Reusable contact lens users are almost four times as likely as those wearing daily disposables to develop a rare sight-threatening eye infection, a study has found.
The researchers suggest people should avoid wearing their lenses while swimming, or in the shower, and that packaging should include “no water” stickers.
The study identified multiple factors that increase the risk of Acanthamoeba keratitis (AK), including reusing lenses or wearing them overnight or in the shower.
AK is a type of corneal infection, and is a condition that results in inflammation of the cornea -the eye’s clear protective outer layer.
Lead author Professor John Dart, of UCL Institute of Ophthalmology and Moorfields Eye Hospital NHS Foundation Trust, said: “In recent years we have seen an increase of Acanthamoeba keratitis in the UK and Europe, and while the infection is still rare, it is preventable and warrants a public health response.
“Contact lenses are generally very safe but are associated with a small risk of microbial keratitis, most commonly caused by bacteria, and which is the only sight-threatening complication of their use.
“Given that an estimated 300 million people across the globe wear contact lenses, it is important that people know how to minimise their risks for developing keratitis.”
Researchers say contact lens use is now the leading cause of microbial keratitis in patients with otherwise healthy eyes in countries in the global north.
While sight loss resulting from microbial keratitis is uncommon, Acanthamoeba, although a rare cause, is one of the most severe.
It is responsible for about half of those contact lens users who develop sight loss after keratitis.
Some 90% of AK cases are associated with avoidable risks although the infection remains rare, affecting fewer than one in 20,000 contact lens wearers per year in the UK.
The most severely affected patients – a quarter of the total – end up with less than 25% of vision or become blind following the disease and face prolonged treatment.
Overall, 25% of people affected require corneal transplants to treat the disease or restore vision, the study suggests.
The study, led by UCL and Moorfields researchers, recruited more than 200 patients of Moorfields Eye Hospital who completed a survey, including 83 people with AK.
They were with 122 people who came to eyecare clinics with other conditions.
The study found that people who wore reusable soft contact lenses (such as monthlies) had 3.8 times the odds of developing AK, compared to people who wore daily disposable lenses.
Showering with lenses increased the odds of AK by 3.3 times while wearing lenses overnight increased the odds by 3.9 times.
Among daily disposable wearers, reusing their lenses increased their infection risk while a recent contact lens check with a health professional reduced the risk.
First author, associate professor Nicole Carnt, of UNSW, Sydney, UCL Institute of Ophthalmology and Moorfields Eye Hospital, said: “Previous studies have linked AK to wearing contact lenses in hot tubs, swimming pools or lakes, and here we have added showers to that list, underlining that exposure to any water when wearing lenses should be avoided.
“Public pools and coastal authorities could help reduce this risk by advising against swimming in contact lenses.”
Prof Dart added: “Contact lens packaging should include information on lens safety and risk avoidance, even as simple as ‘no water’ stickers on each case, particularly given that many people buy their lenses online without speaking to a health professional.
“Basic contact lens hygiene measures can go a long way in avoiding infections, such as by thoroughly washing and drying your hands before putting in your lenses.”
The study, published in Ophthalmology, was funded by Fight for Sight, the NIHR Moorfields Biomedical Research Centre and Moorfields Eye Charity.