New Measurement Tool to Examine the Cornea
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
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
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
For references, please visit
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.
Contact Lens Spectrum, Issue: October 2006