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In a featured paper published free online (www.optvissci.com), Szczotka-Flynn and Diaz1 of the Case Western Reserve University in Cleveland, Ohio, review their findings from a meta-analysis of 23 studies, to evaluate the risks of corneal inflammatory events (CIEs) in users of SH and low-Dk hydrogel extended wear.
Their search included online databases, unpublished meeting abstracts and other cited references presented or published between 1990 and February 2006. A total of 9,336 subjects and 18,537 eyes comprised the sample. Follow-up ranged from four to 36 months, with a median of 12 months.
The authors found a two-fold higher risk for CIEs in users of SH lenses when worn for up to 30 days' extended wear, compared with low-Dk extended wear lenses worn for seven days' extended wear. However, the increased risk could not be definitively linked to SH materials because the effect of material on outcome was confounded by length of wear.
Based on ocular physiological improvements with high-Dk SH lenses and the low severity of CIEs, many of which are asymptomatic, SH lenses remain the authors' favoured choice for extended and daily wear.
SH lenses are often promoted on the basis of their superior oxygen perfomance, but how reliable are the manufacturers' claimed Dk values? Efron et al2 measured the Dk of five SH materials and two conventional hydrogels using a modified polarographic technique and corrected for boundary and edge effects. Water content was measured using the gravimetric method at room temperature and 35°C.
Measured values (±95 per cent CI) of Dk (with manufacturer-claimed values in brackets) were: Focus Night & Day 162.0±9.8 (140) Acuvue Oasys 107.4±7.4 (103) O2 Optix 80.5±4.9 (110) PureVision 75.9±6.6 (91) Acuvue Advance 75.2±9.8 (60) 1-Day Acuvue 21.0±1.0 (21.4) and Seequence 8.2±0.7 (8.5).
The authors conclude that claimed Dk values for Acuvue Oasys and the two reference hydrogels fall within the 95 per cent confidence interval of their measured values but Dk measurements for the other four SH lenses are not in agreement with claimed values. There was a general inverse relation between Dk and water content (both at 35°C) for SH lenses.
Comfort considerations
There are encouraging reports on the latest generation of SHs from two other papers. Young et al3 measured the proportion of soft lens users who wear lenses in challenging environments and evaluated their comfort. A second objective was to determine whether refitting with SH lenses affected lens comfort among subjects who wear lenses in these environments.
Baseline data were collected from 496 soft lens wearers on the frequency of lens use and associated comfort in 12 challenging environments. Subjects were then refitted with one of three second-generation SH lenses: senofilcon A, galyfilcon A or lotrafilcon B. Comfort responses were compared for subjects who 'always' or 'frequently' used lenses in these environments after two weeks of SH daily wear.
More than 80 per cent of subjects used their habitual lenses 'always' or 'frequently' while reading, sitting in an air-conditioned or heated car, using a computer and while driving at night. The least comfortable environments were in dusty, polluted or smoky surroundings.
After using senofilcon A and galyfilcon A lenses for two weeks, comfort in all environments improved significantly except when using a computer with galyfilcon A. After two weeks with lotrafilcon B lenses, there was improved comfort in all environments except driving at night, using a computer and sitting in an air-conditioned or heated car. The authors conclude that daily wear SH lenses improve comfort significantly in most environments.
Biofinity is the latest of the new generation SH lenses. In a study published in the May issue of Contact Lens & Anterior Eye, Brennan et al4 evaluate the clinical performance of the lens for continuous wear compared to other SH lenses. A total of 45 subjects were fitted in one eye with Biofinity (comfilcon A), and either a Focus Night & Day (lotrafilcon A) or a PureVision (balafilcon A) lens in the other eye.
Of 33 subjects who completed the 12-month study, the test lens was preferred to both controls for comfort and overall preference and to the lotrafilcon A lens for vision. Biofinity scored significantly better than both controls for comfort during the day, end-of-day and overall preference, and better than lotrafilcon A for vision quality, night vision and handling. All lenses showed comparable performance for microcysts and corneal staining but there were significant differences for limbal redness and conjunctival staining, with lotrafilcon A showing the greatest effect and balafilcon A the least. These authors conclude that comfilcon A offers advantages over first-generation SH materials for continuous wear.
One final paper in Optometry and Vision Science contributes to the debate on solution-related corneal staining and its significance (Optician, March 2). Carnt et al5 at the Vision CRC, Sydney, examined the relationship between solution toxicity and corneal infiltrative events (CIEs) in soft CL daily wear by conducting a retrospective analysis of clinical trials between May and November 2005. They found that eyes that experienced solution toxicity are more likely to experience a CIE. Alternative solution/lens combinations need to be investigated if toxic staining is detected. ?
To access abstracts and text of these papers go to the journal website or enter www.ncbi.nih.gov/entrez/ and search by author.
References
1 Szczotka-Flynn L and Diaz M. Risk of corneal inflammatory events with silicone hydrogel and low Dk hydrogel extended contact lens wear: a meta-analysis. Optom Vis Sci, 200784:4 247-56.
2 Efron N, Morgan P B, Cameron I D et al. Oxygen permeability and water content of silicone hydrogel contact lens materials. Optom Vis Sci, 200784:4 E328-37.
3 Young G, Riley C M, Chalmers R L et al. Hydrogel lens comfort in challenging environments and the effect of refitting with silicone hydrogel lenses. Optom Vis Sci, 200784:4 302-8.
4 Brennan N A, Coles C, Connor H R M et al. A 12-month prospective clinical trial of comfilcon A silicone hydrogel contact lenses worn on a 30-day continuous wear basis. CLAE, 200730:2 108-18.
5 Carnt N, Jalbert I, Stretton S et al. Solution toxicity in soft contact lens daily wear is associated with corneal inflammation. Optom Vis Sci, 200784:4 309-15.