Daytime variations of tear osmolarity and tear meniscus volume

Abstract

Objectives: The aim of this study was to determine the pattern of variations in tear osmolarity and tear meniscus volume in patients with dry eyes and in healthy control subjects over an 8-hr daytime period.

Methods: Ten normal subjects (5 men and 5 women with a mean age of 27±7 years) and 10 dry eye patients (4 men and 6 women with a mean age of 36±12 years) who had been diagnosed on the basis of having an ocular surface discomfort index >12 and a tear breakup time of <10 sec or Schirmer test score of <5 mm were included. The tear meniscus volumes of the participants were measured using ultrahigh resolution optical coherence tomography (OCT), and tear osmolarity was measured using the TearLab Osmolarity System. Both measurements protocols were conducted on the right eye of each participant every 2 hrs beginning at 8:30 AM and ending at 4:30 PM. The OCT imaging was performed first and was followed by osmolarity testing.

Results: The mean tear osmolarity of the dry eye patients was 304.0±10.8 mOsm/L, and the mean tear osmolarity of the normal subjects was 298.0±14.2 mOsm/L (P>0.05). Over the course of 8 hrs, the average measured osmolarities of the dry eye group varied by approximately 21.9±13.5 mOsm/L (range, 6-43 mOsm/L), and the average measured tear osmolarities of the normal group varied by approximately 21.0±9.2 mOsm/L (range, 8-35 mOsm/L). At 2:30 PM, the average volume of the tear menisci in the dry eye group was significantly lower than that of the subjects in the normal group (P<0.05). No correlations between the tear meniscus volumes and tear osmolarities of either group were observed.

Conclusions: Variations in the tear osmolarities of individual dry eye patients and healthy normal control subjects were documented over the course of 8 daytime hours. No relationships between tear osmolarities and tear meniscus volumes were observed.

Figure 1. Daytime variations in tear osmolarity

For normal subjects, tear osmolarity had fallen to levels that were significantly lower than baseline levels by 12:30 PM. Contrarily, there were no differences in the tear osmolarity values of the dry eye group at any of the 5 time points (P > 0.05). * indicates P < 0.05 compared with the baseline osmolarity value. Bars indicate 95% confidence intervals.

Figure 2. Variations in tear osmolarity

In both the dry eye and normal groups, the differences between the minimum and the maximum tear osmolarities that were measured for each subject during the course of the 8-hour study period fell within a wide range of values. The solid line indicates the average of the maximum differences in the tear osmolarity values of the members of the dry eye group; the dashed line indicates the average of the maximum differences in the osmolarity values of the members of the normal group.

Figure 3. Daytime variations in tear meniscus volume

Both the average total tear meniscus volume (A) and the average lower tear meniscus volume (B) of the dry eye group were significantly less than respective volumes of the normal group at 2:30 PM. In addition, significant differences between the lower tear meniscus volumes of the two groups were apparent at 12:30 PM. (C) There were no significant between-group differences in the mean upper tear meniscus volumes at any time point. TTMV, total tear meniscus volume; LTMV, lower tear meniscus volume; UTMV, upper tear meniscus volume. * indicates P < 0.05 in comparison to the dry eye group. Bars indicate 95% confidence intervals.

Figure 4. Analyses of correlations between tear osmolarity, LTMV and TTMV

No correlations between the tear meniscus volumes and the tear osmolarities were observed for either the dry eye group or the healthy normal subjects (P > 0.05). LTMV: lower tear meniscus volume; TTMV: total tear meniscus volume.