Applanation tonometry has been considered the gold standard for determining IOP for almost 50 years. This technique is less likely than Schiotz tonometry to be influenced by variables such as scleral rigidity.1 Although, in his landmark article,2 Goldmann discussed the influence of variable central corneal thickness on IOP as measured by applanation, he believed that significant variations in central corneal thickness occurred only rarely.

After an optical pachymeter became commercially available, researchers found a positive correlation between central corneal thickness and IOP as measured by applanation.3 This relationship has been confirmed in animal and human studies in which investigators performed intraocular cannulation to experimentally control IOP.4,5

Recently, the Ocular Hypertension Treatment Study determined that central corneal thickness is a significant risk factor for the progression to primary open-angle glaucoma in ocular hypertensive patients.6 This study was the first to prospectively demonstrate that a thinner central cornea predicts the development of primary open-angle glaucoma. Due to the newfound importance of central corneal thickness on IOP measurements, pachymetry measurements are rapidly becoming the standard of care for patients with glaucoma and glaucoma suspects. This article provides an overview of the technology.

ULTRASOUND
Ultrasonic pachymetry has largely replaced optical pachymetry, because ultrasound units are easy to use and accurate. Comparing the two types of pachymeters, Salz et al7 found that the optical pachymeter had two to three times as much intrasession variability as the ultrasound pachymeters. To assess the reproducibility of central corneal thickness measurements by means of ultrasonic pachymetry, Miglior et al8 conducted a study in which 51 volunteers completed three sessions of these measurements. The researchers found that measuring central corneal thickness by ultrasonic pachymetry was highly reproducible.

CHOOSING A PACHYMETER
When on the lecture circuit, I am frequently peppered with questions on the use of the different ultrasonic pachymeters. Which one is the best value for the money? How reliable are the devices? Are fancy extras necessary? Table 1 (click here for a downloadable PDF of Table 1) provides key data on the most popular commercially available ultrasonic pachymeters. The information comes directly from the manufacturers' product listings.

When selecting a pachymeter, one needs to consider the setting in which the unit will be used. For instance, if multiple offices will use the pachymeter, then a smaller, portable unit would be appropriate. Possibilities include the 55 Pachmate or 550 Pachette 2 (both DGH Technology, Inc, Frazer, PA), the Pocket II Precision Pachymeter (Quantel Medical, Bozeman, MT), and the Corneo-Gage Plus (Sonogage, Inc., Cleveland, OH). I have taken the 550 Pachette 2 on research trips to Ghana, West Africa, and have had no trouble with its transportation or durability.

Accuracy
All of the devices reviewed in this article seem to offer similarly excellent accuracy in pachymetry measurements. In 1992, Wheeler et al9 compared the accuracy and reproducibility of the Pach-Pen (Bio-Rad, Santa Ana, CA) and the DGH 1000 (DGH Technology, Inc.) and found the Pach-Pen to be more accurate, with measurements within 3 to 65 µm of the true corneal thickness. Intraobserver reproducibility was excellent with both pachymeters. Recently, McLaren et al10 compared measurements of central corneal thickness with the DGH 1000 and the Sonogage devices and found good agreement between the two.

Price
The cost of the pachymeters reviewed ranges from $2,495 to $8,500. Now that procedural reimbursement is available from most insurance carriers, the purchase of one of these units has become more affordable.

Bells and Whistles
The more expensive units have features that may be useful in select cases. The Advent Pachymeter (Accutome Ultrasound, Malvern, PA) has a 55º-angle probe that may allow for greater visibility under the surgical microscope. This unit's voice-audible mode might be a boon in the OR. The built-in printers with the SP-3000 and AL-3000 units (both from CBD/Tomey, Phoenix, AZ) would be helpful in cases where the accurate compilation of pachymetry measurements is necessary.

CONCLUSION
The Ocular Hypertension Treatment Study showed that measuring central corneal thickness is crucial to assessing a patient's risk for glaucomatous progression. Other studies now show the importance of measuring central corneal thickness in established glaucoma as well to further stratify patients' risk.11 A number of accurate, durable ultrasonic pachymeters are available that will allow the practitioner make this important calculation.

Leon W. Herndon, MD, is Associate Professor of Ophthalmology at the Duke University Eye Center in Durham, North Carolina. He stated that he holds no financial interest in the products or companies mentioned herein. Dr. Herndon may be reached at (919) 684-6622; leon.herndon@duke.edu.

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