I find your comments interesting, TG. I look at overnight OK from the
following perspective.
The incidence of microbial keratitis from sleeping in a soft
(conventional hydrogel) lens is about 1/500 per year, confirmed by
numerous studies. But, loss of vision occurs in only about 13% of
those cases. This is because the location of the ulcer is random, and
probability dictates that it will more likely occur off the visual
axis. The cause of ulcerative keratitis is adherence of bacteria to a
compromised, roughened epithelial surface. The cause of the
compromised epithelium in this case is hypoxia.
With OK, the epithelium is thinner centrally as it is literally pushed
away by the pressure of the lens. The cause of the compromised
epithelium is mechanical. Therefore, the forces likely to cause
epithelial compromise are concentrated on the visual axis, with a
greater likelihood that an infection will occur there, or in very close
proximity. Therefore, if an ulcer does occur, it will certainly do
more damage.
Naturally, the presence of acanthamoeba suggests improper lens hygeine
and the use of tap water to rinse the lenses prior to insertion.
DrG
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