More surgeons measuring corneal thickness after OHTS

Surgeons have become more conscious of the roles corneal thickness, elevated IOP, race, age and family history play in the development of glaucoma.

The results of the Ocular Hypertension Treatment Study in 2002 have changed the way some surgeons manage glaucoma, interviews with several surgeons suggest.

“We now know that corneal thickness is another factor involved in the risk for glaucoma,” Jody Piltz-Seymour, MD, an investigator in the Ocular Hypertension Treatment Study (OHTS) told Ocular Surgery News. “It’s a pretty strong factor when deciding whether to treat or not to treat a patient.”

Rebecca S. Walker, MD, FACS, agreed. “Corneal thickness is another piece of the puzzle that we are factoring into our management of glaucoma patients,” said Dr. Walker, an ophthalmologist in Chalfont, Pa. “The findings on corneal thickness from the OHTS have definitely impacted my practice. Now I check corneal thickness at every glaucoma evaluation.”

Jeffrey R. Brant, MD, a general ophthalmologist in Cartersville, Ga., has also incorporated more pachymetry into his practice routine. “I think that most ophthalmologists have been impacted by the OHTS. It helped us to understand the effects of the applanation tonometry readings. So if the cornea is thicker, you can get a falsely high reading; if it’s thinner, you can get a falsely low reading,” Dr. Brant said.

Targeting IOP

The OHTS was the first large-scale study in North America that demonstrated that lowering intraocular pressure can effectively delay or prevent glaucomatous progression. While surgeons had suspected this for some time, the OHTS confirmed that IOP is an important risk factor for glaucoma.

“OHTS found that after 5 years, 10% of patients who aren’t treated for high IOP are going to go on to develop primary open-angle glaucoma. If you treat these same patients early, only about 5% will develop glaucoma, so close monitoring and earlier treatment reduces the risk by 50%,” Dr. Brant explained.

As a result of the study, the definition of glaucoma has changed, he said. “We used to say that glaucoma was optic nerve damage caused by high IOP,” Dr. Brant said. “Now we learn that’s not completely true. Glaucoma is really an optic neuropathy of which elevated IOP is just one risk factor,” he continued.

“Since the study came out, if a patient’s pressure stays over 25 mm Hg and he has no real evidence of glaucoma-like optic disc or visual field changes, I tend to go ahead and treat him,” Dr. Brant said. In the past, he said, this same patient would have been closely monitored in his practice. Just a few years ago, some healthy patients in his office remained untreated with pressures up to 30 mm Hg, he said.

“We are treating a lot more patients at earlier stages than we used to,” Dr. Brant said. “In the long run, we are hoping to avoid glaucoma progression.”

Corneal thickness as a risk factor

Pachymetry billing code

In light of the necessity for measuring corneal thickness, the American Medical Association has established a billing code for pachymetry. The Corneal Pachymetry CPT/HCPCS code is 0025T. This is a category 3 code, which means it is a new and emerging technology. “We are successfully billing with this code,” said Kristin A. Gray, business office manager of Minnesota Eye Consultants in Minneapolis. “Our payers are recognizing and paying on this code, specifically BSCS, Preferred One and our Medicare carrier, WPS.” According to AMA guidelines, use of Goldmann pachymetry is acceptable for billing every 6 months for a patient who has corneal disease. There must be a reasonable expectation that the outcome of corneal pachymetry will affect the decision- making process in the medical management of the patient. For patients suspected of having glaucoma, the pachymetry test is paid for once per lifetime. “This is a bilateral test, which for a patient would be covered only once,” Ms. Gray said. In her office, Ms. Gray is currently billing $50 for bilateral pachymetry in glaucoma evaluation. However, once RVU’s are set by Medicare, a fee in the range of $75 to $90 per examination may be reasonable, she said.

While keeping IOP levels under control had been an important part of glaucoma management before the study, measuring corneal pachymetry had not.

“One of the most interesting findings of the OHTS was the importance of measuring corneal thickness. It’s quite possibly the most important finding that came out of the study,” Dr. Piltz-Seymour said.

“It’s interesting that we didn’t used to think about corneal thickness too much,” Dr. Brant added. “The OHTS really brought this idea to the forefront.”

The study found that corneal thickness (depending on whether the cornea was unusually thick or unusually thin) affected the accuracy of applanation tonometry readings, giving surgeons false IOP measurements.

“When you use an applanating tonometer, the instrument touches and (flattens) the cornea a little in order to get the reading,” Dr. Brant explained. “So if the cornea tends to be thicker it will be more rigid and the applanation tonometer tip will not displace the cornea as much as with a thinner cornea. So therefore, it will give you an inflated reading.”

The opposite is true for a thin cornea, Dr. Brant said. “Thin corneas are easier to displace. But you’re going to tend to get a lower pressure because of the mechanism that the tonometer uses to measure the pressure,” he said.

“So if a surgeon measures someone who has high pressures and thin corneas, the true pressure could even be higher than measured. If corneas are thin, when you measure the pressure you are underestimating the true IOP. When they are thick, you are overestimating it, so your reading is higher than the true pressure,” Dr. Piltz-Seymour said. As a result, patients with thick corneas and elevated pressure might have normal pressure, and patients with thin corneas and normal pressure might have elevated pressure, which could be a health risk that goes unnoticed.

Traditional applanation tonometers, such as the Goldmann tonometer, do not compensate for varying corneal thickness. According to Drs. Piltz-Seymour and Brant, there are nomograms and calculations that attempt to convert the readings more accurately, but they vary.

“A true method of gaining an accurate reading would be helpful for the general ophthalmologist,” Dr. Brant noted.

Pachymetry in practice

In her practice, Dr. Walker has begun using pachymetry on all of her glaucoma and at-risk glaucoma patients. One of her patients is a young African-American woman with normal pressure, optic disc and visual fields, but she has thin corneas.

“Before last year, I would have said, ‘See you back in a couple of years,’” Dr. Walker said. “Now, knowing that she has the risk of a thin cornea, in addition to being African American and having a family history of glaucoma, I’m going to follow her more closely.”

Conversely, another patient in Dr. Walker’s practice had normal visual fields and a healthy optic nerve, but elevated IOP.

“This patient had an elevated pressure of 24 mm Hg, but when I checked her corneas, they were thick,” Dr. Walker said. “I explained to her that the measurements just may not be accurate because of the thickness of her corneas. Then I took her off drops.”

Multiple variables in glaucoma

Dr. Walker noted that her management of each patient is customized. “The OHTS taught us to include corneal thickness as another risk factor for glaucoma, which includes race, age and family history. Each factor may increase the risk for the development of the disease,” she said.

“The study highlighted the various risk factors,” Dr. Piltz-Seymour agreed. “It showed us to consciously look for the patients who are most likely to go on and get glaucoma and target your therapy towards them. The very high-risk people are those with thin corneas, elevated IOP, but also a large cup-to-disc ratio and increasing age.”

“There are many variables,” Dr. Brant added. “There isn’t a real cookbook or calculator that tells you when to treat a patient and when not to. However, the OHTS study gives us a few more things to consider when we are trying to make a decision.”

For Your Information:

• Jody Piltz-Seymour, MD, can be reached at the University of Pennsylvania, Presbyterian Medical Center, 51 North 39th St., Philadelphia, PA 19104; (215) 662-8715; fax: (215) 243-4696.
• Rebecca S. Walker, MD, FACS can be reached at Eye Associates at Highpoint Professional Building, 700 Horizon Circle, Suite 204, Chalfont, PA 18914; (215) 997-2015; fax: (215) 997-8350.
• Jeffrey R. Brant, MD, can be reached at the Allatoona Eye Institute, PC, 962 JF Harris Parkway, Suite 201, Cartersville, GA 30120; (770) 382-3598; fax: (770) 382-4892; e-mail: jbrant@allatoonaeye.com