This article is more than 5 years old. Information may no longer be current.
Pachymetry increasingly important in glaucoma management
The unreliability of Goldmann applanation tonometry may become a serious issue in the wake of OHTS.
BARCELONA, Spain — In managing glaucoma and glaucoma suspect patients, corneal thickness should be taken into account in order to better understand data on IOP, according to a surgeon speaking here.
James D. Brandt, MD, presenting during the International Glaucoma Society’s 4th International Glaucoma Symposium, said central corneal thickness (CCT) influences the accuracy of tonometry measurements. Further, CCT should be considered in the evaluation of patient response to treatment, and it can be particularly important to consider in patients who have had refractive procedures.
“The bottom line is that Goldmann applanation tonometry is far less accurate than we previously appreciated, and glaucoma diagnosis generally should be based on structure and functional parameters of the optic nerve, not just IOP. This is not an uncommon problem. It is something you should be sensitized to in your practice, and I encourage you to start thinking along these lines as you take care of your patients,” said Dr. Brandt, director of the glaucoma service at the University of California-Davis.
He noted that clinical trials evaluating patient response to medications and other studies should also start considering CCT measurements to better understand the IOP data generated. Nonresponders to medication may in some cases simply be patients with particularly thick corneas, he said.
CCT and IOP variability
According to Dr. Brandt, pachymetry has increased in use over the past several years among glaucoma specialists, contributing to the understanding of corneal thickness and its effect on IOP measurement. Glaucoma diagnosis and management has subsequently improved. At the same time, there has been a grudging recognition that Goldmann applanation tonometry is much less accurate than previously recognized.
Dr. Brandt said the Ocular Hypertension Treatment Study (OHTS) demonstrated that corneal thickness is a factor in judging which ocular hypertensive patients may go on to develop glaucoma.
“I think we have all learned over the last few years that corneal thickness is diagnostically important in ocular hypertensives, patients with normal-tension glaucoma and certainly in refractive surgical patients,” he said.
Dr. Brandt noted that Goldmann designed his applanation tonometer assuming a general corneal thickness of 500 µm, postulating there would be little variance. It was later determined there is significant variability in corneal thickness among the general population, with growing evidence of such variation.
He said corneal thickness is “remarkably” variable, especially among ocular hypertensive patients. He said about 25% of the patients enrolled in the OHTS had corneal thickness measurements above 600 µm.
“We also showed there are racial differences in corneal thickness, with [black] patients having thinner corneas than their white counterparts,” he said.
Work done by Niels Ehlers, MD, in the mid-1970s showed IOP can vary by roughly 5 mm Hg for every 70 µm difference in thickness from the point at which Goldmann tonometry is most accurate, which he said is actually about 520 µm.
Dr. Brandt noted that several other algorithms for correcting for variability in corneal thickness have been less aggressive, but all suggest corneal thickness affects accuracy in measuring IOP.
He said that if the researchers conducting the OHTS had applied the various algorithms to correct for corneal thickness variability, half of the OHTS participants would not have qualified for the study.
“We were clearly misclassifying a significant portion of these patients, based on wrong IOP,” he said.
However, he said, corneal thickness was found to be a strong baseline factor for predicting which patients will go on to develop glaucoma. The study also demonstrated that black patients have thinner corneas and larger cup-to-disc ratios, which meant that race (or ethnicity) was no longer significant in the multivariate analysis when these factors were in the predictive model.
“If you have a thin cornea, regardless of what the IOP was, these patients had much higher rates of developing glaucoma than those with thicker corneas. You could identify groups of patients who had a remarkably high risk of developing glaucoma during that 5-year follow-up period,” he said.
Similarly, an independent dose-response relationship was shown between corneal thickness and cup-to-disc ratio. Using these predictive factors, Dr. Brandt said a patient with a 600-µm cornea with modest elevation in IOP has a relatively low risk of developing glaucoma. If some factors are changed, such as increasing the IOP while the other factors remain stable, the risk remains quite low. However, if the pressure is lowered, the risk increases as the corneal thickness decreases.
CCT and treatment response
According to Dr. Brandt, one of the interesting issues has been whether variation in corneal thickness confounds the surgeon’s ability to assess patient response to treatment, an issue somewhat addressed in the OHTS.
In this study, he said, patients with thick corneas and thin corneas showed a slight difference in response to treatment with beta-blockers. Also, in the Food and Drug Administration phase 3 regulatory trial of Lumigan (bimatoprost ophthalmic solution 0.03%, Allergan), Dr. Brandt said there was a 3-mm Hg difference at 3 months between patients with thick corneas and thin corneas among patients given timolol 0.5% twice daily.
“The thick corneas are in the 600-µm range, while the thin corneas are relatively normal,” he said. “This appears to be a powerful confounder in clinical trials and is something we should probably begin paying attention to in IOP-related studies.”
Regarding corneal thickness and its effect on measurement of IOP, Dr. Brandt said another problem clinicians will face relates to measurement of IOP in patients diagnosed with normal-tension glaucoma who have undergone refractive surgery.
He noted that one patient, a 45-year-old woman who had undergone photorefractive keratectomy, had pachymetry measurements of 425 µm with an IOP reading of 19 mm Hg.
“Her true pressure was probably in the mid to upper twenties. This patient didn’t need to be pigeonholed as having ‘normal’ to explain her findings; se had primary open-angle glaucoma with elevated IOPs,” he said.
Dr. Brandt said PRK and LASIK will also affect surgeons’ abilities to accurately measure IOP.
“For all its faults, tonometry in general practice [remains] the way most patients are detected with glaucoma,” he said.
For Your Information:
- James D. Brandt, MD, can be reached at UC-Davis, Department of Ophthalmology, 4860 Y St., Suite 2400, Sacramento, CA 95817; (916) 734-6676; fax: (916) 734-6992; e-mail: jdbrandt@ucdavis.edu.