Sleeping in contact lenses can lead to an increased risk of severe eye infection, new research suggests.
But new generation contact lenses, the investigation reveals, perform better in this regard than their predecessors.
The University of Manchester study found that wearers who failed to remove their lenses before bedtime had an increased risk of developing keratitis than those who routinely took out their lenses before going to sleep.
The research also found that the type of contact lens worn had a significant effect on a person's chances of developing a severe infection.
The findings, based on a year-long study of patients attending the Royal Manchester Eye Hospital, showed that people who slept in hydrogel lenses were five times more likely to develop keratitis than those sleeping in silicone hydrogel lenses.
No difference between the type of lens worn and the risk of infection was found for normal daily wear.
The research, published in the British Journal of Ophthalmology tomorrow (Tuesday, March 22), was led by Dr Philip Morgan, an optometrist in the University's Faculty of Life Sciences.
He said: "Patients coming to the hospital with acute eye problems were asked to supply details of lens type and pattern of wear, including whether they slept in their lenses.
"Four types of lenses were studied - rigid, hydrogel daily disposable, hydrogel and silicone hydrogel - and patients' eye problems on the cornea were scored according to their severity.
"It was shown that the risk of severe keratitis was increased when lenses were slept in and that this risk varied according to the type of lens worn.
"Those who choose to sleep in lenses should be advised to wear silicone hydrogel lenses, which carry a five times decreased risk of severe keratitis for extended wear compared with hydrogel lenses."
About 3 million people in the UK wear contact lenses, the vast majority of whom use hydrogel lenses. In the catchment area served by the Royal Manchester Eye Hospital, 30,000 of the estimated 55,000 lens wearers have hydrogel contacts compared to just 1,700 silicone hydrogel wearers.
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