A New Measurement Tool to Examine the Cornea
BY KRISTIN GLAVINE
There are many methods we can use to examine the cornea. With a slit lamp, for example, you can assess the health of the various layers of the cornea. You can use pachymetry to determine the the cornea's thickness. Recently, a new way to examine the cornea has emerged, along with an entirely new corneal property.
The Reichert Ocular Response Analyzer (ORA, Reichert Ophthalmic Instruments) is a new type of non-contact tonometer (NCT) that measures corneal properties and takes them into account when reading IOP.
According to a paper published by David Luce (2005) the ORA uses an air puff similar to traditional NCT machines. This instrument also uses the same light beam detection system. However, the ORA produces a bi-directional applanation of the cornea.
Specifically, after the air puff first flattens the cornea, the ORA takes a pressure measurement. Next, as the air puff continues, the cornea becomes indented slightly. At this point the air puff weakens and the cornea is allowed to return to its normal state. During this time, the cornea again becomes flat and the ORA takes a second pressure measurement.
The ORA produces four different measurements in a single procedure. The first is a traditional IOP calculated from the average of the first and second pressure readings.
The next measurement is called corneal hysteresis (CH). CH is independent of IOP in normal eyes and was determined to be a new measurable corneal property. CH measures the cornea's visco-elastic properties, calculated simply as the difference between the first and second pressure measurements.
The third measurement is IOPcc. This is a corneal-corrected IOP measurement and it takes CH into account. The last measurement is corneal resistance factor (CRF), a measure of the corneal resistance to the air puff. You can think of the CRF as a measure of the stiffness of the cornea.
How it Works
The Reichert ORA exam is quick and easy for both patient and practitioner. The ORA automatically aligns with the patient's eye and delivers a quiet puff of air which may minimize patient anxiety. The test results appear on the computer screen in numerical and graphical form. The graph consists of a line plot called a corneal signature. Practitioners can see the two applanation pressures, as well as the measurement of CH on the graph.
The Reichert ORA produces a more accurate measurement of IOP by accounting for corneal resistance. This is important for glaucoma management. However, practitioners can also use the instrument to examine the corneal response to pressure.
Research by Luce (2005) has shown that patients who have keratoconus and Fuch's dystrophy have a decreased CH compared to patients who have normal corneas. Patients also had decreased CH after LASIK. Luce found that the corneal signatures were different for these patient groups.
Practitioners also may find the ORA useful in screening candidates for refractive surgery.
Overall, the Reichert ORA seems to be a valuable tool in several different areas of eye care. As we understand more about the process, it's possible that the ORA will continue to be used for new applications in optometric and ophthalmic practice.
For references, please visit www.clspectrum.com/references.asp and click on document #131.
Ms. Glavine is a second year OD/MS student at the New England College of Optometry. She is currently working with Marjorie Rah, OD, PhD, in contact lens research.