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Post Info TOPIC: ocular Response Analizer


Status: Offline
Posts: 437
Date: Mon May 22 10:36 PM, 2006
RE: ocular Response Analizer

i see. Besides measuring the cornea only properties, it looks like with that devise you can subtract the corneal part and get the true IOL, unaffected by corneal viscosity. Isn't that the idea?


Phase Two

Status: Offline
Posts: 482
Date: Tue May 23 6:47 PM, 2006

Hi Yarsky,

I was told last month that there is a machine which is coming on to the market which does record eye pressure that is not effected by other factors. It seems that this machine can do this.

Although if eye specialist's will rush out to get this machine is another matter... i think the really good specialst's will... as when i did express the problem with the current way they test for eye pressure for kc patients, the specialist i went to see said that they are going to resolve this problem by purchasing an eye pressure machine, which just come on the market, which can take readings independantly from other factors invoved with taking a correct reading, with things like the the softness or thickness of the cornea coming in to play for kc patients.

In general, i think every eye pressure test should be done this way even for people with out kc... as the thickness of the cornea is variable from person to person, with or with out kc, but of course with kc the thckness issues is more of an issue.

Hopfully this way of testing will be the way for most, sooner than later. 

-- Edited by QuintriX at 21:08, 2006-06-14


Phase Two

Status: Offline
Posts: 482
Date: Sun Jul 16 4:10 PM, 2006

Here is a good article on the issues discussed...

Measuring IOP: The Cornea Factor

Studies continue to reveal new connections between corneal characteristics and intraocular pressure.

Leon Herndon, MD, Durham, N.C.

The relevance of central corneal thickness to glaucoma diagnosis isn’t a new idea. It was first discussed 25 or 30 years ago, al­though it wasn’t widely covered in American journals until about 10 years ago. However, the nuances of the relationship between glaucoma and CCT continue to unfold, both in terms of IOP and possible connections between CCT and anatomical features such as nerve fiber layer thickness.

Here, I’d like to provide an update on what we’ve been learning and offer some suggestions for im­proving the accuracy of IOP measurements.


The Biomechanics Factor

The most eye-opening development in the past few years has been the revelation that corneal biomechanics—the physical characteristics of corneal tissue—may affect the accuracy of intraocular pressure applanation measurement as much as, or more than, CCT. Much of the key work in this area has been done by Cynthia Roberts, PhD, a biomedical engineer at Ohio State University.1

Goldmann applanation tonometry measures IOP by flattening the cornea, which is not neutral in this measurement. Dr. Roberts has shown that factors affecting corneal resistance include structural considerations, such as the amount of rigidity produced by the way the collagen beams in the tissue line up. The “bendability” of corneal tissue can also be affected by short-term factors such as the presence of cor­neal edema.

As a result of this new perspective, re­searchers are studying hysteresis, one measure of corneal biomechanical properties. Currently, the only instrument that can measure hysteresis is the Reichert Ocular Response Analyzer, which also measures corneal resistance, CCT and traditional (Goldmann) IOP, and provides a “compensated” IOP, less affected by corneal properties. (Our practice recently purchased a unit, and we’re hoping to successfully incorporate its measurements into our IOP calculations.) Another way of gauging the cornea’s biomechanical properties, currently being looked at by researchers, is Young’s Modulus. This is a measure of stiffness that helps to predict how a material under pressure will behave.

Of course, another approach to this problem is to find a way to measure IOP that is unaffected by the cornea’s thickness or biomechanics. The Pascal Dynamic Contour Tonometer (Zeimer Ophthalmics) is a digital tonometer that uses the principle of contour matching instead of applanation to measure IOP. A re­cent prospective, in vivo clinical study reported at the 2006 Association for Research in Vision and Ophthalmology meeting found that IOP measured by the DCT was within 1 mmHg of the intracameral pressure measured manometrically with a reference pressure sensor, over a wide range of pressures. (A. Weber et al; 2006 ARVO E-abstract 4444.)

The Pascal DCT could be the future gold standard for measuring IOP. However, even if this turns out to be the case, it will take a number of years for the DCT to replace Goldmann tonometry, for several reasons. First, we’re all ac­customed to using the Goldmann instrument. Second, the DCT costs more than $5,000 per unit, so making the switch requires a substantial investment, especially if you plan to place it in multiple lanes. Third, because the DCT readings may differ from the Goldmann readings, it will require switching to a new set of standards; IOP numbers will mean something different from what we’re accustomed to. So for now, clinicians are likely to continue using Goldmann and adjusting the numbers on the basis of CCT.


Making the Most of Goldmann

Until an alternative to Goldmann applanation tonometry becomes popular, here are a few strategies to help ensure that you get the best outcomes using this familiar technology.

Don’t rely too much on conversion tables. In the past few years, many MDs have come to rely on conversion tables that use an algorithm to adjust IOP measured with Goldmann, based on CCT. What we’ve learned about corneal biomechanics indicates that this approach isn’t necessarily reliable. It may produce an accurate IOP for many patients—but there’s no way to know which patients those are. One 580-µm cornea may be altering the IOP reading very differently than another does.

Make the patient blink right up until you measure CCT. If you anesthetize the eye, patients tend to lose their blink response, and it only takes 15 to 30 seconds for a cornea to begin drying out, causing it to become thinner.

The Dynamic Contour Tonometer from Zeimer Ophthalmics (left) uses contour matching instead of applanation to measure IOP, minimizing the impact of corneal characteristics on the measurement. Reichert’s Ocular Response Analyzer (right) measures the cornea’s biomechanical properties, which may make it possible to compensate for their effect on IOP measurement.

Measure CCT before performing gonioscopy. The contact lens you place on the eye to perform goni­oscopy will slightly alter the cornea. So do gonioscopy after you’ve taken your corneal measurements.

Think of CCT as falling into three categories. While corneal biomechanics may make it difficult to be certain of the accuracy of Goldmann measurements, simply noting that a cornea is especially thin, average or thick is useful information. If a cor­nea’s thickness is less than 500 µm we need to be concerned because we know from the Ocular Hypertension Treatment Study that a thinner cornea is more likely to convert to glaucoma over time. (This can be at least partly at­tributed to the fact that a thinner cornea requires less force to flatten, making Goldmann readings underestimate the true pressure.)

If a cornea is thicker than 600 µm, the patient is probably in less danger. Thicker corneas tend to produce artificially higher readings, and the literature suggests that these individuals have a better prognosis. I’ve had pa­tients with very thick corneas come to me with a diagnosis of ocular hypertension, taking two or three medications, but with no nerve damage or visual field loss. I tell them that their pressure is probably not as high as the tonometer indicates, and reduce their therapy. Of course, you still need to follow them expectantly, but you can be fairly sure that their true pressure is lower than what you’re reading off the machine.

A patient’s relative change in IOP is useful information. Despite the fact that corneal biomechanical properties in a given patient may shift Goldmann readings higher or lower than the actual pressure, we can still use these measurements to gauge our treatment and its effectiveness. For example, if a patient measures 24 mmHg using Goldmann, we probably need to strive for a significant pressure reduction on the order of 25 or 30 per­cent—based on the patient’s presentation of glaucomatous damage—and we can judge the effectiveness of the treatment in terms of the relative drop in pressure, whether or not the numbers are absolute.

One CCT measurement may not be good for life. Currently, most ophthalmologists only measure a patient’s CCT once. Aside from the possibility of short-term changes in thickness caused by hydration status or edema, we have data soon to be published in the British Journal of Oph­thalmology showing that corneas can thin over time. We took CCT measurements eight years apart, and found they had thinned 1 to 23 µm, a statistically significant change.

Of course, statistical significance and clinical significance aren’t necessarily the same; this level of change might not alter our recommendations. On the other hand, people can have glaucoma for decades, so it’s probably worth measuring CCT again when considerable time has passed.

We’ve also followed some patients who’ve had trabeculectomies and found that their corneas thin over time. (Notably, endothelial function didn’t change.) We haven’t published the data yet, but it seems possible that lowering the pressure significantly can lead to thinning of the cornea, and consequently falsely low IOP measurements.

You should certainly remeasure CCT if the patient has had any kind of surgical intervention such as refractive surgery, which may alter both the CCT and the corneal biomechanics.

Measure in the center. Studies have shown that there’s about a 3-mm diameter area in the center of the cornea where thickness is fairly uniform. When you measure IOP or cor­neal thickness you need to be centered over the pupil, holding the probe perpendicular, to get an accurate measurement.

CCT should be remeasured following refractive surgery. Here, a Visante anterior chamber OCT image (Zeiss) shows postop flap thickness and residual thickness of the stromal bed. Studies indicate that trabeculectomies, as well as the passage of time, may also affect corneal thickness.
Mark Packer, MD, FACS

View IOP in context with other measures. IOP measurement is im­portant, but we have a lot of other things to look at as well: the nerve; the visual field; OCT scans and other mo­dalities. Pressure is just one aspect of the patient’s overall situation. All the in­formation at our disposal should contribute to determining the best treatment option for the patient.


Headed in the Right Direction

Researchers are now investigating the possibility that a thick or thin cornea may tell us something about the back of the eye. It’s been noted that African Americans have thinner corneas than Caucasians, and often have more advanced glaucoma.2,3 So, a thinner cornea may indicate a thinner or more susceptible lamina cribrosa or optic nerve. Other research has found that ocular hypertensive patients with thinner corneas have thinner nerve fiber layers than ocular hypertensive patients with thicker corneas and healthy control subjects.4 There may be an anatomical correlation, but it’s not well-established yet.

Clearly, we’ve learned a lot about the interaction of CCT and glaucoma during the past 30 years; hopefully it won’t take another 30 to resolve the remaining issues. In any case, we know enough right now to be of real help to our patients, and the future looks even brighter.


Dr. Herndon is associate professor of ophthalmology in the Glaucoma Service at Duke University Eye Center in Durham.

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