Patients who suffer from both Type 2 diabetes and obstructive sleep apnoea are at greater risk of losing their vision within a period of four years, a study has found.

Obstructive sleep apnoea (OSA) is a condition where the walls of the throat relax and narrow during sleep, resulting in snoring and interrupting breathing, and it is common in patients with Type 2 diabetes.

Meanwhile, diabetic retinopathy - the most common form of diabetic eye disease - affects between 40 and 50 per cent of patients with diabetes and is a leading cause of blindness in the western world.

Previous studies have shown a link between OSA and diabetic retinopathy.

However, there had been no published studies assessing the impact of OSA on the progression of diabetic retinopathy in patients with Type 2 diabetes.

"Despite improvements in glucose, blood pressure and lipid levels, diabetic retinopathy remains very common," said Abd Tahrani from the University of Birmingham in the UK.

"Meanwhile, OSA has been shown to be very common in patients with Type 2 diabetes, which is not surprising considering that excess weight contributes to the development of both of these conditions," said Tahrani.

"However, most patients who have OSA are not aware that they have the condition and the disease could go undiagnosed for years," he said.

"We have shown that patients with OSA and Type 2 diabetes, compared to those with diabetes only, are at increased risk of developing advanced diabetic retinopathy over a period of three years and seven months," he added.

The study was carried out at two diabetes clinics and involved 230 patients with Type 2 diabetes.

The patients were assessed for diabetic retinopathy using specialist retinal imaging, while OSA was assessed using a home-based, multi-channel cardio-respiratory portable device.

The results showed that diabetic retinopathy prevalence was higher in patients with OSA (42.9 per cent) compared to those without OSA (24.1 per cent).

The study found that at a follow-up appointment, on average 43 months later, the patients with OSA (18.4 per cent) were more likely to develop moderate to severe diabetic retinopathy compared to those without OSA (6.1 per cent).

It also showed that patients who received treatment for OSA using a machine connected to a face mask that delivers pressure to prevent the blockage of the airways during sleep had a lower risk of developing advanced diabetic retinopathy compared to patients who did not receive the treatment.

"We can conclude from this study that OSA is an independent predictor for the progression to moderate or severe diabetic retinopathy in patients with Type 2 diabetes," said Tahrani.

The study was published in the American Journal of Respiratory and Critical Care Medicine.