Better methods of clinically assessing mucus functions are required

Mucus is an integral part of the tear film with functions including the anchoring of the aqueous components and stabilising their contact with the cornea by rendering structural support to the microplicae, having potential involvement in apoptotic and cell mediating activity as well as combatting pathogens and foreign bodies.1 Ocular mucus helps maintain the wettability of the ocular surface and provides lubrication for lid movements over it.2 In the healthy eye, the glycocalyx, which has previously been referred to as the mucus layer, prevents the epithelial surface from dewetting.3 Consistent with the current understanding of tear function, the fluorescein tear break up time test devised by Norn4 was originally intended to be used as a measure of mucus efficiency or deficiency. This model is supported by the finding that shorter tear break up times have been found to be associated with reduced goblet cell density. Thus short tear break up time findings may be partly a consequence of qualitative and/or quantitative mucus dysfunction. The focus in current dry eye literature could suggest that short tear break up times and other indications of tear instability may be attributed to Meibomian gland dysfunction without consideration of other sources of tear instability such as mucus and/or aqueous deficiency. However, excess mucus production can also interfere with tear functions. For example, giant papillary conjunctivitis is associated with excess mucus production as are other forms of ocular inflammation such as vernal and allergic conjunctivitis.5 Abnormal quantities of mucoid corneal filaments may accumulate in the tear layer in dry eye syndromes6 and contribute to tear instability. Epitheliopathy over the pupil is associated with increased higher order aberrations and backward light scattering7 and any abnormal distribution of mucus over a desiccated area of epitheliopathy could also contribute to tear instability, shorter TBUT, backward light scattering and reduced visual function. Roles for mucus in tear functions and dysfunctions are supported by the finding that in dry eye syndrome