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Editors Selection IGR 20-2

Clinical Examination Methods: Corneal Thickness and IOP

Comment by Andrew Tatham on:

84094 Modified Goldmann prism intraocular pressure measurement accuracy and correlation to corneal biomechanical metrics: multicentre randomised clinical trial, McCafferty SJ; Tetrault K; McColgin A et al., British Journal of Ophthalmology, 2019; 103: 1840-1844

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Although Goldmann applanation tonometry (GAT) remains the standard for measurement of intraocular pressure (IOP), readings are affected by properties of the cornea such as corneal thickness and rigidity, potentially leading to significant measurement error.¹ In March 2018, the United States Food and Drug Administration (FDA) cleared a new Correcting Applanation Tonometry Surface (CATS) prism (Intuor Technologies, Arizona) for use with Goldmann or Perkins tonometers. The CATS prism differs from a standard Goldmann tonometer prism due to the sinusoidal shaped curve of its modified applanating surface.² The prism is designed to partially match the curvature of the cornea, reducing force on the prism face due to corneal deformation during applanation, while also minimizing tear-film induced error.²

Previous studies of the CATS prism have demonstrated that it is less influenced by central corneal thickness (CCT) and corneal hysteresis (CH) than standard Goldmann prisms³ and the CATS prism provides measurements more closely related to IOP measured using a surgically placed intra-cameral pressure transducer.⁴ The present study built on previous work by comparing IOP measurements using the CATS and standard prisms in a study including eyes with thin (< 500 µm) and thick (> 600 µm) corneas.

A calibrated Haag-Streit 900 applanation tonometer was used to measure IOP using the CATS and standard prisms in 243 eyes. IOP was measured at 180 and 90 degrees and averaged for astigmatism correction. Overall there was excellent correlation between IOP measured using the standard GAT and CATS prisms (R² = 0.91); however, the CATS prism reduced the slope of the relationship between IOP and CCT and the slope of the relationship between IOP and CH. Differences in paired IOP measurements were greater in eyes with high or low CCT, and in eyes with high or low CH. Compared to the standard GAT prism, the CATS prism gave higher readings in eyes with thin corneas or low hysteresis, and lower readings in eyes with thick corneas or high hysteresis.

CATS prism may improve the accuracy of GAT measurements, with the advantage that the CATS prism can be fitted to readily available Goldmann-type tonometers

A disadvantage of the study was that IOP was not measured using a two-person technique and though hysteresis was assessed, there was no direct comparison between IOP measurements and corneal-compensated IOP from the Ocular Response Analyzer. It would also be interesting to see a comparison between the CATS prism and dynamic contour tonometer. Nevertheless, the results provide evidence that the CATS prism may improve the accuracy of GAT measurements, with the advantage that the CATS prism can be fitted to readily available Goldmann-type tonometers.

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