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New Orleans—Silicone hydrogel contact lenses may produce fewer complications for patients because of their high oxygen permeability, although their extended wearability could negate that advantage, suggested Peter R. Kastl, MD, PhD, professor of ophthalmology and adjunct professor of biochemistry, Tulane University, New Orleans.
"The new silicone hydrogel lenses deliver a lot more oxygen to the cornea and will probably have fewer complications," said Dr. Kastl, a specialist in anterior segment and corneal disease. "However, as patients start to wear them more-such as for 30 days at a time-the fewer complications may be counterbalanced by the longer wearing time. Only time will tell."
Silicone hydrogel lenses on the market include Acuvue Advance (Vistakon); Focus Night & Day and O2Optix brands (CIBA Vision); and PureVision (Bausch & Lomb).
Another study compared the PureVision lens among patients randomly assigned to 7-day extended wear or 30-day continuous wear and followed for 12 months. Although corneal infiltrates were twice as high in the 30-day group, the trend was not statistically significant. The authors concluded that there was no significant difference in complication rates between the 7- and 30-day groups.
Complications that can occur with the silicone hydrogel lenses include mucin balls, superior epithelial arcuate lesions, giant papillary conjunctivitis, and corneal erosions. Mucin balls (first described about 4 years ago) are spherical, translucent entities that can cause depression into the ocular surface, Dr. Kastl said. They tend to occur with flat silicone hydrogel lenses and have most commonly been observed with the CIBA Focus Night & Day.
Mucin balls are formed from the gradual movement of the lens across the tear film and are more likely to appear when the silicone hydrogel lenses are worn for longer periods, such as 30 days, rather than for 7 days. However, the mucin balls do not seem to have any effect on symptoms or vision, Dr. Kastl said.
The way to treat mucin balls is to fit a steeper lens, he continued.
"The thing to remember about silicone hydrogel lenses is their low water content," he explained. "They will not tighten with wear, so the way you fit them now is the way they are going to continue to fit. You treat this particular complication by fitting a tighter lens."
Loose lenses also may cause superior epithelial arcuate lesions; the lesion typically resolves after the lens is removed.
Microbial keratitis, which is a serious problem with any contact lens, has also been reported with silicone hydrogel brands.
"Most investigators feel, however, that the incidence will decrease with the much larger amount of oxygen getting to the cornea," Dr. Kastl explained. "This may be balanced by wearing the lenses for a full month."
Dr. Kastl also observed that silicone hydrogel lenses might have some therapeutic uses. Because of the high amount of oxygen flowing through the lens, it could be useful for treating corneal disorders.
This theory was explored in a recent prospective, open, nonrandomized study in which the CIBA Vision Focus Night & Day lens was evaluated as a continuous-wear bandage in several corneal diseases. Seventy eyes in 70 patients with anterior segment conditions for which a therapeutic lens was indicated wore the lenses continuously for 7 to 30 days and were followed up to 18 months.
Bullous keratopathy, the most prevalent condition in the patient group (47 cases), improved, while a high rate of healing was achieved in patients who had erosions and postoperative keratoepitheliopathies.
In addition, patients indicated that the lenses did not cause irritation.