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Editors Selection IGR 14-3

Tonometry: Comparison of five tonometers

Comment by James Brandt on:

24059 Comparisons of intraocular pressure measurements: Goldmann applanation tonometry, noncontact tonometry, Tono-Pen tonometry, and dynamic contour tonometry, Hsu SY; Sheu MM; Hsu AH et al., Eye, 2009; 23: 1582-1588

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The role of tonometry in the diagnosis and management of glaucoma has to some extent taken a back seat to structural and functional parameters such as optic nerve analysis and perimetry. This is probably a good thing ‐ clinicians have begun to recognize the limitations of tonometry ‐ our current tonometers provide only a brief snapshot of a continuously varying physiologic parameter (IOP), are confounded to some degree by numerous other factors (e.g., central corneal thickness (CCT), corneal curvature (K), ocular pulse amplitude (OPA), ocular and corneal rigidity, etc.) and have varying reproducibility characteristics. Nonetheless, IOP measurements are used for treatment decisions, and the clinician should recognize the strengths and weaknesses of the tonometry technique they use. Hsu et al. (1048) compared four of the five tonometers in broadest use (Goldmann applanation tonometry (GAT), non-contact tonometry (NCT), Tono- Pen (TP) and dynamic contour tonometry (DCT); pneumotonometry was not studied) in a convenience sample of 62 healthy adults. They compared IOP estimates acquired with each of these instruments to each other and evaluated the impact of confounders such as CCT, K, OPA and other biometric measures such as axial length and lens thickness. The study confirmed that the tonometers are highly correlated to each other, but each is affected by a different set of confounders. All except DCT are influenced by CCT, lending support to the idea that this new form of tonometry is mostly unaffected by corneal characteristics such as thickness and rigidity. One of the more interesting findings is the strong correlation between lens thickness and IOP measures acquired with the DCT. It is hard to postulate a physiologic or biomechanical reason why lens thickness should influence IOP measurements in normal eyes, and this interesting finding is worth further investigation.

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