David Richardson, MD says
There is no study of which I am aware supporting the validity of adding or subtracting mmHg to/from IOP measurements based on corneal thickness. All that can be said with any confidence is that:
- For corneal thickness under 520um: "The measured IOP may underestimate the actual IOP"
- For corneal thickness over 560um: "The measured IOP may overestimate the actual IOP"
Central corneal thickness (CCT) should be used only as one of many risk factors in the assessment of glaucoma. If one wishes to "adjust" IOP then the only validated method of doing so is with corneal hysteresis and the "Cornea Compensated" IOP (IOPcc) available with the Reichert instruments.
Dr. Richardson highlights that you can only make general assumptions about IOP and corneal thickness. The correction factors are averages that apply across a population. However, making a CCT correction to IOP for a specific individual is not appropriate.
It is even possible that an adjustment based on CCT could be the opposite of what is needed for an individual. Therefore, it is recommended that IOP not be adjusted for CCT for individual patients.
The way most FitEyes members address this concern is by having their IOP measured with a Reichert tonometer that can perform the corneal hysteresis (CH) measurement. The 7CR and the ORA can do that. Dr. Richardson has a Reichert ORA in his office. Dr. Ritch had one too.
A Reichert 7CR tonometer, which is what many FitEyes members own, can tell you the difference between your regular IOP (IOPg -- Goldmann correlated) and your corrected IOP (IOPcc -- corneal compensated). Although that difference is not a fixed number, even knowing it from one measurement will give you a better estimated correction factor than a CCT-based adjustment.
As patients, we could all adopt the habit of asking our ophthalmologists if they can check our IOP with one of these modern tonometers:
- Reichert Ocular Responses Analyzer
- Oculus Corvis ST
- Pascal Dynamic Contour
- Reichert Model 30 pneumotonometer (not a modern design, but nonetheless a choice recommended by experts)
It is possible that your ophthalmologist will object for various reasons. One is perceived accuracy.
A recent survey showed that 89% of responding optometrists and 82% of responding glaucoma specialists consider Goldmann tonometry (GAT) to be the most accurate form of tonometry. Junk AK, Chang TC, Vanner E, et al. Current trends in tonometry and tonometer tip disinfection. J Glaucoma. 2020;29(7):507-12.
However, that survey states an opinion, not a fact. It simply means that about 82% of glaucoma specialists are not keeping up with tonometer technology and they hold an incorrect opinion about the accuracy of GAT.
I have also seen objections such as, "even though these tonometers are known to be more accurate than Goldmann tonometry, they are "cumbersome, time-consuming, and more expensive to use."
This is not correct either, however. Other posts on FitEyes cover tonometer technology in more detail, and if you have specific questions about the accuracy of tonometers, please ask a question on that topic. All I will add here is that these modern tonometers are convenient, quick and no more costly in terms of disposables than the best practices that apply to Goldmann Applanation Tonometry (which, increasingly, is away from reusable prisms and alcohol swab disinfection -- because that practice, while cheap, is dangerous).
In a medical specialty where intraocular pressure is the most important treatable risk factor, and the tonometer is the instrument to monitor that all-important risk factor, all professionals should be experts on tonometer technology and they should use the best technology available. If patients begin a dialog with their doctors about tonometers, it may help usher in helpful changes to old habits.
When I did this last, the doctor happily pulled out his Pascal Dynamic Contour and measured my IOP. He simply was not in the habit of using it for every patient. If we all start asking, many ophthalmologists many start paying greater attention to the tonometer they use.