People who wear soft contact lenses for more than 24 hours have a 20 times greater risk of microbial keratitis, in which the cornea, the transparent surface layer of the eye, becomes infected with bacteria, fungi and amoebae, according to Dutch researchers writing in The Lancet.
They conclude: "Use of contact lenses can lead to profound and permanent visual loss in otherwise healthy eyes. The main risk factor for corneal infection is overnight wear."
John Dart, a consultant ophthalmologist at Moorfields Eye Hospital, London, says in a commentary in the journal that the recent lifting of restrictions on extended wear lenses because of improvements in lens materials, may be premature.
Microbial keratitis is rare, affecting just over one person per 10,000 contact-lens wearers a year, but can lead to permanent blindness. The type of lenses and the method of wearing them can significantly affect the chances of getting the disease.
The research found the incidence of microbial keratitis was 1.1 per 10,000 wearers per year for rigid gas permeable lenses, and 3.3 for soft lenses worn for less than 24 hours. Among those who wore soft lenses for over 24 hours the incidence was almost 20 times higher.