Inputs and Outputs of the Lacrimal System: Review of Production and Evaporative Loss

Meta-analyses were carried out of studies of tear production (by fluorophotometry, tear turnover rate [TTR]) and evaporation (from capture of fluid loss from the eye). TTR was reduced in dry eye relative to normal at 9.26 +/- 5.08%/min (0.54 +/- 0.28 mu l/min) vs 16.19 +/- 5.1%/min (1.03 +/- 0.39 mu l/min); with values of 7.71 +/- 1.02%/min (0.4 +/- 0.10 mu l/min) in aqueous deficiency dry eye (ADDE) and 11.95 +/- 4.25%/min (0.71 +/- 0.25 mu l/min) in evaporative dry eye (EDE). Evaporation was increased in dry eye at 21.05 +/- 13.96 x 10(-7)g/cm(2)/s (0.21 +/- 0.13 mu l/min) vs 13.57 +/- 6.52 x 10(-7)g/cm(2)/s (0.14 +/- 0.07 mu l/min) in normals; with values of 17.91 +/- 10.49 x 10(-7)g/cm(2)/s (0.17 +/- 0.1 mu l/min) in ADDE and 25.34 +/- 13.08 x 10(-7)g/cm(2)/s (0.26 +/- 0.16 mu l/min) in EDE. Evaporation rate from tear film thinning was also considered, and possible reasons and consequences for the much higher rates thereby reported are discussed. A new statistical approach determined diagnostic efficacy of cut-offs for dry eye derived from the meta-analyses; sensitivities and specifications ranging from 69.5 to 98.6% and 58.7 to 96.8% (TTR) and 45.5 to 61.2% and 79.8 to 90.6% (evaporation). Indices of tear dynamics were reconsidered, and ratios of evaporation and TTR suggest that an increase of between 2 and 3 times may be associated with dry eye.