TONOMETRY UPDATE

The Goldmann applanation tonometer (GAT) remains the gold-standard in tonometry, but newer devices are currently available or are under development that may help provide a more accurate picture of the effect of IOP-lowering therapy in eyes with glaucoma, said Michele Iester MD, PhD, associated professor of the University of Genoa, Genoa, Italy. “IOP measurement is of fundamental importance in the management of glaucoma because, as numerous large multicentre studies have demonstrated, IOP reduction can reduce the amount of retinal ganglion cell loss in eyes during the follow-up of the disease,” Prof Iester said at a Glaucoma Day session of the XXXI Congress of the ESCRS in Amsterdam.

The GAT mounted on the slit-lamp was the first applanation tonometer to employ variable force and it has remained the gold-standard in tonometry for more than 50 years. However, there are a number of factors that can affect GAT measurements and can lead to an under- or overestimation of the real IOP. They include differences in central corneal thickness, astigmatism, corneal oedema, decentration of the measurement and the variability of tear film properties.

With regard to central corneal thickness, Prof Iester noted that GAT measurements are based on the assumption that the central corneal thickness is 520 microns. Therefore, GAT will give erroneously low IOP in eyes with thinner corneas and erroneously high IOP values in eyes with thicker corneas. However Goldmann and Schmidt predicted that an infinitely thin and flexible cornea would require a correction of 2.5 mmHg, and it seems improbable that the error in Goldmann tonometry induced by an abnormally thin cornea would ever exceed 2 or 3 mmHg. (Goldmann H, Schmidt T. Uber Applanationstonometrie. Ophthalmologica 1957; 134:221–242.)

Are newer technologies necessary?

Prof Iester noted that laboratory research involving human cadaver eyes show that dynamic contour tonometry may provide better accuracy than GAT (Kniestedt et al, Arch Ophthalmol. 2004;122(9):1287- 1293). Moreover another study showed that the dynamic contour tonometry measurements have a close concordance with true manometric IOP measurements obtained through intracameral cannulation during cataract surgery (Boehm et al, Invest Ophthalmol Vis Sci. 2008; 49:2472–2477).

However, superior accuracy in determining the actual IOP values may not necessarily yield clinically superior information, he pointed out. What is most important is that the measurements be reproducible and that changes in the measured IOP values following treatment will correspond to an effect on the glaucomatous pathology. “All the most important clinical trials have used Goldmann applanation tonometry, so even if there was some error in the IOP measurements, the studies’ results showed that the measured IOP reduction was related to a reduction in the worsening of the optic nerve head damage and visual field,” Prof Iester said.

Some of the newer tonometers can offer some advantages in difficult cases. For example, the Tono-penR (Reichert) and the pneumotonometer can be used in eyes with corneal oedema, irregular cornea and corneas with scars, although the pneumotonometer gives readings higher than the Goldmann tonometer.

The IcareR tonometer (Icare) has the advantage of requiring no topical anaesthetic and only a minimum amount of cooperation from the patient. Compared to GAT its readings are higher in children but are lower in older adults. The PascalR dynamic tonometer (Ziemer) has the advantage of providing dynamic measurements that are independent from corneal structure and it also measures ocular pulse. The ocular response analyzer is able to measure, in addition to IOP, corneal biomechanical properties including corneal hysteresis.

Several studies have implicated IOP fluctuation as a factor in disease progression, although other studies have contradicted those findings, he noted. However, measurement of fluctuation to date has been imperfect since even the most reliable method, sleep laboratory, involves placing patients in conditions that are very different from those of most patients’ daily lives.

In the future possible alternative systems able to detect the IOP during all of the day and the night will use the so-called “MEMS” (Micro Electro Mechanical Systems). Actually the SENSIMED TriggerfishR (Sensimed AG), a contact lens-based device designed to record corneal radius change continuously over 24 hours, is under evaluation. It consists of a strain-gauge sensor embedded in a soft silicone contact lens and it detects circumferential changes in the region of the corneo-scleral junction and transmits its measurements wirelessly to portable recorder. However, the device still awaits clinical validation, it is not clear what it measures and is therefore not ready for general use, Prof Iester said.

“Goldmann applanation tonometer is still highly recommended in clinical practice. What we need for management is a baseline value not absolute number but a reproducible IOP measurement to assess the effect of treatment during the followup. Probably in the future we could have a different device able to analyse IOP data all day long,” he added.