Mucin Balls and High Dk Soft Lenses
By Jacqueline Tan, BOptom,
Lisa Keay, BOptom and
Deborah Sweeney, PhD, FAAO
Benign mucin balls have become more common with high Dk soft lenses. Learn how to distinguish them from harmful conditions.
Mucin balls, also referred to as pre-corneal deposits or "lipid plugs," appear in varying numbers in patients wearing both soft and rigid gas permeable lenses. They occur in higher numbers with certain lens materials and also with certain patients. Increased frequency of mucin balls during high Dk silicone hydrogel lens wear has heightened interest in this phenomenon.
Mucin balls form within minutes of lens insertion, but appear to have no adverse effects on ocular health. They do not affect vision and comfort, and they are not associated with any adverse reactions. It is important to distinguish mucin balls from other ocular signs which are similar in appearance but indicate that a cornea is under hypoxic or other stress.
Measuring approximately 40 to 120 µm in diameter, mucin balls are relatively large, generally spherical bodies that we easily observed with the biomicroscope under direct white light illumination (Figure 1), indirect and retro illumination. They differ from both vacuoles and microcysts, which are less visible, smaller in size and require retro illumination to be viewed.
Figure 1: Mucin balls viewed with direct white light (magnification 16x)
Behavior and Detection
Mucin balls remain motionless under the lens, even during blinking, and indent the corneal epithelial surface during contact lens wear. Following lens removal, mucin balls tend to easily dislodge with blinking, leaving an imprint or depression in the cornea which pools tear fluid. View these indentations best by instilling fluorescein, which pools in these areas (Figures 2 and 3), and does not indicate an epithelial breach.
Fluorescein can also be used to distinguish between mucin balls and vacuoles, as the latter shows no response to fluorescein. Do not confuse fluorescein pooling in the mucin ball indentations with macropunctate corneal staining. Indentations resolve spontaneously between 30 minutes and several hours after lens removal. Occasionally mucin balls float briefly in the tear film following lens removal.
We observed mucin balls more frequently in the superior quadrant of the cornea beneath the upper eyelid. They vary greatly in size and number from patient to patient. Over 100 have been observed in some patients (Figure 4).
Figure 4: Over 100 mucin balls have been observed in some patients.
An extended wear clinical trial at the Cornea and Contact Lens Research Unit (CCLRU), University of New South Wales, Sydney, Australia compared the incidence and number of mucin balls in patients with low Dk soft lenses on a six night weekly replacement schedule and high Dk soft lenses on a 30 night monthly replacement schedule for one year. There were no differences in the number of patients with mucin balls between the two groups: the percentage of patients with mucin balls ranged from 40 to 80 percent over 12 months. Mucin balls were found in significantly greater numbers in the high Dk soft lens wearing group. These results suggest that patient characteristics as well as different lens types predispose lens wearers to development of mucin balls.
How Mucin Balls Form
No definite etiology has been established for the development of mucin balls. However, greater numbers of mucin balls are observed with lens materials of higher modulus. The relatively stiffer materials in high Dk silicone hydrogel lenses may exacerbate the mechanical interaction of the lens with the ocular surface.
One hypothesis is that lenses with special surface characteristics can create shear and surface tension forces within the tear film. These forces cause the tear film to roll into beads as the lens moves over the ocular surface.
We've seen that different lens surface treatments stimulate the number of mucin balls above what is seen with current commercial soft high Dk lenses. In addition, predisposition of some patients to mucin balls indicate that as yet unknown patient characteristics are also involved in mucin ball etiology.
Use of high Dk silicone hydrogel materials will increase the occurrence of this benign phenomenon. Practitioners must learn to recognize mucin balls to distinguish them from problematic findings. CLS
Participating Investigators: Isabelle Jalbert, OD and Brien Holden, DSc, FAAO.
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Jacqueline Tan, BOptom is a research optometrist at the Cornea and Contact Lens Research Unit at the School of Optometry, The University of New South Wales, Australia
Lisa Keay, BOptom was Manager of Clinical Research at the Cornea and Contact Lens Research Unit, School of Optometry, The University of New South Wales, Australia and is now Research Optometrist at Visioncare Research Ltd, UK.
Deborah F. Sweeney, PhD, FAAO, an Associate Professor, is Executive Director and Director of Clinical Research at the Cornea and Contact Lens Research Unit at the School of Optometry, The University of New South Wales, Australia