Reichert Ocular Response Analyzer Measures Corneal ... - innova

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Figure 12 – Corneal hysteresis of 15 eyes pre- and post-LASIKIn addition to the possible uses already discussed, someexperts believe that the CH measurement will be usefulin customizing ablation algorithms in order to betterpredict and control LASIK outcomes. While this ispurely hypothetical at the present time, studies are beingpursued to explore this possibility.NeyesAvemmHgRangemmHgSt DevmmHgIOPGNormals 246 15.10 7.03 – 24.92 3.22POAG 289 15.97 4.01 – 55.00 5.26NTG 24 16.07 12.05 – 21.32 2.30CHNormals 246 11.19 7.24 - 15.80 1.63POAG 289 10.08 2.07 - 19.96 2.32NTG 24 8.82 6.80 – 11.59 1.39CRFNormals 246 10.46 5.37 - 15.74 1.75POAG 289 9.30 1.70 – 24.18 2.26NTG 24 8.67 6.35 – 12.60 1.50Table 3 – CH and CRF of normal, keratoconic, and Fuchs’ subjectsCorneal Hysteresis and CRF in GlaucomaRecently, the Ocular Hypertension Treatment Study(OHTS), and other studies on the subject, have broughtto light the importance of Central Corneal Thickness(CCT) in diagnosing and managing glaucoma. Thesestudies have suggested that low CCT (thin cornea)may be an independent risk factor for the developmentand progression of the disease. Many experts believethat corneal parameters other than CCT may provideclues that will aid in the diagnosis and management ofglaucoma. Recent studies suggest that pathology in thelamina cribrosa may be reflected in detectable changes ofcorneal properties. Clinical studies utilizing the OcularResponse Analyzer support this hypothesis.Compared to normals, glaucomatous subjects have asignificantly lower than average Corneal Hysteresis anda much wider range. A comparison of IOP, CH, and CRFvalues for normal, Primary Open Angled Glaucoma(POAG), and “Normal Tension Glaucoma” (NTG)populations is shown in Table 3. A histogram of CH forthe three populations is shown in Figure 13.An interesting observation is that lower-than-average CHis also observed in patients who have been identified asNTG subjects. Currently individuals who have NTG areoften missed during routine IOP screening. Obviouslyit would be a tremendous breakthrough if CornealHysteresis is proven to be a reliable indicator of thisdisease condition.Figure 13 – Comparing the corneal hysteresis distributionof normal and glaucomatous subjectsAlso noteworthy is the fact that signals obtained fromeyes of NTG subjects looks similar to the signalsobtained from keratoconus, Fuchs’, and post-LASIKpatients, reinforcing the theory that glaucomatousdamage, in some manner, presents itself via the cornea.The signal from a NTG subject’s eye is shown inFigure 14.Figure 14 – Signal obtained from the eye of an NTG subject
As previously mentioned, in a normal population, thereis essentially no correlation between CH and IOPG(Page 3, Figure 5). However, in a typical glaucomatouspopulation there is a negative correlation between thetwo parameters (Figure 15). The eyes with severelyelevated pressures have much lower than averageCH. Conversely, these same eyes have higher CRFmeasurements. Investigators theorize that low CHis a result of “corneal remodeling” in response toglaucomatous damage and that high CRF is indicative ofstiffening of the cornea due to the elevated pressure.Figure 16 – Typical relationship between CCT and IOPGin a normal population N=182 eyes.The scatter present in this plot demonstrates that CCTcannot be used to adjust individual measured IOP values.Analysis of this and other IOP vs. CCT data has shownthat adjusting IOP based on CCT could lead not only toerrors in the magnitude of the adjustment, but also in thedirection of the adjustment.Figure 15 – IOPG vs. CH and CRF in a glaucomatous population of289 eyesIOPCC: Eliminating the Corneal EffectBy now nearly everyone recognizes that the currentgold standard for measuring pressure, the Goldmanntonometer, has considerable flaws. The GAT, andtonometers calibrated to “read like” a GAT, are affectedby corneal properties including rigidity, thickness,hydration, curvature, and perhaps other factors not yetidentified. Dr. Goldmann designed his tonometer toprovide accurate measurements in eyes with “average”corneas. But we now know that many corneas varymore significantly from average than previously thought.In theory, thicker corneas overstate IOP values andthinner corneas understate IOP values. Many attemptshave been made to establish a CCT-based correctionalgorithm to adjust Goldmann-obtained IOP values.However, the magnitude of the CCT/IOP relationshipidentified by the various studies is inconsistent. Thisvariation has resulted in numerous algorithms that differsignificantly from one another. A typical IOP vs. CCTplot for a population of 182 normal eyes is shown inFigure 16.Evidence in the literature continues to mount leadingexperts to question whether the Goldmann tonometeris sensitive and specific enough to be used for thecritical purpose of measuring IOP in the diagnosis andmanagement of Glaucoma. Refractive surgery, perhapsmore than anything else, has exposed how seriouslythe Goldmann Tonometer is compromised by cornealproperties.It is well-documented that there is an apparent, butartificial, reduction in post-LASIK GAT-measuredIOP. However, this anomaly cannot be explained by, orcorrected for, using CCT data or any currently knownparameter. This is because the LASIK procedure notonly thins the cornea, it induces complex biomechanicalchanges that are not measurable by any previouslyavailable device.Clearly, we need a new method of measuringIntraocular Pressure that takes corneal biomechanicsinto consideration. Only the Ocular Response Analyzeris capable of measuring these properties. IOPCCtakes corneal biomechanical properties into account,enabling the Ocular Response Analyzer to providepressure measurements that are less affected by cornealproperties. As evidence, a plot of IOPCC vs. CCT for a182-eye normal population is shown inFigure 17.
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Reichert Ocular Response Analyzer Measures Corneal ... - innova

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