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Central corneal thickness (CCT) is a variable that significantly affects intraocular pressure (IOP) measurements on indentation and applanation tonometry, and accounting for CCT during IOP measurement remains a challenge in glaucoma. In addition, most available tonometers measure the “force” needed to deform or applanate the cornea, and hence indirectly provide the intraocular pressure. Dynamic contour tonometry (DCT) may solve both these problems. DCT measures IOP after contour-matching between the human cornea and the pressure-sensitive transducer tip of the tonometer occurs. This allows the measurement of an IOP that is less dependent, or perhaps independent of CCT. Theoretically, this tonometer measures “pressure” rather than force, and hence directly measures IOP. In addition, the device incorporates the measurement of the ocular pulse amplitude (OPA), a variable that has controversial significance in the diagnosis and management of glaucoma. Many centers have studied its use in glaucoma with varying results; however, this new technique may hold promise for IOP measurement in glaucoma management. The human sclera and cornea are neither uniformly elastic nor thin and flexible, and hence most tonometers, including the widely accepted gold standard Goldmann applanation tonometer (GAT),1,2 approximate the intraocular pressure using averaged values of tissue properties, such as corneal elasticity and central corneal thickness (CCT). However, CCT seems to vary considerably between healthy individuals and groups at risk for or diagnosed with glaucoma, as shown by multicenter studies. The Ocular Hypertension Treatment Study reported a mean (+/– SD) CCT of 573 (+/– 39.0) μm,3 and the Rotterdam Study described a mean (+/– 2SD ) CCT of 537.4 (+/– 3.6) μm, with a range of 193 μm, and a maximal difference between eyes of 42 μm.4 Corneas tend to be thicker in patients with ocular hypertension, which may be explained, in part at least, by the fact that some of these eyes are misclassified owing to IOP overestimation.3 Argus5 described a mean ± SD CCT in his ocular hypertension group of 610 ± 33 μm. Patients having primary open-angle glaucoma (POAG) show slightly thinner corneas than those of control subjects.4,5 Most strikingly, normal-tension glaucoma (NTG) is associated with CCT in the low 500-μm range.6 These inter-individual/inter-group variations in CCT may serve as a source of error for GAT and other methods of IOP measurement.7 Thicker corneas require greater applanation forces compared to thinner corneas, and hence result in IOP overestimation. Conversely, very thin corneas may cause an underestimation of the IOP. Different studies have shown variations from 0.11 mm Hg7 to 0.71 mm Hg8 (with an average of 0.25 mm Hg) for every 10 μm of CCT change. Established methods of measuring intraocular pressure use indirect methods of measuring the IOP. The Goldmann, Schiotz, Perkins, Mackay-Marg, Draeger and all non-contact (“air-puff”) tonometers might be classified as “force tonometers” as they measure the force required to generate a defined amount of deformation of the cornea. They then indirectly estimate the IOP after taking into consideration a set of material constants which are assumed to be uniform for all eyes. A tonometer which could measure the true “pressure” directly and continuously, without being affected by CCT or corneal elasticity would be useful in clinical practice. Manometry would be able to measure the true IOP directly, but is invasive and impractical for routine use. The pneumotonometer according to Langham, another type of indirect device, allows a continuous measurement of IOP, displaying a pressure curve that reveals pulsatile short-term fluctuations in IOP. Dynamic contour tonometry (DCT) may solve some of these problems, and its principle and practical use are described in this article and accompanying video.

New Investigations: Contour-Matching Tonometry in the World of Glaucoma Diagnostics