May 11, 2009 (Fort Lauderdale, Florida) — The dynamic contour tonometer (DCT) gives a repeatable and reproducible measure of intraocular pressure (IOP) on central corneal thickness that is superior to the current gold standard in IOP measurement, the Goldmann applanation tonometer (GAT), according to results presented here at the Association for Research in Vision and Ophthalmology (ARVO) 2009 Annual Meeting.
"GAT is still considered the reference standard for measuring IOP," said investigative team member Aachal Kotecha, PhD, from the National Institute for Health Research, Biomedical Research Centre for Ophthalmology at Moorfields Eye Hospital NHS Foundation Trust and University College London's Institute of Ophthalmology in the United Kingdom, during her presentation. "However, it is common knowledge that its accuracy is affected by corneal thickness, with Goldman underestimating and overestimating true IOP." She explained that GAT underestimates with a thin cornea and overestimates with a thick cornea.
In a previous study, Dr. Kotecha and colleagues found that the inter- and intraobserver variability of a prototype DCT device was almost twice that of GAT. The primary purpose of this study was to reassess the measurement precision of a production DCT compared with GAT, and to assess the precision of corneal compensated IOP (IOPcc) measurements in the relatively new ocular response analyzer (ORA), a noncontact device. Their secondary objective was to examine the agreement among the devices.
IOP measurement precision from GAT, DCT, and ORA was assessed in 100 eyes of 100 patients (mean age, 53.2 years) by 2 of 3 experienced observers. The majority of these patients had glaucoma or glaucoma was suspected.
A total of 3 good-quality ORA scans were obtained in a way that allowed the observer to assess the raw ORA plot but to remain masked to the IOP values. Two GAT and 3 DCT readings were then made in a randomized order. The GAT readings were obtained in a masked fashion, and only DCT readings of quality 2 or better were saved; the first reading discarded. Device reproducibility and agreement were determined using Bland-Altman analysis.
At the end of the study, the investigators found that on average, the IOP measurements from GAT read about 3 mm Hg lower than those from the DCT and ORA. DCT gave a repeatable and reproducible measure of IOP that was superior to GAT, whereas the ORA IOPcc showed repeatability acceptable for a noncontact device and a slightly better agreement with the DCT than with GAT.
The repeatability of measures for observer 1 was 2.5 mm Hg for GAT, 1.7 mm Hg for DCT, and 4.5 mm Hg for ORA IOPcc; and 2.3 mm Hg, 2.0 mm Hg, and 4.1 mm Hg, respectively for observers 2 and 3 combined.
For reproducibility, the mean difference in IOP measurements between observer 1 and the combination of observers 2 and 3 was 0.0 mm Hg (95% limits of agreement [LoA], –4.9 to 4.8) for GAT; –0.1 mm Hg (95% LoA, –4.2 to +4.0) for DCT; and –0.3 mm Hg (95% LoA, –7.3 to +6.7) for ORA IOPcc.
IOP measures with both the DCT and ORA were higher than measures made with GAT. The mean difference between GAT and DCT measures was –2.0 mm Hg (95% LoA, –6.0 to +2.1), between GAT and ORA was –1.9 mm Hg (95% LoA, –7.3 to +3.6), and between ORA IOPcc and DCT was –0.1 mm Hg (95% LoA, –4.9 to +4.7).
"The DCT displays excellent measurement precision that appears to be less affected by central corneal thickness," summarized Dr. Kotecha. "ORA precision was acceptable for a new device. And while we eyeballed its measurements, its new software with waveform quality control may make it even more precise."
Dr. Kotecha told Medscape Ophthalmology that the study's number 1 takeaway message is the importance of measurement precision. "It's important to look at how much variability there is in measurements due to the device itself and due to different servers using it. There haven't been that many studies that have looked at measurement precision. Yet one of the things we study is intraocular pressure over time, so we need to know how precise our measurements are. With the various new devices that are being introduced, I think it's important that we don't forget the variability that can occur."
She concluded: "Of the 3 devices we studied, and there are many others out there, DCT is the most precise. It has good repeatability and good reproducibility. . . . It is absolutely one we should watch."
"I liked the study," said session moderator J. Crawford Downs, PhD, from the Ocular Biomechanics Laboratory at Devers Eye Institute in Portland, Oregon, after the presentation. "I think that it is interesting in the sense that it does really start to elucidate the lengths between the accuracy in precision of DCT vs the ORA-based IOP measurement and [GAT], which is the accepted gold standard."
However, "I think one of the issues with the study is that they never truly measured what the real IOP was. But I do think it's a nice study. And I think there's some emerging evidence that dynamic contour tonometry is maybe a better measurement than [GAT]."
When asked if he thought DCT could eventually become the accepted standard, Dr. Downs replied: "That's a good question, but a big leap right now. [DCT] is a very, very nice way of doing things. And everybody knows that central corneal thickness as measured by pachymetry is involved in errors in the [GAT] measurement, which then becomes a risk factor for glaucoma's progression, [because of] the underestimation of IOP with [GAT] with thin corneas. So that's a situation that needs to get corrected."
This study was funded by the National Institute for Health and Research (UK). Dr. Kotecha and Dr. Downs disclosed no relevant financial relationships.
Association for Research in Vision and Ophthalmology (ARVO) 2009 Annual Meeting: Abstract 5219. Presented May 7, 2009.
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Cite this: ARVO 2009: Emerging Evidence for Superiority of Dynamic Contour Tonometry to Goldmann Method for Measuring Intraocular Pressure - Medscape - May 11, 2009.
