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[Comparative evaluation of tonometry methods in keratoconus]
S E Avetisov1,2, V V Averich1,3, A V Volzhanin1
1Krasnov Research Institute of Eye Diseases, Moscow, Russia.
None:
The chronic course of keratoconus (KC) and the associated structural and functional changes in the cornea complicate the interpretation of intraocular pressure (IOP) measurements. A promising approach is to analyze tonometric data with consideration of the variability of corneal biomechanical (viscoelastic) properties and thickness rather than disease stage. This is supported by clinical observations showing a wide variation in IOP values even within the same stage of KC.
Purpose:
This study aimed to comparatively evaluate tonometric readings obtained by different devices in KC based on corneal biomechanical characteristics and thickness.
Material And Methods:
A total of 167 patients (334 eyes) with bilateral KC (mean age 31.0±8.8 years) were examined. Three subgroups were formed according to data from bidirectional pneumotonometry (Ocular Response Analyzer, ORA) and non-contact tonometry (Corvis ST with Scheimpflug imaging), taking into account corneal hysteresis (CH), corneal resistance factor (CRF), and central corneal thickness (CCT). IOP was measured by pneumotonometry, bidirectional pneumotonometry (ORA), rebound tonometry (RT), dynamic contour tonometry (DCT), and Maklakov tonometry.
None:
The analysis assumed that, excluding structural changes in the cornea in young subjects, differences in IOP values between subgroups would be minimal. Thus, the observed deviations were attributed mainly to biomechanical changes in KC, where lower tonometric readings reflected greater dependence of IOP on the degree of corneal deformation.
Results:
Statistically significant differences (p<0.05) in tonometry readings between subgroups confirmed the substantial impact of corneal biomechanical parameters and thickness on IOP measurements in KC. The greatest variability (39-50%) was observed with pneumotonometry and bidirectional pneumotonometry (Goldmann-correlated IOP, IOPg, by ORA). In these cases, lower IOP values correlated with poorer biomechanical status of the cornea and lower CCT. For other methods (corneal-compensated IOP (IOPcc) by ORA, RT, DCT, and Maklakov tonometry), the variability was less pronounced and did not exceed 26%.
Conclusion:
Regardless of the method used to assess intraocular pressure, alterations in corneal biomechanics in KC result in underestimation of IOP values.
