Corneal hysteresis measurement useful in refractive surgery, glaucoma

Issue: September 2007

ByMarc R. Bloomenstein, OD, FAAO

Marc R. Bloomenstein, OD, FAAO
Marc R. Bloomenstein

For years we have been talking about the ramifications of corneal biomechanics as well as the alteration of the central cornea on measuring intraocular pressure. It is well known that Goldmann tonometry is based on the Imbert-Fick law, which, in turn, does not take into account corneal changes. Corneal refractive surgery can have grave consequences on the accuracy of an IOP measurement. Moreover, assessing the cornea for healthy, viable refractive surgery candidacy is a challenge that still haunts physicians.

Fortunately, the diagnosis of glaucoma is not solely based on IOP measurement, and topographical analysis can provide details about consistency. A new corneal value, hysteresis, may play an important role in both refractive and glaucoma evaluations.

Measuring corneal hysteresis

Hysteresis is defined as the lag between making a change, such as increasing or decreasing power, and the response or effect of that change. Reichert Ophthalmic Instruments (Depew, N.Y.) has developed the Ocular Response Analyzer (ORA) to measure the biomechanics of the cornea and specifically the corneal hysteresis.

The Reichert ORA employs a dynamic bidirectional applanation process to measure IOP and the cornea’s biomechanical properties. According to Luce, ORA uses a rapid air impulse and an advanced electro-optical system to record two applanation pressure measurements: one while the cornea is moving inward and the other as the cornea returns. Because of its complex viscoelastic structure, the cornea resists the dynamic force of the air pulse, causing a delay in the inward and outward applanation events, resulting in two different pressure values. The average of these two pressure values provides a repeatable, Goldmann-correlated IOP measurement. The difference between these two pressure values is corneal hysteresis.

The corneal hysteresis measurement indicates the viscous damping in the cornea or the ability of the tissue to absorb and dissipate energy. The repeatability of the corneal hysteresis provides practitioners an opportunity to quantify the biomechanical nature of the cornea before refractive surgery and thus detect any anomalies. This measurement and average IOP can also lead to earlier diagnosis of glaucoma.

An ARVO poster by Luce describes two additional parameters provided by the ORA: corneal resistance factor (CRF), another new measurement of the cornea’s biomechanical properties, and corneal compensated IOP, a measure of IOP that is less influenced by corneal properties than today’s tonometry, including Goldmann.

According to Medeiros, corneal compensated IOP is unaffected by central corneal thickness (CCT) and corneal biomechanical properties. In addition, Kirwan has reported that corneal compensated IOP remains essentially unchanged after LASIK. Alternatively, CRF appears to indicate overall corneal stiffness and is affected by IOP.

Cornea’s effect on glaucoma

The Ocular Hypertension Treatment Study (OHTS) and other studies have brought to light the importance of corneal parameters in diagnosing and managing glaucoma. These studies have shown that a thin cornea is an independent risk factor for the development and progression of the disease.

Research has led us to believe that corneal parameters other than CCT may provide clues that will aid in the diagnosis and management of glaucoma. Evidence suggests that the cornea may reflect the condition of the lamina cribrosa. Congdon has reported that low corneal hysteresis is an independent indicator of glaucomatous damage and progression. Moreover, Shimmyo has observed lower-than-average corneal hysteresis in subjects who have been identified as normal tension glaucoma patients.

Corneal evaluation for refractive surgery

These new metrics may provide the elusive information needed to determine who will develop ectasia, similar in appearance to keratoconus, prior to LASIK. Ectasia is arguably the worst possible post-LASIK complication.

Currently, we evaluate central corneal thickness and topographical appearances to rule out any patient who may potentially become ectatic. However, even when these factors are well within normal limits, patients have still developed the characteristic inferior steeping and decrease in vision associated with ectasia.

Shimmyo and colleagues compared corneal hysteresis and viscoelastic response to CRF in normal eyes, eyes with keratoconus and pre- and post-LASIK eyes (206 normal, 60 keratoconus, 100 pre- and post-LASIK). Their results indicate that corneal hysteresis and CRF were significantly reduced in both keratoconic and post-LASIK eyes.

Pepose and colleagues evaluated a number of IOP measurement methodologies: Goldmann applanation tonometry, the ORA and the Pascal Dynamic Contour Tonometer (Ziemer USA, Wood River, Ill.). The authors found that LASIK produced a significant decline in CH and CRF.

Predicting ectasia

According to Shimmyo, these data suggest that corneal hysteresis can help practitioners identify eyes at risk of developing ectasia preoperatively because they can clearly distinguish between normal and pathologic corneas. Considering the differences in corneal hysteresis and CRF found when comparing normal and compromised corneas, it is possible that these measurements may better characterize the cornea’s biomechanical state than CCT.

The cornea is a powerful optical instrument that can provide the clinician with necessary information to help make informed decisions and diagnose disease early. We appear to be on the threshold of harvesting this information and thus preventing unnecessary postsurgical complications. Although still early, corneal hysteresis and the other new measurements obtained from the ORA may help bridge that gap.

For more information:

Marc R. Bloomenstein, OD, FAAO, is a member of the Editorial Board of Primary Care Optometry News and director of optometric services at Schwartz Laser Eye Center, 8416 E. Shea Blvd., Ste., C-101, Scottsdale, AZ 85260; (480) 483-3937; e-mail: drbloomenstein@schwartzlaser.com. Dr. Bloomenstein has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.

The Ocular Response Analyzer is available from Reichert Ophthalmic Instruments, 3374 Walden Ave., Depew, NJ 14043; (716) 686-4500; fax: (716) 686-4545; Web site: www.reichertoi.com.

References:

Alfonso R. The value of the Ocular Response Analyzer in the assessment of ectasia risk in LASIK patients. Medcompare. Available at http://www.medcompare.com/spotlight.asp?spotlightid=227. Accessed July 27, 2007.

Congdon NG, Broman AT, Bandeen-Roche K, Grover D, Quigley HA. Central corneal thickness and corneal hysteresis associated with glaucoma damage. Am J Ophthalmol. 2006;141:868-875.

Kirwan C, O’Keefe M. IOP measurement using an Ocular Response Analyzer after keratorefractive surgery. Paper presented at: American Society for Cataract and Refractive Surgery; April 27-May 2, 2007; San Diego, CA.

Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg. 2005;31:156-162.

Luce DA. Methodology for cornea compensated IOP and corneal resistance factor for the Reichert Ocular Response Analyzer. Poster presented at: Association for Research in Vision and Ophthalmology; April 30-May 4, 2006; Fort Lauderdale, FL.

Medeiros FA, Weinreb RN. Evaluation of the influence of corneal biomechanical properties on intraocular pressure measurements using the ocular response analyzer. J Glaucoma. 2006;15:364-370.

Pepose JS, Feigenbaum SK, Qazi MA, Sanderson JP, Roberts CJ. Changes in corneal biomechanics and intraocular pressure following LASIK using static, dynamic and noncontact tonometry. Am J Ophthalmol. 2007 Jan;143(1):39-47.

Shimmyo M, Hayashi NI, Orloff PN. Intraocular pressure compensated for corneal effects, corneal hysteresis and corneal resistance factor in normal, open-angle and normotensive glaucoma eyes M. Presented at the American Society of Cataract and Refractive Surgery; April 28, 2007; San Diego.