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CORNEAL pachymetry is now an essential tool for glaucoma diagnosis and management and is likely to find increasing clinical applications in detecting the disease in patients who have under-gone refractive surgery, according to James Brandt, MD.
"Pachymetry over the last few years has really come of age as a major aspect of our understanding of the management and diagnosis of glaucoma.

Over the last decade or so, there has been an increased recognition that Goldmann applanation tonometry is a lot less accurate than we previously recognised," Dr Brandt told the 4th International Glaucoma Symposium.

The results of the Ocular Hypertension Treatment Study (OHTS) showed that central corneal thickness was an important factor measured at baseline that helped predict who would progress to glaucoma among the ocular hypertensive patients in the study.

The observed effect associated with thin corneas may have been due to the inability of conventional tonometry to provide accurate IOP measurements.
When Goldmann designed applanation tonometry, he assumed that most eyes had a corneal thickness of around 500 microns with little variability.
However, in the 1970s, Niels Ehlers MD cannulated 29 eyes undergoing cataract surgery and discovered there was significant variability in corneal thickness and, as a result, corresponding variability of Goldmann tonometry.

Dr Ehlers therefore proposed a conversion table to correct for the misleading pressure lowering effect of a thin cornea. Essentially, his proposition was that a variance of 70 microns from the normal corneal thickness of around 520 microns would change the applanation measured IOP by 5.0mmHg.Data from OHTS indicates that corneal thickness is even more variable than had been previously suspected. Among the 1,636 patients enrolled in the trial, most had corneal thickness above normal and over 25% had corneal thickness above 600 microns, Dr Brandt noted.

"If you apply Ehler’s algorithm fully, half the OHTS participants would not have even qualified to be in the study if we had used corrected IOP. So we may have been misclassifying a significant portion based on their IOP," Dr Brandt said.
Another aspect of corneal thickness is that it can help explain the increased risk for glaucoma among African Americans. The OHTS data showed that African Americans have on average thinner corneas than Caucasians, but they also have larger cup-to-disc ratios. As a result, race ceases to be an independent risk factor in a multivariate analysis.

Dr Brandt noted that the effect of thin corneas on IOP measurements has several important clinical implications for individual ophthalmologists.
He described a patient who presented at his own centre who had undergone very aggressive glaucoma therapy, including filtration surgery at a different centre, for what appeared to be a very refractory elevated IOP. However, corneal pachymetry revealed that she had extremely thick corneas and in fact didn’t have glaucoma at all.

Another patient came to his centre who had been previously diagnosed with low-tension glaucoma. Tonometry gave an IOP measurement of 19mmHG. However she had undergone PRK four years earlier and pachymetry showed her corneal thickness to be 425 microns. Therefore her real IOP was probably in the mid to upper 20s and her diagnosis was not normal tension glaucoma but simply glaucoma.

Dr Brandt predicted that corneal refractive procedures like PRK and LASIK would increasingly diminish the ability of glaucoma specialists to accurately measure IOP.
What makes the situation more urgent is that most patients who undergo such procedures are myopic. Several studies including the Blue Mountain Eye Study showed that, independent of all other factors, myopia conferred a two to three-fold risk of glaucoma compared with non-myopes

"We are creating from 50,000 to 100,000 patients a year who are genetically predestined to get glaucoma and in whom tonometry will not be accurate. Many of us are predicting that we will see an epidemic in the next few years of pseudo-normal tension glaucoma in these patients and they will be presenting with quite advanced field loss," Dr Brandt explained.

Two recent papers, the Early Manifest Glaucoma Trial (EMGT) and the Bermuda Eye Survey, failed to show a relationship between CCT and glaucoma. However, their findings may not be in direct contradiction to those of the OHTS study, Dr Brandt said.
The EMGT researchers defined glaucoma and entered the patients based on presence of damage regardless of IOP. The patients had already demonstrated that their optic nerves were sensitive to whatever pressure they happened to have, making corneal thickness irrelevant.

Meanwhile the Bermuda Eye Survey involved a very racially homogenous population, with a high risk of glaucoma and high IOP. The genes responsible for glaucoma in this group may have dwarfed any effect of corneal thickness.

"The practical implications are that corneal thickness does influence the accuracy of tonometry and this extends to assessment of treatment response in clinical trials.
"Therefore, when evaluating response to medication and other therapies, glaucoma specialists really should start to consider corneal thickness measurements to better understand their IOP data," Dr Brandt said.

Several of the participants at the congress questioned whether the increased risk for glaucoma in patients with thin corneas is solely a result of underestimated IOP.
"The risk elevation of developing glaucoma in those with thinner corneas in OHTS was about three-and-a-half times larger than we could explain away by the artefact on pressure. Those who developed glaucoma also had more disc than field changes.

"What I want to see in the continuation of OHTS is whether many of the disc changes were just mechanical deformations which will be reversible when the pressures are brought down. For now I think we need to perform pachymetry in ocular hypertensives because corneal thickness was as strong a risk factor as IOP," said US specialist Paul Palmberg MD, PhD.

Paul Kaufman MD added: "If one did have to hypothesise that there was another effect than on the IOP, one could think about how a thin cornea may be related to what is happening at the lamina cribrosa and its effect on connective tissue support for the optic nerve head when its assaulted by pressure or other factors. But that's pure speculation."
"It is important that we realise it’s more than just the thickness of the cornea we should be interested in. It is also the biomechanical behaviour of the cornea and the whole eye, the lamina cribosa and the whole scleral shell," said Harry Quigley MD.

Dr Quigley also told EuroTimes in an interview that refractive surgeons should screen patients requesting LASIK and PRK more rigorously than they currently do for early signs of glaucoma. In addition, he pointed out that LASIK increases the IOP for a short period of time and some patients have reported a worsening of field after undergoing the procedure.

On the other hand, the thinning of the cornea from laser ablation is unlikely in itself to increase a patient’s glaucoma risk, he noted.
"I would speculate based on my experience that there is probably no chance that thinning the cornea is actually going to cause glaucoma," Dr Quigley said.

However, LASIK does cause a sort of false lowering of IOP. Dr Quigley therefore advises his glaucoma patients who are undergoing LASIK to undergo IOP measurements two or three times both before and after undergoing the refractive procedure. In that way they can establish a new baseline and a new target IOP.
He also recommends to his colleagues that they use disc and field exams rather than tonometry to screen such patients for glaucoma.

"This is another of those examples pointing up to why it is foolhardy to use only eye pressure measurement as a way of detecting glaucoma," Dr Quigley noted.

James D Brandt MD
University of California, US
Email: jdbrandt@ucdavis.edu

Paul Palmberg MD
Bascom Palmer Eye Institute, Miami, Florida, US
Email: ppalmberg@med.miami.edu

Harry Quigley MD
Wilmer Eye Institute, Baltimore, US
Email: hquigley@jhmi.edu

Paul L Kaufman MD
University of Wisconsin Medical School, US
Email: kaufmanp@mhub.ophth.wisc.edu