinopathy: a population-based, cross-sectional and longitudinal study over 12 years

The Scottish Care Information Diabetes Collaboration system

The Scottish Care Information Diabetes Collaboration (SCI-DC) data set holds information on the demographic characteristics of each patient diagnosed with DM, such as the postcode of residence, registration of general practitioner (GP), year of birth and date of diagnosis of DM. The SCI-DC data set also includes clinical parameters, which are measured at least once a year after the diagnosis of diabetes: HbA1c, cholesterol levels, systolic blood pressure (sBP) and diastolic blood pressure (dBP). We obtained the most recent DR status, HbA1c, cholesterol, sBP and dBP levels for this study.

Socioeconomic deprivation

We used the Scottish Index of Multiple Deprivation (SIMD) 2012 score as a measure of area-based deprivation.⁸ The SIMD score for each geographical data zone is a combination of 38 indicators of deprivation across seven broad domains: education, crime, geographic access, income, skills and training employment, health, and housing. We used the residential postcodes to assign each patient an individual SIMD 2012 score. The higher the SIMD 2012 score, the more deprived the area of residence. The SIMD score is used to group the data zones into quintiles: 0–20% most deprived, 20–40%, 40–60%, 60–80%, 80–100% least deprived.

Scottish Diabetic Retinopathy Screening protocol

The protocol for Scottish Diabetic Retinopathy Screening has been widely published.⁹ To summarise, all patients with diabetes in the East of Scotland undergo free annual DR screening; this is provided by two mobile units equipped with digital fundus cameras which travel to every GP practice, and a static camera site in Dundee.⁶ Grading of DR and maculopathy is performed from digital retinal photographs and outcomes stored in the regional screening database linked to the SCI-DC. The use of digital retinal photographs for DR screening in Scotland has been previously validated.^{10 11}

The primary outcome of the study was presence of background DR (R1) or worse, in either eye.

Strengths and weaknesses of the study

The strengths of our study lie in the large sample size and longitudinal cohort of patients who have been followed up systematically using the validated standardised yearly Scottish diabetic screening protocol. Through the integrated electronic patient record system, we were able to obtain measures of diabetic control, such as HbA1c levels, BP readings and cholesterol levels.¹⁵ However, one weakness of the study was the use of the most recent biochemistry and clinical data instead of the overall mean values. We were unable to attain information on the changes in postcode of the patients or account for mortality bias in this study.¹⁶ Another limitation to the study is the inherent bias in using screened populations. Patients who have not engaged with healthcare services or the screening programme would not have been included in the analyses. In addition, some patients would have presented with severe retinopathy and been treated under the care of the hospital and hence, were not included in the screening programme. This could result in selection bias if factors, such as socioeconomic deprivation, affect the likelihood of a patient being screened.

Comparison with other studies

Previous studies have examined the influences of socioeconomic deprivation on DR in other parts of the UK. However, these studies did not account for other major risk factors such as duration of disease, HbA1c levels, lipid profile and BP control, all of which are known risk factors for development of DR. In the Gloucestershire cohort, socioeconomic deprivation was associated with sight-threatening DR, but not with non-sight-threatening DR.¹⁷ Similarly, in Avon and Somerset, patients with lower education levels were more likely to develop DR than those with higher education.¹⁸ However, in a Southampton cohort, Litwin et al found no association between relative affluence of residence and presence of retinopathy at time of diagnosis of type 2 DM.¹⁹

From a European perspective, the EURODIAB IDDM Complications study showed a higher prevalence of proliferative DR in men with lower educational qualifications compared with those with higher levels of education.²⁰ In Spain, socioeconomic status was not independently associated with DR.²¹

Comparisons of development of retinopathy and socioeconomic deprivation between studies have to be carefully interpreted due to differences in measures of deprivation. Data compiled from self-reported questionnaires on income and educational attainment may have reporting biases.¹⁸ Moreover, most studies have only looked at baseline or cross-sectional screening outcomes and have not analysed these longitudinally over time.^{22 23}

Meaning of the study

Eye healthcare in Scotland is provided free of charge, and with the use of mobile screening units, socioeconomic deprivation is not independently associated with increased prevalence of DR in type 2 DM. The lack of association between socioeconomic deprivation and DR in patients with type 2 DM in our study echoes the findings in a study conducted by Guthrie et al.²⁴ They found that socioeconomic variations in the care of type 2 diabetes in Tayside have been largely eliminated under the pay-for-performance scheme (Quality and Outcomes Framework (QOF) 2004). However, younger patients were less likely to receive systematic care and have poorer control of intermediate outcomes. This is a reflection of complexity in the management of diabetes, which is beyond achieving ‘QOF targets’. Patient engagement and health behaviours play an important role. Indeed, the QOF was an effective instrument to incentivise care for older patients with type 2 DM, but its lack of efficacy in the control of DM in younger patients shows that it is not a ‘one-size-fits-all’ solution. This could be a possible explanation to the observation that SIMD has a significant relationship with prevalence of retinopathy in type 1 but not in type 2 DM.

It is worrying that despite free access to eye care in a well-developed diabetes care system, there is still an independent association between socioeconomic deprivation and increased prevalence of DR in type 1 DM. Whether socioeconomic deprivation is a cause or an effect of DR in patient with type 1 DM remains unclear. A possible explanation is that environmental factors within deprived areas may trigger immune responses in a genetically susceptible individual predisposed to type 1 DM and accelerate the development of complications.²⁵ Another explanation could be a poorer control of diabetes in patients with lower socioeconomic background—we have previously shown that missed appointments in the diabetes retinal screening programme is associated with younger patients living in more deprived areas.⁶ On the other hand, the development of complications, such as retinopathy, may impact on the socioeconomic status of patients with type 1 DM. Poor diabetic control has been known to negatively affect academic achievement in school-aged children and reduce future career opportunities for patients with type 1 DM.²⁶

Our study underscores the importance of targeted interventions for those patients most at risk of developing DR, in particular those with type 1 DM, and also the need for better allocation of resources to tackle inequality in eye healthcare. We need to find innovative ways to engage with our patients, especially with this younger age group, and support local health authorities in setting priorities for eye health initiatives to reduce inequalities.

These data are available for analysis, thanks to the hard work and dedication of NHS staff across Scotland who enter the data and people and organisations (the Scottish Care Information—Diabetes Collaboration (SCI-DC) Steering Group, the Scottish Diabetes Group, the Scottish Diabetes Survey Group, the management clinical network managers and staff in each Health Board) involved in setting up, maintaining and overseeing SCI-DC.