by Patricia McBroom
A common bacterium known to cause eye infections on injured corneas has now been found to damage the healthy eye if it stays in contact long enough, raising concerns about the use of long-wear contact lenses.
Contacts that can be worn for 30 days or longer are poised to enter the commercial market, but these new findings are a cause of concern, said Suzanne Fleiszig, a microbiologist at the School of Optometry.
Fleiszig has found that several strains of the bacterium Pseudomonas aeruginosa are more aggressive than had been thought. Scientists had believed that healthy surface cells of the cornea were all but impervious to infection by this bacterium.
"Now we know that is not true," said Fleiszig who is also an optometrist and an expert in corneal physiology.
"We are concerned that if these bacteria get trapped under a long-wear lens, they could infect the cornea," she said.
Moreover, the damage progresses rapidly, according to Fleiszig's tests on eyes removed from rats. The bacteria were able to damage the cornea within three hours. This evidence suggests that the bacterium would also cause infection in an intact human eye, said Fleiszig, but tests to prove the point cannot be done.
It is known from past experience with humans, however, that once the cornea becomes infected by these bacteria, the organisms can perforate the cornea within 24 hours, making this a rapidly progressing disease.
In December, Fleiszig received the Irvin M. Borish Outstanding Young Researcher Award from the American Academy of Optometry for her research, part of which was published in January in the quarterly journal of the Contact Lens Association of Ophthalmologists.
Coauthors on the paper are Ellen Lee, Christine Wu, Renisa C. Andika, Vicky Vallas and Marta Portoles from Fleiszig's Berkeley laboratory and Professor Dara Frank of the Medical College of Wisconsin, Milwaukee.
Publication came in time for the association's annual scientific meeting in Las Vegas Jan. 29 to 31, where it alerted eye doctors to the potential threat of these aggressive strains of bacteria.
Currently, the FDA limits the use of extended wear to seven days at a time for the more than 20 million contact lens users in the United States.
Approval for 30-day wear of contact lenses had been granted in 1981 but was rescinded in 1989 when the rate of infection turned out to be quite high, roughly one in a thousand users, according to Fleiszig. Since then, industry scientists have created new lenses to allow more oxygen to reach the eye, hoping this would create a safer lens for the longer period of 30 days or more.
"The new lenses should represent a significant improvement in reducing many of the other complications associated with extended wear of contact lenses," said Fleiszig, "but we do not know if they will reduce the risk of infection. Oxygen deprivation is not the only issue that needs to be addressed."
She said that extended wear of lenses also interferes with the movement of tears across the eye, with the result that dangerous forms of bacteria could be allowed to stay in contact with the cornea too long.
"In effect, the lenses would interfere with the eye's natural garbage disposal system," she said.
The infectious potential of P. aeruginosa may have gone unnoticed before because the topsides of corneal cells resist infection. But Fleiszig's research has shown that the undersides of the cells are vulnerable. The undersides of the cells are exposed during the process of exfoliation, when they peel off to make way for new cells.
"Normally, tear movement would cleanse the eye and wash away this garbage," said Fleiszig.
But tear movement is extremely limited beneath soft contact lenses, according to work by Fleiszig's collaborator, Professor of Optometry Kenneth Polse. Fleiszig and Polse are now working with others at the school in a new tear-mixing laboratory to find ways of improving fluid dynamics under contact lenses.
Fleiszig expressed concern that her research might cause people who want an alternative to glasses to opt for new laser refractive surgery instead of contact lenses.
"I don't want my research to drive the public toward laser refractive surgery," she said. "We don't know enough about the biology of the cornea to say this kind of surgery is completely safe, and you can't reverse it."
She recommended that until more data is made available, the public continue daily wear of contact lenses.
"Decades of research attest to the safety of contact lenses that are removed every day," said Fleiszig.