Part 2
This first synopsis of the 2007 AAO meeting summarizes papers and presentations on the physiological effects of silicone hydrogel lenses, as well as case reports relating to the use of silicone hydrogel lenses for therapeutic needs.
Physiological effects
During a course of treatment with therapeutic contact lenses, it may be necessary to measure intraocular pressure without removing the patient’s contact lens. Muriel Schornack and Melissa Rice (Mayo Clinic) compared intraocular pressure measurements through a galyfilcon A contact lens to measurements made with no lens in place. Results of this study showed no clinically significant difference between these measurements.
Barbara Fink and colleagues (Ohio State University) evaluated the physiological performance of four silicone hydrogel contact lens materials. They found that lens material, lens power and wearing condition have a significant effect on corneal oxygen shortfall (OSU). The mean OSU was significantly lower when wearing -1.00D compared to +4.00D lenses, and static wear produced significantly higher levels of oxygen shortfall. However lens material or power did not affect tear exchange.
Gary Gunderson and colleagues (Illinois College of Optometry) reported the results of a study aimed at determining whether lens modulus influences higher optical aberrations and Strehl ratios (expression of image quality) in patients wearing silicone hydrogel contact lenses. They found no difference in the amount of spherical aberration, coma, astigmatism, root square mean and Strehl ratios among any of the lenses. There was also no significant statistical difference between measurements taken directly on the cornea and measurements taken over lenses. Placing a lens on the cornea did not increase or decrease aberrations or image quality.
Contact lens wear is known to affect the morphology of conjunctival epithelial cells. Chantal Coles-Brennan and colleagues (Brennan Consultants) studied the impact of daily wear silicone hydrogel lenses on conjunctival morphology, taking conjunctival impressions from habitual lens wearers and non-lens wearers. Goblet cell percentages, cell sizes and nucleus to cytoplasm ratios all varied significantly with lens type. Hydrogel lens wear induced a decrease in goblet cell percentage and increase in cell size, and all lenses caused a decrease in nucleus to cytoplasm ratio. The inferior conjunctiva had significantly larger cell sizes and reduced nucleus to cytoplasm ratio compared to the superior conjunctiva. DW of silicone hydrogel lenses does not seem to produce as great an impact on conjunctival morphology as traditional hydrogel materials, from the perspective of goblet cell density and overall cell size.
Jill Woods and colleagues (Centre for Contact Lens Research) investigated whether existing signs of hypoxia in highly myopic(>-7D), long term hydrogel lens wearers changed after being refitted with a custom silicone hydrogel lens. Compared to baseline, comfortable wear time was higher at one month and there was an increase in days of lens wear. Preference for the study lens grew during the study. Limbal hyperemia had decreased by one month. Corneal vascularisation assessments showed significant reduction in the inferior, nasal and temporal quadrants at three months. This study demonstrates that some highly myopic hydrogel lens wearers may experience minor adaptation issues when refitted with custom silicone hydrogel lenses, and that the adaptation is complete by one month.
Phillip Morgan (Eurolens Research) reported the results of a study aimed at deriving threshold central and peripheral Dk/t values for soft contact lenses for the avoidance of corneal swelling during daily wear. Results showed that silicone hydrogel lenses (galyfilcon A) offer sufficient Dk/t to avoid both peripheral and central corneal swelling, compared to hydrogel materials which induce swelling either peripherally or both centrally and peripherally.
Robert McIlroy and colleagues (Brennan Consultants) assessed the effect of oxygen transmissibility on limbal redness following overnight wear of silicone hydrogel contact lens materials. There was a change in limbal redness with galyfilcon (0.53 ±0.09), lotrafilcon B (0.43±0.07), senofilcon A (0.26±0.05) and no lens (0.04±0.06), with this change inversely related to measured peripheral lens Dk/t.
Therapeutic uses for silicone hydrogels: case reports
Susan Gromacki (St.Joseph’s Mercy Hospital) and Sandy Doud (OD) reported that fitting post-radial keratometry (RK) patients with silicone hydrogel lenses does not compromise corneal health. They described a case study in which a 48-year-old female patient who experienced corneal moulding after being fitted with reverse geometry GP lenses post-RK. Once fitted with silicone hydrogels, she demonstrated excellent eye health with no neovascularization for one year post full-time wear of these lenses.
Pediatric patients with unilateral aphakia are often required to wear contact lenses rather than spectacles. Deborah Jones and Lyndon Jones (University of Waterloo) described the case of a paediatric patient diagnosed with unilateral congenital cataract as an infant and operated on successfully at age six months. He was initially fitted with a rigid lens, which provided poor comfort. A silicone elastomer lens, with which he was refitted at age four, provided inferior visual acuity, poor comfort, was relatively immobile and collected lipid deposits. Most recently, a custom O2Optix lens provided superior comfort, excellent visual acuity, and masked 1.00D of astigmatism in addition to inducing no edema.
Navpreet Johar (New England College of Optometry) and Stephen Byrnes (OS) reported on the use of a silicone hydrogel lens as a piggyback lens for a 34-year-old patient with keratoconus. Worn in conjunction with the Mini Scleral Design GP lens, the silicone hydrogel acted as a buffer, eliminating issues such as corneal desiccation, trapped bubbles and induced astigmatism.
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Alisa Sivak, MA, DipEd