spotlight on technology & technique
The Ocular Response Analyzer
This instrument offers a new technique in tonometry measurement.
By Leslie Goldberg, Associate Editor

Progress often involves fine-tuning an existing technology. If a new approach is effective and offers advantages over the previous technology, it may eventually take its place next to, or even surpass, the original. This may be the case with the new Ocular Response Analyzer from Reichert (Depew, NY). The Ocular Response Analyzer utilizes a patented applanation process to provide a new measurement called corneal hysteresis. Hysteresis is an indicator of the viscoelastic properties of the cornea and can be used, according to Reichert, to enable a more accurate tonometry measurement. Clinical research has shown that this measurement may be a valuable tool for identifying and classifying conditions such as corneal ectasia and Fuch's Dystrophy.

How the Ocular Response Analyzer Works

The Ocular Response Analyzer uses a rapid air impulse to apply force to the cornea, and then an advanced electro-optical system monitors the deformation. In one simple, fast measurement, the instrument records two applanation events. The precisely metered collimated-air-pulse causes the cornea to move inwards, past applanation and into a slight concavity. Milliseconds after applanation, the air pump shuts off and the pressure declines in a smooth fashion. As the pressure decreases, the cornea begins to return to its normal configuration. In the process, it once again passes through an applanated state. The applanation detection system monitors the cornea throughout the entire process, which takes only milliseconds. Two independent pressure values are derived from the inward and outward applanation events.

It might be expected that these two pressure values would be the same. However, due to its viscoelastic material characteristics, the cornea resists the dynamic force of the air pulse, causing a delay in the inward and outward applanation events, resulting in two different pressure values. The average of these two pressure values provides a highly accurate, repeatable, Goldmann-correlated IOP measurement. The difference between these two pressure values is corneal hysteresis. (See Figure, page 94). Reichert says that the ability to measure this effect is the key to understanding the biomechanical properties of the cornea.

Understanding Hysteresis

Unlike conventional tonometers, the Ocular Response Analyzer makes a dynamic measurement which includes two applanation events, enabling the device to gain information about the cornea's response to the air pressure pulse. The corneal hysteresis phenomenon is a result of viscoelastic dampening in the cornea. In other words, the tissue's ability to absorb and dissipate energy. Studies have shown that subjects whose corneas exhibit low corneal hysteresis, which can be thought of as having a "soft" cornea, are probable candidates for a variety of ocular diseases and complications.

Benefits of Hysteresis Measurement

► It has been shown that the elastic and viscoelastic properties of the cornea are related, making possible the use of the hysteresis measurement to arrive at a more accurate measurement of IOP.

► Reichert researchers believe that because the corneal hysteresis measurement appears to present a complete characterization of the cornea's biomechanical state, it has potential uses in screening refractive surgery candidates and predicting/controlling outcomes.

► In addition this new metric appears to be useful in the diagnosis and management of glaucoma.

► The hysteresis measurement enables the calculation of a new pressure measurement called IOPCC (corneal compensated). This measurement is less influenced by corneal properties such as central corneal thickness (CCT) and does not appear to drop artificially post-LASIK.

Corneal Pathologies and Corneal Hysteresis: Identifying and Classifying Various Conditions

Reichert research has found that because the Ocular Response Analyzer is capable of assessing the biomechanical properties of corneal tissue, for the first time it is possible to identify and categorize various corneal conditions by means of a measurable and repeatable metric. Comparing the corneal hysteresis measurements of eyes with known corneal conditions to normal subject's measurements reveals significant differences. A comparison of the corneal hysteresis values of three unique populations is shown in Table 1.

Figure. The difference between the "inward" applanation and the "outward" applanation is called corneal hysteresis.

Corneal Hysteresis and LASIK

The potential clinical applications of the corneal hysteresis measurement in the area of refractive surgery are evident. Currently, CCT is the primary factor used for screening candidates for refractive surgery. Patients with thinner corneas are considered to be at higher risk for developing post-LASIK corneal ectasia. This complication is a concern for both doctors and patients. Due to the large and easily identifiable differences in hysteresis between normal and compromised corneas, Reichert believes that this metric provides a more complete characterization of the biomechanical state of the cornea than does the measure of CCT. This observation, coupled with the fact that corneal hysteresis is only weakly correlated with CCT, leads Reichert researchers to believe that the corneal hysteresis measurement will be a useful tool for eliminating LASIK candidates who are at risk of developing post-LASIK ectasia. Studies investigating this subject are currently ongoing.

The corneal hysteresis measurement also has potential uses in post-LASIK follow up. Clinical trials have shown significant post-LASIK changes in corneal hysteresis. While these results are preliminary and not yet fully understood, it appears that the reduction in post-LASIK hysteresis is universal. In a population of 26 LASIK patients who had bilateral LASIK, the average pre-LASIK corneal hysteresis was 10.54 mm Hg and average post-LASIK hysteresis was 8.18 mm Hg.

Clinical investigators are hypothesizing that the reduction in corneal hysteresis is not primarily a function of reduction in corneal tissue, but rather a result of a weakening of the structure due to the flap. The hysteresis measurement enables ophthalmologists to quantify this biomechanical material change, which may provide a more complete understanding of lower post-LASIK measured IOP.

Corneal Hysteresis and Glaucoma

Recently, the Ocular Hypertension Treatment Study (OHTS) and other studies on the subject have brought to light the importance of CCT in diagnosing and managing glaucoma. These studies have suggested that low CCT may be an independent risk factor for the development and progression of the disease.

Evidence suggests that the cornea may reflect the condition of the lamina cribrosa at the back of the eye. Clinical studies utilizing the Ocular Response Analyzer support this hypothesis.

Compared to normal subjects, glaucomatous subjects have a significantly lower average corneal hysteresis and a much wider range. A comparison of the hysteresis values for normal and glaucomatous populations is shown in Table 2. Perhaps the most interesting observation is that lower-than-average corneal hysteresis is also observed in patients who have been labeled normal-tension glaucoma subjects (Table 3).

In addition, the fact that the signal obtained from the eye of an normal-tension glaucoma subject looks similar to the signals obtained from keratoconus, Fuch's, and post-LASIK patients, reinforcing the theory that glaucomatous damage, in some manner, presents itself via the cornea.

Comments from the Field

Cynthia Roberts, Ph.D., associate director at Ohio State's Biomedical Engineering Center, says that the Ocular Response Analyzer is the only existing device that allows users to measure corneal biochemical properties in vivo. "The Ocular Response Analyzer is an easy-to-use non-contact tonometer and its ability to measure corneal hysteresis may prove to be a very effective device in evaluating risk for glaucoma progression," says Dr. Roberts.

"The Ocular Response Analyzer has some useful advantages over a standard Goldman tonometer," says Jay Pepose, M.D., Ph.D., and director at the Pepose Vision Institute. "The instrument has two formulas for IOP. One gives an IOP readout that closely correlates with the Goldman IOP and the other attempts to compensate for corneal characteristics. This second IOP measurement, for example, does not show as significant a drop after LASIK surgery." In addition, says Dr. Pepose, "the device takes advantage of the viscoelastic qualities of the cornea, which allows a readout of corneal hysteresis."

"The magnitude and shape of the hysteresis curve gives important information about corneal biomechanics, and may help identify patients with ectasia, keratoconus, pellucid marginal degeneration, and other corneal conditions. This signature of corneal biomechanics is a unique feature and the strength of the device," says Dr. Pepose.

James Brandt, M.D., director of Glaucoma Services at UC Davis, says that the Ocular Response Analyzer is an advancement in tonometry, "whereas the Goldmann tonometer does not account for differences in corneal thickness and elasticity, the Ocular Response Analyzer provides a better understanding of the biomechanics of the cornea. The Ocular Response Analyzer helps doctors to make better sense of patients whose standard tonometry readings were misleading, such as patient's who have had LASIK surgery."

Dr. Brandt considers the more accurate understanding of "true" pressure provided by the Ocular Response Analyzer to be a significant advancement and a useful research tool.

Appearing Soon

The Ocular Response Analyzer will be launched in Europe at the September European Society of Cataract and Refractive Surgeons show in Lisbon, Portugal and in the United States at the American Academy of Ophthalmology in Chicago this October.

LASIK data courtesy of David Castellano, M.D. and Jay Pepose, M.D. Glaucoma data courtesy of Misugu Shimmyo, M.D. Keratoconus data courtesy of Sunil Shah, M.D. Normal data courtesy of Clifford Scott, O.D.

Table 1. Hysteresis of normal, keratoconic and Fuch's subjects.

Table 2. Hysteresis of normal and glaucomatous subjects.

Table 3. Hysteresis and IOP of 24 normal-tension glaucoma eyes.