Article

Corneal Hysteresis in Glaucoma Management

A valuable tool to optimize intervention and follow-up.

Corneal hysteresis (CH) enables glaucoma surgeons to further risk stratify glaucoma patients and glaucoma suspects and gauge the level of intervention, as well as the timing and frequency of follow-up. In the last several years, I incorporated CH into my clinical practice, and I depend on this tool to help individualize my care for each patient. An interesting and powerful benefit from the Ocular Response Analyzer (ORA; Reichert Technologies) is that it provides a corneal-compensated intraocular pressure (IOP) measurement that is useful in certain challenging situations. The purpose of this article is to provide an overview of what CH is and what it tells us about the eye, to provide a brief overview of fundamental studies that have contributed to our knowledge base in terms of CH and glaucoma risk, and to provide clinical examples of where CH and the corneal-compensated IOP (IOPcc) have been particularly useful for optimizing the quality of glaucoma clinical care in my practice.

Measuring Corneal Biomechanics

Corneal hysteresis is the only in-vivo measurement of the corneal and ocular biomechanics of the eye. Its value reflects the ability of the corneal tissue to dissipate energy.1,2 While the exact method for measuring CH is beyond the scope of this article, one can read the full details in the literature.1-4 Clinically, CH reflects an eye’s shock-absorbing ability. Eyes that have good shock absorption (high CH) are less likely to develop glaucoma, and their glaucoma is less likely to progress. Conversely, eyes that have poor shock absorption (low CH) are more likely to develop glaucoma, and their glaucoma is more likely to progress. Corneal hysteresis reflects how an eye responds to stress (elevated IOP) and whether it experiences the brunt of the stress (low CH) or whether it is able to dissipate the energy and protect the eye and the optic nerve from the stress (high CH). As a point of reference, the CH population average for most ethnicities is around 10.2-4 A simplistic guide is that a CH above 10 is “good,” and a CH below 10 is “bad.”

Corneal Hysteresis and Visual Field Progression

Medeiros et al demonstrated in a well-designed, prospective clinical trial that baseline CH has a significant effect on the rate of visual field progression over time in patients with glaucoma.4 Specifically, his group found that over time, glaucomatous eyes with a CH ≥10 did not have any rapidly progressive visual field loss; however, in the eyes with a CH <10, there were several cases of rapid progression, all else being equal. This study also revealed that for eyes with lower CH, the impact of IOP was significantly greater than in eyes with higher CH. In fact, in their multivariate model, CH was more than 3 times more commonly associated with an increased rate of visual field progression than was central corneal thickness. Numerous other studies have also reported similar findings along this line.3,5,6 Murphy et al demonstrated not only that low CH a risk factor, but also that high CH is protective against glaucoma progression.5

Susanna et al followed up with a similar study, evaluating whether CH was a risk factor for predicting development of glaucoma in glaucoma suspects. In this study, eyes with a lower-than-average CH had a cumulative probability of developing glaucoma that was nearly 3-fold greater than eyes that had a greater-than-average CH.7 Medeiros and his group have essentially demonstrated, in well-designed, prospective, randomized trials, that CH is tightly linked to a patient’s risk of developing glaucoma and progressing from glaucoma.

Measuring Corneal Hysteresis

Corneal hysteresis refers to the specific output number by the ORA. The output from this device is very clinically useful. In addition to a CH measurement, it also reveals a predicted Goldmann applanation value (the value that the machine predicts one’s Goldman applanation will read), as well as a corneal compensated IOP, which is a Goldmann-correlated IOP measurement that is calculated using a patented algorithm intended to minimize the influence of corneal properties. Of these measurements, I find the CH value and the IOPcc to be essential for caring for my patients with glaucoma or at risk for glaucoma.

In my practice, Goldmann applanation and the IOPcc measurements are in close agreement when the CH is average and above average. However, when the CH is low, I have found that there is sometimes a discrepancy between the IOPcc and Goldmann, with Goldmann usually reading a lower IOP and IOPcc nearly always reading a higher value. These are usually the cases that frustrate glaucoma specialists. These patients have eyes that seem to have visual field progression despite a Goldmann applanation of 11 mmHg. In these cases, I find that the CH is invariably low and the IOPcc is often in the mid- to upper teens. I tend to put more weight on the IOPcc in these cases and treat based on the IOPcc instead of Goldmann applanation.

The following cases highlight examples of how CH and IOPcc can facilitate an optimal risk stratification and treatment plan.

Case 1: Patient With Visual Field Defect and Suspicious Optic Nerves

A 59-year-old Asian male with no past medical history and a past ocular history of low myopia presents for glaucoma evaluation. His optometrist detected a visual field defect, as well as mildly elevated IOP and suspicious nerves. His vision is 20/20 in both eyes. His CCTs were 520 OD and 523 OS. His cup-to-disc ratio OD was 0.7, tilted with peripapillary atrophy (PPA) and inferior thinning of the nerve fiber layer (Figure 1A). His cup-to-disc ratio OS was 0.75 with PPA and inferior thinning of the nerve fiber layer (Figure 1B). He has a nasal step on his right visual field (Figure 2A) with a corresponding focal loss of nerve fiber layer on optical coherence tomography OD (Figure 3). His visual field OS is relatively normal (Figure 2B). On his ganglion cell analysis, there is focal loss OD, which also corresponds to his visual field defect and nerve fiber layer defect (Figure 4). His optical coherence tomography from 2016 is essentially unchanged (Figure 5). The patient’s IOPs at every follow-up over 3 years have ranged from 16 mmHg to 20 mmHg in both eyes. The output from the ORA showed the patient’s CH was 11.7 OD and 11.9 OS, and his IOPcc was 13.9 OD and 12.1 OS. The patient has a high (protective) CH, and his IOPcc readings are actually lower than predicted by applanation.

Figure 1. Cup-to-disc ratio OD (A) and OS (B) in a 59-year-old Asian male with visual field defect, mildly elevated IOP, and suspicious nerves.

Figure 2. Visual field showing a nasal step on the right (A) and a relatively normal reading on the left (B) in a 59-year-old Asian male.

Figure 3. Optical coherence tomography from 2019 showing focal loss of nerve fiber layer in a 59-year-old Asian male.

Figure 4. Ganglion cell analysis showing focal loss OD, which also corresponds to visual field defect and nerve fiber layer defect in a 59-year-old Asian male.

Figure 5. Optical coherence tomography from 2016 of a 59-year-old Asian male, demonstrating little change.

Based on the Ocular Hypertension Treatment Study (OHTS) calculator, his 5-year risk of developing primary open angle glaucoma is high, greater than or equal to 33%. While this patient is not the classic OHTS patient, I think it is informative to use various models to help predict the patient’s risk of developing glaucoma. These cases of patients with myopic discs and visual field defects are always difficult to manage. This case is a perfect example of how CH can help me further risk stratify a patient, and given the patient's higher than average CH, I feel comfortable following him on a 9-month basis, despite the findings from the OHTS study.

Case 2: Patient With Relatively Low IOP and Gradual Progression Over Several Years

A 78-year-old white male presented with a past medical history of hypertension and a past ocular history of chronic angle-closure glaucoma treated with laser peripheral iridotomy in both eyes in 2005. He subsequently underwent phacoemulsificaiton and IOL implantation, as well as Trabectome (Microsurgical Technology) and endocyclophotocoagulation (ECP; BVI) in both eyes in 2014. His maximum IOP prior to cataract surgery was 28 mmHg. His CCT was 507 OD and 525 OS.

Since the time of the cataract and glaucoma surgeries in 2014, the patient’s IOP was seemingly controlled in the 10 mmHg to 12 mmHg range on Goldmann applanation on no medications. Despite excellent IOPs, I observed slow but definite progression of nerve fiber layer and Humphrey visual field loss over several years. Figure 6 demonstrates a slow progressive loss of nerve fiber layer over time, classic for a subtle but progressive glaucomatous optic neuropathy. Figure 7 demonstrates slow progressive Humphrey visual field loss over time, consistent with the area of progressive thinning on optical coherence tomography (superior nerve fiber layer thinning and inferior Humphrey visual field loss). Figure 8 demonstrates the IOPs over the past few years, confirming unmedicated pressures in the low teens on applanation.

Figure 6. Slow progressive loss of nerve fiber layer over time in a 78-year-old white male OD (A) and OS (B).

Figure 7. Slow progressive Humphrey visual field loss over time in a 78-year-old white male OD (A) and OS (B), consistent with the area of progressive thinning on optical coherence tomography.

Figure 8. Intraocular pressure readings for a 78-year-old white male over several years, confirming unmedicated pressures in the low teens on applanation.

Figure 9 shows the patient’s optic nerve photos which demonstrate subtle progressive cupping over time, especially superiorly and temporally. One can appreciate the slight change in some of the vessel curvature OU.

Figure 9. Optic nerve photos of a 78-year-old white male demonstrating subtle progressive cupping over time (OD 2013 [A] and 2019 [B]; OS 2013 [C] and 2019 [D]) especially superiorly and temporally. There is a slight change in vessel curvature in both eyes.

At the patient’s May 2019 visit, we obtained CH and IOPcc measurements. This test demonstrated that the patient’s IOPcc was actually in the upper teens despite Goldmann applanation reading in the low teens. I started the patient on latanoprost ophthalmic solution OU at bedtime. Upon his return in July 2019, his IOPcc decreased to 12.6 mmHg and 15.2 mmHg despite the fact that his Goldmann applanation did not detect a change. Interestingly, his CH measurement increased slightly OU, which can be seen once a patient’s IOP is better controlled (Figure 10). There have been reports of latanoprost increasing a patient’s CH and predicting a response to topical prostaglandin treatment; however, it is unclear whether this finding is due to a drug effect or the eye having a more controlled IOP and perhaps a more reliable CH measurement.9

Figure 10. Corneal hysteresis over time, before and after treatment, showing a decrease in corneal-compensated intraocular pressure.

This case is an example of a patient who can be found to slowly progress despite having seemingly controlled IOPs. In these cases, I have found that the CH is slightly low, and there is a discrepancy between IOPcc and Goldmann applanation, with Goldmann applanation usually reading lower. Interestingly, the Goldmann applanation did not change with treatment despite the IOPcc dramatically decreasing.

Conclusion

Corneal hysteresis and IOPcc are essential glaucoma vital signs that enable glaucoma specialists to further elucidate risk stratification for patients. Measurements can be found using IOPcc that indicate patients may need intervention in cases where Goldmann applanation alone might not suggest they need treatment. In my practice, I do not make treatment decisions without knowing patients’ CH and IOPcc. GP

References

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  9. Agarwal DR, Ehrlich JR, Shimmyo M, Radcliffe NR. The relationship between corneal hysteresis and the magnitude of intraocular pressure reduction with topical prostaglandin therapy. Br J Ophthalmol. 2012;96(2):254-257.