Outflow of tears and its influence on tear secretion and break up time (B.U.T.)

Schirmer's test shows that a retarded outflow of tears causes reduction of the tear secretion. The B.U.T. is independent of the outflow of tears in patients with epiphora (119 eyes with mechanical obstruction, functional obstruction, or normal passage). Mucomimetics prolong the B.U.T. and retard the outflow of tears, whereas ointments reduce both the B.U.T. and the outflow (74 eyes). The B.U.T. becomes reduced when tears are absorbed by filter paper in Schirmer's test, but remains uninfluenced by filter paper inserted in the fornix. The B.U.T. is a valuable clinical test. It is the resultant of many different factors (tear volume, mucus, fat, etc.).