Background
Diabetes is characterized by high levels of blood glucose (sugar circulating in the blood) and is classified as either type 1 or type 2, depending on the underlying cause of increased blood glucose. A common complication in people with diabetes is diabetic retinopathy, often called 'diabetic eye disease,' which affects the blood vessels in the back of the eye. Diabetic retinopathy is a major cause of poor vision and blindness worldwide among adults of working age. Research has shown that control of blood glucose reduces the risk of diabetic retinopathy and prevents worsening of the condition once it develops. However, the current diabetes epidemic suggests that the rates of new and worsening diabetic retinopathy will increase without effective means of prevention and treatment in addition to blood glucose control. Simultaneous treatment to reduce blood pressure among diabetics has been suggested as one approach.
Study characteristics
We found 15 randomized controlled trials, conducted primarily in North America and Europe, to investigate the effects of methods to lower blood pressure (drug-based in 14 trials; lifestyle change in 1 trial) in 4157 type 1 and 9512 type 2 diabetics, ranging from 16 to 2130 participants in individual trials. The follow-up period ranged from one to nine years for included trials. Of the 15 trials, six were funded in full by one or more drug companies. Seven more studies received drug company support, usually in the form of study medications. The remaining two studies were conducted with support from government-sponsored grants and institutional support. The evidence is current to April 2014.
Key results
Overall, the included trials provided modest support for lowering blood pressure to prevent diabetic retinopathy, regardless of diabetes type or baseline blood pressure level. However, the evidence did not indicate that lowering blood pressure kept diabetic retinopathy from worsening once it had developed or that it prevented advanced stages of diabetic retinopathy that required laser or other treatment of affected eyes. Treatment to reduce the blood pressure of people with diabetes is warranted for other health reasons, but the available evidence does not justify reduction of blood pressure solely to prevent or slow diabetic retinopathy.
Quality of the evidence
Overall, the quality of the evidence was low to moderate based on the reported information. The quality was downgraded mainly because some studies did not report outcomes for all or most participants at follow-up time points, and results from different studies were not highly consistent.
Authors' conclusions:
Hypertension is a well-known risk factor for several chronic
conditions in which lowering blood pressure has proven to be
beneficial. The available evidence supports a beneficial effect of intervention
to reduce blood pressure with respect to preventing diabetic
retinopathy for up to 4 to 5 years. However, the lack of evidence to
support such intervention to slow progression of diabetic retinopathy or to prevent other outcomes considered in this review,
along with the relatively modest support for the beneficial effect on
incidence, weakens the conclusion regarding an overall benefit of
intervening on blood pressure solely to prevent diabetic retinopathy.