title: Associations between diurnal changes in Goldmann tonometry, corneal geometry, and ocular response analyzer parameters.
abstract: OBJECTIVE
  To investigate the possible covariations between diurnal changes in Goldmann applanation tonometry (GAT) estimates of intraocular pressure (IOP), central corneal thickness (CCT), central corneal radius, and ocular response analyzer measures of corneal hysteresis and corneal resistance factor (CRF).
  
  
  METHODS
  Twenty-five healthy normal subjects (age, 21 ± 2 years) had central corneal radius, ocular response analyzer, GAT, and CCT measurements performed in 1 eye at multiple times for more than 24 hours. Measurements were taken at 17:00 and repeated every 2 hours until 23:00. Subjects then slept uninterrupted overnight for approximately 8 hours. On awakening, readings were taken at a 20-minute frequency from 7:00 to 9:00 inclusive, followed by 2 hourly intervals until 17:00.
  
  
  RESULTS
  The maximum variation in GAT for more than 24 hours was 2.5 mm Hg, and these fluctuations were predicted by the diurnal variations of CCT (0.37 mm Hg ΔGAT/10 μm CCT, P < 0.01) and CRF (0.68 mm Hg ΔGAT/mm Hg CRF, P < 0.01). The diurnal changes in corneal hysteresis and CRF both covaried with those of CCT (0.16 and 0.30 mm Hg/10 μm ΔCCT, respectively, both Ps < 0.01). None of these associations was statistically different during and after the nominal period of resolution from overnight corneal swelling (2-way interactions, all Ps > 0.05).
  
  
  CONCLUSIONS
  If the IOP is assumed to be independent of corneal parameters, the clinically significant 24-hour variation in GAT being partially predicted by the changes in CCT and CRF suggests that the diurnal variation in corneal parameters affects tonometry measurements. CRF may also be dependent on the level of IOP.
authors.affiliation: School of Optometry and Vision Science, University of New South Wales, Sydney, Australia. wm.lau@unsw.edu.au
authors.forename: William
authors.initials: W
authors.lastname: Lau
authors.forename: David C
authors.initials: DC
authors.lastname: Pye
mesh_terms: D001696:Biomechanical Phenomena; D002940:Circadian Rhythm / Q000502:physiology*; D003315:Cornea / Q000033:anatomy & histology* / Q000502:physiology*; D004548:Elasticity / Q000502:physiology*; D005260:Female; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D014065:Tonometry, Ocular; D055815:Young Adult