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Editors Selection IGR 15-2

Clinical examination methods: IOP and CCT

Comment by Aachal Kotecha & David Garway-Heath on:

12446 Comparison of dynamic contour tonometry with goldman applanation tonometry over a wide range of central corneal thickness, Doyle A; Lachkar Y, Journal of Glaucoma, 2005; 14: 288-292

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The Pascal® dynamic contour tonometer (DCT) is emerging as a tonometric device that may be able to measure the intraocular pressure (IOP) largely independent of corneal properties. The principle of the DCT is based on the principle of contour matching. The hypothesis behind contour matching is that if the eye were enclosed by a contoured, tight-fitting shell that allowed the eye to maintain its natural shape with no distortion, the forces generated by IOP would act on the shell-wall. Replacing part of the shell-wall with a pressure sensor would enable measurement of this pressure. The DCT tonometer has a contoured tonometer tip, with integrated pressure sensor, that fits closely to the corneal contours, facilitating a direct transcorneal measurement of the anterior chamber pressure.

In a study by Doyle and Lachkar (467), the DCT was compared with the Goldmann applanation tonometer (GAT) in seventy-five treated glaucoma patients. In order to remove the confounding effect the use of topical hypotensive therapy would have on their study results, the authors studied the effect of CCT on the differential of IOP measurements made with the GAT and DCT. The study found that in patients with 'thin' corneas (mean &plm; standard deviation: 491 &plm; 19 µm), GAT significantly underestimated DCT IOP measurements, to the order of 0.7 mmHg per 10 µm reduction in CCT, in patients with 'thick' corneas (615 &plm; 22 µm), GAT over-estimated DCT-measured IOP by 0.2 mmHg per 10 µm increase CCT and in patients with 'regular' corneas (552 &plm; 16µm) , no difference between IOP measuring devices was found. Although the authors found that agreement between GAT and DCT was

'excellent' overall (mean difference (95% confidence intervals (CI)): -0.1 (-3.1 to +2.9) mmHg), when stratifying the patient data into CCT groups, they found that with 'thick' corneas the limits of agreement between the two measuring devices was almost doubled (mean (95 % CI): -0.06 (-6.5 to +4.2) mmHg). The study concluded that, although in some patients the variability of the DCT was larger than desirable, the instrument may give a more accurate assessment of the true IOP in patients with thin corneas.

The authors used an interesting method to analyse the data. Patients were stratified into three distinct CCT groups, classified as 'thin', 'regular' and 'thick', and for each, the CCT/IOP-measurement differential relationship was assessed. As a result, the power to detect the CCT/IOP-measurement differential relationship for each group is low. Therefore, it is difficult to strongly support the conclusions drawn regarding the usefulness of the DCT with different CCT's. Indeed, the large variability of the DCT in patients with thicker corneas may explain the insignificant CCT/IOP-measurement differential relationship found for this particular group. The authors graphically present the CCT/IOP-measurement differential for all 75 eyes, however, and it is clear that GAT underestimates DCT in patients with thin corneas, and this trend reverses with increasing CCT.

Another factor that may affect the strength of relationships found in this study is the type and duration of topical hypotensive therapy that the patients were taking. Prolonged treatment with particular types of medication may result in changes in corneal biomechanics, which in turn may affect the IOP-measurement with GAT. It would have been interesting to have analyses evaluating the effect of therapy on the CCT/IOP-measurement differential relationship.

The variability of the DCT has been commented upon in other papers using the device, and may be due to the relative unfamiliarity both clinicians and patients have with the technique, particularly with respect to the prolonged corneal contact required when using the DCT compared with GAT (minimum five seconds for an adequate DCT reading versus approximately one second for a GAT reading). This variability may diminish with regular use, although further studies are required to investigate this phenomenon. It may be that the instrument performs less well in patients with thicker corneas, as this paper suggests, and further work on this hypothesis is required before such conclusions can be drawn.

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