Accessing medical records
For this purpose they analysed data from the Clinical Practice Research Datalink, which includes medical records on about 10 million patients in the U.K., about 7% of the country’s population. Enrolled patients are considered representative of the country with regard to age, sex and geographic distribution.
The patients were all diagnosed using a standardised coding system, the READ Codes, which include occupation; social circumstances; clinical signs and symptoms; laboratory tests and results; diagnoses; diagnostic, therapeutic or surgical procedures performed; as well as administrative items.
In order to be considered diabetic for this study, a patient had to have a READ code for diabetes mellitus and two or more prescriptions for medications for diabetes recorded within the 6 months prior to, and up to 1 year after, the first-time diagnosis of diabetes. Cataract was defined simply by the READ code for diagnosis or extraction.
The researchers only looked at patients 40-years-of-age or older at the time of the diabetes diagnosis, and only considered those diagnosed from January 2000 to December 2015. To these patients, they matched patients without diabetes to create a comparison group similar in age, sex, general practitioner and year of diabetes diagnosis.
The researchers excluded individuals with a diagnosis of congenital cataract, cancer, HIV, or alcoholism; patients with a diagnosis of glaucoma, patients had had glaucoma surgery, one or more prescriptions for drugs to treat glaucoma, or traumatic or secondary cataract; and individuals with binocular blindness, cataract, or cataract extraction recorded before the time point when they met the researchers’ diabetes definition.
They identified 56,510 patients meeting their definition of diabetic. The mean age at diagnosis was 60.1 years.
Among patients with diabetes, the incidence of cataract diagnosis or surgery was 20.4 per 1,000. Among patients without diabetes, the incidence was 10.8. Among men with diabetes the incidence was 16.5, among men without diabetes it was 8.5. For women with diabetes it was 24.9, and for women without diabetes it was 13.4.
For those with diabetic macular oedema, the incidence was 59, and among those with retinopathy it was 26.3.
The incidence of cataract increased considerably around the age of 70 years, but the incidence rate ratio—comparing those with diabetes to those without—was highest in patients 45 to 54 years of age.
Examining subpopulations, the researchers did not find an increased risk of cataract with obesity, smoking or antidiabetic drug treatment. But the risk of cataract rose with a higher HbA1c level; it was 20% higher among those with the highest HbA1c level. Diabetes duration was also associated with an increased cataract risk.
The study findings corresponded to those of previous studies, both in the U.K. and in the United States, the researchers reported.