The Ocular Response Analyzer is the only device used to measure corneal hysteresis
Dr Jessica Steen
Nova Southeastern University College of Optometry, Florida USA
Glaucoma risk assessment and identification of patients who may benefit from intraocular pressure (IOP) lowering therapy is a complex process.
Well-established risk factors for glaucoma include IOP, age, central corneal thickness (CCT), family history and ethnicity, which help guide our treatment plan and evaluate general risk assessment of the development and progression of glaucoma.
Corneal hysteresis (CH), a corneal biomechanical property easily measured non-invasively in-office, is a relatively new parameter that may provide additional information to aid in clinical decision-making.
Hysteresis is an inherent biomechanical property of the cornea, which measures the cornea’s ability to dampen a force when applied. Although CCT plays a role, CH may be a better indicator of how the cornea, as well as other ocular tissues, possibly including the lamina cribrosa, respond to short- and long-term pressure fluctuations.1 CH is a measure of tissue function, rather than just a structural parameter like CCT.
The Ocular Response Analyzer (ORA-Reichert Ophthalmic Instruments, Buffalo, NY, USA) is the only device used to measure CH. An air impulse applies a force to the cornea in a similar fashion to a non-contact tonometer. Corneal applanation is measured at two moments in time: at an ‘inward’ bending and an ‘outward’ bending point over a total period of 20 milliseconds. The difference between the two endpoints, the inward and outward applanation pressure, reflects corneal hysteresis, measured in mmHg. The ORA also provides an estimation of objective Goldmann IOP (IOPg) as well as cornea compensated IOP (IOPcc) which incorporates CH into an adjusted IOP value.2
Clinical correlation of CH
Corneal hysteresis is related to cellular and structural properties of the cornea. Independent factors that affect CH include age, CCT, IOP, glaucoma diagnosis and glaucoma severity.3 In general, lower CCT values correspond to lower CH.4 With increased age and IOP, CH is also lower.4 Additionally, as glaucoma severity increases, corneal hysteresis decreases.3 Corneal hysteresis is stable throughout the day and is unrelated to corneal radius or spherical equivalent.5 African Americans have the lowest CH (and CCT) compared to Hispanics and Caucasians.6 Data show that women have a higher CH than men but exhibit no difference between IOPg or IOPcc.7
CH has been identified as an independent risk factor in glaucoma progression.8 It is thought that eyes with a higher CH, or a greater ability to dampen IOP fluctuations, may be less susceptible to the development of glaucoma. In contradistinction, eyes with a low CH, or less ability to dampen such fluctuations in IOP, may increase the risk of glaucomatous optic neuropathy.3 It has been established that eyes with primary open angle glaucoma, exfoliative glaucoma as well as glaucoma with statistically normal pressure have lower CH than eyes of ocular hypertensives and normal people. These eyes with lower CH seem to progress more quickly than those with higher CH.3,8,9
Eyes with low corneal hysteresis at initiation of treatment with topical prostaglandin analogue showed a greater reduction in IOP than those patients with higher baseline CH.10 Data show that corneal hysteresis may increase following the initiation of topical therapy.11
Currently, CH may be somewhat quantifiable as a potential risk factor for the progression of suspects progressing to glaucoma as well as the progression in previously diagnosed glaucoma patients. A normal range of CH has not been well established but seems to range between 8 and 14 mmHg.6,9,10
Questions remain regarding the true importance and clinical implications of CH in practice. We know that as IOP is lowered, CH increases. However, it is unknown whether this is a mechanical result, an indicator of possible recovery or true slowing of progression of the glaucomatous disease process. Additionally, due to the lack of longitudinal studies at this period of time, uncertainty remains whether CH is variable over a patient’s lifetime in the case of individuals with established glaucoma, which clinically correlates to defining how frequently the test should be administered.
Clinical applications of CH
The combination of CH and CCT in evaluating risk assessment of glaucoma appears to provide more information than using either factor in solitude. As such, CH is not likely to replace CCT as a standard of care in glaucoma risk assessment but will continue to become an adjunct to CCT in glaucoma risk assessment, identifying those patients who may progress more quickly as well as providing an explanation for why some patients have greater IOP lowering with topical prostaglandin therapy than others. Measurement of corneal hysteresis including the adjusted IOP measurement provided by the ORA may be of additional importance in patients following keratorefractive surgery or with corneal pathology.9
Understanding CH and the effect that it may have on an individual’s variability in disease or disease progression and incorporation of this data into glaucoma risk assessment may allow for earlier diagnosis of glaucoma as well as more appropriate long-term management of our patients with glaucoma.
Figure 1. Corneal hysteresis (CH) is the difference in the inward and outward pressure values obtained during the dynamic bi-directional applanation process employed by the Ocular Response Analyzer, as a result of viscous damping in the cornea
Figure 2. The Ocular Response Analyzer uses a bi-directional applanation process to measure biomechanical properties of the cornea and the intraocular pressure of the eye
1. Wells AP, Garway-Heath DF, Poostchi A et al. Corneal hysteresis but not corneal thickness correlates with optic nerve surface compliance in glaucoma patients. Invest Ophthalmol Vis Sci 2008; 49: 8 :3262-3268.
2. Luce D. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg 2005; 31: 156-162.
3. Pensyl D, Sullivan-Mee M, Terros-Monte M, Halverson K, Qualls C. Combining corneal hysteresis with central corneal thickness and intraocular pressure for glaucoma risk assessment. Eye 2012; 26, 1349-1356.
4. Congdon NG, Broman AT, Bandeen-Roche K, Grover D, Quigley HA. Central corneal thickness and corneal hysteresis is associated with glaucoma damage. Am J Ophthalmol 2006; 141: 5: 868-875.
5. Fontes BM, Ambrosio R Jr, Alonso RS, Jardim D, Velarde GC, Nose W. Corneal biomechanical metrics in eyes with refraction of -19.00 to +9.00 D in healthy Brazilian patients. J Refract Surg 2008; 24: 9: 941-945.
6. Haseltine SJ, Pae J, Ehrlich JR, Shammas M, Radcliffe NM. Variation in corneal hysteresis and central corneal thickness among black, Hispanic and white subjects. Acta Ophthalmol 2012; 90: 8: 626-631.
7. Allam RS, Khalil NM. Evaluation of sex differences in corneal hysteresis. Eur J Ophthalmol 2015; 25: 5: 391-395.
8. 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.
9. Sullivan-Mee M, Billingsley SC, Patel AD et al. Ocular response analyzer in subjects with and without glaucoma. Optom Vis Sci 2008; 85: 463-470.
10. Agarwal DR, Erlich JR, Shimmyo M, Radcliffe NM. The relationship between corneal hysteresis and the magnitude of intraocular pressure reduction with topical prostaglandin therapy. Br J Ophthalmol 2012; 96: 254-257.
11. Tsikripis P, Papaconstantinou D, Koutsandrea C et al. The effect of prostaglandin analogs on the biomechanical properties and central thickness of the cornea of patients with open-angle glaucoma: a 3 year study on 108 eyes. Drug Des Devel Ther 2013; 7: 1149-1156.
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