Tear-film lipid layer turnover rates

Article Outline

 

The newest model of the tear film consists of a thick aqueous-mucin layer covered by 2 lipid layers with opposing polarities. There are other important components within these layers, such as lipocalin proteins, cholesterol, fatty acids, and wax esters. Various studies have well documented the flow of the aqueous-mucin layer, but no major studies have documented the turnover rate of the lipid layer. The researchers studied the turnover rate of the aqueous-mucin layer and lipid layers of the tear film.

Researchers used fluorescein sodium to determine the turnover rate of the aqueous layer and 5-dodecanoylaminofluorescein (DAF) to evaluate the tear lipid layers. DAF is a free fatty acid, lipophilic conjugate of fluorescein. The study included 6 male and 6 female volunteers between the ages of 21 and 47 years. None of the volunteers had any known ocular disease. Each participant received a routine eye examination and dry eye evaluation, including Schirmer testing and tear break up time. The 2 dyes were studied on different days. On each day of testing, either the DAF mixture or fluorescein sodium mixture was placed in the right eye of the 12 volunteers. After several blinks, the fluorescein intensity of the central cornea was measured using a commercial slit lamp fluorophotometer. The fluorescein sodium intensity was measured every minute for 10 minutes. The intensity was measured every 5 minutes for 50 minutes with DAF because of its longer decay. DAF testing was done on 3 different days on 3 different subjects to ensure the results were repeatable.

The results were calculated by plotting the fluorescence intensity of each agent against time in a semilog plot and expressed in percent per minute (%/min). The results showed that the DAF had a straight regression line. In a few cases the fluorescein sodium had an initially fast turnover rate flowed by a slower, more stable rate. In these few instances the second slower rate was used for the study. The turnover rates for DAF were 0.93 %/min and 10.3%/min for fluorescein sodium. There was also a positive correlation between the turnover rate of DAF and that of fluorescein sodium.

Researchers found the turnover rate of lipids is much slower than that of the aqueous portion of tears. The turnover rate of lipids, however, is associated with the tear aqueous flow in healthy adults. The study was limited by the use of a DAF emulsion, which may disrupt tear film stability. The tears also have multiple types of lipids, and their respective turnover rates may differ. The data presented in this study were obtained from healthy adults. Past research has revealed that lipid turnover in tears does differ in older subjects or in patients with ocular surface disease such as meibomian gland dysfunction.