This document addresses corneal hysteresis (CH). CH is a measure of the viscous damping characteristics of corneal tissue. It is calculated by deflecting the cornea with a rapid air pulse and then using an advanced electro-optical system to obtain two intraocular pressure measurements. The first measurement is taken as the cornea moves inward and the second as it returns to baseline. The difference in the values is defined as CH. Measurement of CH has been proposed as an alternative to the gold standard of intraocular pressure (IOP), the Goldmann applanation tonometer (GAT).

Note: Please see the following related documents for additional information.

• MED.00006 Ophthalmologic Techniques for Evaluating Glaucoma
• SURG.00009 Refractive Surgery

Investigational and Not Medically Necessary: 

Measurement of corneal hysteresis including, but not limited to, its use in the evaluation and management of glaucoma, refractive eye surgery, and keratoconus is considered investigational and not medically necessary.

The scientific literature is inadequate to validate the clinical role for measurements of CH. The Ocular Response Analyzer (ORA) (Reichert, Inc., Depew, NY) received 510(k) clearance from the U.S Food and Drug Administration on January 20, 2004. The device is indicated for  the measurement of intraocular pressure (IOP) and assessment of the biomechanical response of the cornea in the diagnosis and monitoring of individuals with glaucoma. The basic output of the ORA includes a measure of CH. The 510(k) clearance process does not require the submission of extensive safety and efficacy data. In addition, measurement of CH by the ORA has been proposed as a method to evaluate the potential for post-surgical complications in those who are being considered for refractive surgery and for assessing the biomechanical properties of the cornea in keratoconus.

The majority of published peer reviewed literature consists of studies evaluating correlations and associations between CH and established measures of intraocular pressure and central corneal thickness (CCT) (Congdon, 2006; Kotecha, 2006; Martinze-de-la-Casa, 2006; Medeiros, 2006; Shah, 2006; Shah, 2009a; Shah, 2009b: Wells, 2008). These studies do not demonstrate that the measurement of CH can be used to enhance the management of an individual's care or improve outcomes.

The American Academy of Ophthalmology (AAO) (2005) does not mention measurement of CH in its Preferred Practice Pattern for the evaluation and management of Primary Open Angle Glaucoma (POAG).

Ortiz and colleagues (2007) compared the biomechanical properties of normal, post-laser in situ keratomileusis (LASIK), and keratoconic corneas. These were evaluated by CH and the corneal resistance factor measured with the ORA in a prospective case series study. Two hundred fifty eyes were divided into 3 groups: normal (control group), post-LASIK, and keratoconus. The corneal biomechanical properties were measured with the ORA. The main outcome measures were intraocular pressure, CH, and the corneal resistance factor. The control group had 165 eyes; the LASIK group, 65 eyes; and the keratoconus group, 21 eyes. In the control group, the mean CH value was 10.8 mm Hg +/- 1.5 (SD) and the mean corneal resistance factor, 11.0 +/- 1.6 mm Hg. The CH value was lower in older eyes, and the difference between the youngest age group (9 to 14 years) and oldest age group (60 to 80 years) was statistically significant (P = .01, t test). One month following LASIK, the CH and corneal resistance factor decreased significantly, from 10.44 to 9.3 mm Hg and from 10.07 to 8.13 mm Hg, respectively. In the keratoconus group, the mean CH was 7.5 +/- 1.2 mm Hg and the mean corneal resistance factor, 6.2 +/- 1.9 mm Hg. There were significant differences in both biomechanical parameters between keratoconic eyes and post-LASIK eyes. The authors concluded CH and corneal resistance factor values were significantly lower in keratoconic eyes than in post-LASIK eyes. Future studies are needed to determine whether these differences are useful in detecting keratoconus when other diagnostic tests are equivocal.

Shah and colleagues (2007) compared CH in normal and keratoconic eyes. The study consisted of 207 normal and 93 keratoconic eyes. Volunteers were recruited from the staff and relatives of patients attending a single ophthalmology clinic in a teaching hospital. The hysteresis was measured by the ORA and the data were recorded by Generation 3 software for the ORA. CCT was measured with a handheld ultrasonic pachymeter in the midpupillary axis. Study results demonstrated the mean hysteresis was 10.7 +/- 2.0 (SD) mm Hg (range, 6.1-17.6) in normal eyes compared with 9.6 +/- 2.2 mm Hg (range, 4.7-16.7) in keratoconic eyes. The difference was statistically significant (P < 0.0001, unpaired t-test). Mean CCT in the normal and keratoconic eyes was 545.0 +/- 36.4 microm (range, 471-650) and 491.8 +/- 54.7 microm (range, 341-611), respectively; the difference was significant (P < 0.0001, unpaired t-test). The overall data analysis showed that CH was significantly higher in normal than in keratoconic eyes. The study also demonstrated there was no variation in the slopes found for CCT versus hysteresis in the normal and keratoconic groups. Analysis showed that CCT was the predominant factor for severity, although there was an effect of hysteresis. The author noted further work is required to assess the importance of these factors.

In a another study, Lam and colleagues (2007) compared IOP obtained from the ORA versus GAT on a group of 125 normal young Chinese subjects aged 4 to 18 years with one eye per person randomly selected for this study. Each eye was first measured with the ORA, followed by the GAT and ultrasound pachometry, in a randomized order. Four readings were taken from the ORA and three measurements were taken with the GAT. The mean was used for analysis. The ORA provided a Goldmann-correlated IOP (IOPg) and a corneal-compensated IOP (IOPcc). Three CCT values were measured using an ultrasound pachometer, and the mean was used for analysis. Study results showed the IOP obtained from the ORA was similar to that from the GAT (IOPg minus GAT: mean difference = 0.33 mm Hg, 95% limits of agreement = 4.55 to -4.44 mm Hg; IOPcc minus GAT: mean difference = 0.24 mm Hg, 95% limits of agreement = 4.83 to -5.07 mm Hg). CCT was positively associated with CH (r2 = 0.30, p < 0.01), corneal resistance factor (r2 = 0.38, p < 0.01), GAT (r2 = 0.09, p < 0.01) and IOPg (r2 =0.16, p < 0.01). IOPcc was not associated with CCT (r2 = 0.01, p = 0.33). The authors concluded that although ORA can provide in vivo measurement of some corneal biomechanics, its clinical usefulness requires further investigation.

Kotecha (2007) stated current evidence suggests that the importance of corneal biomechanics to the glaucoma clinician rests primarily with its effects on IOP measurement. However, the possibility that corneal biomechanics may give an indication of the structural integrity of the optic nerve head cannot be completely excluded. The author also noted further population and longitudinal studies are needed to clarify whether current in vivo measures of corneal biomechanical properties, including corneal hysteresis, prove to be independent predictors of glaucoma susceptibility.

At the present time, there is insufficient evidence available from the peer-reviewed literature to validate the clinical role for measurement of CH. Further investigation is required before the clinical usefulness of this procedure is proven.

According to the National Institutes of Health, glaucoma is a leading cause of blindness in the United States and is a group of diseases which can damage the eye's optic nerve and result in vision loss and blindness. Primary open angle glaucoma (POAG) is the most common type of glaucoma. POAG is associated with a build up of aqueous fluid pressure within the eye, which can lead to visual field loss and optic nerve damage usually without any associated pain or discomfort. In the management of POAG, the goal is to reduce the intraocular pressure (IOP) to retard the development of optic nerve damage.

Keratoconus is a noninflammatory condition of unknown etiology affecting the central cornea characterized by thinning and bulging of the cornea. It may significantly affect vision due to irregular astigmatism and corneal scarring. Keratoconic eyes are known to be less rigid and more elastic than normal eyes and possibly may have a different hysteresis than normal eyes. One possible measure of ocular rigidity in keratoconus is hysteresis using the ORA (Shah, 2007).

The preferred method of measuring IOP is using a contact applanation method such as a Goldmann tonometer. Central corneal thickness (CCT) affects the accuracy of IOP measurement by applanation techniques (AAO, 2005a). A thin central cornea causes an underestimate of the true IOP as measured by applanation. This can lead to loss of visual field in an individual with apparently "normal" IOP. Conversely, a central cornea that is thicker than normal causes the pressure reading to be overestimated, and may explain why some individuals do not lose visual field even with an "abnormal" IOP. Use of the CCT measurement provides a correction factor to allow the true IOP to be determined.

Measurement of CH has been proposed as a new measurement of the biomechanical properties of the cornea. During the measurement, an air pulse applies force to the cornea. The cornea moves inwards, past applanation, and into a slight concavity. Milliseconds after applanation, the air pump shuts off and the pressure applied to the eye declines. As the air pulse pressure decreases, the cornea returns to its normal shape while passing through the applanated state. Two independent pressure values are derived from the inward and outward applanation events. Due to the dynamic nature of the air pulse, viscous damping in the cornea causes delays, resulting in two different pressure values. The average of these two pressure values provides a repeatable, Goldmann-correlated IOP value. The difference between these two pressure values is CH. Corneal compensated IOP, derived from the CH measure, has been suggested as a superior measurement of IOP compared to the Goldmann tonometer measurement.

Applanation: the flattening of the cornea by external pressure

Tonometer: an instrument that measures the pressure inside the eye

The following codes for treatments and procedures applicable to this document are included below for informational purposes.  Inclusion or exclusion of a procedure, diagnosis or device code(s) does not constitute or imply member coverage or provider reimbursement policy.  Please refer to the member's contract benefits in effect at the time of service to determine coverage or non-coverage of these services as it applies to an individual member.

When services are Investigational and Not Medically Necessary:

Peer Reviewed Publications:

1. Congdon NG, Broman AT, Bandeen-Roche K, et al.  Central corneal thickness and corneal hysteresis associated with glaucoma damage. Am J Ophthalmol. 2006; 141(5):868-875.
2. Kotecha A, Elsheikh A, Roberts CR, et al. Corneal thickness- and age-related biomechanical properties of the cornea measured with the ocular response analyzer. Invest Ophthalmol Vis Sci. 2006; 7(12):5337-5347.
3. Kotecha A. What biomechanical properties of the cornea are relevant for the clinician? Surv Ophthalmol. 2007; 52 Suppl 2:S109-114.
4. Lam A, Chen D, Chiu R, Chui WS. Comparison of IOP measurements between ORA and GAT in normal Chinese. Optom Vis Sci. 2007; 84(9):909-914.
5. Martinez-de-la-Casa JM, Garcia-Feijoo J, et al. Ocular response analyzer versus Goldmann applanation tonometry for intraocular pressure measurements.  Invest Ophthalmol Vis Sci. 2006; 47(10):4410-4414.
6. Medeiros FA, Weinreb RN. Evaluation of the influence of corneal biomechanical properties on intraocular pressure measurements using the ocular response analyzer.  J Glaucoma. 2006; 15(5):364-370.
7. Ortiz D, Piñero D, Shabayek MH, et al. Corneal biomechanical properties in normal, post-laser in situ keratomileusis, and keratoconic eyes. J Cataract Refract Surg. 2007; 33(8):1371-1375
8. Shah S, Laiquzzaman M. Comparison of corneal biomechanics in pre and post-refractive surgery and keratoconic eyes by Ocular Response Analyser. Cont Lens Anterior Eye. 2009a; 32(3):129-132.
9. Shah S, Laiquzzaman M, Bhojwani R, et al. Assessment of the biomechanical properties of the cornea with the ocular response analyzer in normal and keratoconic eyes. Invest Ophthalmol Vis Sci. 2007; 48(7):3026-3031.
10. Shah S, Laiquzzaman M, Cunliffe I, Mantry S. The use of the Reichert ocular response analyser to establish the relationship between ocular hysteresis, corneal resistance factor and central corneal thickness in normal eyes. Cont Lens Anterior Eye. 2006; 29(5):257-262.
11. Shah S, Laiquzzaman M, Yeung I, Pan X, Roberts C. The use of the Ocular Response Analyser to determine corneal hysteresis in eyes before and after excimer laser refractive surgery. Cont Lens Anterior Eye. 2009b; 32(3):123-128.
12. Wells AP, Garway-Heath DF, Poostchi A. Corneal hysteresis but not corneal thickness correlates with optic nerve surface compliance in glaucoma patients. Invest Ophthalmol Vis Sci. 2008;49(8):3262-3268.

Government Agency, Medical Society, and Other Authoritative Publications: 

1. American Academy of Ophthalmology (AAO). Glaucoma panel: primary open angle glaucoma suspect. Preferred Practice Pattern. Revised September 2005a. Available at: http://one.aao.org/CE/PracticeGuidelines/PPP.aspx?p=1. Accessed on March 16, 2010.
2. American Academy of Ophthalmology (AAO). Primary Open angle glaucoma. Preferred Practice Pattern. Revised September 2005b. Available at: http://one.aao.org/CE/PracticeGuidelines/PPP.aspx?p=1. Accessed on March16, 2010.
3. Centers for Medicare and Medicaid Services. Glaucoma Screening Overview. Modified December 14, 2005. Available at: http://www.cms.hhs.gov/GlaucomaScreening/01_Overview.asp. Accessed on March 16, 2010.
4. U.S. Food and Drug Administration 510(k) Premarket Notification Database. Reichert Inc. Non-Contact Tonometers, Ocular Response Analyzer. No. K032799. Rockville, MD: FDA. January 24, 2004. Available at: http://www.accessdata.fda.gov/cdrh_docs/pdf3/K032799.pdf. Accessed on March 16, 2010.
5. U.S. Food and Drug Administration 510(k) Premarket Notification Database. Reichert Inc. Ocular Response Analyzer. No. K081756. Rockville, MD: FDA. Available at: http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfPMN/pmn.cfm?ID=28199. Accessed on March 16, 2010.

1. National Institutes of Health, The National Eye Institute. Glaucoma- What you should know. Available at: http://www.nei.nih.gov/health/glaucoma/glaucoma_facts.asp. Last reviewed September 2009. Accessed on March 16, 2010.
2. The Glaucoma Foundation. Available at: http://www.glaucomafoundation.org/. Accessed on March 16, 2010.

Corneal Hysteresis
Ocular Response Analyzer

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