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Joycelyn Niimi, Bo Tan, Jenny Chang, Yixiu Zhou, Avanti Ghanekar, Wing Li, Annie Lee, Michelle Wong, Meng C. Lin; Diurnal Variation of Tear Osmolarity and Its Relationship with Central Corneal Thickness Over a 14-hr Period. Invest. Ophthalmol. Vis. Sci. 2012;53(14):560.
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This study aims to identify diurnal variations of tear osmolarity and corneal de-swelling pattern after overnight eye closure and to investigate the relationship between tear osmolarity and corneal thickness.
Tear osmolarity and central corneal thickness (CCT) were measured using the Tear Lab Osmolarity System and Bioptigen OCT, respectively, on a total of 15 healthy subjects (mean age = 22.15 +/- 2.58 SD, range 19-29 yrs) who did not experience ocular dryness. One randomly selected eye of each subject was patched overnight. The patch was removed immediately before the A.M. awakening measurement. Tear osmolarity and CCT of both eyes were measured at bedtime (baseline), immediately upon awakening, as well as 20 min, 40 min, 1 hr, 2 hrs, 4 hrs, and 8 hrs post-awakening. All subjects slept in the Clinical Research Center.
Mean ± SD tear film osmolarity at baseline (an average of 14 hrs from awakening time) was 299±20 mOsms/L. Upon awakening, mean ± SD osmolarity was 265±14 mOsms/L, a significant drop of 11 % from baseline (P<0.001), and elevated back to baseline at 40 min after eye opening (296±20 mOsms/L). 1-, 2-, 4-, and 8-hr mean ± SD tear film osmolarity in mOsms/L were 292±11, 288±8, 283±8, and 282±12, respectively. Mean ± SD CCT at baseline was 552.7 ± 38.8 μm and increased to 570.0 ± 41.8 μm immediately after overnight sleep, equivalent to 3.13% overnight corneal swelling. Cornea returned to baseline thickness 2 hrs after awakening (p =0.876) and remained stable throughout the day. Higher CCT diurnal variation was significantly associated with higher osmolarity diurnal variation (p=0.0194) within the first 2 hrs of awakening. No statistically significant association between osmolarity and CCT was observed during the rest of the measurement period.
There is a diurnal variation of tear osmolarity. At awakening, the tear film is the most hypotonic and reaches its peak in tonicity within 40 min. Throughout the day, tear osmolarity gradually reduces and again rises at 14 hrs post-awakening. In contrast, the central corneal thickness is greatest at initial eye opening, deswells completely and plateaus around 2 hrs, and remains at that same thickness throughout the day. The positive correlation between corneal thickness and tear film osmolarity is most apparent in the first 2 hrs post-awakening among normal individuals.
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