Article Date: 9/1/2005

glaucoma
Pachymetry — Is Now the Time?
Central corneal thickness values are the greatest predictor for determining which patients are at risk for glaucoma.
BY ROBERT WOOLDRIDGE, O.D., Salt Lake City, Utah


ILLUSTRATION BY JOEL AND SHARON HARRIS

So, Doctor, are you still sitting on the sidelines when it comes to pachymetry? Haven't decided whether to invest in an instrument? Already decided you don't need one? Or perhaps you have purchased one, but you're not sure when to use it? Let's look at the issues, pro and con, for pachymetry and its impact on your practice.

Clinical standard of care?

The Ocular Hypertension Treatment Study (OHTS) in 2002 evaluated risk factors for the development of glaucomatous damage in OH patients, as measured by visual field progression or visible optic nerve damage. In a multivariate risk analysis, higher intraocular pressure (IOP), old age, large cup-to-disc ratio, greater pattern standard deviation and thin central corneal thickness (CCT) as measured by ultrasonic pachymetry were found to be significant risk factors. Of these factors, CCT provided the greatest predictive power for determining which patients will develop glaucomatous damage. Patients with a corneal thickness of 555μm or smaller were three-times more likely to develop POAG, compared with those who had a thickness of more than 588μm. This landmark study, the largest and perhaps best-designed investigation of risk factors for glaucoma in ocular hypertensives, advises that pachym-etry be a standard part of the evaluation of ocular hypertension patients. The authors conclude that central corneal thick- ness provides new information about the risk of developing POAG and recommend provid-ers measure it in evaluating patients with ocular hypertension.

The OHTS authors further recommend that such patients be evaluated for risk of damage according to the information in the table on page 52 (OHTS Guidelines).

The right optic nerve appears healthy, while the left shows inferior notching.

   

Other factors

So, if you think race, family history, refractive error and general health are key in your patients with OH, you should know that none of these variables come close to CCT when it comes to assessing a patient's risk of damage.

Keep in mind that OHTS only evaluated patients with ocular hypertension. Researchers at the University of Washington in Seattle investigated the association between corneal pachymetry and visual field progression in patients with open-angle glaucoma. A retrospective case-control study evaluated 88 glaucoma patients and followed them for an average of eight years. Patients were matched for race, type of glaucoma and age at pachymetry. Primary open-angle glaucoma (POAG), pigmentary glaucoma (PDG), pseudoexfoliative glaucoma (PXG) and normal tension glaucoma (NTG) were included in the study. Visual field progression and pachymetry were defined as the main outcome measures. The researchers found that the mean CCT in patients with visual field progression was significantly lower than that of patients who did not progress (529 versus 547). Variables included age at diagnosis and at pachymetry, compliance, myopia, CCT, mean and maximum IOP, months of follow-up, years of disease, presence of disc hemorrhage and presence of PXG. Of these risk factors, only CCT was found to be a significant predictor of progression in a multivariate analysis. Investigators determined that for every 40 microns of corneal thinning, there is a 44% increase in risk of progression. This study, together with OHTS, conclude CCT is an important factor in determining the risk of progression in both OH and glaucoma patients.

Numerous additional studies have validated that CCT should considered to properly interpret Goldmann applanation tonometry. Clearly, pachymetry is rapidly becoming a standard of care in the evaluation of ocular hypertension and glaucoma patients.

OHTS Guidelines

HIGH RISK MODERATE RISK LOW RISK
IOP (mmHg) > 25.75 > 23.75 to < 25.75 < 23.75
CCT (μm) < 555 > 555 to < 588 > 588
Vertical C/D > 0.5 > 0.3 to < 0.5 < 0.3

Popularity gains

How has the ophthalmic marketplace responded to this revelation? Tammy Evans, pachymetry product manager for Heidelberg Engineering, estimates that more than 9,000 pachymeters have been sold in the United States since 2002. She further estimates that around 54% of optometrists and 90% of ophthalmologists in the country own a pachymeter.

Let's look at a case as an example of how important CCT can be to the overall evaluation of a glaucoma suspect. A 70 year-old male of mixed African American/Caucasian heritage is seen as a glaucoma suspect. He has no visual complaints, unremarkable general health history and no family history of glaucoma. Visual acuity measured 20/20 in each eye. IOP was 20mmHg O.S., 21mmHg O.D. Further examination revealed mild cataracts O.U., left afferent papillary defect and a normal retinal exam. Images of this patient's optic nerves are seen on page 52.


The OCT RNFL evaluation is normal in the right eye. Note the thicker than average inferior NFL. However, the left NFL is thinned inferiorly and the asymmetry between eyes is obvious.

The patient appears to have early glaucomatous cupping in the left eye, along with visual field loss consistent with glaucoma despite having only borderline IOP. Pachymetry reveals CCT of Right: 486μm and Left: 471μm. While this patient's IOP falls below the pressure range evaluated in OHTS, clearly his very thin corneas are an important factor in his overall evaluation.

The patient reports that he has had an eye examination the past two years with no mention of possible glaucoma until his most recent exam revealed glaucomatous visual field loss. Had the previous doctors been aware of his very thin CCT values? Might they have been more proactive in assessing his risk and taking appropriate steps to establish baseline values for visual fields and optic nerve photos?

Perhaps further evaluation with HRT, OCT, GDx and matrix visual fields would have detected damage at an earlier stage. At the very least, both doctor and patient would have been better informed in regards to his risk of developing glaucomatous vision loss. Earlier treatment may have prevented the damage seen now.

There are numerous nomograms for calculating a "corrected IOP" based upon the CCT. While they do not exactly coincide, most would indicate an adjustment in this case of approximately 6mmHg. His adjusted IOP would then be 27mmHg, raising the level of concern in his case.

Excuses, excuses

I sometimes hear doctors justify not purchasing a pachymeter because their glaucoma patients are referred to specialists anyway. However, consider this: Knowing the CCT of a patient with borderline IOP and/or larger-than-average cup-to-disc ratios better enables you to determine which patients need to be referred for further evaluation and which may be safely followed within your own practice. In such cases, pachymetry may be the key component in deciding whether to refer. The case we reviewed here is an example of such a patient.

Another rationalization for not owning a pachymeter: "I can't afford it." Prices for pachymeters today generally range from $2,000 to $3,500. Medicare reimbursement for measuring CCT (using CPT code 92154) averages around $12. Even with a conservative estimate of performing the procedure on ten patients a month, a pachymeter will pay for itself in two years. While it is not a large source of profits, the clinical value of the information gained makes it a worthwhile procedure at any price.

Ageless wisdom

So doctor, who needs pachymetry? You and your glaucoma patients do. Benjamin Franklin once said, "You may delay, but Time will not." Don't let time and standards of care pass you by. I urge you to purchase a pachymeter today and utilize it in the evaluation and care of your patients.

Dr. Wooldridge is the clinical director of the Eye Foundation of Utah. He is also an adjunct clinical professor at several schools and colleges of optometry, as well as a founding member of the Optometric Glaucoma Society. E-mail Rpwod@aol.com.

 

On HRT evaluation, there is mild vertical elongation of the right cup but the disc is within normal limits. The left HRT clearly shows overall enlargement of the cup with severe thinning of the inferior rim tissue.

 



Optometric Management, Issue: September 2005