Debarati Ghosh, Bachelor of Optometry
Optometrist, Dr. Shroff’s Charity Eye Hospital, New Delhi.
Ocular Response Analyzer testing set up.
Glaucoma is one of the leading causes of visual impairment and blindness worldwide. (1, 2) The elevated Intra-Ocular Pressure (IOP) is the only known modifiable risk factor. (3, 4) Certain conditions like normal tensive glaucoma has raised questions about factors other than IOP in the pathophysiology of glaucoma. Goldmann Applanation Tonometry (GAT) is regarded as the reference standard to measure IOP. GAT is known to be influenced by the corneal properties, such as corneal curvature and Central Corneal Thickness (CCT). (5)
Reichert’s Ocular Response Analyzer (ORA), a new, non-invasive device analyses corneal biomechanical properties simply and rapidly. (6) The principles of ORA are based on noncontact tonometry, in which the IOP is determined by the air pressure required to applanate the central cornea. This instrument measures Corneal Hysteresis (CH) and Corneal Resistance Factor (CRF), that describe the cornea’s viscoelastic properties. Research suggests that ORA helps to understand the relationship between corneal biomechanics and the pathophysiology of glaucoma, (7) and in some cases, to predict the risk of ectasia after corneal refractive surgery. (8) Studies also report that CH and CRF are lower in Keratoconus than in normal and post-LASIK corneas.(9)
INDICATIONS OF ORA
- Pre refractive surgery
- Post corneal transplantation
- Fuch’s corneal dystrophy
OPERATING THE ORA
ORA consists of four key components (Fig.1). An air pump, an infrared light emitter, a light intensity detector and a pressure transducer. As we start the test sequence, the infrared light emitter directs a beam of light onto the cornea .The air pump delivers a stream of air directly onto the eye.
Figure 1: Illustration of ORA’s sensor; Consists of an infrared light emitter, a light detector, and a pneumatic tonometer
As the cornea moves inwards, the reflected light beam begins to converge directing an increasing amount of light onto the detector. When applanation is achieved the detector registers a signal peak, causing the pressure transducer to record the applanation pressure. This event is recorded as peak (P1) on the ORA’s signal plot. As the air pulse continues to direct air upon the cornea causing it to deflect into a state of slight concavity. The light realigns with the infrared detector after the air pulse is discontinued and the cornea passes through a second applanation event (recorded as P2) (Fig 2). At the end of the test, the cornea returns to its convex shape.
Figure 2: Illustration of the measurement of biomechanical properties of the cornea.
ORA provides four measurements, two IOP values and two corneal biomechanical values. The first IOP value is an estimate of Goldmann IOP (IOPG) and corresponds to the IOP value at the first applanation point in the ORA waveform (P1). The second wave value, IOPCC, is an estimate of IOP corrected for the biomechanical properties of the cornea. The two biomechanical values are CH and CRF. CH is a quantification of the cornea’s ability to absorb energy. It is calculated as the difference between in pressure of the two corneal applanation events (P1 and P2) in the waveform. Corneal viscoelasticity (CV) and CRF appear to decrease minimally with increasing age in healthy adults.(10)
The CH measurement gives clinicians a new tool to identify risk of glaucoma progression and to determine the treatment course. CH and CRF were found to be significantly reduced in the eyes of Exfoliative Glaucoma (EXG) patients, which might result in decreased support of peripapillary scleral structure and increased damage to the optic nerve during IOP increase.(11) As more research are going on to explore the ORA measurements which in turn will improve the understanding of ocular biomechanics. The additional information on corneal properties might help to assess patient’s susceptibility to progressive optic nerve damage and visual field loss in presence of elevated IOP.
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