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Editors Selection IGR 9-1

Intraocular pressure: CCT

Comment by Teruyo Kida on:

16899 Diurnal variation of central corneal thickness and Goldmann applanation tonometry estimates of intraocular pressure, Hamilton KE; Pye DC; Aggarwala S et al., Journal of Glaucoma, 2007; 16: 29-35

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Clinical measurements of intraocular pressure (IOP) can be influenced by corneal biomechanical properties including central corneal thickness (CCT). In cross-sectional population studies, a high IOP is associated with a large CCT. Since both IOP and CCT fluctuate during a 24-hour period in each individual, can a high IOP be due to a large CCT? Hamilton et al. (25) demonstrated that IOP and CCT fluctuated in distinctive patterns during the 16-hour diurnal/wake period in 25 healthy young adults. They showed that IOP measured using an applanation tonometer was positively correlated with the overnight corneal swelling of 1.6% - 6%; a strong correlation existed between the IOP change and the CCT change within 2 hours after awakening. The authors proposed that this CCT variation may interfere with the accuracy of IOP measurement. Should clinicians be wary of IOP measurements using the applanation tonometer until the overnight increase in CCT has resolved? The answer is: probably not. It is known that 24-hour peaks of sitting IOP and CCT often occur during the nocturnal/sleep period in healthy young adults. In a recently published study¹, the nocturnal peak CCT appeared earlier than the nocturnal peak IOP.

There is no evidence that the 24-hour IOP pattern in healthy young adults is associated with a change in the known corneal biomechanical properties including CCT

Measurements of IOP in that study were performed using the non-contact tonometer which may be influenced by CCT more than the applanation tonometer. Mathematical data simulations confirmed that the phase timings for the 24-hour rhythms of IOP and CCT were different. Although there was a positive correlation between the 24-hour mean CCT and the 24-hour mean corneal hysteresis (an indicator of overall corneal viscoelasticity) among different individuals, no correlation existed between time-dependent changes of CCT and corneal hysteresis in the same eye. The 24-hour change of corneal hysteresis was not significant. Therefore, there is no evidence that the 24-hour IOP pattern in healthy young adults is associated with a change in the known corneal biomechanical properties including CCT.

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