Advances in pachymetry are offering practitioners easy-to-use, inexpensive ways to obtain corneal thickness data for diagnosing and managing patients with a variety of conditions.
Practitioners are using both ultrasound and optical pachymeters for various applications. Todays ultrasound pachymeters can be as small as a laptop computer, making them portable for those times when your patient flow renders it easier to move the instrument to the patient rather than move the patient to the instrument.
I think pachymetry should become more standard of care in optometric practices. Just like a topographer is standard equipment to me, and I cant imagine practicing without it, I view pachymetry as becoming that, too, said Louise A. Sclafani, OD, FAAO, director of optometric services and associate professor of clinical ophthalmology at the University of Chicago Medical Center.
Pachymetry readings can be used in managing glaucoma patients and glaucoma suspects when determining the correlation between the patients corneal thickness and the patients intraocular pressure.
Pachymetry has become truly essential in the glaucoma practice, said Robert E. Prouty, OD, FAAO, center director of Omni Eye Specialists in Denver. It has much greater use than ever before in the optometric office.
With the release of the Ocular Hypertensive Treatment Study, which found that central corneal thickness is a factor in the development of glaucoma, some guidelines have been set forth as a mechanism for knowing whether or not the cornea is influencing IOP measurements. However, assessing the thickness of the cornea in relation to the patients IOP is still subjective.
I use the Ultrasonic Pachymeter by Alcon (Fort Worth, Texas) on all glaucoma patients and glaucoma suspects, Dr. Sclafani explained. Anything between 500 µm and 530 µm is questionable. Above 530 µm is considered a thick and safe cornea. If a patients cornea is thinner than 500 µm with borderline pressure readings, I might be more aggressive in starting treatment for glaucoma patients.
Because corneal thickness plays a part in the pressure measurements, Loretta Szczotka-Flynn, OD, MS, associate professor in the Department of Ophthalmology, at Case Western Reserve University and director of the Contact Lens Services at University Hospitals of Cleveland, explained that she guages average corneal thickness at about 555 µm. If a patient has a significantly thicker cornea of about 580 µm or above, the pressure reading may be artificially increased due to the corneal thickness. Also, if the patient has thin cornea, the pressure may measure artificially lower.
Refractive surgery comanagement
A pachymeters corneal thickness data can also help determine the candidacy of refractive surgery patients.
Dr. Szczotka-Flynn uses both the pachymetry readings from the Orbscan (Bausch & Lomb, Rochester, N.Y.) and from an ultrasound pachymeter, the Corneo-Gage (Sonogage, Cleveland), for such patients.
The Orbscan, a larger instrument, offers an optical pachymetry reading along with other measurements.
The Orbscan finds the thinnest point on the cornea very quickly and easily, Dr. Szczotka-Flynn said. But, I also use the ultrasonic pachymeter to verify.
While a thin cornea may in itself rule out a patient for refractive or cataract surgery, the corneal thickness data can be valuable on many levels when pre-screening. The measurements can also warn of keratoconus, Fuchs dystrophy, inferior steepening or warpage of the cornea, swelling or a weakened endothelium.
For cataract surgery, someone who has a weakened endothelium is going to have a much higher potential of corneal edema, postoperative inflammation and swelling. There may also be potential to set off a full decompensation of the cornea following cataract surgery, Dr. Prouty explained.
Following surgery, pachymetry readings are also used to make sure complications are not developing. In the best-case scenarios, we do post-LASIK pachymetry readings at about 1 month following the surgery, and then we will monitor the patient annually, unless there are complications or unexplained vision loss, Dr. Sclafani said.
Corneal transplant patients
Because of the risk of rejection, thickening of the cornea in post-penetrating keratoplasty patients raises concern.
In these patients, Dr. Sclafani said she finds it helpful to look at both the elevation maps and the pachymetry maps obtained from the Orbscan. I find it very helpful to look at the maps comparing the elevation with the thickness, because it gives me a much better sense of whether the elevation is due to thinning and protrusion from internal forces, or if the cornea is actually thicker in one area, Dr. Sclafani explained.
Thickening of the cornea in a transplant patient may also indicate that the graft is failing in cases older than 15 years.
In corneal transplant patients with older grafts that are beyond the traditional early-phase risk of rejection, Dr. Szczotka-Flynn said she continues to use pachymetry to provide an assessment of how much life may be left on the graft.
Pachymetry readings may also help warn of keratoconus during a pre-screening for refractive surgery.
According to Dr. Prouty, you would not want to perform LASIK on a cornea with a progressively thinning disease such as keratoconus since you would thin it further.
It is helpful to look at the elevation data along with the corneal thickness data for this condition.
I dont just base it on thinness, Dr. Sclafani said. Often, a cornea is stable at 500 µm, but in a case where the periphery has 750 µm and the center is only 500 µm, there may be suspicion of keratoconus, she explained.
Overnight lens wear
A pachymeter can help monitor extended-wear contact lens patients, safeguarding against inferior steepening or warpage of the cornea that may occur from long-term wear.
As the patient population is increasingly wearing more continuous wear lenses, Dr. Sclafani said the need to use the pachymeter is increasing.
Personally, I started out in 1-day lenses and now am in 30-day lenses. I have built my patients into those lenses as I have built myself into them, she said. As more of my patients are becoming 30-day lens wearers, I feel the need to do more pachymetry on them.
Dr. Sclafani also obtains pre-fit pachymetry data on her patients with bifocal fits, highly astigmatic eyes and questionable refractive error, and follows up annually with those readings.
Optical vs. ultrasound readings
Which is better, optical or ultrasound? Optical and ultrasound pachymeters are just different ways to do the same thing, explained Dr. Prouty, who uses both the Orbscan and several different ultrasonic pachymeters from various manufacturers. Previously, all pachymeters were optical before the current ultrasound technology became available, he said.
One difference between ultrasound and optical pachymetry is that an ultrasound pachymeter needs to touch the cornea to obtain the reading, possibly creating a slight indentation and compression of the tissue and providing a slightly thinner reading than an optical pachymeter.
I have found that, in general, the Orbscan will measure 20 to 30 µm thicker than an ultrasound pachymeter, Dr. Sclafani said.
The ultrasound pachymeters are fast and portable and a different technique to get the same rough estimate, explained Dr. Prouty, Certainly there is some variability when you are down to 500 µm of measurement, he said.
Also, the more surgically altered the eye, the less consistent an optical pachymetry reading may be. In these cases, it may be beneficial to compare the two different pachymetry readings for the most accurate measurements.
Ultrasound pachymeters are less expensive and very fast methods of obtaining corneal thickness measurements. If more expensive equipment is not available, these pachymetry readings are just as valuable, Dr. Szczotka-Flynn explained.
For Your Information:
- Louise A. Sclafani, OD, FAAO, can be reached at 5841 S. Maryland Ave., Chicago, IL 60637; (773) 702-6953; fax: (773) 703-0830; email@example.com.
- Robert E. Prouty, OD, FAAO, can be reached at 55 Madison St. #355, Denver, CO 80206; (303) 377-2020; fax: (303) 377-2022; RProutyOD@Juno.com.
- Loretta Szczotka-Flynn, OD, MS, can be reached at 11100 Euclid Ave., Cleveland, OH 44106; (216) 844-5272; fax: (216) 844-7117; e-mail: firstname.lastname@example.org.
- Neither Dr. Sclafani, Dr. Prouty nor Dr. Flynn has any direct financial interest in the products mentioned in this article, nor are they paid consultants for the companies mentioned.