Dynamic contour tonometer versus goldmann applanation tonometer.

To the Editor: I read with interest the thoughtful editorial by Drs Schneider and Grehn in the February 2005 issue of Journal of Glaucoma and appreciated the comments concerning the intraocular pressure measurement (IOP). We have some agreements with the authors. Besides this we have some objections to their study. Drs Schneider and Grehn mentioned that in clinical practice, advantages from dynamic contour tonometer (DCT) can be expected for cooperative patients, outpatients, and patients with sufficient bilateral ocular fixation, whereas Goldmann applanation tonometry (GAT) measurements are more reliable in case of patients with inadequate cooperation, poor vision, or nystagmus. We think this idea does not reflect the trues and needs to be revised. As a measurement method there is nearly no difference between GAT and DCT. Measurement with the 2 methods can be applicable under topical anesthesia and the tips of the instruments need to be contact with corneal surface. DCT through its concave elastic tip that is made from medical-grade silicone does not applanate the cornea and fits the corneal contours. Bilateral ocular fixation is not necessary for DCT measurement. Only monocular steady fixation on primary position is enough for the measurement of the same eye. Actually this is also necessary for true IOP measurement with GAT. Furthermore measurements with DCT can be carried out in patients with poor vision. We think that the only disadvantage of the DCT is longer time needed to contact with corneal surface because of waiting minimum of 3 full, undisturbed heartbeat cycles required. It is hard to measure IOP not only with DCT but also GAT in patients with inadequate cooperation and nistagmus. GAT is still the most used and the gold standard method for IOP measurements. Although the GAT is reliable and accurate through a wide range of IOPs, errors in measurement can arise from a number of factors, including those that the central thickness of the cornea affects IOP readings and repeated tonometry reduces IOP, causing an underestimation of the true level. IOP measurements with Pascal DCT (Swiss Microtechnology AG, Port, Switzerland) are not influenced by central corneal thickness and other variations in corneal properties. In routine clinical examinations it will appear to take place of GAT. The functioning principle of Applanation Tonometers in particular the GAT is based on measuring the force required to achieve a specified deformation of the cornea (3.06mm in diameter). Repeated IOP measurements with GAT results falsely hypotony because of the force necessary to applanate the cornea. Drs Schneider and Grehn were performed IOP measurements in the following sequence: GAT and DCT in their study. We think that this sequence might be wrong. There was an approximately linear decrease in IOP with successive measurements with GAT, though the difference between the first and second readings was the largest. So IOP measurement with DCT after GAT may be resulted with falsely low IOP measurements. Instead of GAT, if initial IOP measurements would be done by DCT or in randomized order, it would be more suitable choice. DCT fits the corneal contours with the aid of its silicon tip so that there is little or negligible force on cornea and repeated IOPs does not affect the actual IOP levels.