New instrumentation offers advance for IOP measurement

Lisbon, Portugal - Newer tonometry technology seems to offer an improvement on Goldmann tonometry, which has been the gold standard for measuring IOP during the last 50 years, said Jay Pepose, MD, PhD, professor of clinical ophthalmology and visual sciences, Washington University, School of Medicine, St. Louis, MO, United States.

Dr. Pepose reported the results from a prospective study using Goldmann applanation tonometry (GAT), the Pascal Dynamic Contour Tonometer (DCT, SMT Swiss Microtechnology AG), and the Ocular Response Analyzer (ORA, Reichert) to measure IOP pre- and postmyopic LASIK. For the ORA, both an ORA-G value, which has been shown to correlate with the GAT value, and an IOP-CC value, which is a corneal compensated measure generated using a new third-generation nomogram, were analyzed.

The results showed there was a greater mean reduction in postLASIK IOP measured with the Goldmann tonometer and when using the ORA-G value compared with the DCT. Both the GAT and ORA-G IOP values showed a relationship with pachymetry whereas the IOP generated by the DCT was independent of corneal thickness. However, the IOP reduction after LASIK as measured with the ORA appeared to be minimized using the IOP-CC.

Analyses of percentage changes in tonometry results relative to the preoperative value showed the DCT and ORA-CC had the lowest percentage absolute change and smaller standard deviations as compared with the GAT and ORA-G values.

“Understanding the cornea is critical for assessing glaucoma risks since many of the standard tests used to assess glaucoma risk, including laser polarimetry and tonometry, are filtered through the optical and biomechanical properties of the cornea,” Dr. Pepose said. “These biomechanical properties can be profoundly changed after LASIK flap creation and laser ablation.”

“Although there have been proposal to create linear algorithms to modify IOP values obtained with Goldmann tonometry, there is no scientific basis for any linear correction of IOP because the positive correlation between Goldmann IOP and central pachymetry is very weak,” Dr. Pepose added. “Therefore, more accurate measures of IOP will require instruments, such as these devices, that perform dynamic rather than static applanation measurements and assess and compensate for corneal biomechanics.”