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Definition
Hypoxia
A contact lens forms a barrier to the oxygen supply of the cornea. The
resulting corneal hypoxia induces metabolic changes in the epithelium,
which decreases glycogen stores, and increases sensitivity and adhesion.
Corneal hypoxia is associated with lens intolerance, corneal neovascularization,
and refractive error changes
Lens Intolerance
This condition presents in a seemingly successfully fit contact lens patient
as some or all of the symptoms of: pain, photophobia, ocular injection,
swollen lids, increased corneal thickness, abrasion, flush, anterior chamber
flare, decreased visual acuity, and an apparently tight (steep) fitting
lens.4 All of these symptoms are caused by a decrease in oxygen supply
to the cornea. Patients that exhibit these symptoms may be treated by
refitting with a rigid gas permeable (RGP) lens with higher oxygen transmissibility
or with a more loosely (flat) fitting lens.
Corneal
Neovascularization
Corneal neovascularization is a potentially sight threatening physiological
change in the cornea associated with lens overwear and extended wear lenses.
It is characterized by the growth of blood vessels into the normally avascular
cornea to supply oxygen and nutrients to this tissue. The condition may
resolve with cessation of lens wear, compliance to the regimen of wear
set forth by the eye care practitioner, or fitting with an RGP lens.
Refractive
Error Changes
Refractive error changes associated with hypoxia are largely due to corneal
stroma edema. Also contributing to refractive error changes is corneal
warpage, which is a change in corneal shape not associated with edema.5
The cause of corneal warpage may be lens mechanical action on the cornea
or interference with normal corneal metabolism
Overwear
The complications associated with overwear of daily wear contact lenses
are very similar to those associated with extended wear lenses. The daily
lens wearer who does not allow for proper corneal recovery periods and
who compounds this problem by occasionally sleeping in his or her lenses
has, in effect, self prescribed extended wear lenses. The major complications
that present due to these conditions of wear are attributable to hypoxia
and lens deposits.
CLKC
The Contact Lens-Related Superior Limbic Keratoconjunctivitis (CLKC) condition
begins after 2 months to 3 years of lens wear, whereupon the patient may
begin to complain of burning, itching, red eyes with increased photophobia
and secretions.
The clinical
signs of CLKC encompass:
- intense
injection of the superior bulbar conjunctiva
- punctate
staining of the superior limbus and cornea
- epithelial
and subepithelial opacities
- superior
corneal vascularization
- fine
papillary hypertrophy of the superior tarsus.
The corneal
changes seen with CLKC can reduce visual acuity because the inflammatory
events may encroach into the pupillary zone. The corneal changes often
start at the superior limbus and progress in a V-shaped wedge, with its
apex toward the pupil.
Thimerosal
Thimerosal, a preservative that was commonly found in soft contact
lens solutions, is a sodium ethylmercuric thiosalicylate, a compound of
organic mercury and thissalycilic acid. Thimerosal may cause ocular delayed
hypersensitivity reactions. When thimerosal hypersensitivity is diagnosed,
lens wear should be temporarily discontinued and the lenses should be
replaced.
Because thimerosal
is an effective preservative, it has found many uses in the United States
and abroad. This widespread availability of thimerosal in topical and
other types of preparations can potentially result in a large pool of
patients sensitive to this preservative.
Today, thimerosal
use is very low, because most practitioners have educated their patients
and are using thimerosal-free preparations. Nevertheless, thimerosal-containing
solutions are still sold, and the patient looking for "bargains" in lens
care solutions is apt to switch products and inadvertently begin using
thimerosal-containing ones.
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