Study Of The Aqueous Tear Film And Implications Of Contact Lens Wear

This book summarizes current understanding of the scientific, clinical, and technical issues surrounding the use of contact lenses. It discusses the special occupational conditions experienced by military personnel, particularly in extreme environments, that give rise to the question of whether or not to use contact lenses. Experts in optometry, ophthalmology, visual psychophysics, and engineering describe recent developments in design and use; and representatives of the military services provide examples of actual situations in aerospace settings. Considerations in Contact Lens Use Under Adverse Conditions will be of particular interest to those involved in the design of contact lenses and those responsible for occupational safety and health matters in the private sector.

Contact lens wear is known to disrupt the tear film and cause tear film instability, yet there is insufficient research to quantify these changes. This study aimed to investigate how contact lens wear and materials influence tear film stability by obtaining quantifiable metrics of the prelens tear film and its effect on visual performance and subjective contact lens comfort. Lipid layer thickness, measured by interferometry, was used to quantify tear film stability. Visual performance was assessed with high and low contrast Bailey-Lovie charts. The Contact Lens Dry Eye Quesionnaire-8 (CLDEQ-8) was issued to assess subjective contact lens comfort. 10 subjects participated in the study. At the initial visit the baseline precorneal tear film lipid layer thickness was measured. The subject was then fit with one of the 2 contact lenses chosen for the study, Dailies Total1 or 1-Day Acuvue Moist. Lipid layer thickness and high and low contrast visual acuity was measured 5 and 15 minutes after contact lens insertion. The CLDEQ-8 was issued to assess initial contact lens comfort. The subject wore the contact lens for 6 hours, then returned for repeat measurements of lipid layer thickness, high and low contrast visual acuity, and the CLDEQ-8 assessment. After a washout period, the subject was fit with the other contact lens and the same procedures were performed.

Abstract: Successful contact lens wear is dependant on many factors including the tear film. Adequate quality and quantity of tears must be present to provide a stable, uniform coating over the contact lens making it a smooth optical surface, and to provide comfort and lubrication. Placing a contact lens on the eye disrupts the normal tear film dynamics and increases symptoms of dry eye. Dryness is one of the main factors leading to a decrease or discontinuation of contact lens wear. Contact lens induced dry eye (CLDE) is considered to be evaporative in nature although the exact mechanisms are not yet understood. Tear film interferometry is a non-invasive technique used to measure the thickness and thinning rate of the tear layer in front of the contact lens also known as the pre-lens tear film (PLTF). Using interferometry the thickness and thinning rate of the PLTF were collected and compared under evaporative (open air) and non-evaporative (closed chamber) conditions to determine the relation between PLTF thinning and CLDE, and the effect of evaporation on the tear film of dry eye (DE) and non-dry eye (NDE) subjects. One hundred and sixty-nine subjects (169) completed Part 1 of this study examining the relation between CLDE and rate of PLTF thinning. Forty (40) of the original 169 subjects returned for the second, Part 2, visit to investigate what amount evaporation contributes to PLTF thinning and symptoms of dry eye in contact lens (CL) wearers. No significant difference was found between the PLTF thinning rates of dry eye (DE) and non-dry eye (NDE) subjects in Part 1 of this study (Z = -0.13, p = 0.90). A significantly higher rate of PLTF thinning in DE subjects was found in Part 2 of this study (Z = -2.84, p 0.004). These inconsistent results may indicate high inter-subject variability. Under non-evaporative conditions, the average rate of PLTF thinning decreased greatly and there ceased to be a significant difference in PLTF thinning rates of DE and NDE subjects (Z = -0.90, p 0.93). This suggests that the majority of PLTF thinning is due to evaporation, and that evaporation may play a larger role in dry eye subjects than in non-dry eye subjects.

During the past decade a significant international research effort has been directed towards understanding the composition and regulation of the preocular tear film. This effort has been motivated by the recognition that the tear film plays an essential role in maintaining corneal and conjunctival integrity, protecting against microbial challenge and preserving visual acuity. In addition, research has been stimulated by the knowledge that alteration or deficiency of the tear film, which occurs in countless individuals throughout the world, may lead to desiccation of the ocular surface, ulceration and perforation of the cornea, an increased incidence of infectious disease, and potentially, pronounced visual disability and blindness. 7 To promote further progress in this field of vision research, the International Conference on the Lacrimal Gland, Tear Film and Dry Eye Syndromes: Basic Science and Clinical Relevance was held in the Southampton Princess Resort in Bermuda from November 14 to 17, 1992. This meeting was designed to assess critically the current knowledge and 'state of the art' research on the structure and function of lacrimal tissue and tears in both health and disease. The goal of this conference was to provide an international exchange of information that would be of value to basic scientists involved in eye research, to physicians in the ophthalmological community, and to pharmaceutical companies with an interest in the treatment of lacrimal gland, tear film or ocular surface disorders (e. g. Sjogren's syndrome).

The most anterior optical surface of the eye has the greatest refractive power. Therefore, the quality of this surface is critical to retinal image quality and visual performance. Most optical texts identify the cornea as the first optical surface but in fact it is the tear layer. A smooth tear layer is considered optically essential and superior to the microscopically irregular corneaZ epithelium. Unlike the other optical surfaces within the eye, the tear film can vary tremendously from second to second as tear' film changes occur between blinks. Although numerous studies have evaluated the change in tear film "quality" during periods of non-blinking little is know about the optical and visual effects resulting from changes in the tear film layer. The classic tear film models suggest that the precorneal tear film is composed of three layers, the superficial lipid layer derived from the meibomian glands, the middle aqueous layer provided by the major and minor lacrimal glands maintains ocular wetting, among other physiological effects, and the inner most layer overlies the corneaZ and conjunctival epithelial cells with mucous like material. Recent studies suggest that the tear film is composed of a mucin "gel" topped by a thin lipid film.