Maximal tear secretion evoked by controlled stimulation of corneal sensory nerves in healthy individuals and dry eye subjects.

Abstract

To measure, the tear flow changes evoked in healthy subjects and dry eye disease (DED) patients by controlled sensory stimulation of the eye surface with i-Onion™, a new stimulation device. Sensory corneal nerves were stimulated with an instrument (i-Onion™) that ejects puffs of CO2 gas (99.9%) at 200 ml.min-1 for 3s, delivered 5 mm from the cornea. Using Schirmer test strips, tear volumes were measured over 3 min in the cornea of one eye before (basal tear volume -BTV) and in the other eye after stimulation of the sensory nerves with CO2 (stimulated tear volume -STV). These measurements were obtained from a control group of adults of either sex (17 students aged 20-30 and 29 subjects without signs of dry eye aged 25-61), a cohort of DED patients (aged 34-75) that included 12 asymptomatic, suspected DED subjects (Schirmer <7 mm and/or TBUT <10s), and 30 Sjögren's syndrome (SS) patients. CO2 stimulation significantly increased the tear volume (BTV = 14.6 ± 1.0 mm, STV = 19.0 ± 1.1 mm: n = 46) in 78% of control subjects, reflecting a mean tear reserve volume (TRV = STV-BTV) of 4.4 ± 0.8 mm. Individual differences were wide, and while no increase in reflex tearing was evoked in 30% of subjects with a BTV >10 mm, the remaining 70% responded vigorously to stimulation, even those with a BTV >18 mm. Asymptomatic DED subjects displayed weaker responses to CO2 stimulation, with lower STVs. Both the BTV and STV of SS patients were low, significantly below those of the healthy controls. Measuring the rise in reflex tearing volume evoked by controlled corneal stimulation provides objective information about the tear glands' secretory capacity in health and disease.