ocular response analyser disadvantages
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Abstract
Until recently, corneal biomechanical properties could not be measured in vivo. The ocular response analyzer is a new, noninvasive device that analyses corneal biomechanical properties simply and rapidly. The ORA allows cornea compensated IOP measurements and can estimate corneal hysteresis (CH) and corneal resistance factor (CRF). It is designed to improve the accuracy of IOP measurement by using corneal biomechanical data to calculate a biomechanically adjusted estimate of intraocular pressure. This review critically evaluates the technology and its implications in current day glaucoma management.
Keywords: Hysteresis, Ocular response analyser, Corneal biomechanics, Tonometry.
INTRODUCTION
Glaucoma is one of the leading causes of visual impairment and blindness worldwide.1-3 Lowering intraocular pressure (IOP) is the only proven means to slow or halt disease progression, as shown by studies of those at high risk of developing glaucoma [ocular hypertension treatment study (OHTS)],4 those with early to moderate glaucoma [collaborative initial glaucoma treatment study5 and early manifest glaucoma trial (EMGT)]6 and those with more advanced glaucoma [collaborative initial normal-tension glaucoma study7 and advanced glaucoma intervention study (AGIS)].8 Across all randomized, controlled trials, lowering IOP resulted in reduction in rates of worsening of glaucoma over 5 years.8,9 These studies confirm that one of the pathophysiological basis for glaucoma is elevated IOP.
Goldmann applanation tonometry (GAT) is regarded as the reference standard by which to measure IOP. GAT is known to be influenced by factors related to the corneal properties, such as corneal curvature and central corneal thickness (CCT).10,11 Although GAT may be less prone to biomechanical influence than Schiotz tonometry, it is clearly affected by corneal biomechanical influences, such as hydration, elasticity, hysteresis and rigidity. Achieving accurate estimates of intraocular pressure remains difficult. Even though increased IOP is the only proven means to delay or halt the development of glaucoma or progression of established disease, there remains the paradox of normal tension glaucoma with so-called ‘normal’ IOP and ocular hypertension with raised IOP but no disease. This has raised questions about factors other than IOP in the pathophysiology of glaucoma.
Until recently, corneal biomechanical properties could not be measured in vivo. The ocular response analyzer (ORA; Reichert Ophthalmic Instruments, Inc., Buffalo, NY, USA) is a new, noninvasive device that analyses corneal biomechanical properties simply and rapidly.12,13 The ORA allows cornea compensated IOP measurements and can estimate corneal hysteresis (CH) and corneal resistance factor (CRF). It is designed to improve the accuracy of IOP measurement by using corneal biomechanical data to calculate a biomechanically adjusted estimate of intraocular pressure. The ORA generates two separate IOP output parameters: Goldmann-correlated IOP (IOPg) and the corneal-compensated IOP (IOPcc).
ORA
Reichert has produced an instrument, the ocular response analyzer, which measures the corneal response to indentation by a rapid air pulse. The principles of the ORA are based on those of noncontact tonometry, in which the IOP is determined by the air pressure required to applanate the central cornea. A fully automated alignment system positions an air tube to a precise position relative to the apex of the cornea. Once aligned, a 25 millisecond air pulse applies pressure to the cornea. The air pulse causes the cornea to move inward, past applanation and into a slight concavity before returning to normal curvature. Corneal deformation is recorded via an electro-optical infrared (IR) detection system (similar to the classical air-puff tonometers).
The ORA acquires corneal biomechanical data by quantifying this differential inward and outward corneal response to an air pulse over a time span of approximately 20 milliseconds. Once the air pulse induces the desired indentation/applanation, it symmetrically reverses, which allows the cornea to resume its original shape. Because, a time lag is necessary to activate the reversal of the air pulse, the cornea actually indents mildly beyond the intended applanation point. This action permits the detection of a second applanation point, as the cornea returns from its overapplanated state. Using the first applanation pressure point (P1) and the second applanation pressure point (P2), the ORA generates two separate IOP output parameters
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