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[Central and peripheral corneal thickness: influence on the iop measurement by Tonopen]
Willer Otávio Guimarães Amaral1, Roberto Márcio Batista Teixeira, Leandro Moulin Alencar
1Serviço de Glaucoma, Hospital São Geraldo, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil.
Purpose:
To assess the influence of central and peripheral corneal thickness on the IOP measurement by Tonopen.
Methods:
The IOP of 58 eyes from suspected and glaucomatous patients was evaluated by Goldmann's tonometry (GT) and, by Tonopen XL on the central (TC) and peripheral (TP) regions of the cornea. Using the DGH 5100e pachymeter, we also measured the corneal thickness in its central (CCT) and peripheral (at 6:00 h) regions (PCT). Patients with any corneal changes were excluded. The mean values of all measurements was comparatively analyzed (GT x TC; GT x TP; TC x TP; CCT x PCT) by the paired t test and by regression and correlation analysis.
Results:
The means (+/- standard deviation) of IOP obtained with GT, TC and TP were respectively 14.6 (+/-) mmHg; 18.9 (+/-2.9) mmHg and 20.7 (+/-3.7) mmHg with a statistical highly significant difference (p<0.001). The means (+/- standard deviation) of central and peripheral corneal thickness were respectively 526 (+/-42.3) and 639 (+/-52.9) microm, also with a statistical highly significant difference (p<0.001). The correlation coefficients (r) between the measurements were 0.71 (p<0.001) for GT and TC; 0.65 (p<0.001) for GT and TP and 0.64 (p<0.001) for TC and TP.
Conclusions:
There was a positive correlation between the measurements performed with Tonopen XL and Goldmann's tonometer at the central region of the cornea and the mean IOP was higher with Tonopen XL. The mean IOP obtained by TP was higher than that obtained by TC. These data showed that Tonopen XL, besides providing higher IOP readings than those with Goldmann's tonometer, is influenced by the corneal thickness presenting higher IOP readings in the periphery than in the central region of the cornea.
