Ocular Surface Characteristics of the Asian Eye

Deborah Sweeney - BOptom (UNSW) 1980 PhD (UNSW) 1992 Professor Deborah Sweeney is the Chief Executive Officer of the Vision Cooperative Research Centre. Her major research area has been corneal physiology, and her work has been instrumental in developing an understanding of the physiology of the human cornea and the effects of contact lens wear on corneal function characteristics. Professor Sweeney is also active in national and international optometric and ophthalmic organizations, including Executive roles in the International Society for Contact Lens Research, The Keratoprosthesis (KPro) Study Group and the International Association of Contact Lens Educators.

Goodlaw (1946) was one of the first individuals to hypothesize that contact lenses act as a barrier to the eyes' anterior oxygen supply.

We are now well aware that inadequate oxygen, or hypoxia, can result in the development of a number of contact lens-related complications. With the introduction of silicone hydrogel lenses, the contact lens industry has succeeded in producing materials that are able to provide enough corneal oxygen to virtually eliminate hypoxia-related complications, while at the same time providing a surface that is hydrophilic enough to permit comfortable wear. Most contact lens wearers who wear new silicone hydrogels experience fewer corneal problems such as edema, epithelial microcysts, polymegethism, neovascularization and ocular bulbar and limbal injection.

According to Morgan et al silicone hydrogels now account for 30 percent of lenses prescribed as new daily wear fits in the United States. With silicone hydrogels released into the large Japanese market, 4 percent of all contact lens fits are with silicone hydrogels. These latest results suggest that we are rapidly accepting silicone hydrogel materials for the fitting of daily wear patients, coupled with wide-scale availability of a range of silicone hydrogel products.

So, who should we fit silicone hydrogels to?

Should we not be considering refitting our asymptomatic patients, especially as we hope that they're going to be wearing lenses for many years? Even though we don't see any problems now, using silicone hydrogels could prevent future problems.

And what about children - today we are fitting not only teenagers but tweens (11 to 14 year olds) and children as young as 8, with contact lenses. With increases in life expectancy these patients could be wearing their contact lenses for the next 85 years. Fitting lenses that provide sufficient high levels of oxygen and do not cause any compromise to the cornea will be necessary to ensure happy long-term wear.

One of the potential future problems that could face these long-term wearers is corneal exhaustion syndrome, CES, or corneal fatigue.

This is a term used to describe the loss of tolerance to contact lens wear. The reason may not be evident, especially when the patient is examined without their lenses, but we believe it is caused by endothelial dysfunction related to chronic corneal hypoxia and acidosis after many years of low Dk contact lens wear. Endothelial polymegethism (variation in cell size) occurs not only with age but also is associated with wear of low Dk lenses on both daily and extended wear regimens. The long-term effects on the corneal endothelium are very long lasting,probably permanent. It has been known for some time now that changes in endothelial morphology probably compromises endothelial function; with the result that the cornea has less adaptive reserve to meet subsequent stress. For instance, it has been reported that patients who exhibit high levels of polymegethism before undergoing intraocular surgery face greater a than normal risk of serious corneal edema and reduced vision post-operatively.

Also with CES, patients complain that they cannot wear their lenses for any significant period of time without discomfort. The patient is frustrated because of their previous record of satisfactory tolerance and this is often a sudden change. They also report symptoms of blurred or fluctuating vision, increased lacrimation and photophobia. On examination eyes are often hyperemic and an excessive open-eye edema response can be observed. In addition, signs and symptoms of past events of uveitis are common, as is pigment deposition on the endothelial surface and/or bedewing. Generally the endothelial mosaic is unclear and poymegethous.

Systemic factors are associated with corneal fatigue such as chronic fluid retention and hormonal imbalance or emotional or physical stress may be also described by patients.

It is likely that individual susceptibilities may influence the degree of endothelial compromise induced by chronic hypoxia or acidosis, and may therefore determine whether individual patients develop significant endothelial dysfunction. CES seems to occur more often in lens wearers with higher than average requirements for oxygen and in those such as moderate to severe hypermetropes who effectively have lenses of lower Dk/t.

In addition some have reported patterns of regression where patients can experience recurrence of the signs and symptoms after several years having been refitted e.g. patients moved from low Dk to moderate Dk lenses needed to be refitted eventually with high Dk materials to allow successful continuation of lens wear. This highlights the question of whether fitting all patients with as high a Dk lens that is suitable and available for them should be the standard of care we offer all of our patients – whether they are symptomatic or not!

As one practitioner has put it "The way I see it, it's more effective to switch patients now into better lenses rather than to risk their dropping out of lens wear, leaving us with the cost of attracting new patients to the office."