Oxygen may be the culprit in nuclear
WASHINGTON, DC - A new understanding of the role of oxygen in post-surgical
cataract formation should lead to a better understanding of how
cataracts form and how they might be prevented, David C. Beebe PhD
told a Research to Prevent Blindness seminar.
He explained that the natural lens exists normally in a very low
oxygen environment and the vitreous appears to have a sheltering
role, keeping the oxygen away from the lens.
When the vitreous body liquefies prematurely, as in high myopia
or vitrectomy, nuclear cataracts can develop and progress.
Dr Beebe cited research indicating that patients repeatedly exposed
to high levels of oxygen over a long period of time would develop
nuclear cataracts. Work at his own laboratory and others has shown
that oxygen is toxic to the lens.
"We believe that as oxygen diffuses from the blood vessels
in the retina, it uses up oxygen at a very high level. So if the
vitreous gel is present in the eye, the oxygen levels are high only
very close to the source - the retinal vessels.
"We also know that people who have vitrectomies are at a very
high risk of developing nuclear cataracts," Dr Beebe said.
He believes this oxygen hypothesis is a reasonable explanation for
why people get cataracts after vitrectomy.
He predicts that if retinal surgery could be performed without vitrectomy,
post-vitrectomy nuclear cataracts would not occur.
There is clinical support for this idea. For example, in one study,
pre-retinal membranes removed without vitrectomy did not lead to
post-vitrectomy cataract or myopic shift after two years of follow-up,
Dr Beebe predicts that, as may be the case after vitrectomy, when
the vitreous body breaks down, circulation will start in the eye
and carry oxygen from the retina to the lens. Consequently, patients
who lose much of their vitreous to breakdown will be at risk from
In a study reported earlier this year at the annual meeting of the
Association for Research in Vision and Ophthalmology (ARVO), Dr
Beebe and colleagues evaluated whether liquefaction of the vitreous
gel was related to maturity-onset nuclear cataract.
An analysis of 83 donor lenses confirmed that increased vitreous
liquefaction was associated with increased age-related nuclear cataract.
The study showed a statistically significant association between
the amount of vitreous breakdown and the extent of cataract.
"This is the first time that we've been able to connect changes
in the environment around the lens to cataracts. It gives us an
understanding of the potential mechanisms that lead from changes
in the eye to cataract formation.
"We have known of no other mechanism to understand each step
in the process that would lead from a particular change in the body
to a cataract.
"So, the idea that breakdown of the vitreous body or its destruction
during vitrectomy leads to cataract is the first time we've really
had a causal chain of events, Dr Beebe told EuroTimes.
Stuart J. McKinnon MD, PhD said he found Dr Beebe's hypothesis interesting
and sensible. He added that he would like to see more data on oxygen
concentration in various parts of the eye.
"I would like to see the researchers show that there is a difference
in oxygen concentration after vitrectomy, that it is actually higher
around the lens and then perhaps to artificially create a condition
where more oxygen is around the lens investigate whether or not
that would produce a cataract," Dr McKinnon.
He noted that the research suggests the possibility of using an
oxygen-scavenging molecule to prevent cataracts in that situation.
Moreover, there are similar implications in other areas of ophthalmic
surgery, such as glaucoma filtration, where changing the normal
physiology can alter the way the lens behaves and can induce cataracts.
Dr Beebe agreed there is a need to better understand the mechanism
of the disease before therapies can be designed for its prevention.
The current study at least suggests that preservation or replacement
of the vitreous gel might protect patients from nuclear cataract.
"I think the advances we are making, if verified by other studies,
will lead to a better understanding of the mechanism of the disease.
We feel that others will certainly take this up and look at ways
to prevent the process," he said.