10 - Mucin Balls

Abstract

Approximately 50% of patients who wear silicone hydrogel lenses on an extended-wear basis display a peculiar phenomenon known as mucin balls. These formations can be observed in the post-lens tear film as small discrete particles, or ‘plugs’, and are similar in appearance to tear film debris. In some patients, as many as 200 mucin balls can be observed. At high magnification (×40), mucin balls can be seen to be of variable size and to take on a characteristic ‘flattened doughnut’ shape, with a thin circular annulus and broad central depression. They are observed in greater numbers in patients who sleep wearing silicone hydrogel lenses. Mucin balls are immovable beneath the lens and appear to be stuck to the epithelium. A higher number of mucin balls is associated with a looser lens fit. Mucin balls generally increase in number over the first months of lens wear and remain constant thereafter. Mucin balls cause no discomfort or loss of vision and appear to be of no immediate consequence with respect to ocular health. Confocal microscopy has shown that mucin balls can penetrate the full thickness of the epithelium, leading to activation of the keratocytes in the underlying anterior stroma. Lens-induced mucin ball production may represent a compromise to the protective role of the mucin layer. Mucin balls are composed primarily of collapsed mucin and some lipid and tear proteins. The mechanism by which mucin balls form beneath the lens may, in part, be related to a physico-chemical phenomenon caused by the plasma-treated surface of silicone hydrogel lenses, creating a shearing force that has the effect of rolling up tear mucus into small spheres. After lens removal, some mucin balls remain ‘stuck’ to the epithelium, and some are washed away with blinking but leave behind pits in the epithelial surface, which then fill with tear aqueous. The fluid-filled pits give rise to the optical phenomenon of un-reversed illumination.