Study: Most soft contact lenses produce only minor corneal changes

Natural diurnal variations in corneal thickness were typically larger than changes caused by silicone hydrogels, the study authors said.

Issue: November 2010

In a study of four different types of soft contact lenses, researchers concluded, “The corneal swelling and curvature changes seen in this study … are comparable with those seen after overnight sleep and are not likely to affect the wearer’s fit, comfort or vision.”

The study included 12 healthy subjects, 21 to 32 years old, and used four different types of soft lenses on four different days. The lenses were either toric or spherical, had powers of -3 D or 7 D and were made of hydroxyethyl methacrylate (HEMA) or silicone hydrogel.

Regional corneal thickness, as well as anterior and posterior corneal topography, were measured twice daily before and after lens wear, and within 5 to 10 minutes of lens removal, using the Pentacam HR system (Oculus, Germany).

All contact lenses caused posterior corneal steepening, which researchers found correlated weakly to central and peripheral corneal swelling. The HEMA toric lenses — the only non-silicone hydrogel lenses used in the study — caused the most corneal thickening, which the study authors attributed to its low oxygen permeability.

“These areas of swelling corresponded to the horizontal periphery of the [HEMA] lens where the thickness profile is greatest due to the stabilization zones,” lead researcher and PhD student, Garima Tyagi, said in an interview with Primary Care Optometry News. “These effects were not evident with silicone hydrogel toric lenses of identical design, indicating that the increased oxygen permeability of these modern materials significantly improves oxygen supply to the underlying cornea.”

Wearing silicone hydrogel lenses for 8 hours resulted in less corneal thickening (0.4 µm to 4.5 µm) than the natural thinning of the cornea over the same period of time (8 µm to 9 µm), according to the researchers.

“In our study, measurements were always collected at least 2 hours after subjects woke in the morning, to ensure that the large (natural) diurnal thinning of the cornea after waking did not confound the results,” Ms. Tyagi said. “However, we also found that a small thinning of the cornea continued to occur throughout the day into the afternoon (a magnitude of 8 µm to 9 µm) when subjects weren’t wearing contact lenses. Contact lens wear interferes with this natural diurnal fluctuation in the cornea by inducing a small degree of corneal swelling.”

This is the first study to investigate the effects of soft toric lenses on corneal thickness and topography, as well as the effects of “open eye” contact lens wear on posterior corneal shape, the study authors said.

Ms. Tyagi recommended that these recent findings be considered in future studies of contact lens-induced corneal swelling.

“We confirmed that the natural diurnal variations in corneal thickness, which are greatest in the morning, should be considered when analyzing corneal thickness changes to allow for accurate conclusions to be drawn,” she said.

However, because the study included only young subjects in a narrow age range, Ms. Tyagi believes an older study population could alter the results.

“Other studies have reported that aging corneas are more prone to hypoxia and take a longer time to recover from its effects, therefore, we might expect to see bigger changes in corneal thickness in response to contact lens wear in an older population,” she said. – by Courtney Preston

References:

Polse KA, Brand R, Mandell R, et al. Age differences in corneal hydration control. Invest Ophthalmol Vis Sci. 1989;30:392-399.

Siu AW, Herse PR The effect of age on the edema response of the central and mid-peripheral cornea. Acta Ophthalmol.1993;71(1):57-61.

Tyagi G, Collins M, Read S, et al. Regional changes in corneal thickness and shape with soft contact lenses. Optom Vis Sci. 2010;87(8):567-575.

Garima Tyagi can be reached at Queensland University School of Optometry, Kelvin Grove, Brisbane, Queensland 4059, Australia; 61-7-3138-5716; g.tyagi@qut.edu.au.