IMPORTANT YARDSTICK

Ponometers with measurements that are less affected by corneal thickness and rigidity are likely to replace Goldmann applanation tonometry (GAT) eventually, but a greater consensus is needed in terms of how to interpret the measurements of the new devices in the management of the glaucoma patient, said Aachal Kotecha PhD, Moorfields Eye Hospital, London, UK, at a Glaucoma Day session of the XXXII Congress of the ESCRS in London.

Since its introduction in 1957, GAT has become the gold standard for intraocular pressure (IOP) measurement. It has been used in epidemiological population studies and randomised controlled trials comparing different glaucoma treatment strategies.

It is also the standard in terms of accuracy and precision against which any new tonometer is compared. Thus, all our knowledge regarding IOP is based on the GAT.

However, the accuracy of GAT is inherently limited by the principle on which it is based, the Imbert-Fick law of applanation, a speculative theory empirically derived in order to explain the workings of the GAT. A large body of research shows that the curvature and the thickness, rigidity and elasticity of the cornea all influence the accuracy of IOP measurements performed with GAT.

“If you have a rigid, steep cornea and you use the Goldmann applanation tonometer you overestimate the IOP; conversely if you have a soft flat cornea it will underestimate the IOP,” Dr Kotecha said.

Two of the main alternatives to GAT introduced over the past decade, to address the inaccuracy and imprecision of GAT, are the Pascal® Dynamic Contour Tonometer (Ziemer) and the Ocular Response Analyzer® (ORA, Reichert). The measurements of both devices are less influenced than GAT by the cornea’s thickness, curvature or elasticity.

Studies have also shown that the Pascal device measurements are very close to those obtained with intracameral measurements in patients undergoing cataract surgery. A consistent finding with both the ORA and Pascal tonometer is that they measure IOP as being 2.0mmHg higher than that measured with GAT, suggesting a need for a re-evaluation of what is ‘normal’ and ‘abnormal’ IOP.

In terms of reproducibility, she noted that research she and her associates have conducted showed that the differences between individual clinicians’ measurements with GAT, Pascal and ORA in the same patient varied on average by less than 0.5mmHg with all three tonometers.

However, the range of the differences can be as high as 5.4mmHg for GAT, 7.1mmHg for Pascal and 4.0mmHg for ORA IOPcc.

“The Dynamic Contour Tonometer and the Ocular Response Analyzer are possible contenders to replace GAT, but clinicians need a little bit more information to reach a consensus agreement about how we interpret their measurements,” Dr Kotecha said.

Aachal Kotecha: aachalkotecha@gmail.com