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Corneal hysteresis (CH) determination by air impulse stimulation for the diagnosis and management of glaucoma and corneal disorders is considered experimental, investigational and/or unproven.
Glaucoma is a group of diseases that damage the eye’s optic nerve and is a leading cause of blindness. Intraocular pressure measurements are used in the management of glaucoma, it is not the only criteria by which glaucoma is diagnosed however, intraocular pressure is a risk factor that can be modified. (1)
Accurate intraocular pressure measurements began in 1950 with the Goldman applanation tonometer and it is still the most used tonometer in the world. The principle on which applanation tonometers are founded is in a thin-walled sphere; the internal pressure can be closely approximated by the force in grams necessary to flatten (applanate) a specific area. Other physical properties of the thin-walled sphere can affect the force needed to flatten the wall such as elasticity (or its opposite, rigidity), thickness, and compressibility.(1)
A version of tonometry is a non-contact tonometry such as the Ocular Response Analyzer®. The Ocular Response Analyzer measures corneal hysteresis (CH) which is an indication of the biomechanical properties of the cornea. This device is essentially an air puff tonometer, in which a column of air keeps increasing in force until the cornea is indented. The force of the air column is decreased until applanation is again achieved. The difference in the pressures at the two applanation points is a measure of corneal elasticity (hysteresis). (1) Measurement of the biomechanical properties of the cornea allows the device to provide a corneal compensated intraocular pressure. One of the clinical applications proposed for the Ocular Response Analyzer is that the CH measurement provides clinicians a new method to assist in identifying those patients at risk of glaucoma progression.(2)
Through the U.S. Food and Drug Administration (FDA) 510 (k) process Ocular Response Analyzer -ORA (Reichert, Inc.) received clearance in January 2004 for the following indicated use:
To measure intra-ocular pressure of the eye and the biomechanical response of the corneal for the purpose of aiding in the diagnosis and monitoring of glaucoma.(3)
A search of peer reviewed literature through September 2014 was performed. The following is a summary of the key literature to date.
In a systematic review by Cook et al. the objective noted was to assess the agreement of the Goldmann applanation tonometer (GAT), the most commonly accepted reference device with tonometers available for clinical practice. Eight tonometers were represented: dynamic contour tonometer, noncontact tonometer (NCT), ocular response analyzer, Ocuton S, handheld applanation tonometer (HAT), rebound tonometer, transpalpebral tonometer, and Tono-Pen. The following results of 102 studies, including 130 paired comparisons were noted by the authors: The agreement (95% limits) seemed to vary across tonometers: 0.2 mmHg (-3.8 to 4.3 mmHg) for the NCT to 2.7 mmHg (-4.1 to 9.6 mmHg) for the Ocuton S. The estimated proportion within 2 mmHg of the GAT ranged from 33% (Ocuton S) to 66% and 59% (NCT and HAT, respectively). Substantial inter- and intraobserver variability were observed for all tonometers. Conclusions noted by the authors were NCT and HAT seem to achieve a measurement closest to the GAT. However, there was substantial variability in measurements both within and between studies. (4)
In a prospective cross-sectional study, Kaushik et al, evaluated corneal biomechanical properties across the glaucoma spectrum. The relationship between these measurements and intraocular pressure measured by Goldmann applanation tonometry (GAT-IOP) and central corneal thickness (CCT) were studied. Participants (71 normal, 101 glaucoma suspect [GS], 38 ocular hypertension [OHT], 59 primary angle-closure disease [PACD], 36 primary open-angle glaucoma [POAG], and 18 normal-tension glaucoma [NTG]) who had received no ophthalmic treatment were included in the study. The authors noted the following results: Corneal hysteresis (CH) measurements were significantly less in primary open-angle glaucoma (POAG) and normal-tension glaucoma (NTG) compared to normal subjects (P=.034 and P=.030 respectively), regardless of the intraocular pressure. The corneal resistance factor (CRF) was significantly less in NTG and maximum in POAG and ocular hypertension (OHT). Regression analysis with CH as dependant variable showed significant association with GAT-IOP and CRF (P < .001) but not CCT, persisting on multivariate analysis (adjusted R(2) = 0.483). GAT-IOP correlated strongly with Goldmann-correlated IOP on the ORA (IOPg) (r = 0.82; P < .001), but limits of agreement between the measurements were poor. Conclusions noted by the authors included: CH and CRF may constitute a pressure-independent risk factor for glaucoma. CRF appears to influence GAT-IOP measurements more than simple geometric thickness measured by CCT. However, IOP measurements from the Ocular Response Analyzer (ORA) are not interchangeable with, and are unlikely to replace, Goldmann applanation tonometry at the present time. (5)
Nessim et al. using multiple tonometry devices investigated the relationship between measured intraocular pressure (IOP) and central corneal thickness (CCT), corneal hysteresis (CH) and corneal resistance factor (CRF) in ocular hypertension (OHT) primary open-angle (POAG) and normal tension glaucoma (NTG) eyes. The following four devices were used in measuring the IOP: Goldmann applanation tonometry (GAT); Pascal dynamic contour tonometer (DCT); Reichert ocular response analyser (ORA); and Tono-Pen XL. The following results were reported by the authors: Compared to the GAT, the Tonopen and ORA Goldmann equivalent (IOPg) and corneal compensated (IOPcc) measured higher IOP readings (F=19.351, p<0.001), particularly in NTG (F=12.604, p<0.001). DCT was closest to Goldmann IOP and had the lowest variance. CCT was significantly different (F=8.305, p<0.001) between the 3 conditions as was CH (F=6.854, p=0.002) and CRF (F=19.653, p<0.001). IOPcc measures were not affected by CCT. The DCT was generally not affected by corneal biomechanical factors. The authors concluded that the study suggested that measurements from any tonometer should be interpreted with care, particularly when alterations in the corneal tissue are suspected, the true pressure of the eye cannot be determined non-invasively. (6)
Mubig et al. investigated how modern screening methods support the diagnosis of keratoconus.
A prospective study that included 93 eyes of 93 keratoconus patients and 107 eyes of 107 healthy subjects (control group) was conducted. Exclusion criteria for both groups included previous eye surgery, cross-linking therapy, glaucoma, uveitis or other inflammatory diseases of the eye. As well as all patients with a thyroid disorder were excluded from the control group. Devices used to examine participants’ eyes included; TMS-5 topographer, Pentacam and Ocular Response Analyzer (ORA). The authors note in their results that all parameters showed statistically highly significant differences between the keratoconus and control group (p ≤ 0.0001). The ORA indices Corneal Hysteresis (CH), Corneal Resistance Factor (CRF) and Keratoconus Match Index (KMI) showed slightly poorer performance with CH (8.22/11.48/0.909), CRF (7.25/11.20/0.951), and KMI (0.31/1.05/0.909). In this study, the authors concluded tomography and topography was more reliable in diagnosing keratoconus than evaluating the biomechanical properties of the cornea. Surface Asymmetry Index (SAI) and Keratoconus Severity Index (TMS) as well as Topographic keratoconus Classification (TKC) and Index of Surface Variance (Pentacam) showed improved recognition rates compared to the Keratoconus Match Index (ORA). However, individual parameters alone are not sufficient for the diagnosis of keratoconus.(7)
In another study concerning keratoconic eyes, the relationship of corneal biomechanical properties to refraction and corneal aberrometry were evaluated by Pinero et al. Three groups, noted as mild, moderate and severe, were differentiated according to the severity of keratoconus. Evaluations made included visual acuity, refraction, corneal topography, and corneal aberrations. Additionally corneal biomechanics were analyzed in relation to two parameters: corneal resistance factor (CRF) and corneal hysteresis (CH). Results noted by the authors included: CH and CRF in the severe keratoconus group were significantly lower than those in the other two groups (P < or = 0.01). A significant difference in CRF was found between mild and moderate cases (P = 0.04). A moderate correlation was found between the CRF and mean keratometry in the overall sample (r = -0.564). In addition, multiple regression analysis revealed that CRF correlated significantly with keratometry and the corneal spherical-like RMS (R(2) = 0.40, P < 0.01). The authors concluded the following CRF correlates with the magnitude of corneal spherical-like aberrations, especially in severe keratoconus and should be considered an additional factor in keratoconus grading, (9)
Intraocular pressure monitoring is used in the management of glaucoma patients. Accurate IOP measurements are necessary in that intraocular pressure is a risk factor that may be modified to help manage the progression of glaucoma. The measurement of corneal hysteresis has also been investigated in other corneal disorders. At this time there is insufficient evidence available from the peer reviewed literature to demonstrate that clinical management or that health outcomes are improved by the measurement of corneal hysteresis.
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1. Stamper, Robert L. MD. A History of Intraocular Pressure and Its Measurement. Optometry and Vision Science: January 2011Volume88, Issue 1. pp E16-28.
2. Clinical Applications of the Ocular Response Analyzer: Glaucoma (Product information). Available at: <www.reichert.com>. Accessed on 2014 October 14.
3. FDA – Summary Ocular Response Analyzer. Food and Drug Administration. Available at www.fda.gov. Accessed 2014 October 14.
4. Cook, J., Botello, A., et al. Systematic review of the agreement of tonometers with Goldmann applanation tonometry. Ophthalmology. 2012 Aug; 119(8): 1552-7. Epub 2012 May 10.
5. Kaushik S., Pandav, S., et al. Relationship between corneal biomechanical properties, central corneal thickness and intraocular pressure across the spectrum of glaucoma. Am J Ophthalmol. 2012 May: 153(5):840-849.e2.
6. Nessim M., Mollan S., et al. The relationship between measurement method and corneal structure on apparent intraocular pressure in glaucoma and ocular hypertension. Cont Lens Anterior Eye. 2013 Apr; 36(2):57-61.
7. Mubig, L., Zemova, E., et al. A comparison of Device-Based Diagnostic Methods for keratoconus. Klin Monbl Augenheilkd. 2014 Jul 15. [Epub ahead of print].
8. Facts about Glaucoma. NIH. National Eye Institute. Available at <www.nei.nih.gov> Accessed on 2014 October 15.
9. Pinero, D., Alio J., et al. Corneal biomechanics, refraction and corneal aberrometry in keratoconus: an integrated study. Invest Ophthalmol Vis Sci. 2010 Apr: 51(4): 1948-55.
|9/1/2016||Reviewed. No changes.|
|1/1/2015||New medical document. Corneal hysteresis (CH) determination by air impulse stimulation for the diagnosis and management of glaucoma and corneal disorders is considered experimental, investigational and/or unproven.|