Human tear-production rate from closed-eye Schirmer-strip capillary dynamics

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Physicochemical and engineering aspects, 2017-05, Vol.521 (C), p.61-68
• Main Authors: Telles, R., Li, W., Dursch, T.J., Lin, M.C., Radke, C.J.
• Format: Article
• Language: English
• Subjects: BASIC BIOLOGICAL SCIENCES; Capillary imbibition; Human lacrimal-production rate; Schirmer-tear test; Tear evaporation
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/j.colsurfa.2016.08.027

[Display omitted] •Human-eye health, specifically dry eye, is diagnosed by inserting a Schirmer strip into the lower fornix and measuring the wetting length after 5min.•Wicking theory is applied to predict wetting dynamics of a Schirmer strip. Agreement of theory with clinical observation is excellent.•A linear-length time regime is predicted in which the wetted strip length varies linearly in time. The slope of wetting length in this regime is directly proportion to tear production rate. A new clinical procedure is suggested to obtain quantitative lacrimal-production rates from the Schirmer tear test (STT). A Schirmer tear test (STT) is commonly used to gauge human tear production, especially when dry-eye symptoms present. In an STT, the rounded tip of a standardized paper strip is inserted into the lower fornix of the eye, and the wetted length extending out from the lower lid is recorded after 5min of eye closure. Longer wetted lengths suggest higher tear production rates. To date, however, there is no methodology to transform STT transient wetting lengths into basal tear- production rates. We develop a physical model to elucidate wetting kinetics in a Schirmer strip. Tear evaporation from the exposed portion of the strip and gravity are accounted for. Careful consideration of the initial depletion of tear in the closed-eye tear prism reveals an initial fast increase in wetted length followed by slower growth. Excellent agreement of the proposed model is achieved with experimental observation. When evaporation is negligible, the slow-growth regime exhibits a linear increase of wetted length in time. The linear-length-growth time regime permits simple calculation of quantitative tear-production rates. We suggest measuring several dynamic wetting lengths along a sheathed Schirmer strip and near the 5-min insertion duration followed by fitting to a straight line. The slope of the length-versus-time data gives the basal lacrimal-supply rate.