Corneal Compensated IOP: A Game Changer?

A tool to enhance the early prediction of glaucomatous damage.

By Marc Bloomenstein, OD

We are constantly in need of more advanced methods to diagnose and manage ocular disease. The use of technology to determine the state of a patient’s disease becomes increasingly crucial as our emphasis moves from treatment to prevention. The chronicity of some common eye pathologies can be slowed or thwarted by recognizing risk factors and identifying signs of early disease and other mediators of the condition. There is no more onerous a disease to diagnose early than glaucoma.

A recent meta-analysis estimated that the worldwide incidence of glaucoma is going to almost double in the next 3 decades from 64.3 million to 111.8 million.1 The challenge is that eye care providers must predict the progression of this disease prior to the damaging sequelae.

As someone who has spent his entire career (I’m old—you don’t have to ask how long) working in and around refractive surgery patients, I have known the challenges of diagnosing this population with glaucoma. You do not want to wait to start treatment after the condition has already induced irreparable consequences, and waiting for nerve damage in the absence of elevated intraocular pressure (IOP) helps no one. There has to be a better way to provide the clinician with an earlier predictor of damage. I can attest that, in my practice, the ability to measure corneal hysteresis (CH) and to measure IOP with CH—a new metric known as corneal compensated IOP (IOPcc)—has been a game changer.

IMPRECISE MEASUREMENTS

The cornea is a fickle tissue that can be molded and massaged for different curvatures to adapt to new refractive distances. Corneal refractive surgery emphasizes the curvature and thickness of the cornea to obtain a reliable result, but it was evident early on that this process reduces the accuracy of the Imbert-Fick method of measuring IOP. With previous refractive surgery patients aging and propelling toward a possible glaucomatous future, IOP measurements need to be more precise. You may remember when we attempted to use the Tono-Pen AVIA (Reichert) off the axis of the central cornea to provide a Goldmann-equivalent measurement in a postsurgical cornea. We were just skipping around the modified curvature and thickness in hopes of better accuracy.

IMPORTANCE OF THE CORNEA

The cornea itself became even more relevant when data from the Ocular Hypertension Treatment Study revealed a list of primary risk factors: age, cup-to-disc ratio, visual field pattern standard deviation, IOP, family history, and the addition of central corneal thicknes.2 Great! Now, not only is the Goldmann IOP manifestly affected by changing the cornea, but the cornea is also further highlighted by noting the thickness as an independent risk factor.

It appears that the Ocular Hypertension Treatment Study uncovered a previously unknown finding. The cornea’s unique biomechanical structure (specifically the thickness, according to this early study) is contiguous with the collagen of the eye and can be used to elucidate optic nerve vulnerability. But that was in 2001; there is ample evidence today to support the assessment of the corneal viscoelastic properties as identified by the CH measurement for the prediction of the development and progression of glaucoma.3,4 Moreover, since this value is more predictive of visual field changes than any other factors, it stood to reason that I needed this in my practice.

CORNEAL HYSTERESIS

CH is obtained by the inward and outward displacement of the cornea via a patented bi-directional application implemented by the Ocular Response Analyzer (ORA). It is a really sophisticated noncontact device. My original desire to use this equipment in my practice had been to obtain a more efficacious IOP measurement. The ORA uses the CH value to provide an IOP—in this case, IOPcc—that is less influenced by nefarious changes to the cornea, both natural and unnatural. The quick measurement, ease of use, and consistent pressures quickly became the gold-standard to assess IOP in my practice on previous refractive surgical patients. The use in my practice started to increase as the age of the population and of previous surgical patients progressed.

TO THE POINT

New tools to measure IOP fluctuations may provide data more reliable than those that are available now, perhaps leading to earlier and more accurate identifications of glaucoma.

Above-average CH was shown by Shah et al to have a protective role in patients with elevated IOP.5 Higher CH appears to indicate a greater integrity of all ocular tissue and, thus, insulates the optic nerve head in patients with ocular hypertension or glaucoma-like optic discs. Conversely, lower CH is significantly associated with increased risk of developing glaucoma and progressing more rapidly in the disease.

RIGHT DIAGNOSIS, RIGHT TIME

All of us can agree that we want to err on the side of caution and provide our patients with treatment that is necessary. However, we do not want to provide an unnecessary diagnosis fraught with expense and worry. Glaucoma diagnosis has been an elusive and oftentimes subjective determination prior to any objective damage. We need reassurance.

In a longitudinal study of glaucoma suspects, each 1 mm Hg lower CH was associated with a 22% increase in the risk of developing glaucoma during follow-up. Lower CH is more related to the development of glaucoma than all other risk factors evaluated,6 suggesting that we need to have all information available at all visits.

A NEW WAY TO MEASURE

The changes to the cornea induced by refractive surgery highlight the limitations we have with traditional glaucoma assessment. The use of the ORA and the addition of CH as another metric in my diagnosis has created a greater sense of confidence. The ability to obtain an IOP that is less influenced by external factors, is easy to use, and provides an overall snapshot of the biomechanics of the eye makes this measurement a must for all my patients.

1. Tham YC, Li X, Quigley HA, Aung T, Cheng CY. Global prevalence of glaucoma and projections of glaucoma burden through 2040: a systematic review and meta-analysis. Ophthalmology. 2014;121:2081-2090.

2. Gordon MO, Besier JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002;120:714-720.

3. Medeiros FA, Meira-Freitas D, Lisboa R, et al. Corneal hysteresis as a risk factor for glaucoma progression: a prospective longitudinal study. Ophthalmology. 2013;120(8):1533-1540.

4. De Moraes CVG, Hill V, Tello C, Liebmann JM, Ritch R. Lower corneal hysteresis is associated with more rapid glaucomatous visual field progression. J Glaucoma. 2012;21(4):209-213.

5. Shah S, Laiquzzaman M, Mantry S, Cunliffe I. Ocular response analyser to assess hysteresis and corneal resistance factor in low tension, open angle glaucoma and ocular hypertension. Clin Experiment Ophthalmol. 2008;36(6):508-513.

6. Zhu F, Diniz-Filho A, Weinreb R, Zangwill L, Medeiros F. A prospective longitudinal study to investigate corneal hysteresis as a risk factor for predicting development of glaucoma. Poster presented at: The Annual Meeting of the Association for Research in Vision and Ophthalmology; May 1-5, 2016; Seattle, WA.

Marc Bloomenstein, OD, FAAO
• director, optometric services, Schwartz Laser Eye Center, Scottsdale, Ariz.; cofounder, Optometric Council on Refractive Technology
• financial disclosure: consultant, Abbott, Allergan,
Akorn, Bausch + Lomb, Lunovus, OcuSoft
mbloomenstein@gmail.com