Clin Exp Ophthalmol. Dec;34(9) Dynamic contour tonometry: principle and use. Punjabi OS(1), Kniestedt C, Stamper RL, Lin SC. The PASCAL Dynamic Contour Tonometer (DCT) from Ziemer is a slitlamp mounted tonometer for measuring intraocular pressure IOP independent of corneal. The gold standard for assessing IOP is Goldmann applanation tonometry (GAT). Recently, the dynamic contour tonometer (DCT) has become.
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Copyright American Medical Association. Dynamic contour tonometry and GAT intraocular pressure differences significantly increased with older age slope, 0. Elevated intraocular pressure IOP is generally regarded as one of the major risk tonometrt for glaucoma, and its reduction is the most frequently used surrogate of successful management of risk factors. For that reason, accurate IOP is a fundamental variable in clinical practice.
However, its accuracy depends on many factors, including corneal thickness and other biomechanical properties. Several studies 4 – 7 have reported that most patients confour ocular hypertension have a high CCT, which may lead to a spuriously high IOP measured using an applanation device rather than truly elevated IOP.
Other studies 68 – 10 found that patients with normal-tension glaucoma have thinner corneas than those of the general population. Therefore, patients with normal-tension glaucoma may have a higher IOP than measured. Furthermore, a thin cornea may be an independent risk factor for the conversion to primary open-angle glaucoma POAG in patients with ocular hypertension, 11 although such an effect has been called into question. For this reason, it is controversial whether CCT is an independent risk tonometdy for progression of established glaucoma.
According to the manufacturer 18 and recent studies 1920 on cadaver eyes, DCT measurements are minimally dependent on structural properties of the cornea, particularly CCT. The concave surface of the tonometer tip matches the contour of the cornea; this contour matching creates equilibrium between capillary force, rigidity force, appositional force, and force tonomegry on the cornea by Cynamic.
A piezoelectric sensor integrated into the contoured surface of the tip measures IOP without systematic errors caused by these forces or by changes in the corneal biomechanical properties. In a prospective single-center study, consecutive patients with POAG were recruited from the glaucoma unit of the Department of Ophthalmology, University Hospital Basel, Basel, Switzerland, during a 6-month period between November 1,and April 30, Excluded were patients with pseudoexfoliation, a history of trauma, pigmentary dispersion, narrow or closed iridocorneal angle, evidence of any secondary glaucoma, any type of preceding refractive surgery and corneal disease, and chronic or recurrent inflammatory eye disease eg, scleritis or uveitis.
In addition, patients with poor cooperation, poor quality of DCT readings, and unreliable measurements due to astigmatism greater than 2 diopters were also excluded. The mean IOP reading for each measurement method was recorded.
PASCAL® Dynamic Contour Tonometer from Ziemer Group
The right eye was always measured first. After application of topical anesthesia to the cornea, a paper stripe impregnated with fluorescein was used to stain the precorneal tear film immediately before IOP measurement. The patient was asked to blink before measurement to ensure equal distribution. Goldmann applanation tonometry was performed using a slitlamp Haag-Streit, Koeniz, Switzerland with a tonometer calibrated according to the manufacturer’s guidelines.
Intraocular pressure readings by DCT were computed and displayed by the instrument, thereby tpnometry possible observer bias. Dynamic contour tonometry provides 5 different quality levels, with 1 being the best and 5 being the poorest.
As recommended by the manufacturer, only measurements of quality 3 or less were evaluated and included in the study. Because neither of the 2 methods can at the outset be assumed to be superior to the other, the difference between GAT and DCT was plotted against the mean of the 2 methods for analysis of individual pairs according to the method by Bland and Altman.
Mixed-effects models incorporated fixed and random effects. The patient was the random factor, and the eye side right vs left was dnamic fixed factor varying within the patient. Central corneal thickness and mean age were covariates; CCT varied within dynaic patient, but age did not. All patients were treated with monotherapy or combined topical dynamlc.
Dynamic contour tonometry: principle and use.
This indicates parallelism between the 2 methods. Previously published data dynmaic nonglaucomatous patients undergoing laser in situ keratomileusis suggested that the new nonapplanation DCT device depended less on CCT than GAT.
This may have in part contributed to the present results. Furthermore, the corneal rigidity in patients with glaucoma may be altered primarily or secondarily to topical drugs, possibly affecting IOP measurements, as some antiglaucomatous drugs may modulate the extracellular matrix.
Intraocular pressure measured using DCT was dynamlc higher compared with GAT measurements in human cadaver eyes, 19 in eyes undergoing refractive surgery, 2528 and in healthy eyes.
At present, it is unclear why the difference in this study population is so large. However, a study 29 differentiating between patients who had glaucoma and those who did not have glaucoma did not find different results between the 2 groups. In the present study, all patients with POAG had been receiving 1 or 2 topical antiglaucoma eyedrops for months or years.
Therefore, we cannot exclude that the use of eyedrops may have had an effect on corneal biomechanics and accordingly on IOP measurement. However, we did not subgroup patients with POAG according to different medication use because of the heterogeneity of the drugs used and the statistical weakness of such stratification. In previous studies, 3435 no significant relationship between CCT and the use of some topical IOP-lowering drugs has been reported.
Nevertheless, it remains to be elucidated whether various topical glaucoma medications might confound the relationship between CCT and IOP.
Dynamic contour tonometry: principle and use.
Likewise, whether there is an effect of topical drugs on biomechanical properties of the cornea other than CCT needs further validation, as there is evidence that some IOP-lowering drugs may alter tissue 3637 by stimulating the degradation of extracellular maxtrix eg, modulation of matrix metalloproteinases in conjunctival and subconjunctival tissue.
A thicker cornea does not necessarily mean higher rigidity of the cornea in all patients, and corneal factors other than CCT may play a role in the corneal biomechanics affecting IOP readings, such as hydration state, 2038 curvature, 38 and age of the patient. In this analysis, we used an interocular study design that has not been described previously, to our knowledge. The advantage of an interocular dtnamic intraindividual comparison is minimal interference from other nonocular factors in different individuals.
Corneal structure or biomechanics varies among patients even when CCT is the same. From a statistical point of view, the interocular design increases efficiency, with greater degrees of freedom than an interindividual dynxmic.
Seven percent of patients were excluded from the analysis because of the inability to obtain good-quality DCT. Some patients were unable to sit completely still, breathe quietly, and avoid slight movements of the eye or head. However, the promising advantages of DCT are the short learning curve, the ease of use of DCT in most patients in our experience, and the low intraobserver and interobserver variability because of semiautomatic recording.
April 13, ; final revision received November 2, ; accepted November 3, Relationship between corneal thickness and measured intraocular pressure in a general ophthalmology clinic. The Ocular Hypertension Treatment Study: Ocular surface inflammatory changes induced by topical antiglaucoma drugs: See More About Glaucoma Ophthalmology.
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