CCT may affect results of IOP measurement

"Results from the Ocular Hypertension Treatment Study increased awareness that central corneal thickness affects IOP and led to the quest to develop new tonometers that might provide more accurate measurements," said Dr. Nessim, specialist registrar at Birmingham and Midland Eye Centre and Heart of England NHS Foundation Trust, Birmingham. "Our study has limitations, including the relatively small number of eyes evaluated and the fact that some participants were on IOP-lowering treatment. However, we believe the results support the suggestion that, in eyes with an extremely thin or thick central cornea, it is important to recognize the limitations of GAT and use an alternative method to check IOP."

The study was an observational consecutive case series that over 3 months enrolled 87 eyes with ocular hypertension (OHT), 34 eyes with normotension glaucoma (NTG), and 92 eyes with primary open-angle glaucoma (POAG).

The measurements were performed by four experienced ophthalmologists. IOP and CCT were evaluated bilaterally, but no significant difference was observed between measurements in fellow eyes and, hence, only the right eye data were used for statistical analyses. For the ORA, the corneal compensated IOP value (IOPcc) was used for analysis.

The results of the CCT measurements showed significant differences between the three groups. Mean CCT was thickest in the OHT eyes (560 μm), intermediate in the POAG eyes (535 μm), and thinnest in the NTG eyes (517 μm).

Patients were divided according to diagnostic subgroup, and the IOP results obtained with each technique in each patient were plotted against that individual's CCT. The resultant regression lines showed that, of the four IOP measurement techniques, the PDCT measurement was least affected by CCT in the OHT eyes, which had the thickest corneas, on average, whereas in the NTG eyes that had the thinnest average corneas, IOP was least affected by CCT when measured by the ORA.

In the POAG group, the IOP measurements obtained with the ORA, PDCT, and Tono-Pen were all affected less by CCT than were the results obtained with GAT, he said.

Further analyses compared the difference in mean IOP measured using the various tonometers versus GAT.

In the OHT eyes, the GAT value was significantly higher than the Tono-Pen and PDCT values and significantly lower than the IOPcc. In the NTG eyes and the POAG eyes, the GAT-determined IOP was significantly lower than that measured with each of the other three techniques.

In choosing among these different IOP measurement techniques, several other factors should be considered, according to Dr. Nessim.

"Results obtained with all of the automated techniques are free from user bias, and because they are either non-contact methods or use disposable caps, the ORA, PDCT, and Tono-Pen all have the benefit of decreased risk of cross-infection," he said. "However, the PDCT measurement takes a little longer than GAT, and so patients need to be relatively more cooperative, and experience shows that the PDCT and ORA may not be able to measure all eyes, such as those with keratoconus and other abnormal corneas."

Dr. Nessim also reminded his colleagues that GAT has been around for almost 50 years, but the PDCT and ORA are still new devices and have been available for about 2 years. "Further refinements in those technologies may be expected," he said.