title: The relationship between measurement method and corneal structure on apparent intraocular pressure in glaucoma and ocular hypertension.
abstract: OBJECTIVE
  To analyse the relationship between measured intraocular pressure (IOP) and central corneal thickness (CCT), corneal hysteresis (CH) and corneal resistance factor (CRF) in ocular hypertension (OHT), primary open-angle (POAG) and normal tension glaucoma (NTG) eyes using multiple tonometry devices.
  
  
  METHODS
  Right eyes of patients diagnosed with OHT (n=47), normal tension glaucoma (n=17) and POAG (n=50) were assessed. IOP was measured in random order with four devices: Goldmann applanation tonometry (GAT); Pascal dynamic contour tonometer (DCT); Reichert ocular response analyser (ORA); and Tono-Pen XL. CCT was then measured using a hand-held ultrasonic pachymeter. CH and CRF were derived from the air pressure to corneal reflectance relationship of the ORA data.
  
  
  RESULTS
  Compared to the GAT, the Tonopen and ORA Goldmann equivalent (IOPg) and corneal compensated (IOPcc) measured higher IOP readings (F=19.351, p<0.001), particularly in NTG (F=12.604, p<0.001). DCT was closest to Goldmann IOP and had the lowest variance. CCT was significantly different (F=8.305, p<0.001) between the 3 conditions as was CH (F=6.854, p=0.002) and CRF (F=19.653, p<0.001). IOPcc measures were not affected by CCT. The DCT was generally not affected by corneal biomechanical factors.
  
  
  CONCLUSIONS
  This study suggests that as the true pressure of the eye cannot be determined non-invasively, measurements from any tonometer should be interpreted with care, particularly when alterations in the corneal tissue are suspected.
authors.affiliation: Birmingham and Midland Eye Centre, City Hospital, Birmingham B18 7QH, UK.
authors.forename: Maged
authors.initials: M
authors.lastname: Nessim
authors.forename: Susan P
authors.initials: SP
authors.lastname: Mollan
authors.forename: James S
authors.initials: JS
authors.lastname: Wolffsohn
authors.forename: Mohammad
authors.initials: M
authors.lastname: Laiquzzaman
authors.forename: Subramaniam
authors.initials: S
authors.lastname: Sivakumar
authors.forename: Stephanie
authors.initials: S
authors.lastname: Hartley
authors.forename: Sunil
authors.initials: S
authors.lastname: Shah
mesh_terms: D000368:Aged; D003315:Cornea / Q000000981:diagnostic imaging*; D003430:Cross-Sectional Studies; D005260:Female; D005500:Follow-Up Studies; D005901:Glaucoma / Q000175:diagnosis* / Q000503:physiopathology; D006801:Humans; D007429:Intraocular Pressure; D008297:Male; D033401:Microscopy, Acoustic; D008875:Middle Aged; D009798:Ocular Hypertension / Q000175:diagnosis* / Q000503:physiopathology; D011446:Prospective Studies; D015203:Reproducibility of Results; D014065:Tonometry, Ocular / Q000379:methods*