Wearing lenses overnight could cause blindness

Bacteria in the eye can cause a serious infection

Leaving contact lenses in overnight carries a hugely increased risk of an infection that can cause permanent sight loss, say researchers.

Contact lens use is already known to be most significant cause of microbial keratitis, a condition in which the outer layer of the eye, the cornea, becomes inflamed after infection by bacteria, fungi or amoebae.

This risk is 80 times that of the non-wearer.

But the Netherlands study, published in "The Lancet", has found that the type, and duration, of contact lens use increases or reduces the risk of infection.

People who wore hard contact lenses had about a 1 in 10,000 chance per year of picking up the condition.

Vastly increased risk

Wearers of soft contact lenses using them for 24 hours at a time or less trebled that risk.

However, people who wore soft contact lenses for longer than this faced a risk of microbial keratitis 20 times higher than hard contact lens wearers.

The research concluded: "Use of contact lenses can lead to profound and permanent visual loss in other healthy eyes.

"The main risk factor for corneal infection is overnight wear, which should be discouraged."

The risks of infection are high because contact lenses stop the cornea getting enough oxygen, with the water layer on top stagnated, leading to deposits of bacteria and dirt on the lens surface.

New soft lenses being launched have increased permeability to oxygen, but John Dart, from Moorfield Eye Hospital in Hospital suggests this may not be enough to cut the risk entirely.

Safety not yet known

He said: "Pre-marketing studies are too small to be able to warn of uncommon but serious problem, so whether the increased oxygen permeability of the silicone hydrogels makes extended-wear contact lenses safe may not be known for a long while.

"Another experiment at the expense of the public health may not be forgiven."

There are currently about 1.7m contact lens wearers in the UK.

The Dutch team used information provided from 379 opthalmologists, who revealed 92 cases of microbial keratitis.