Christoph Kniestedt

Dynamic contour tonometry: presentation of a new tonometer.

With tonometers currently in use intraocular pressure is indirectly determined by measuring a physical quantity related to a specified deformation of the cornea. We present a new principle of direct, continuous, and transcorneal intraocular pressure measurement, describe its theoretical foundation, and evaluate its application on the basis of an in vitro model. Methods:On a living human eye an optimized pressure-sensitive contact surface was determined by performing pressure measurements with differently shaped tonometer heads. Based on these results and on the theoretical model, a Dynamic Contour Tonometer was constructed and validated on eye bank bulbi against a manometric reference pressure. Results:A concave contact surface with a radius of curvature of 10.5 mm creates a distribution of forces between the central contour matching area of the tip and the cornea that equals the forces generated by the internal pressure of the eye. A sensor integrated into the surface having the same contour measures the intraocular pressure closely to the manometric reference pressure in human cadaver eyes. The accuracy of the tonometer appears to be unaffected by variations in corneal properties. Conclusion:Dynamic Contour Tonometry eliminates most of the systematic errors arising from individual changes of corneal properties that adversely influence all types of applanation tonometers. The advantage of measuring the true pressure in combination with the capability of registering dynamic pressure fluctuations discloses new tonometric opportunities to diagnose and classify different types of glaucoma.

Correlation between intraocular pressure, central corneal thickness, stage of glaucoma, and demographic patient data: prospective analysis of biophysical parameters in tertiary glaucoma practice populations.

PurposeTo determine the correlation of central corneal thickness (CCT) to Goldmann applanation tonometry (GAT) and dynamic contour tonometry (DCT, PASCAL®), and to glaucoma stage as assessed by cup-to-disc ratio (CDR). DesignProspective, cross-sectional tricenter observation study. Patients and MethodsFrom three glaucoma specialty practices a sample of 406 independent eyes was included. After ultrasound pachymetry, intraocular pressure was measured using PASCAL® and Goldmann applanation tonometry and cup-to-disc ration was reassessed. Demographic data were included in the multivariate analysis. ResultsMean corneal thickness was 540 μm. African Americans and normal-tension glaucoma patients showed the lowest values (518 μm and 522 μm, respectively). These values were significantly thinner than the central corneal thickness of Caucasians (549 μm) and ocular hypertensives (564 μm). Intraocular pressure assessed by Goldmann applanation tonometry shows a significant correlation with central corneal thickness (r2=0.068, P<0.001), whereas PASCAL® is not significantly associated with central corneal thickness (r2<0.001, P=0.997). Increased IOP is significantly correlated with large ocular pulse amplitudes (r2=0.13, P<0.001), which is predominantly seen in ocular hypertensives. A significant negative correlation was detected between cup-to-disc ratio and central corneal thickness (r2=0.102, P<0.001). ConclusionGlaucoma patients with thin central corneal thickness are more likely to be found at an advanced stage of the disease and among those with normal-tension glaucoma and black African ancestry. Underestimation of intraocular pressure by Goldmann applanation tonometry could be one causative factor.

Visual acuity and its measurement

Despite its critical importance to our daily life, the most common measurement of visual function, visual acuity, is a relatively crude and narrow one testing only a small portion of the broad range of visual functions. Visual acuity is the measurement of the ability to discriminate two stimuli separated in space at high contrast relative to the background. Clinically, this is measured by asking the subject to discriminate letters of known visual angle. The visual acuity is represented as the reciprocal of the minimal angle of resolution (the smallest letters resolved) at a given distance and at high contrast. Other measurements of visual acuity also exist, including Vernier acuity. Newer charts, such as the ETDRS chart, use letters of equal recognition difficulty and use the log of the minimal angle of resolution; these charts have significant advantages over the old Snellen-type charts. This article reviews visual measurements in children and in patients with low vision, and it reviews factors affecting visual acuity, such as pupil size, refractive error, media opacities, and pharmacologic agents.

Dynamic Contour Tonometry

Dynamic contour tonometry (DCT) is a new technology for noninvasive and direct measurement of intraocular pressure (IOP); its results are thought to be influenced less than those of other methods by structural characteristics of the eye. The curvature of the pressure sensing device is concave and only slightly flatter than that of the human cornea. The cornea adapts to the curvature of the transducer head, and the sensor in the centre of the adapted area measures the pressure on the other side of the cornea in the force-free range. Studies published so far suggest that DCT is less heavily dependent than applanation tonometry on the biomechanical properties of the cornea . A further advantage of DCT over other forms of tonometry is the capability of dynamic measurements over time. It is also possible to measure both the diastolic and the systolic IOD and determine the difference between the two, i.e. the ocular pulse amplitude (OPA). OPA is an indirect indicator of choroidal perfusion and reflects the condition of the arterial vascular system and the action of the heart. It could be important in the diagnosis and treatment of glaucoma.

Dynamic contour tonometry: principle and use

Interindividual variability of central corneal thickness has been found to be a source of error for conventional Goldmann applanation tonometry. The dynamic contour tonometer represents a potentially new technology for non‐invasive and direct intraocular pressure (IOP) measurement, and has been proposed to accurately measure the true IOP irrespective of the corneal thickness. It is based on the principle that when the tip of the device exactly matches the contour of the cornea, the pressure measured by a transducer placed on its tip is an accurate indicator of the true IOP. This device is also capable of measuring the ocular pulse amplitude, a variable that has controversial significance in the diagnosis and management of glaucoma. Even though this technique seems to be very promising, further studies are required to conclusively determine the effectiveness of the dynamic contour tonometer in patients having an abnormal or irregular corneal contour.

Intraocular Pressure and Ocular Pulse Amplitude Comparisons in Different Types of Glaucoma Using Dynamic Contour Tonometry

Purpose: To compare the intraocular pressures (IOP) and ocular pulse amplitudes (OPA) in patients with different types of glaucoma, ocular hypertension (OHT), and normal controls (NC) using dynamic contour tonometry (DCT) and the goldmann applanation tonometry (GAT). Methods: 906 eyes of 501 adult patients in the following five groups were included in this cross-sectional study: primary open angle glaucoma (POAG), normal tension glaucoma (NTG), Pseudoexfoliative Glaucoma (PXG), OHT, and NC. The following tests were performed simultaneously during a single visit: IOP using DCT and GAT; OPA using DCT and central corneal thickness (CCT) using ultrasound pachymetry. Mixed effects regression models were used to compare the DCT and GAT IOP measurements in the five groups; the effect of CCT on IOP and the relationship between OPA and IOP within each group. Results: DCT consistently had higher IOP values than GAT in POAG, PXG, NTG, and controls (p < 0.001) but not in OHT (p = 0.84). DCT IOP did not change while GAT IOP showed a non-significant increase (p = 0.09) with increased corneal thickness in each group. OPA was found to be highest in OHT (3.61 mmHg) and lowest in the control group (2.86 mmHg) and significantly increased with IOP in all groups. Conclusions: DCT measures an IOP that is significantly higher than GAT IOP in glaucoma and control subjects but not in ocular hypertensives. Furthermore, the DCT may measure an IOP that is independent of the CCT, which may not be true for the GAT, which increases with the CCT. OPA was highest in OHT and may be affected by the IOP.

Tonometry Through the Ages

The definition of glaucoma has changed over the decades from a simple ocular pressure disease to a systemic disorder of multivariate etiology. Glaucoma may be defined for the individual eye as a chronic ocular disease with various underlying pathophysiologic disorders. However, elevated intraocular pressure (IOP) is still the most important risk factor for an untreated glaucomatous eye to progress to a more severe stage of the disease. As the main risk factor within therapeutic reach, IOP and its appropriate measurement deserve our ongoing interest. Not only has our understanding of glaucoma changed but also our approach to the measurement of the IOP. In this article we focus our attention on the various developments in tonometry from the simple force-tonometers of the late 19th century to the high-technology pressure tonometers that were recently introduced for clinical use.

Role of central corneal thickness on baseline parameters and progression of visual fields in open angle glaucoma

PURPOSE To evaluate the relationship of central corneal thickness (CCT) to baseline visual field parameters and visual field progression in patients with primary open-angle glaucoma (POAG). METHODS Charts of consecutive patients with POAG were reviewed to obtain visual field data. Visual field was measured by standard threshold static perimetry. Variables analyzed included mean deviation (MD) and pattern standard deviation (PSD). RESULTS A total of 121 eyes examined over 4 years were evaluated. A significant negative relationship between CCT and PSD (correlation coefficient: -0.02, p<0.05) was found. Analyses comparing CCT to change in PSD and MD (visual field progression) were statistically not significant. CONCLUSIONS Patients with thinner corneas initially present with a greater visual field defect, indicating that thin corneas may contribute to advanced glaucomatous damage at the time of diagnosis. However, CCT does not seem to be a significant risk factor for progression of the disease.

Prevalence of plateau iris syndrome in young patients with recurrent angle closure.

Background:  To assess and describe the prevalence and clinical features associated with plateau iris syndrome (PIS) in young individuals with recurrent angle closure despite initial therapy.

Tonometrie im Wandel

ZusammenfassungDas dynamische Konturtonometer ist das erste und einzige nichtinvasiv, kontinuierlich und direkt messende Tonometer. Durch Aufsetzen des drucksensiblen Tonometerköpfchens wird die Hornhaut schonend in die Form gebracht, die sie natürlicherweise annimmt, wenn der Druck auf beiden Seiten gleich ist. Zwischen Tonometerköpfchen und Hornhaut entsteht ein Kräftefeld, das genau dem Augeninnendruck entspricht. Ein in die Kontaktfläche integrierter piezoresistiver Drucksensor misst kontinuierlich und präzise den Augeninnendruck und erfasst auch dessen zeitliche Schwankungen als okuläre Pulsamplitude (OPA). Die Abhängigkeit des Tonometers von den biomechanischen Parametern der Hornhaut ist deutlich geringer als bei den herkömmlichen, auf Applanation oder Indentation beruhenden Messverfahren.AbstractThe dynamic contour tonometer (DCT) is the first and only noninvasive contact tonometer that is capable of measuring intraocular pressure (IOP) directly and continuously. The touch of the pressure-sensitive tonometer tip induces the cornea to gently assume a shape (contour) which it will naturally assume when pressure on both sides is equal. A force field establishes between tonometer tip and cornea, that corresponds exactly to IOP. A piezoresistive pressure sensor, integrated into the surface of the tonometer tip, precisely measures IOP continuously and therefore also records time-dependent modulations of IOP as “ocular pulse amplitude” (OPA). Dependence of the DCT on biomechanical properties of the cornea is substantially smaller than with traditional tonometers that applanate or indent the cornea.