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J. M. Tiffany, A. Varatharaj; Direct Evaporation Measurements on Cornea and Bulbar Conjunctiva. Invest. Ophthalmol. Vis. Sci. 2007;48(13):432.
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© ARVO (1962-2015); The Authors (2016-present)
Previous studies (ARVO 2005 Poster Abstract No 4401) suggested that the evaporation rate from the tear film over the bulbar conjunctiva ("sclera") was substantially higher than from the cornea, and whole-eye measurements might overestimate the rate from the cornea. However the technique of deducing evaporation rates from the rate of humidity rise inside a goggle involved several estimates of area, preventing reliable apportioning of rates of loss between cornea and sclera. A more direct technique is needed.
The probe of the direct-reading ServoMed Ep 1C evaporimeter was modified by replacement of the central 12mm "chimney" containing the humidity sensors by a chimney of 8mm diameter with the entry tip extended downwards, and placement of a small drying sachet of silica gel at the top of the chimney. In vitro tests showed that reliable readings were obtained from moist surfaces at any surface-tip distance in the range 0-20 mm. The readout was multiplied by 2.25 to correct for narrowing of the sampled area. The probe was mounted on a spectacle frame and readings taken on supine subjects over the cornea (direct upward gaze) and sclera (with sideways gaze). Subjects were 12 normal undergraduates (7M, 5F, age range 20-23, mean 20.5). The mean of 2 readings was taken from both surfaces on the right eye only.
The corneal evaporation rate was 0.75 ± 0.19 ug/cm2/sec for cornea and 0.81 ± 0.18 ug/cm2/sec for sclera (mean ± SD). The difference is significant at the level p<0.005 using the Wilcoxon Signed Rank test. On a typical eye, comparing palpebral height of 8mm in downgaze with 15mm in upgaze, the total rate of evaporative loss is 250% greater in upgaze.
The modified ServoMed evaporimeter gives a simple direct measurement of evaporative loss rate from the eye, independent of room humidity, and would be suitable for use in clinical assessment of the dry eye. Measuring from an 8mm-diameter spot, it can discriminate between corneal and scleral (bulbar conjunctival) rates, and demonstrates the protective effect of downgaze in e.g. computer screen use.
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