IOP CONTROL

Goldmann Applanation Tonometer (GAT) has long been considered the gold standard for iOP measurement in glaucoma management, but that privileged status is under threat thanks to a new generation of tonometric devices on the market, according to a number of presentations at the World Glaucoma Congress. 'Goldmann tonometry provided repeatable measurements that were less dependent on scleral rigidity and therefore more accurate than the schiotz tonometer that preceded it,' said Robert L stamper MD. 'This allowed it to become the most commonly used tonometric technique in the world, and allowed comparability of readings. however, as several studies have now shown, the accuracy of Goldmann tonometry is seriously affected by factors such as corneal thickness, irregularity of the cornea and high astigmatism,' he said.

Dr stamper, professor of clinical ophthalmology at the University of California, san Francisco, Us, said that it is important to bear in mind that tonometry has limited use as a diagnostic tool given that only about 25 per cent of the population with intraocular pressure measurements over 35 mmhg actually have glaucoma. 'iOP is not such a good screening device even though it has been the traditional one, and the reason is clearly that false positives are frequent. Only about one per cent of people with elevated intraocular pressures per year actually develop glaucoma. And false negatives are also frequent with somewhere between 25 per cent and 50 per cent of people who ultimately develop glaucoma recording a normal iOP on screening exam,' he said. Dr stamper noted, however, that the importance of iOP measurement escalates dramatically when it comes to glaucoma management. 'This is because we know from multiple studies that elevated iOP is a major, if not the major risk factor, for progression from ocular hypertension to glaucoma. Elevated iOP is also a major risk factor for progression of primary open angle glaucoma (POAG) and fluctuation of intraocular pressure is another known risk factor for progression of POAG,' he said.

Risk reduction
The Advanced Glaucoma interventional study (AGis) showed that lowering iOP delays the visual consequences of glaucoma and reduces the risk of progression, said Dr stamper. 'Therefore in chronic glaucoma it is very important to know the mean iOP, the maximum iOP, and the variability in iOP, and glaucoma management should be aimed at reducing all of those factors,' he said. Dr stamper pointed out that the most accurate means of measuring iOP – manometry – is also unfortunately the least practical and least acceptable to patients since it involves sticking a needle directly into the eye. Goldmann applanation tonometry, purportedly based on the imbert-Fick law which states that the force required to applanate the cornea is proportional to the iOP, is the most widely used indirect method of measuring iOP, said Dr stamper. Nevertheless, despite its ubiquity, there are a long list of factors that can impact on the Goldmann tonometer's accuracy including corneal thickness, corneal irregularity, astigmatism, excess or inadequate fluorescein, calibration errors, and observer bias, among others, he said.

Dynamic contour tonometry (PAsCAL, Ziemer Group), which works on the basis of contour matching instead of applanation, represents an interesting alternative to GAT, said Dr stamper. 'Dynamic contour tonometry is less dependent on corneal thickness than Goldmann devices and therefore provides more accurate readings in real-time. it can also calculate ophthalmic pulse amplitude, and no fluorescein is needed,' he said. Dr stamper said that studies conducted by Dr Kniestedt and colleagues at the University of California have shown that dynamic contour tonometer readings are relatively independent of corneal thickness and provides the most accurate tonometric measurements in thin corneas. The device also provides accurate iOP in oedematous corneas, is relatively accurate after LAsiK, and is also independent of corneal radius of curvature, he added. The enhanced accuracy and precision of dynamic contour tonometry compared to Goldmann tonometry was also stressed by Ted Garway-heath MD in a separate presentation.

'Accuracy is how close to the truth the measurements are, and precision is all about the repeatability of the measurements,' said Dr Garway-heath, iGA professor of ophthalmology at University College London and glaucoma theme lead at the NihR Biomedical Research Centre at Moorfields Eye hospital. he noted that a number of published studies have shown how iOP measured by Goldmann tonometry varies with corneal thickness. 'They are pretty much in agreement that for every 100 microns of cornea thickness difference there is about a 2.5 mmhg difference in measured iOP. But central corneal thickness is not the only factor of the cornea that affects the measurement accuracy. Corneal curvature, and the material properties or stiffness of the cornea (Young's modulus), and its viscoelastic properties, play a role as well,' he said. Dr Garway-heath cited a study by Andreas Boehm and colleagues at the University of Dresden which showed that measurements with the Pascal DCT showed good concordance with intracameral iOP, and that corneal thickness exerted a statistically significant effect on measurements with the DCT, although the size of the effect is less than half that seen in Goldmann tonometry. Another population-based study, the Los Angeles Latino Eye study, also showed that measurements of DCT iOP are affected by corneal thickness, but to a much less degree than that seen with Goldmann applanation.

Excellent precision
in another recent study carried out by Aachal Kotecha et al at Moorfields Eye hospital, the Pascal DCT showed excellent measurement precision and displayed better repeatability and reproducibility compared to the Goldmann tonometer and also the Ocular Response Analyzer (Reichert Ophthalmic instruments), said Dr Garway-heath. Tests of another tonometric device, the icare rebound tonometer (icare Finland Oy), carried out by Martinez de la Casa et al, found that it tended to overestimate iOP compared with Goldmann applanation tonometry. Another study by Nakamura et al also demonstrated that the icare tonometer was also significantly more affected by corneal thickness than Goldmann tonometry.

Summing up, Dr Garway-heath said that overall Pascal DCT and ORA measurements are less affected than Goldmann tonometry by central corneal thickness, while i-care measurements are more affected than Goldmann by corneal thickness. With respect to reproducibility, Pascal DCT measurements are as repeatable as, and more reproducible than GAT iOP measurements, while the Ocular Response Analyzer iOP measurements are less repeatable and reproducible than Goldmann iOP measurements.