Diurnal Variability of Human Tear Volume and Production Rate: An Exploratory Study

The purpose of this study was to compare the diurnal patterns of tear meniscus height (TMH), traditional Schirmer strip test (TSST) wetted length, modified Schirmer strip test (MSST) wetted length, and tear aqueous production (TAP) rate.

One hundred fourteen subjects were evaluated between 7:00 and 16:00 hours. TMH (mm) was measured with the Oculus Keratograph 5M. TSST wetted length (mm) was measured without strip sheathing and topical anesthetic. The MSST with topical anesthetic and strip sheathing was used to measure wetted length (mm) and TAP rate (µL/min). The diurnal pattern was analyzed with linear mixed models which used the first visit time period as the reference and adjusted for demographic and clinical factors.

TMH was relatively stable throughout the day (P values > 0.05). TSST wetted lengths fluctuated throughout the day, but the differences were insignificant (P values > 0.05). MSST wetted length increased significantly during 13:01 to 14:00 hours and 14:01 to 15:00 hours, with adjusted mean change of 3.75 mm (95% confidence interval 0.15-7.34, P = 0.04) and 3.97 mm (95% confidence interval = 0.24-7.70, P = 0.04), respectively. TAP rate also increased significantly during 13:01 to 14:00 hours, with an adjusted mean change of 0.63 µL/min (95% confidence interval = 0.03-1.22, P = 0.04).

By adjusting for demographic and clinical confounders, this study found a distinct diurnal pattern in TAP rate and MSST wetted length. In contrast, TMH and TSST wetted length did not change significantly throughout the day. Study results accentuate the limitations of TMH and TSST wetted length for evaluating tear production.

TAP rate has the potential to provide valuable insight into tear production by evading the limitations of conventional tear volume measurements.