Implications of Refractive Surgery
Dr. Bloomenstein: In making their calculations, the creators of the Goldmann tonometer assumed an average corneal thickness of 520 µm.9 In our practice, where the primary focus is on refractive surgery, the measurement of corneal thickness has been standard of care. Thickness has always been a parameter for assessing risk involved in refractive surgery.14 (See “Using the ORA for Refractive Surgery and Corneal Care” above.) What's interesting is that the standard deviation is up to 35 µm thicker and thinner than the average cornea. In most patients, you're not going to find 520 µm. To me, GAT has always been questionable. Also, because I see many pre- and post-op refractive surgery patients, I've always wondered how affecting the shape of the cornea translates to disease diagnosis. Regardless of whether we're flattening or steepening the cornea, we're certainly altering its thickness and curvature.
In a post-operative patient, I wonder how many millimeters of mercury we're off when we measure IOP with a Goldmann tonometer. How much further away are we from really knowing the pressure in these patients?
Dr. Gaddie: Dr. Thimons, what's the effect on IOP that you see after LASIK?
Dr. Thimons: We've performed about 60,000 cases at this juncture. Our general conclusion is that you can't really use GAT to measure IOP in a post-operative LASIK patient. The numbers vary considerably. There has to be another factor involved besides CCT measurements, which we've found we can't use to create nomograms. Corneal biomechanics has always been a large factor, which is why we've found the ORA to be so important in clinical practice. It gives us the ability to assess biomechanics. The ORA uses a rapid air impulse and an advanced electro-optical system to record two applanation pressure measurements; one measurement while the cornea is moving inward, and another measurement as the cornea returns.
Because of its biomechanical properties, the cornea resists the air puff, causing delays in the inward and outward applanation events, resulting in two different pressure values.
The average of these two pressure values provides a repeatable, Goldmann-correlated IOP measurement (IOPg). Meanwhile, the difference between the inward and outward applanation measurements provides us with a corneal hysteresis (CH) measurement. CH is a measurement of corneal tissue properties that results from viscous damping in the corneal tissue. This measurement provides us with information related to the biomechanical properties of the cornea.
The CH measurement also permits the device to provide a corneal-compensated IOP measurement referred to as IOPcc. IOPcc is less affected by corneal properties than other methods of tonometry, such as GAT.
Dr. Gaddie: With these factors in mind, let's focus on corneal dystrophies and situations in which biomechanics are a factor. Consider these scenarios:
• A patient with Fuchs' dystrophy presents with the typical thick cornea, as measured by pachymetry.
• Another patient, by contrast, has keratoconus, with classic thinning of the cornea. How might these factors affect IOP?
Dr. Wooldridge: These are great examples that drive home the point that all corneas are not the same. A young, healthy cornea that is 600 µm thick is not the same as the cornea of an older patient with a great deal of guttata and the same thickness. The Fuchs' cornea may be thick, but it's also floppy and weak, causing the GAT-measured IOP to be understated. I also think IOP in keratoconus is represented by GAT as being lower than the actual level. We have to look beyond corneal thickness, which is an oversimplification of the biomechanical properties of the cornea as they relate to IOP measurement.
Variability of GAT
Dr. Bloomenstein: Another issue beyond accuracy in these abnormal corneas is the variability of GAT that is seen in all patients. We all work with students, technicians and other clinicians. Taking a pressure with GAT isn't easy. You may have too much or too little fluorescein in the eye, thus giving false high and low readings. There are so many factors that affect GAT measurements, we really need a simplified, less cumbersome, more reliable measurement.
Dr. Gaddie: You make a good point. I would guess that if the four of us had a patient here and we measured his IOP, we'd find consistent differences among our findings. The conundrum we find ourselves in today is that everything we do in glaucoma that's associated with treatment revolves around lowering IOP.3,15 We're using a very inaccurate, if not completely inconsistent, device to measure it.
Dr. Wooldridge: Think about the advanced technologies we use today, yet we're still relying on Goldmann tonometry, a 60-year-old test. Unfortunately, the standard of care changes very slowly. We need to help people understand how poorly the Goldmann performs. We have better technology available today.
Close-up on Corneal Biomechanics
Dr. Gaddie: I think one of the highlights from the OHTS was that statistically thin corneal pachymetry, in the setting of ocular hypertension, was identified as an independent risk factor for conversion to glaucoma.2 The cornea is reflecting glaucoma susceptibility. I'd like to get away from discussing corneal thickness and focus more on corneal biomechanics, both from a scientific standpoint and a clinical standpoint. My question is how do we measure corneal biomechanical properties such as corneal hysteresis?
Dr. Thimons: Clinicians and researchers have discussed corneal biomechanics for several years, but we haven't been able to learn much because no tool was available to measure these properties. I think the ORA is an instrument that's shown itself to be very capable of meeting this objective.16
Dr. Gaddie: Would anyone like to define corneal hysteresis (CH) to help advance this discussion?
Dr. Bloomenstein: CH is, quite simply, a value obtained by the ORA. As Dr. Thimons mentioned earlier, it's a value that's very important because the cornea is made of material that's not very well understood. It consists of complex viscoelastic tissue, with properties that are similar to a foam mattress. When you indent the cornea rapidly, it absorbs some of the energy of the indentation. The reaction of the material doesn't directly respond to the applied force. There's a delayed effect. The ORA measures this reaction, representing the hysteresis value. CH isn't a property, but a measurement output that represents the biomechanical properties of the cornea.
Dr. Gaddie: What do we know about CH from the literature?
Dr. Wooldridge: In general, we know that CH represents an aspect of corneal biomechanics, it's somewhat correlated to thickness and is found to be lower in patients with keratoconus17 and Fuchs' dystrophy.18 It's also reduced after ablative refractive surgery and, perhaps, most relevant to this discussion, it is lower in glaucoma patients — particularity in patients with significant progression.19
Dr. Thimons: One of the factors that I believe makes CH so interesting is that it seems to be associated with the structural integrity of the lamina cribosa. We tried to make that association with corneal thickness, but I don't think the correlation was highly relevant.
I think CH represents the whole eye, suggesting that the relationship between glaucoma and corneal thickness/corneal biomechanics is complex. CH seems to represent an indirect measurement of the integrity and strength of the lamina cribosa. Beyond a representation of corneal biomechanics, it's also a biomarker for glaucoma susceptibility.
Applying Hysteresis to Practice
Dr. Gaddie: I've found that CH is predictive of which patients with ocular hypertension are more likely to convert to glaucoma. Also, those patients who have been diagnosed with glaucoma and have low hysteresis are more likely to be quick progressors. I've found CH to be remarkably predictive of both of those situations.
Now, let's consider a clinical scenario in which a patient is identified as a glaucoma suspect and all the other typical test findings — cup-to-disc ratio, visual field, retinal nerve fiber layer — are equivocal for glaucoma. Have any of you reached the point when you look at CH and say, “This is the one factor that has motivated me to start treatment?”
Dr. Wooldridge: In such cases, I never treat on the basis of one finding alone, and I suspect you don't, either. However, I think CH is an important issue in a list of risk factors that we need to consider, recognizing the importance of the information in our decision-making.
Dr. Bloomenstein: Low CH is extremely predictive, indicating that something is going on, such as glaucoma, keratoconus or Fuchs' dystrophy. This is almost like seeing early tendencies in a child. The lower CH just implies that we should seek the diagnostic condition that is affecting the patient.
Dr. Gaddie: I agree. In my practice, if someone has normal cup-to-disc ratio, no family history and normal IOP, and I see that the CH is low, I interpret that finding as being noncontributory to the patient's examination. However, I've seen how predictive low CH is of progression in patients diagnosed with glaucoma. Therefore, I note the low CH finding in the patient's chart, where I've already indicated a diagnosis of glaucoma. I want to watch this metric to see what happens.
I also find low CH very valuable when other factors point to glaucoma. Let's say, for example, that a patient has some optic nerve cupping without an overt notch in the rim. The patient has some mild diffuse retinal nerve fiber layer loss, but the OCT shows nothing definitive. Your findings suggest glaucoma, but you can't be certain enough to make a definitive diagnosis. I would look at the CH in such a patient. If it's significantly low, then I feel more confident that I have glaucoma on my hands. CH could be the factor that pushes the diagnosis over the edge.
Dr. Thimons: What I find is that CH may be able to identify rapid progressors, much as I found in the past when considering the difference between dynamic contour tonometry (DCT) and GAT.20
I anticipate that hysteresis will add yet another layer to this assessment, providing a much more relevant value than the difference between DCT and GAT. It will give me another reason to say, “That's a potential rapid progressor. I need to watch this patient more carefully.”