continuous wear colloquy
Evaluating the Performance Of Silicone Hydrogel Lenses
BY N. REX GHORMLEY, OD, FAAO

Recently I had the opportunity to talk with Kathy Dumbleton, MSc, MCOptom, FAAO, about the clinical performance of silicone hydrogel lenses when worn on a continuous wear schedule. Kathy is a senior research optometrist at the Centre for Contact Lens Research at the University of Waterloo in Canada. One of her major areas of clinical research is continuous wear with high-Dk silicone hydrogel contact lens materials.

Understanding Mucin Balls

Rex Ghormley (RG): With the biomicroscope I occasionally see mucin balls under a silicone hydrogel lens. What are mucin balls? What is their clinical significance?

Kathy Dumbleton (KD): Mucin balls are spherical, translucent particles or plugs of debris that form between the epithelium and the posterior surface of the contact lens. They're mainly composed of mucin, tear proteins and lipids. Mucin balls do not move as the lens moves and are blinked away following lens removal, but they often leave temporary depressions or imprints in the epithelium.

They form when decreased aqueous in the tear film from overnight wear forms a mucin-rich viscous substance that rolls up into spheres because of intrinsic surface forces associated with the cornea/lens interaction. Mu-cin balls do not affect the physiological response, comfort or ability to wear lenses on a continuous wear basis. Patients may reduce the number of mucin balls by using lubricating drops at night before sleep and reducing their consecutive nights of continuous wear.

Distinguishing Microcysts

RG: Why do I sometimes observe microcysts when refitting a low-Dk lens patient into silicone hydrogel lenses?

KD: Microcysts may occur in some of these patients because of a "rebound" effect. The microcysts are temporary and their numbers will significantly decrease within a month of starting silicone hydrogel wear. Clinicians must learn to distinguish between mucin balls and microcysts.

Microcysts display typical "reversed illumination" during slit lamp observation because of their greater refractive index compared with the surrounding tissue. They will show little or no epithelial staining.

Mucin balls may leave depressions in the epithelium on lens removal that fill with tears and demonstrate "unreversed illumination." They'll readily pool with fluorescein, but this isn't true staining.

Dealing with Hyperopic Shift

RG: Have you seen a refractive error shift when fitting patients with silicone hydrogel lenses?

KD: When refitting patients from hydrogel contact lenses into silicone hydrogel contact lenses, a small hyperopic shift may result. This is in fact a "rebound" or reversal of the myopic shift that can result from long-term hydrogel lens wear.

While the average decrease in myopia following extended wear with low-Dk lenses is small (0.37D), the degree of change may be significant for some individuals. For this reason, you should carefully refract all patients several weeks after refitting because their lenses may have too much minus power or too little plus power. This could result in near vision problems, especially for patients on the verge of presbyopia. Myopic patients may appreciate the reduced prescription, but you will need to educate your hyperopic patients about the health benefits associated with their apparent increase in prescription.

Dr. Ghormley is in private practice in St. Louis, MO. He is a past president of the American Academy of Optometry and a Diplomate of its Cornea & Contact Lens Section. He is also the team optometrist for the St. Louis Rams (NFL) and the St. Louis Blues (NHL).