Profiling the changes in the tear physiology with contact lens wear

• Main Author: Thai, Lee Choon
• Published: Glasgow Caledonian University 2005
• Subjects: 617.764075
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.443226

The aim of these studies was to investigate the longitudinal physiological effects of contact lens wear on the tear film. This differed from previous studies which have explored the effect of contact lens wear by looking at tear physiology at particular phases of lens wear. A series of investigations were designed to examine the effect of contact lens wear on neophytes in the initial period, during wear and following the cessation of contact lens use. The studies scrutinised the biophysical changes, by evaluating the structure, production, evaporation rate, stability and osmolality, and the protein composition of the tear film. Since wearers experience contact lens induced dry eyes (CLDE), subjective symptoms were assessed by means of questionnaires. The consequence of lens wear on visual performance was studied by measuring the contrast sensitivity in relation to the tear film stability. In order to improve the acceptability of contact lenses, further studies were conducted to investigate the biocompatibility of five different hydrogel lens materials and the use of an ocular lubricant, hydroxypropyl methylcellulose (HPMC), in contact lens care solutions. The results indicated that biophysical tear disruption was initiated by contact lens insertion. This disruption persisted during the contact lens wear, but these were not significant to affect the tear film osmolality and turnover rate. The tear film appeared to recover immediately upon cessation of contact lens wear, without any long-term effect. No significant change in the different tear protein concentrations occurred during the initial phase, but the protein concentrations became elevated with continued wear and remained elevated after cessation of lens use. This suggests the possibility of low-level anterior ocular inflammatory response, which supported by the persistence of symptoms after contact lens removal. Visual performance was shown to be reduced when the pre-lens tear film (PLTF) broke up. This indicates that the break-up of the PLTF could account for the complaints of inferior vision with contact lens compared to spectacles. There was no significant difference in the pre-lens biophysics and the surface wettability between the five different hydrogel lenses. However, the new biomimetic lens showed improved performance over the conventional lenses. Some measures of PLTF physiology were also improved, when multipurpose solutions containing HPMC were used. These studies confirm that the physiological changes of the tear film are interdependent and they trigger a chain reaction of factors that may lead to CLDE and reduce visual function. Even though the tear film is a fragile microenvironment, these studies illustrate that it is, in many respects, robust and quick to recover from external intrusion, such as contact lens wear