ORTHOKERATOLOGY

Recent studies suggest that orthokeratology, using night-wear rigid contact lenses to reshape the cornea, is effective for correcting low to moderate myopia, and may slow progression by reducing axial length growth by about 45 per cent over three years, Helen Swarbrick PhD told the XXXI Congress of the ESCRS in Amsterdam.

However, while nine published studies since 2005 all found that orthokeratology reduces axial length growth, most were not truly randomised, noted Dr Swarbrick of the School of Optometry and Vision Science, University of New South Wales, Sydney, Australia. Further research is needed to confirm long-term efficacy and evaluate the potential for rebound after temporary treatment, she said.

Reverse geometry

Orthokeratology utilises rigid contact lenses of a “reverse geometry” design, which incorporate a steeper “reverse” curve at the mid-periphery joining the flatter central base curve to the alignment curve in the periphery. Worn overnight, the lenses temporarily reshape the cornea, flattening and reducing power in the central optical zone, Dr Swarbrick explained. The corrective effect increases over about the first 10 nights of wear, and patients generally regress up to 0.5 D as the cornea reshapes over the course of each day, which can be compensated by slightly overcorrecting, Dr Swarbrick said. Orthokeratology is also reversible, with the cornea reverting to its original shape when lens use ceases.

The lenses can correct up to -4.0 dioptres of sphere, but can be used up to -6.0 dioptres, and reduce astigmatism about half, Dr Swarbrick said. They are removed in the morning and there is no lens wear or use of any visual aid during the day if the treatment works well. New designs are also being used to correct hyperopia and presbyopia.

The impact on myopic progression is less certain. In a 12-month contralateral eye study involving 26 patients that she conducted, axial length declined slightly at three and six months in eyes treated with overnight orthokeratology while it increased in fellow eyes using conventional day-wear rigid contacts. When lens-eye combinations were reversed, axial length again fell in the orthokeratology eyes, but increased about twice as fast in the previously treated eyes, suggesting a rebound effect, Dr Swarbrick said (Swarbrick et al 2010).

Safety

Orthokeratology works at least in part by thinning the central corneal epithelium, by about 15 to 20 microns over 90 days (Alharbi and Swarbrick Invest Ophthalmol Vis Sci. 2003 Jun;44(6):2518- 23). Could this compromise the epithelial barrier, increasing the risk of microbial keratitis?

Several studies suggest not, Dr Swarbrick said. Analysis of 129 cases of keratitis in orthokeratology patients reported in the literature from 2001 to 2007 found most occurred in children, and Pseudomonas and Acanthamoeba were the most common causes. “This suggested one of the issues was exposure to contaminated water in lens care.” (Watt and Swarbrick, Eye Contact Lens. 2007 Nov;33(6 Pt 2):373-7; discussion 382.)

Further, about half the cases were reported in 2001 in Southeast Asia, when orthokeratology was largely unregulated in the region. Follow-up since 2007 shows a decrease in reported cases worldwide, suggesting improved prevention practices, Dr Swarbrick said. A study of 2,600 US patients also found an overall infection rate of 7.7 per 10,000 patient treatment years, with a slightly higher risk for children (Bullimore et al. Optom Vis Sci. 2013 Sep;90(9):937-44). “Safety is similar to other overnight contact lens modalities,” she said.