IOP: Corneal Hysteresis

Core Messages

››Corneal hysteresis (CH) is a measurement that reflects the viscoelastic properties of the cornea and gives an indication of its biomechanical integrity.

››Large variations from normal values for CH can profoundly affect the measurement of intraocular pressure (IOP).

››Central corneal thickness (CCT) and CH values are independently associated with underand overestimation of IOP. Therefore, CCT and CH are important clinical parameters for the clinician to consider when estimating true IOP.

››The dynamic contour tonometer estimates IOP most independently of CCT and is ideal for use in LASIK-thinned corneas.

11.1  What is Corneal Hysteresis and How Does it Influence IOP Measurement?

Corneal hysteresis (CH) is a measurement that reflects the viscoelastic properties of the cornea and its biomechanical integrity. Hysteresis is expressed in units of mmHg, just like intraocular pressure (IOP). Unlike IOP, CH shows no significant variation throughout the day [1]. The Ocular Response Analyzer (ORA) (Reichert

Corp. Buffalo, NY) is an instrument that measures CH. It uses a rapid air pulse to record two applanation pressures (Fig. 11.1) [2]. One applanation pressure is recorded while the cornea is moving inward, during indentation, and the second is measured while the cornea is moving outward (while recovering from indentation). The difference between the two pressures is CH. These two pressures are different because of corneal resistance properties delaying the inward and outward applanation.

IOP, on the other hand, is derived indirectly from a force measurement, and is based on a number of assumptions about corneal deformability. Corneal deformability represents a summation of the actual IOP, surface tension, the cornea’s curvature, and elastic properties [3]. The cornea’s elasticity is affected by many properties, including its thickness, collagen composition, and packing density of collagen fibrils, hydration, and extracellular matrix among other factors that undoubtedly vary from individual to individual. The corneal properties just listed may dwarf the effect of CCT on the accuracy of IOP estimation.

L. A. Marzette ( )

Duke University Eye Center, Box 3802 DUMC, Durham, NC 27710, USA

e-mail: lionel.marzette@duke.edu

Fig. 11.1  The ocular response analyzer (Reichert Corp.). From teaching files of Leon Herndon

In a biomechanical model of the cornea, Liu and Roberts demonstrated that corneal biomechanics can have a tremendous impact on IOP [3]. The model uses a value called Young’s modulus, also called the modulus of elasticity. Young’s modulus is the ratio of stress (load per area) to strain (displacement per unit length) [3]. Materials with higher Young’s modulus values are harder to deform than those with lower values, and thus, steel has a higher Young’s modulus than that of wood.

Young’s modulus values for the human cornea are thought to vary widely, from 0.01 to 10 MPa [3], and there is evidence that refractive surgery can significantly change an individual cornea’s value [4]. Significant variations in Young’s modulus (i.e., the corneal biomechanics) can have very large effects on IOP measurements. Liu and Roberts used Young’s modulus values between 0.1 and 0.9 (which are thought to be physiologic values) in their corneal model while keeping CCT and corneal radius of curvature constant [3]. The difference in predicted IOP measurements was 17 mmHg using 0.1 vs. 0.9 for Young’s modulus. This effect was far greater than the effect of CCT or radius of curvature on IOP when Young’s modulus was kept constant [3]. There is also evidence to support that a low CH value is a risk factor for underestimation of IOP and that a high CH value is a risk factor for overestimation of IOP [5].

Summary for the Clinician

››Corneal hysteresis is a measurement of the viscoelastic properties of the cornea. It can be likened to the spring effect of the cornea.

››Corneal elasticity is affected by corneal thickness, collagen composition, hydration, and extra­ cellular matrix.

››Corneal hysteresis appears to have a greater effect on measured IOP than on CCT or corneal radius of curvature.

››Corneal hysteresis is measured by the ocular response analyzer.

››Low CH may underestimate IOP and high CH may overestimate IOP.

11.2  What Are Typical Corneal

Hysteresis Values?

Patients with primary open-angle glaucoma (POAG) and normal tension glaucoma (NTG) have decreased values of CH than do normal patients. A study by Bochmann et al. [6] showed that POAG patients with acquired pit of the optic nerve (APON) had significantly lower CH values compared with those of glaucoma patients without APON (mean CH 8.89 and 10.2 mmHg, respectively). In another study by Congdon et al. [7], a lower CH was associated with progressive visual field worsening.

Normal children have similar average CH values to adults – 12.5 mmHg. In pediatric cases of glaucoma, patients have also been shown to have lower CH values. A study by Kirwan et al. showed congenital glaucoma patients to have a mean CH of 6.3 mmHg compared with 12.5 mmHg in their normal cohorts [8].

Summary for the Clinician

››Average normal CH is around 12.5 mmHg.

››Normal children have CH values similar to normal adults.

››Decreased CH values are associated with glau­ coma.

››Glaucoma patients with lower CH values have greater progressive visual field worsening.

11.3  What Is the Relationship Between CCT, IOP, and Corneal Hysteresis?

A great deal of clinical decision making in glaucoma is based around IOP, and we would like to have as accurate a measure of it as possible. It is well known that there are large variations in corneal thickness among individuals, and that corneal thickness can affect pressure readings taken with the Goldmann applanation tonometer (GAT). In their seminal paper, Goldmann and Schmidt [9] acknowledged that when large variations occur in CCT the accuracy of GAT readings can be affected.

Corneas that are thicker than normal require greater force to flatten, and thinner corneas require less force. This means that thicker corneas yield an overestimation of IOP, whereas thinner corneas give an underestimation. In the 1970s Ehlers [10–12] performed a number of studies assessing the effect of CCT on IOP. They cannulated 29 otherwise normal eyes undergoing cataract surgery and correlated corneal thickness with errors in GAT. They found that GAT most accurately reflected “true” intracameral IOP when CCT was 520 µm, and that deviations from this value resulted in an overor underestimation of IOP by as much as 7 mmHg per 100 µm.

There is also evidence of a positive correlation between increased CCT and increased CH values [13]. The evidence suggests that thicker corneas possess greater viscoelastic properties, meaning that they may inherently be less elastic [14]. It has also been noted that increased CH values and CCT are individually associated with less likelihood of glaucomatous disease. A high or low CH value can cause an overor underestimation of IOP, respectively, the impact of which can be very significant theoretically (see Sect. 11.1). It has been suggested too that corneal biomechanics may reflect the structural integrity of the optic nerve head [14].

Summary for the Clinician

››CCT impacts the accuracy of IOP measurement when taken by GAT, with thicker CCT leading to an overestimation and thinner CCT leading to an underestimation.

››CH impacts the accuracy of IOP measurement; lower CH values are associated with an underestimation of IOP and higher CH values are associated with an overestimation of IOP.

11.4  Should I Invest in Newer Devices to Measure IOP that Claim Less Influence of CCT?

[15] is least affected by corneal biomechanics. It is particularly useful in patients who have had their corneas thinned by previous LASIK surgery. DCT is, therefore, a relatively good investment for ophthalmologists who want to measure IOP relatively independent of corneal factors. However, it must be recognized that studies using IOP estimated by DCT to follow glaucoma progression are lacking at this time.

DCT is a continuously recording type of tonometer that reports IOP and ocular pulse amplitude (OPA). The OPA is a measurement of blood flow entering the eye. The OPA is pulsatile and has maximum and minimum measurements, in association with the heart’s pumping action.

The DCT has a central gauge surrounded by a contoured plastic tip (Fig. 11.2). This tip contacts the cornea and creates a tight-fitting shell, but does not applanate the cornea. The DCT compensates for all forces exerted upon the cornea. An electronic sensor measures IOP independently of corneal properties. A pilot study [16] was conducted that compared IOP measurements using GAT with DCT in patients who had undergone firsttime LASIK with a median ablation of 90 µm. Pre­ operative and postoperative IOPs were measured. GAT was found to postoperatively underestimate IOP by as much as 5 mmHg, whereas DCT measurements did not change from the preto postoperative periods. In another study [17] using human cadaver eyes, DCT values were found to be no more than 0.50 mmHg above manometric readings. GAT readings were as much as 3.48 mmHg below manometry. DCT readings also showed no significant changes between hydrated and dehydrated corneal conditions, whereas GAT and pneumatonometry both showed significant variations.

Based on supporting evidence, it appears that the dynamic contour tonometer (DCT; Ophthalmic Deve­ lopment Company (ODC), AG Zurich, Switzerland)

Fig. 11.2  The dynamic contour tonometer (Ophthalmic Deve­ lopment Company). From teaching files of Leon Herndon