OHTS findings generate broader utility for pachymetry

The correlation between corneal thickness and IOP has led to an enhanced role for pachymetry in glaucoma management.

One of the many changes resulting from the Ocular Hypertension Treatment Study has been an increased emphasis on pachymetry in all aspects of ophthalmology.

“Previously considered the province of corneal and refractive surgeons, pachymetry has now found a broader utility in the area of glaucoma management,” said Wesley K. Herman, MD, of Dallas. “Due to the findings of the OHTS, pachymetry has become a standard of care across all disciplines of ophthalmology and even optometry. A thinner cornea will inadvertently measure a lower pressure than it really is, due to the softness of that tissue. Pachymetry is going to come into play more and more in the future.”

In choosing from among the many available pachymeters, practitioners are advised to consider the augmented role of pachymetry in glaucoma management. Several of these pachymeters are described here.

Accutome Pach IV, Advent Pachymeters

The Pach IV is portable, easy to use and extremely accurate, with an accuracy of ± 5 µm and a resolution of ± 1 µm. It allows the user to obtain readings quickly. Accutome offers a straight or angled probe with a range of 300 µm to 999 µm. It has adjustable speed of sound to accommodate velocities from 1,000 to 2,000 m/sec. The Pach IV offers a bright LCD and audible tone definition with foot pedal control or a continuous-read option. This pachymeter qualifies for ADA Tax Credit, according to the manufacturer.

Wallace E. Ruminson MD, of Porterville, Calif., said the Pach IV is easy to use and portable.

“The things I think are nice about this pachymeter are that it is not very expensive and it is simple to use. It doesn’t take my techs any time to move it and set it up. I can quickly take readings on a patient, and then I am out of there,” he said.

Dr. Ruminson has also found the instrument to be helpful in dealing with glaucoma suspects and patients with glaucoma.

“I find it is giving the information on which the Goldmann tonometry reading can be biased. And I think once we learn how to interpret the bias and the variation in corneal thickness, we are going to be able to alter our treatment to an even greater degree,” he said.

Dr. Ruminson said although Goldmann tonometry was originally based on the assumption of a 520-µm cornea, in reality corneal thickness tends to vary.

“That change in corneal rigidity may produce a higher or lower readout from Goldmann tonometry. We are finding that some individuals may tolerate a 22-mm or 23-mm pressure reading on the Goldmann because their pressure may be lower than that,” he said.

What is not known is how a deviation from the 520-µm cornea might translate in terms of mm Hg pressure, he added.

“Once we get that correlation, we will be that much more accurate. But this pachymeter has made our Goldmann tonometry interpretation more accurate. It has produced a bias,” he said.

Accutome also makes the Advent pachymeter. This device is compact and portable, with measurement capability ranging from 1,000 µm to 90 µm. For lamellar procedures the Advent measurement capability is from 400 µm to 90 µm. It has four templates available for customized mapping. The voice audible pachymetry mode (VAP) serves as an assistant by immediately pronouncing measurement values. The Advent has a 55° handle on the probe specifically designed to be used under the operating room microscope, and the VAP allows hands-free operation in the OR. It has an internal printer.

DGH Pachette 2

The Pachette 2 (DGH Technology Inc) is an ultrasonic pachymeter that obtains multiple measurements at a single location while simultaneously displaying the current measurement. The display feature also includes an average of all measurements taken, as well as the standard deviation. In a single applanation, the Pachette 2 automatically stores up to 50 measurements.

Leon Herndon, MD, an associate professor of ophthalmology at Duke University, said his Pachette 2 has proven to be a reliable instrument that yields valuable information.

“I think it is excellent due to its ease of use, its reproducibility and its portability. I like DGH’s commitment to research. They donated the pachymeters for the OHTS to all the sites,” he said.

Dr. Herndon said he expects pachymetry to play a pivotal role in glaucoma management in the future. He credits James Brandt, MD, with emphasizing the importance of corneal thickness measurements during the OHTS.

“We had known about the effect of having a thick cornea on eye pressure for some time, but pachymetry was not included initially,” he said. “Only through the persistence of Dr. Brandt did the investigators go back and measure corneal thickness, and DGH was right there with them.”

Dr. Herndon said the question of reproducibility for pachymetry measurements is still being answered.

“We are looking at that right now,” he said. “That is an issue for third-party providers and payers. The question is, should a patient have pachymetry done only once in their lifetime? So we are looking at patients we measured many years ago with the same instrument, to see if that has changed over time.”

Dr. Herndon said the Pachette 2 seems to yield reproducible results.

“On a day-to-day basis, if you are measuring corneal thickness today and then tomorrow, my sense is that the accuracy is excellent and the reproducibility is good,” he said.

The convenience of the instrument is another valuable feature, he added.

“It is so easy; it averages the readings for you,” he said. “And it is really very durable; we’ve put it to work. We’ve traveled to Ghana with it, and it has survived the trip there and back several times. It’s a great little unit.”

Haag-Streit OLCR

The Haag-Streit group recently gained Food and Drug Administration approval for its new Optical Low Coherence Reflectometry (OLCR) pachymeter. This instrument uses invisible light beams to allow for measurement of ± 1 µm of corneal thickness.

The measuring head of the OLCR emits two red LED aiming beams that converge at the point of focus of the invisible OLCR beam. The operator places the aiming beams on the cornea at the point to be measured and moves the slit lamp back and forth until the two beams merge. The operator is guided by audible signals.

As soon as the system is focused, the OLCR automatically records the corneal thickness and displays the result on the control panel.

For glaucoma applications, the operator enters a previously measured tonometry reading and the system provides a corrected IOP according to the newly determined corneal thickness. The OLCR is noncontact and requires no dilation.

Stephen L. Trokel, MD, a professor of ophthalmology at Columbia University, has been using the OLCR pachymeter under investigation limitations for almost a year. Dr. Trokel said he is pleased with the instrument so far.

“The real advantage of this technique is that it is a noncontact technique and does not require that the eye be touched,” he said. “No topical anesthetic drops are required, and it takes a few seconds to produce a series of measurements that are reported as an average and a standard deviation. The number of measurements made can be varied in the software.”

In addition, Dr. Trokel said, he has found the information obtained from the OLCR to be reproducible.

“It produces the same number over a period of time. It is difficult to separate physiological variation from machine variation. But I have measured my own corneas over a period of months, producing tightly spaced numbers,” he said.

Dr. Trokel said the thickness measurements of the OLCR are comparable to the numbers produced by the alternative technologies.

“The advantage is the extremely tight standard deviation, usually about 1 µm,” he said.

Nidek US-1800 Pachymeter-A-Scan

Sylvia L. Hargrave, MD, director of the Hargrave Eye Center at Methodist Hospitals of Dallas, said she uses the Nidek US-1800 Pachymeter-A-Scan primarily for glaucoma management.

The US-1800 provides corneal thickness measurements with a touch of the probe, according to company information. A thermal printer is built in.

“I use it mainly to screen glaucoma patients, especially since there has recently been increasing importance placed on the correlation between IOP and corneal thickness,” she said. “Thinner corneas can mask changes in IOP, so if you have a patient who has seemingly normal pressures, you have to make sure your cornea isn’t abnormally thin.”

Dr. Hargrave said she generally approaches treatment differently in the case of a patient with thinner corneas.

“For patients with corneal thickness below about 530 µm, I usually will be more aggressive treating seemingly normal IOPs than I would for people with normal-thickness corneas. OHTS has provided us with a risk model where central corneal thickness was shown to be a powerful predictor for the development of primary open-angle glaucoma,” she said.

Dr. Hargrave said the US-1800 has provided her with a good deal of valuable information.

“It is the only pachymeter I have, and the only one I have used. I have been very happy with it so far,” she said. The US-1800 received FDA approval as this issue of Ocular Surgery News went to press.

Orbscan II for pachymetry

The Bausch & Lomb Orbscan II is another instrument frequently used to take pachymetry measurements, particularly for refractive surgery patients.

The Orbscan II analyzes elevation and curvature patterns on both the anterior and posterior surfaces of the cornea. These readings allow the instrument to calculate corneal thickness pachymetry at multiple points.

Daniel S. Durrie, MD, of Overland Park, Kan., said although he sometimes uses ultrasound pachymetry, he uses the Orbscan for this purpose much more frequently.

“We use the Orbscan for most of our pachymetry measurements. I would say we use it about 95% of the time,” he said.

Dr. Durrie said the Orbscan uses a slit-scanning methodology, similar to a slit lamp.

“When you do a slit-lamp exam and bring it in at an angle, it is going to reflect light from the cornea where it goes from the air-cornea interface,” he said. “And then there is another change in direction when it goes from the cornea into the aqueous. So when you bring in the slit, you can tell where the front of the cornea is and the back of the cornea is.”

“The advantage is that it gives you the pachymetry across the whole cornea, so you can know what the thick spots and thin spots are. That is very important when you are evaluating for refractive surgery,” he said.

“This would not be something you would use for situations where precision is required. If I am going to make an incision there, I will do an ultrasound to make it specific. On the other hand, ultrasound doesn’t give me nearly the amount of information the Orbscan does,” he said.

According to Bausch & Lomb, there is no evidence available that Orbscan is inferior to other pachymetry methods, none of which would be suitable for glaucoma.

Quantel Medical Pocket Pachymeter

The Quantel Medical Pocket Pachymeter can now measure from as little as 125 µm to as much as 1.3 mm with an accuracy of 5 µm, according to company information.

At the portable size of 8 inches by 4 inches by 2 inches, the Pocket Pachymeter can be used in the office and the operating room for measuring corneal thickness in a wide variety of applications, including LASIK, contact lenses, radial and arcuate keratotomy and glaucoma.

“I liked this instrument so much that I purchased a total of 10 of them for our various locations,” Dr. Herman said. “I like it because it is small, portable, and you have the option of battery power. We can take it to satellite clinics and we can take it in and out of the operating room.”

Dr. Herman frequently performs relaxing incisions after corneal transplants, and he said he has found the Pocket Pachymeter to be useful in that capacity.

“To do that you need accurate, almost dynamic pachymetry. I have found this instrument to be very accurate,” he said.

Dr. Herman said he also uses the instrument for LASIK as well as for glaucoma.

“It compares very nicely to some of the other forms of pachymeters we use. You get a very nice endothelial cell picture, and a central patient-fixated optical pachymetry reading that correlates very well with ultrasound,” he said.

Dr. Herman said he is comfortable with using optical pachymetry due to its reproducibility and the significant advantage of endothelial cell morphology.

“In our clinic, we have compared corneal pachymetric measurements using the Orbscan and ultrasounds and optical pachymetry using specular microscopy,” he said. “All of the measurements are comparable. And I think knowing that, I am very happy doing optical pachymetry, because it appears to be very reproducible.”

For Your Information:

• Wesley K. Herman, MD, can be reached at 5421 LaSierra Drive, Dallas, TX 75231; (214) 361-1443; fax: (214) 691-3299. Dr. Herman has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Wallace E. Ruminson, MD, can be reached at 444 West Putnam, Porterville, CA 93257; (559) 781-2079; fax: (559) 781-4310. Dr. Ruminson has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Leon W. Herndon, MD, is an associate professor of ophthalmology at Duke University. He can be reached at Duke University Medical Center-Ophth, Box 3802, Durham, NC 27710 (919) 684-6622; fax: (919) 681-8267; e-mail: hernd012@mc.duke.edu. Dr. Herndon has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Stephen L. Trokel, MD, is a professor of clinical ophthalmology at Columbia University. He can be reached at 16 East 60th Street, New York, NY 10032; (212) 326-3313; fax: (212) 326-3313; e-mail: trokel@columbia.edu. Dr. Trokel has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Sylvia L. Hargrave, MD, is director of the Hargrave Eye Center at Methodist Hospitals of Dallas. She can be reached at 221 West Colorado Blvd., Suite 728, Dallas, TX 75208; (972) 572-6262; fax: (972) 572-0423; e-mail: drhargrave@sbcglobal.net. Dr. Hargrave has no direct financial interest in the products mentioned in this article, nor is she a paid consultant for any companies mentioned.
• Daniel S. Durrie, MD, can be reached at Durrie Vision, 5520 College Blvd., Suite 200, Overland Park, KS 66211; (913) 491-3737; fax: (913) 491-9650; e-mail: ddurrie@durrievision.com. Dr. Durrie has a direct financial interest in the Orbscan II mentioned in this article, and is a paid consultant for Bausch & Lomb.

• Accutome, Inc can be reached at 263 Great Valley Parkway, Malvern, PA 19355; (610) 889-0200; fax; (610) 889-3233; e-mail: eyeopenr@voicenet.com; Web site: www.accutome.com.
• DGH Technology Inc can be reached at 110 Summit Drive, Exton, PA 19341; (800) 722-3883; fax: (610) 594-0390; Web site: www.pachymeter.com.
• The Haag-Streit Group can be reached at 5500 Courseview Drive, Mason, OH 45040; (800) 787-5426; (513) 336-7255; fax: (513) 336-7260: Web site: haag-streit.com.
• Nidek can be reached at 47651 Westinghouse Drive, Fremont, CA 94539; (800) 223-9044; fax: (510) 226-5750.
• Bausch & Lomb can be reached at 1400 N. Goodman St., Rochester, NY 14609; (585) 338-5212; fax: (585) 338-0898; Web site: www.bausch.com.
• Quantel Medical can be reached at 601 Haggerty Lane, Bozeman, MT 59717; (888) 660-6726; fax: (406) 586-2924.