The problems of a lack of precision and accuracy with Goldmann tonometry, still the gold standard for better or for worse, have been discussed for decades. Although pointing out its shortfalls and contemplating the development of more accurate and precise methods to indirectly measure IOP are important, there are other major issues to consider when advocating the replacement of Goldmann tonometry.
First of all, clinical research showing that errors in Goldmann tonometry have serious clinical consequences is lacking, even though it is well understood that its error in measuring true IOP increases as IOP increases and that it has errors in estimating IOP in eyes with corneal characteristics outside of “normal.” We physicians must also acknowledge that major factors in the international acceptance of Goldmann tonometry as the ubiquitous method for measuring IOP are its relative ease of use, affordability, and efficiency. Almost every slit lamp in most ophthalmic practices incorporates Goldmann tonometry, and more than likely, any new technology that truly replaces it is going to have to be incorporated into clinical use in a similar fashion in order to be widely accepted.
If indeed a more accurate and precise method of measuring IOP is developed for clinical practice, could it be demonstrated that the new method will significantly change clinical management? This is important to consider, because any new method may well be more expensive than current methods, and the current emphasis in this country on the cost of clinical care must be considered. We would all agree that the addition of corneal thickness measurements has greatly improved how we think about IOP measurements clinically, and this information is extremely valuable in glaucoma management for several other reasons. However, we might be hard-pressed to agree that a more accurate method of IOP measurement would have similar implications. Any new gold standard for measuring IOP would likely not be perceived as being as clinically important as pachymetry measurements, even if it eliminated the need for pachymetry, which helps place Goldmann IOP measurements in a more accurate context.
A more reasonable way to think about how a new type of measurement would be incorporated is to identify cases in which standard Goldmann tonometry is unable to obtain measurements or its readings are highly variable or otherwise suspect. A new method could be used in those cases to “calibrate” or adjust measurements with the Goldmann tonometer or Tono-Pen (Reichert Ophthalmic Instruments, Inc., Depew, NY). Pneumatonometry has been suggested as an ideal technology to be used in this fashion, and it is indeed used in many glaucoma practices in this way, as are Tono-Pen measurements sometimes. However, as Dan L. Eisenberg, MD, points out, pneumotonometry needs to be modified to make it much more user friendly and less expensive, and improvements in Tono-Pen measurements are yet to be validated. Various other forms of IOP measurement described in Dr. Eisennberg's article are currently much more expensive than Goldmann tonometry and would not likely be significantly helpful in a large percentage of cases where IOP measurements were highly suspect using either the Tono-Pen or Goldmann methods.
In conclusion, all of us would like measurements of IOP to become more accurate and reliable, and ultimately, a new gold standard method of IOP measurement may be appropriate to consider. We need, however, an analysis of the issues of cost, clinically meaningful reasons for making a major change in methods, and research not funded by industry that explores these issues. Other matters involving IOP measurements may be more clinically relevant and thus more worthy of limited resources for study. These might include real-time, 24-hour monitoring of changes in IOP (especially nocturnally), their relationship to ocular perfusion pressure, and their correlation to glaucomatous progression.
Adam C. Reynolds, MD, is a glaucoma specialist with Intermountain Eye and Laser Centers in Boise, Idaho. He acknowledged no financial interest in the products or companies mentioned herein. Dr. Reynolds may be reached at (208) 373-1200; adamreynolds@cableone.net.
