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.
Notes for editors:
Keratitis is a term used to define a wide variety of corneal infections, irritations and inflammations, commonly caused by bacterial or fungal invasions following superficial corneal abrasions.
The findings are based on a study of 118 patients with acute eye problems attending the Royal Manchester Eye Hospital between January 25, 2003, and January 24, 2004.
Among the 80 patients defined as having non-severe keratitis 18 slept in their lenses. Of the 38 patients with severe keratitis, nine did so.
The rate for severe keratitis when sleeping in hydrogel lenses was 96 per 10,000 wearers per year compared to 20 per 10,000 per year for wearers of silicone hydrogel lenses. These compare to 6.4 wearers per year for daily wear of hydrogel lenses.
Silicone hydrogels are the most commonly prescribed form of contact lens for 'extended wear' (sleeping in lenses). They were launched in 1999 in the UK and allow a much higher level of oxygen to reach the eye during contact lens wear.
This is the first published study to present the relative risks of all current forms of contact lens types – including silicone hydrogels.
Dr Curtis Dobson, also at The University of Manchester, is currently developing new anti-bacterial compounds whose first application is likely to be in the coating of contact lenses to prevent eye infections. (Please see associated press release attached.)
For further information contact:
Dr Philip Morgan
Eurolens Research
The University of Manchester
Tel: 0161-306-4441
Email: philip.morgan@manchester.ac.uk
Or
Aeron Haworth
Press Office
The University of Manchester
Journal
British Journal of Ophthalmology