title: Diurnal Pattern of Tear Osmolarity and Its Relationship to Corneal Thickness and Deswelling.
abstract: OBJECTIVE
  To identify the diurnal variations of tear osmolarity (TO) and its relationship with central corneal thickness (CCT) and corneal deswelling over a 14-hour period.
  
  
  METHODS
  TO and CCT were measured using the TearLab Osmometer and Bioptigen spectral domain optical coherence tomography, respectively, on 38 healthy neophytes (mean age, 21.5 ± 2.2 years). TO and CCT were measured at bedtime (baseline), upon awakening, 20 minutes, 40 minutes, 1 hour, 2 hours, 4 hours, and 8 hours after awakening. Deswelling rate was estimated and expressed as percent recovery per hour (PRPH). Mixed-effect linear regression models describe the relationships among TO, CCT, and PRPH.
  
  
  RESULTS
  The tear film upon wakening (264 ± 14 mOsm/L) was hypoosmotic compared with baseline (297 ± 15 mOsm/L, P < 0.001). TO (in mOsm/L) at 20 minutes, 40 minutes, 1 hour, 2 hours, 4 hours, and 8 hours were 287 ± 10, 292 ± 16, 293 ± 12, 292 ± 10, 289 ± 10, and 286 ± 10, respectively. CCT (mean ± SD) at baseline was 552.2 ± 35.9 µm and increased to 572.0 ± 38.7 µm after sleep. CCT returned to baseline thickness 4 hours after awakening (P < 0.000) and remained stable throughout the day. A small but statistically significant association was found between higher TO and lower CCT (P < 0.0001) and between lower baseline TO and higher PRPH (faster deswelling; P < 0.0001).
  
  
  CONCLUSIONS
  The diurnal pattern of TO has been established. The association of TO with corneal thickness and deswelling suggests that the tear film tonicity may be partly responsible for corneal hydration control; however, the effect may not be of clinical significance in a normal study cohort.
authors: Clinical Research Center, School of Optometry, University of California, Berkeley, CA.
authors: Joycelyn
authors: J
authors: Niimi
authors: Bo
authors: B
authors: Tan
authors: Jenny
authors: J
authors: Chang
authors: Yixiu
authors: Y
authors: Zhou
authors: Avanti
authors: A
authors: Ghanekar
authors: Michelle
authors: M
authors: Wong
authors: Annie
authors: A
authors: Lee
authors: Meng C
authors: MC
authors: Lin
mesh_terms: D000293:Adolescent; D000328:Adult; D001696:Biomechanical Phenomena; D002940:Circadian Rhythm / Q000502:physiology*; D003315:Cornea / Q000033:anatomy & histology* / Q000502:physiology; D004548:Elasticity / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D009994:Osmolar Concentration; D061286:Osmometry; D010865:Pilot Projects; D015203:Reproducibility of Results; D013666:Tears / Q000737:chemistry* / Q000502:physiology*; D041623:Tomography, Optical Coherence; D055815:Young Adult