Spherical indentations of human and rabbit corneal epithelium following extended contact lens wear

Isabelle Jalbert

Isabelle Jalbert gained her optometry degree in Canada in 1989 and worked there in academic and private optometric practice before moving to Australia in 1995. Isabelle was Manager of Clinical Research for the Cornea and Contact Lens Research Unit until 2000. After submitting her PhD thesis completed at the University of New South Wales earlier this year, she accepted a position as Project Director of Contact Lens Studies for the Vision Cooperative Research Centre. Isabelle’s main research interests include the effects of contact lenses on the corneal physiology and the epidemiology and management of anterior segment disease.

The CLAO Journal 28(4):177-180, 2002

Ladage PM^{1, 2}, Petroll WM¹, Jester JV¹, Fisher S¹, Bergmanson JPG², Cavanagh HD¹

¹ Department of Ophthalmology, University of Texas South-western Medical Center, Dallas Texas

² College of Optometry, University of Houston, Houston, Texas

The authors of this paper used high magnification observations of the spherical indentations produced by mucin balls during extended wear of contact lenses in order to better characterise this phenomenon.

Until recently, reports of mucin balls have been relatively sparse; however, over the past few years the number of reports has increased and mucin balls are now recognised as being associated with extended wear of lenses made from conventional and, more commonly, silicone hydrogel materials.^{1, 2}Despite the increased number of reports, relatively little is known about this phenomenon. Very recently, high magnification observations of mucin balls were provided in vivo through confocal microscopy with the lens in place, and^{3, 4} mucin balls collected using capillary tubes were examined in vitro by light microscopic histochemistry, scanning electron microscopy and electron microscopic elemental analysis.⁴ Such observations revealed that mucin balls were perfectly spherical, optically dense structures formed mostly of glycoproteins that, when large enough, were able to indent the underlying cornea well into Bowman’s layer and the anterior stroma.^{3, 4}

The paper reviewed here takes an original approach, investigating the spherical indentations left behind by mucin balls on removal of the contact lens rather than the balls themselves. Cumulative in vivo confocal microscopy observations in two silicone hydrogel extended wearers and one subject wearing hydrogel extended wear over a 12 month period provided detailed characterisation of the indentations. They ranged in diameter from 33.9 to 78.8 m m (mean 57.9 ± 14 m m), were perfectly spherical and extended to various depths throughout the epithelium. This range is comparative to the 20 to 50 m m range described with confocal microscopic observations of the balls themselves.⁴

This paper also describes the first observation of mucin balls in an animal model. Corneas from rabbit eyes were harvested after 5 days of continuous wear with a medium Dk/t rigid gas permeable lens (14 mm diameter). Immunocytochemistry was performed on two fixed excised corneas with a Ki-67 proliferation marker, and three-dimensional observations were obtained using laser scanning confocal microscopy. Spherical indentations were observed in 2 of 7 rabbit corneas and immunocytochemistry confirmed that no epithelial cell nuclei were visible within the indentations. Some of the larger indentations extended deeply into the corneal epithelium, beyond the basal layer. More interestingly, there appeared to be a local proliferation of stromal cells and an increase in cell density directly underneath the spherical indentations. This reaction is particularly interesting because keratocytes do not divide unless stimulated during a stromal wound healing response. Therefore more work is needed to identify whether these proliferating stromal cells directly underneath the indentations are indeed dividing keratocytes and, if so, the mechanism by which proliferation occurs.

Although this paper provides no definitive explanation for the phenomenon of mucin balls it raises interesting questions as to their significance, particularly when observed in large sizes and numbers. Clearly, mucin balls are dense enough to exert levels of pressure sufficient to either kill or simply shift epithelial cells to the side. There was, however, no evidence of epithelial cells bunching up around the spherical indentations in the rabbit model, indicating that epithelial cells are not being displaced. The significance of these combined findings is unknown but they certainly warrant further investigation, as the authors suggest that such disturbance may lead to impaired barrier function of the epithelium and ultimately contact lens related infection.

Although studies such as these indicate that mucin balls potentially may have deleterious effects on corneal integrity, there have been no reports of mucin balls associated with any adverse event. Even patients who are prone to very high numbers of mucin balls still remain asymptomatic. However practitioners should always remain vigilant to the potential for infection or any adverse event. It has been our experience and that of others that saline rinses or lubricants can help to reduce the number of mucin balls, and these may be of use to patients who are predisposed to consistently present with high numbers.

1. Pritchard N, Jones L, Dumbleton K, Fonn D. Epithelial inclusions in association with mucin ball development in high-oxygen permeability hydrogel lenses. Optom Vis Sci 2000;77:68-72.
2. Tan J, Keay L, Jalbert I, Naduvilath TJ, Sweeney DF, Holden BA. Mucin balls with wear of conventional and silicone hydrogel contact lenses. Optometry and Vision Science 2003;80:291-7.
3. Jalbert I, Stapleton F, Papas E, Sweeney DF, Coroneo M. In vivo confocal microscopy of the human cornea. Br J Ophthalmol 2003;87:225-36.
4. Millar TJ, Papas EB, Ozkan J, Jalbert I, Ball M. Clinical appearance and microscopic analysis of mucin balls associated with contact lens wear. Cornea 2003;22:740-5.