Modern approaches to interpretation of Maklakov tonometry results

Based on the presumption that the average values of ocular rigidity determined ex vivo are sufficient for clinical evaluation of intraocular pressure (IOP), calibration of the Maklakov tonometer is done without taking into due account the biomechanical properties of the tunics of the eye, which can affect the results of IOP measurements. Corneal-compensated IOP (IOPcc) allows evaluating patients' individual structural features of the fibrous tunic of the eye, and according to literature sources, for glaucoma it is a parameter with higher diagnostic value. Comparison of Maklakov tonometer readings and IOPcc from the same patients can improve our understanding of the diagnostic value of tonometry. This article aims to determine based on sufficient clinical data the ranges of corneal-compensated IOP corresponding to applanation tonometry readings performed with a 10-g Maklakov tonometer, and to reveal the dependency between the tip diameter and the IOP with consideration of the variability of biomechanical properties of the fibrous tunic in the population. The comparison study analyzed the readings of 10-g Maklakov tonometer and corneal-compensated intraocular pressure in 14 440 eyes of 7 220 patients (mean age 60.1±10.8 years old) with primary open-angle glaucoma and suspected glaucoma. For analysis, IOPcc values of 6 to 35 were included, with a prerequisite of corneal thickness/ hysteresis data being of high quality. The tip diameter was measured with a Vernier caliper within the accuracy of 0.1 mm according to the instructions from the tonometer manual. It was established that with equal tip diameter the readings of applanation tonometry can still vary significantly, which is associated with population variability in the structure of the fibrous tunic of the eye. A measuring scale was derived consisting of a computation chart with markings denoting the borders of confidence intervals for IOPcc values for 10-g Maklakov tonometer. Mean IOPcc is connected to the applanation tip diameter and the cornea by the following equation: IOP=4.14×D2 -62.4×D+248, which can be used for calibrating Maklakov tonometer in the lower and upper ranges of IOP values. The results of 10-g Maklakov tonometry can be presented as a range of IOP values, which with a certain probability includes the individual IOP level of the patient.

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