Does the key to solving contact lens discomfort lie in tear film assessment?

Vicki Evans PhD BOptom (Hons) Chief Clinical Scientist, Institute for Eye Research (IER). As Head of Clinical Research from December 2006 to February 2009 Vicki lead a team of 30 people conducting commercial and academic clinical trials of contact lenses, solutions and spectacles before recently taking up the role of Chief Clinical Scientist for IER. Vicki has a particular research interest in the clinical performance of silicone hydrogel lenses and their interaction with lens care solutions.  An optometry graduate from UNSW, Vicki has worked in clinical practice in Sydney, in contact lens research at the CCLRU, and in tear film and dry eye research at CRCERT and Oxford University.  Vicki has also worked in contact lens industry for CIBA Vision both in the UK and Australia.  These roles have allowed Vicki to develop a strong understanding of the interfaces between academia, industry R&D, regulatory affairs, professional education and clinical practice.   Vicki is member of the British Contact Lens Association, Association for Research in Vision and Ophthalmology, Tear Film and Ocular Surface Society, Optometric Association of Australia and Contact Lens Society of Australia.

Review of paper entitled:

Effect of Contact Lens Materials on Tear Physiology

Lee Choon Thai, Alan Tomlinson, Marshal G Doane.  Optometry and Vision Science, 2004, 81 (3): 194-204.

Finding objective and repeatable measures to identify and address the mechanism behind the reduction in comfort with lens wear has presented an ongoing challenge to contact lens researchers.   Contact lens wearers tell us they are not happy with lens wear by the end of the day, but without objective measures to assess the impact of the lens materials on the ocular surface, and in particular the tear film, the mechanisms underlying these symptoms  are hard to pin down.
Understanding the mechanism underlying contact lens discomfort, and ultimately providing a solution for this problem is particularly important for contact lens users who may be described as ‘symptomatic’ during lens wear but do not show any particular signs of dryness and discomfort without lens wear.  Despite typically good comfort on lens insertion, these wearers describe more than the usual reduction in comfort and increase in dryness over the course of the day. 
Keys to the comfort puzzle  may lie in the changes in tear lens physiology which are observed between non-wearers and contact lens wearers, between ‘normal’ and dry eye patients or between successful contact lens wearers and symptomatic wearers (described by the authors as “Contact Lens Dry Eye”, CLDE).  A further approach is to look for changes in tear film physiology during lens wear of different lens materials (or lens solution combinations) to examine whether the benefits claimed for a lens material or solution actually manifest on-eye as improved physiology or comfort in CLDE sufferers.  

Whilst differences in symptoms between normal, CLDE and dry eye populations can be measured with  validated questionnaires, it has unfortunately proven difficult to identify tear film parameters which consistently differ between these groups. It has proven even harder to identify measures of the tear film that systematically correlate with the symptomatology reported by the wearer – thus linking the clinical picture to the symptoms. For these reasons, the challenge the authors set themselves of identifying differences in tear variables between contact lens materials was not inconsiderable.

The paper reported on variables such as pre-corneal and pre-lens tear film evaporation rate and tear thinning time as well as interferometric  grading of tear film structure, wetting capability and tear film elimination rate.  The development of grading scales based on Doane’s system of assessing interference patterns for the pre-corneal and pre-contact lens tear film formed a large part of this paper.  A normative study, not mentioned in the abstract, was conducted on 74 healthy individuals to describe the distribution patterns of the new grading scales in a normal population. Subsequently a second, randomized, double masked trial was conducted comparing tear variables at baseline and 30 minutes of wear of hydrogel lenses from each of the four FDA groups and one silicone hydrogel lens in 20 healthy contact lens wearers.

The authors demonstrated a substantial deterioration of pre-lens tear film characteristics compared to the pre-corneal tear film characteristics observed in no lens wear. There was no difference between any of the lens types for tear evaporation rate or for tear thinning time after 30 minutes of wear. The authors commented that the large inter-subject variability may have made it difficult to detect differences between lens types. They showed some difference using their new interferometric grading scales for tear film structure and for tear elimination rate but not for lens wettability.
There was detailed discussion of the interpretation of interferometric results and of which components of the tear film (lipid, aqueous, mucin) is being observed in different interference patterns. The authors concluded that despite one lens material holding some promise, the difference in lens materials was outweighed by the decrement of the tear film in lens wear compared to no lens wear. 

One aspect of this paper which would be good to explore further would be an attempt to link the observed differences in tear film physiology with differences in contact lens comfort in this data set or to measure the variables at the end of the wearing day, when arguably more changes or symptoms may have been evident.

Given the positive performance claims made for particular lens materials and solutions in CLDE, development of objective tear film measurements is an important avenue of enquiry – to submit these claims to objective scrutiny but more importantly to identify further options for improvement in lens and solution composition and to provide fitting recommendations for a better outcome in CLDE.