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S. MISHIMA, A. GASSET, S. D. KLYCE, J. L. BAUM; Determination of Tear Volume and Tear Flow. Invest. Ophthalmol. Vis. Sci. 1966;5(3):264-276.
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The dynamics of tear flow were studied for human subjects using fluorescein as an indicator. A new fluorophotometer attachable to a slit lamp was designed to determine fluorescein concentration of tears in situ. After the instillation of about 1 µl of fluorescein solution (1.0 Gm. L.-1) into the cul-de-sac, its concentration in the tears ivasfound to decay in a single exponential pattern. In most cases, the turnover rate was initially fast and became slower afterabout five minutes. The initial faster turnover rate was interpreted as the result of stimulation of lacrimation due to theapplication of solution, and the subsequent slower decay interpreted as the physiologic turnover. The initial turnover showed individual variation and was lower in older than in younger persons. The physiologic turnover rate was fairly constant among normal subjects, the average being about 16 per cent min-1. The tear volume in the cul-de-sac was measured with two methods. The dilution method consisted of instilling 16.2 µl of fluorescein solution (0.10 Gm. L.-1) sampling after blinking and determining the dilution ratio. This method was found to be subject to large errorsdue to lacrimation. The second method involved the construction of a semilog plot of concentration decay after the application of a known amount (about 1 µ;l) of fluorescein solution (1.0 Gm. L.-1), the extrapolation of the decay curve to zero time, and the computation of the tear volume from the zero time concentration. The latter method gave veryconsistent results. The average tear volume obtained was 7.0 ± 2.0 µl, with no significant difference betweenage groups, sexes, and fellow eyes. This value agreed well with the probable tear volume calculated from anatomical considerations. The combination of the tear volume and turnover rate determinations gave an average tear flow of 1.2 µl min-1 with a range of 0.5 to 2.2 µl min-1. The tear volume was found to increase with increasing tear flow. Since the tear volume obtained with the zero time method corresponds to an initial faster tear flow, the normal tear volume with a normal tear flow was estimated from the volume-flow relationship; the average normal tear volume was 6.2 ± 2.0 µl.
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