Ingo Wilmanns

Effect of Timolol on Optic Nerve Head Autoregulation

Healthy subjects show, in a special pressure compliance test of the optic nerve head, signs of autoregulation of optic nerve head circulation. In this test visual evoked response (VER) amplitudes were recorded during stepwise increased intraocular pressures. In the resulting VER amplitude/pressure curves glaucoma patients did not show the sign of autoregulation of optic nerve head circulation which is seen in healthy subjects. It is known that topically applied timolol maleate may affect optic nerve head circulation. Therefore the pressure compliance test was performed in 20 healthy eyes before and after 3 days of topically applied timolol maleate 0.5%. The aim of the study was to test the effect of topically applied timolol on optic nerve head autoregulation. In addition we determined the ocular perfusion pressures, heart rate and mean arterial blood pressure. While the ocular perfusion pressures show a slight, but statistically not significant, decrease under timolol, the systemic parameters and the autoregulation behavior did not change.

Ein neues Verfahren zur Kalibrierung elektrophysiologischer Untersuchungseinheiten

The method described in this paper permits determination of amplification, band-pass properties, and electrode impedance in electrophysiological experimental setups. The battery powered test-signal generator drives squarewave current pulses through a 2 ohm resistance, which acts as a source of 100 μV p-p pulses. The 2 ohm resistor is wired in series with one of the electrode leads and remains there permanently. For calibration of the above-mentioned properties, one need only turn the generator on, without changing the measurement setup. By observing the shape of the square waves after passing through the system, it is possible to judge the band-pass characteristics of the entire system. Electrode impedance may be measured by putting a (variable) resistor in parallel with the preamplifier input. The test signal current is of the same order of magnitude as currents from biological signal sources. This method makes working with electrophysiological measurement systems which are used in electroretinography, electrooculography, and recording of visually-evoked cortical potentials in ophthalmology much easier. Mit dem vorgestellten Verfahren können Verstärkung, Bandpaß und Elektrodenimpedanz in einem elektrophysiologischen Versuchsaufbau bestimmt werden. Der Prüfsignalgeber ist ein Rechteckimpulsgenerator, dessen Signalspannung von 100 μVSS an einem Widerstand von 2 Ohm abfällt. Dieser Widerstand liegt während der ganzen Untersuchung in Serie mit einer der Elektroden. Zur Kalibrierung der genannten Größen wird lediglich die Stromversorgung des Prüfsignalgebers eingeschaltet, sonst bleibt der Versuchsaufbau unverändert. Anhand der Verformung der Rechtecksignale kann der Bandpaß des ganzen Systems beurteilt werden. Mit Hilfe eines Widerstandes bekannter Größe parallel zum Vorverstärkereingang wird die Elektrodenimpedanz bestimmt. Die Ströme, die während des Prüfvorganges fließen, liegen in der Größenordnung der Ströme aus den biologischen Signalquellen. Das Verfahren erleichtert wesentlich die Arbeit mit elektrophysiologischen Meßsystemen wie sie für die Elektroretinographie, die Elektrooculographie und für die Registrierung visuell evozierter corticaler Potentiale in der Ophthalmologie verwendet werden.The method described in this paper permits determination of amplification, band-pass properties, and electrode impedance in electrophysiological experimental setups. The battery powered test-signal generator drives square-wave current pulses through a 2 ohm resistance, which acts as a source of 100 muV p-p pulses. The 2 ohm resistor is wired in series with one of the electrode leads and remains there permanently. For calibration of the above-mentioned properties, one need only turn the generator on, without changing the measurement setup. By observing the shape of the square waves after passing through the system, it is possible to judge the band-pass characteristics of the entire system. Electrode impedance may be measured by putting a (variable) resistor in parallel with the preamplifier input. The test signal current is of the same order of magnitude as currents from biological signal sources. This method makes working with electrophysiological measurement systems which are used in electroretinography, electrooculography, and recording of visually-evoked cortical potentials in ophthalmology much easier.

Automated corneal topography: computerized analysis of photokeratoscope images

The analysis of corneal topography has recently become popular, mainly as a consequence of the rapid development of refractive surgery techniques. To date, computer-assisted evaluation of photokeratoscope pictures has been the most popular method used to assess corneal topography. The processing system described previously by other authors requires hand-digitizing of the photokeratoscope pictures and therefore the constant presence of an operator for a considerable amount of time. We have introduced the use of a video camera to digitize the photokeratoscope images, making the processing automatic and quicker. Moreover, precision glass spheres have been used to calibrate the system in order to minimize the intrinsic errors even further. The data obtained from the computerized analysis are presented in three different ways. Besides having the numerical values in diopters, corneal maps with various shades of grey are used, together with the quantitative, three-dimensional representation of corneal astigmatism. An automated system for the analysis of corneal topography was tested for both experimental models and clinical conditions. It was easy to use and showed high precision (less than 1‰ error when processing photographs of precision glass spheres).

Changes of the current pathways in the eye due to coating agents during electroretinography

This study suggests a model by which we try to explain why coating agents habitually applied in electroretinography can change the amplitude of electroretinographic responses. The model is based on the results of experiments described in this paper. The accuracy of measurement in clinical and experimental electroretinography can be enhanced by applying this model. Es wird ein Modell vorgeschlagen, mit dem wir zu erklären versuchen, warum elektroretinographische Antworten durch die üblicherweise verwendeten Kontaktflüssigkeiten verändert werden können. Das Modell wird begründet durch die Resultate der in diesem Artikel beschriebenen Experimente. Es erscheint möglich, daß durch die Anwendung dieses Modells die erreichbare Messgenauigkeit in der klinischen und experimentellen Elektroretinographie erhöht werden kann.

The influence of coating agents on the electrode circuit in electroretinography

In electroretinography, coating agents are usually applied to improve the contact between the contact-lens electrode and the eye. These agents influence electroretinographic responses. In this experimental study we show that the characteristic properties of the electrode circuit are not influenced by the coating agent. Thus we conclude that these coating agents influence the eye itself. The mechanism of this interaction may be physical, or may be an unknown pharmacologic drug side effect. Bei der Elektroretinographie werden Augentropfen verwendet, die den Kontakt zwischen der Corneoscleralschalenelektrode und dem Auge verbessern. Seit kurzem ist bekannt, daß diese Tropfen die elektroretinographischen Antworten verändern. Die hier vorgelegten Resultate unserer Experimente zeigen, daß die charakteristischen Größen des Elektrodenstromkreises durch diese Augentropfen nicht verändert werden. Es muß sich deshalb um eine direkte Einwirkung auf das Auge handeln, die durch einen physikalischen Effekt oder eine bisher unbekannte Arzneimittelnebenwirkung hervorgerufen wird.

Nd:YAG Laser Shock Waves in Artificial Eyes

Nd:YAG laser surgery is performed by inducing optical breakdown in the interior of the eye. The optical breakdown is accompanied by shock waves which expand throughout the entire eye. In our experiments, we measured the amplitudes of shock waves at the walls of artificial eyes caused by single and multiple pulses of optical energy originating from an Nd:YAG laser at a wavelength of 1.064 micron. We have shown that the effects observed are not caused by mechanical resonance. We believe that similar results can be expected for human eyes. The peak pulse amplitudes in artificial eyes of different length were found to increase linearly as a function of the energy transferred into the eye. The peak amplitudes of pulse sequences were found to be only half of those for single pulses of comparable energy. In laser surgery, such sequences can be preferable if they achieve the same effect.

Kontrastmessungen am menschlichen Augenhintergrund

Bei der Ophthalmoskopie konnen wir die verschiedenen Strukturen des Augenhintergrundes nur deshalb erkennen, weil sie ein unterschiedliches Reflexionsvermogen zeigen und damit einen Kontrast aufweisen, durch den sie dem Betrachter unterscheidbar sind. Bei der Photographie des Augenhintergrundes (der Einfachheit halber erfolgt hier eine Beschrankung auf die Schwarz-Weis-Photographie) bewirkt der Kontrast eine verschiedene Schwarzung des Filmes. Um das begrenzte Vermogen photographischer Schichten, Leuchtdichteunterschiede getreu darzustellen, optimal nutzen zu konnen, empfiehlt es sich, Aufnahmesituation und photographischen Prozes aufeinander abzustimmen. Eine der Voraussetzungen dazu ist die Messung des Kontrastes, hier also am Augenhintergrund.

Bandpass measurements in the electroretinographic electrode circuit.

In the experiments in this study band-pass measurements in electroretinographic electrode circuits were done for the first time, according to the information presently available to us. Our experiments show that in the much faster band-pass assessment by analysis of sawtooth and square waves no major error is involved. It may be concluded from our results that the recordings of the ERG potentials are not changed by the characteristic properties of the electrode circuit in the frequency range of interest. In den hier beschriebenen Experimenten wurden zum ersten Mal, soweit uns bekannt ist, Bandpaßmessungen im elektroretinographischen Elektrodenstromkreis durchgeführt. Die Resultate unserer Experimente lassen erkennen, daß bei der wesentlich schnelleren Beurteilung des Bandpasses anhand geeigneter Testsignale keine gröberen Fehler gemacht werden.