Evaluation of Goldmann applanation tonometry, rebound tonometry and dynamic contour tonometry in keratoconus.

Dear Editor,

We read with great interest the article by Özcura et al., concerning the evaluation of Goldmann applanation tonometry (GAT), rebound tonometry (RT) and dynamic contour tonometry (DCT) in patients affected by keratoconus, suggesting that DCT may be the most appropriate tonometer to use in keratoconus for the measurements of intraocular pressure (IOP), because DCT does not appear to be dependent upon corneal thickness (CCT) and radius (CR).

This paper confirms previous finding in normal patients, as Lanza et al. found that DCT showed higher values of IOP, mainly related to CCT and volume, concluding that, if in the future DCT should be considered the gold standard, higher values of IOP could still be considered normal.

In our opinion, in cases of keratoconus the difference between DCT and GAT could also be related to the corneal biomechanical changes, such as corneal hysteresis (CH) and corneal resistance factor (CRF). The same happens in corneas that underwent refractive surgery, where the changes in corneal thickness and radius make GAT and IOL power calculation unreliable.3, 4, 5, 6, 7

Moreover in these patients a significant decrease of CH and CRF immediately after myopic PRK that remains stable over the follow-up has been described, compared to normal eyes, in which these parameters have been shown to be related to the corneal shape and thickness. So these parameters could in addition influence the unreliability of GAT, making the DCT potentially better.8, 9

A similar behavior therefore could be present in patients affected by keratoconus, where CH and CRF changes too could influence the differences between GAT and DCT, increasing or decreasing such differences.

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