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H. Higashihara, N. Yokoi, K. Mruyama, S. Kinoshita, K. Horie, N. Tsuge, S. Imai, N. Shiomi; Effect of Lachrymatory Factor from Onion on Human Tear Secretion . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3896.
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Background: Lachrymatory factor from onion is well known to induce reflex tear secretion, however, its precise mechanism is not clearly demonstrated. This study aimed to investigate the effects of synthesized onion lachrymatory factor (Thiopropanal S–Oxide) on reflex tear secretion in healthy young males. Methods: Enrolled were six healthy volunteers [all males, aged 21.7±1.7 (mean±SD)] and their unilateral eyes were exposed to the 3µl lachrymatory factor instilled to a paper in a specially designed eyecup until their limit in endurance against irritation. After the exposure, examinations were performed as follows: 1) measurement of time until the feeling of irritation (TFI), 2) measurement of tear volume increase by measuring the increase in the tear meniscus radius (mm) [ΔR=R(2 minutes after the exposure) – R(baseline)] using a video meniscometer (Yokoi, Cornea, 2000), at the central lower lid margin in both eyes every minute after the exposure, 3) evaluation of the effects of soft contact lens (SCL) wear during the lachrymatory factor application, the topical anesthesia before the lachrymatory factor, and 4) scoring of ocular irritation (0 to 4) once just at the limit of endurance of irritation and once more just after the end of examination. Results: TFI in naked eye, SCL wear, and topical anesthesia were 9.8±1.5 (seconds), 14.5±5.9 and 13.3±2.1, respectively, showing significantly longer time in SCL wear and topical anesthesia, as compared to that in the naked eye (p<0.05, in every two comparison). Delta R in the naked eye, SCL wear, and topical anesthesia were 0.25±0.14 (mm), 0.24±0.19, and 0.15±0.09, respectively. Three eyes out of the 6 naked eyes could be exposed to the lachrymatory factor for more than 20 seconds and produced reflex tears resulting in an overflow from the lid margin, and also in the contralateral non–exposed eyes, ΔR increased 0.3 mm in average. The mean irritation scores in the naked eye, SCL wear, and topical anesthesia just at the limit of endurance of irritation were 3, 3, and 1.5, respectively, and they became 0 soon after the end of examinations. Conclusions: The lachrymatory factor from onion could yield transient tear volume increase. The present results show that TFI is considerably prolonged in SCL wear and after application of topical anesthesia suggesting a possible association of corneal sensitivity with the lachrymatory factor–induced tear secretion.
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