Tear volume and tear film stability in relation to soft contact lens wear in Hong Kong-Chinese

Pao Yue-kong Library Electronic Theses Database

Tear volume and tear film stability in relation to soft contact lens wear in Hong Kong-Chinese

 

Author: Chui, Wan-sang
Title: Tear volume and tear film stability in relation to soft contact lens wear in Hong Kong-Chinese
Degree: M.Phil.
Year: 2001
Subject: Tears
Contact lenses -- Complications -- China -- Hong Kong
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Optometry and Radiography
Pages: vi, 130 leaves : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1566828
URI: http://theses.lib.polyu.edu.hk/handle/200/2514
Abstract: The criteria for a number of clinical tests for the diagnosis of dry eye are well documented. However, practitioners have noted that many contact lens wearers complaining of dryness after commencing lens wear may be free of any signs and symptoms before the commencement of contact lens wear. It seems that conventional tear tests such as the Schirmer test (ST) and the fluorescein tear break-up time (TBUT) test are not sensitive enough to screen out such 'marginal' dry eye patients. Modified techniques of tear tests, namely, the non-invasive tear break-up time (NITBUT) test and the cotton thread test (phenol red thread (PRT) and self-prepared cotton thread test (SP-CTT)) now allow non-invasive and less invasive assessments of the tear film stability and tear volume respectively. They have been claimed to be more valid than the conventional versions of these tests and are thus believed to be more sensitive, and able to detect a milder form of dry eye. Since 1993, Cho and co-workers have performed a series of studies on the tear film stability and tear volume in Chinese eyes. Their results suggested that the NITBUT test and the SP-CTT are reliable in the assessment of tear film stability and tear volume respectively in Chinese eyes. Cho and Yap (1995) found that soft contact lens wear could cause a transient reduction of NITBUT, but had no effect on SP-CTT value (the PRT was not available in Hong Kong (HK) during the period of their study). When used in isolation, the SP-CTT appeared to be more useful in predicting contact lens wearers who may have symptoms after wearing soft contact lenses while the NITBUT was not. However, due to the small number of subjects in their study, it is necessary to confirm their findings with a larger sample size. Aims The current study was therefore designed with the following aims: a) to investigate the effect of contact lens wear on the tear stability(NITBUT) and tear volume (SP-CTT and PRT values) in a larger group of HK-Chinese (measurements made after removal of contact lens), b) to determine whether the tests, in isolation or in combination, can he used to predict successful contact lens wear, and if so, c) to determine the optimal cut-off criterion and the corresponding accuracy of these tests, in isolation or in combination, in predicting successful contact lens wear. d) to investigate the seasonal change on the test values, and dry eye related problems in contact lens wear. Methods Seventy-eight asymptomatic non-contact lens wearers were prescribed with a pair of soft contact lenses and were requested to wear the lenses for 28 weeks on a daily wear basis. The baseline NITBUT, SP-CTT and PRT values were measured. In 51 of these subjects, the test values were obtained at weeks 2, 9 and 28 after the commencement of soft contact lens wear. After 28 weeks, 15 of these subjects were enrolled into the humidity study, and were given two pairs of new lenses; one pair for use during mid-autumn (dry season) and another pair for use during mid-spring (humid season). Results and Discussion We confirmed that the tear volume, as measured by the SP-CTT and PRT test, was not affected by soft contact lens wear. Cho and Yap (1995) reported an initial transient decrease in the NITBUT value at the commencement of soft contact lens wear. However, we did not find any statistically significant change in the NITBUT value (measured after lens removal) after commencement of soft contact lens wear. After 28 weeks of contact lens wear, the subjects were classified according to their maximum wearing time which did not cause any dry eye-related signs and symptoms. The baseline NITBUT and PRT values were unable to 'predict' success of soft contact lens wear. However, the SP-CTT gave a sensitivity and specificity of 81.3% and 55.9% respectively in the prediction of successful contact lens wear when a critical value of 12.5 mm/15 s was used. The positive and the negative predictive values using this cut-off point were 63.4% and 76.0% respectively. We have defined a wetting value by the summation of the NITBUT value and the SP-CTT or the PRT values. However, none of these wetting values were able to provide any useful information on the success of soft contact lens wear. Seasonal change in Hong Kong did not appear to have a significant impact on the signs and symptoms of dry eye during soft contact lens wear. This was probably because the relative humidity in Hong Kong when the study was conducted was generally high. Also, most of our subjects worked indoors, where the humidity were relatively constant. The result of the main study was therefore unlikely to be affected by seasonal change.

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