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Tear flow and evaporation in patients with and without dry eye
1Department of Ophthalmology, University of Iowa Hospials and Clinics, Iowa City, 52242-1091, USA.
Purpose:
To evaluate the steady-state tear flow and evaporation from the ocular surface of patients with and without dry eye.
Methods:
Two groups of patients, 21 with dry eye and 34 without dry eye, with similar age distributions were selected by criteria based on tear osmolarity, Schirmer test, meibomian gland loss, and dry eye symptoms and were compared for tear flow, tear volume, percent turnover, and surface evaporation.
Results:
Tear flow averaged 0.10 +/- 0.08 microliters/minute in patients with dry eye versus 0.15 +/- 0.12 microliters/minute in patients without dry eye (P = 0.002). Tear volume averaged 2.13+/- 1.3 microliters in patients with dry eye versus 2.23 +/- 2.5 microliters in patients without dry eye (P = not significant) and tear turnover averaged 5.3 +/- 2.9% in patients with dry eye versus 8.2 +/- 4.3% in patients without dry eye (P = 0.019). Evaporation averaged 25 +/- 35 X 10(-7) g/cm(2)/second in patients with dry eye versus 13 +/- 6 X 10(-7) g/cm(2)/second in patients without dry eye (P = 0.003).
Conclusions:
Measured tear flow was significantly lower than previously determined in patients with and without dry eye. Evaporation was increased in patients with dry eye and accounted for the majority of the tear loss in patients with dry eye. Normal tear osmolarity can be maintained, even with low tear flow, if evaporation is kept within the normal range.