Franz Fankhauser

STATIC PERIMETRY: ACCURACY AND FLUCTUATIONS

Threshold fluctuations are divided into short‐term and long‐term effects applying statistical methods. Sixteen normal and pathological visual fields were analysed in order to obtain the numerical values for the short‐term, long‐term and total fluctuation. Special attention is paid to fortuitous variations between the results obtained at successive examinations. Finally the concept of spatial correlations is introduced.

Clinical studies on the efficiency of high power laser radiation upon some structures of the anterior segment of the eye

A series of pathological conditions such as pupillary block glaucoma, angle block glaucoma, acoria, pupillary membranes and goniosynechias of various origins have been treated successfully by means of pulses emitted from a Nd: YAG Q-switched laser above optical breakdown threshold. Since linear absorption is not required, non-pigmented or entirely transparent structures are easily dissected by this method. We have also opened the supraciliary and suprachoroidal space in a series of cases of wide angle glaucomas. In all cases however, the clefts closed by secondary wound reactions. This method may be applied to extracapsular cataract surgery for opening the anterior capsule before lens extraction and to the hydrophthalmic eye.

Clinical studies on high and low power laser radiation upon some structures of the anterior and posterior segments of the eye

A Q-switched Nd:YAG laser apparatusManufactured by LASAG AG, CH-3600 Thunhas been used in the treatment of a number of pathological conditions of the anterior and posterior segments of the human eye. This laser system may be driven in a free running mode, thereby allowing variation of pulse duration over a range of 12 ns to 10 ms. With increasing pulse duration, non-linear, mechanical damage patterns characteristic of the biological effects at the lower exposure duration range become unimportant when compared to the so-called thermal effects met at the upper pulse duration range. The thermal damage mechanisms may be useful when applied together with the mechanical damage mechanism, for instance when iris perforations are intended. Because the Wise-Witter irradiation method for the treatment of open angle glaucoma depends on low power, thermal mechanisms, this instrument may also be useful for the treatment of this disease entity. This same irradiation mode may also be applied to at least some irradiation tasks of the retina and choroid.

On automation of perimetry

A method enabling a fully automated computer analysis of the visual field is described. Starting from the most probable assumptions about the sensitivity distribution within the visual field, this distribution is approximated in 4 steps. Decisions are based on probability theory. Every analytical step builds up on the conclusions reached in the preceding one. The theoretical limits of the method in regard to the degree of approximation of the true sensitivity function are discussed in detail. The spatial resolution is determined by the density of the questions per area of visual field. For a grid constant of 3°, the spatial resolution of sensitivity defects varies between 4 and 7°. Otherwise, the quality of the approximation varies within large limits and depends on prior knowledge of the expected sensitivity distribution and on the noise in the system. This includes the patients threshold fluctuation and the reliability of the answers. The whole analytical program was tested on a large number of artificial visual fields contained in computer storage which were then tested by the main program. It is shown that even in the presence of large sensitivity fluctuations and a considerable fraction of erroneous answers by the patient the method is still able to extract the data which are essential from the clinical point of view. Es wird eine Methode der vollautomatisierten Perimetrie beschrieben. Die Empfindlichkeitsverteilung im Gesichtsfeld wird, ausgehend von der wahrscheinlichsten Annahme, in 4 Schritten angenähert. Den Entscheiduugen liegen wahrscheinlichkeitstheoretische Überlegungen zugrunde, wobei jeder analytische Schritt auf den Schlüssen des vorausgehenden aufbaut. Die theoretischen Grenzen der Methode hinsichtlich der Approximationsgüte werden in allen Einzelheiten geschildert. Die räumliche Auflösung hängt von der Dichte der Testpunkte im Gesichtsfeld ab. Bei einer Gitterkonstanten von 3° schwankt das räumliche Auflösungsvermögen für Scotome zwischen 4 und 7°. Die Approximationsgüte schwankt in weiten Grenzen. Sie hängt von der Vorkenntnis der Empfindlichkeitsverteilung im Gesichtsfeld und dem Rauschen im neuro-visuellen System ab. Schwankungen der Empfindlichkeitsschwelle und Konzentrationsstörungen des Patienten, die zu falschen Antworten führen, setzen die Approximationsgüte herab. Das ganze analytische Programm wurde an einer großen Zahl künstlicher Gesichtsfelder geprüft, die im Computer gespeichert wurden. Dabei zeigte sich, daß die Methode selbst bei einer hohen Patientenstreuung und einem hohen Prozentsatz falscher Antworten infolge von Konzentrationsstörungen noch in der Lage ist, aus einem komplexen Gesichtsfeld die Informationen zu extrahieren, die vom klinischen Standpunkt aus von Bedeutung sind.A method enabling a fully automated computer analysis of the visual field is described. Starting from the most probable assumptions about the sensitivity distribution within the visual field, this distribution is approximated in 4 steps. Decisions are based on probability theory. Every analytical step builds up on the conclusions reached in the preceding one. The theoretical limits of the method in regard to the degree of approximation of the true sensitivity function are discussed in detail. The spatial resolution is determined by the density of the questions per area of visual field. For a grid constant of 3°, the spatial resolution of sensitivity defects varies between 4 and 7°. Otherwise, the quality of the approximation varies within large limits and depends on prior knowledge of the expected sensitivity distribution and on the noise in the system. This includes the patients threshold fluctuation and the reliability of the answers. The whole analytical program was tested on a large number of artificial visual fields contained in computer storage which were then tested by the main program. It is shown that even in the presence of large sensitivity fluctuations and a considerable fraction of erroneous answers by the patient the method is still able to extract the data which are essential from the clinical point of view.

Some aspects of the automation of perimetry

Manual perimetry has certain inherent limitations stemming from both the patient and the examiner. Automated perimetry, if properly developed, can offer the advantages of exact reproducibility of the examination strategy, accuracy achieved through averaging of repeated determinations, and systematic application of previous data to the current examination. The history, concepts and clinical practically of automated perimetry are discussed. Several systems capable of performing functions resembling human intellectual activity are commerically available; one, the OctopusTM, is described in detail.

Ultrastructural changes of the trabecular meshwork of the monkey (Macaca speciosa) following irradiation with argon laser light

An attempt to elucidate the mechanism of the pressure-lowering effect after treatment of the chamber angle according to Wise and Witter with argon laser light is presented. The ultrastructural effects upon the trabecular meshwork of fourMacaca speciosa monkeys were analyzed at planned intervals by scanning and transmission electron microscopy.The ultrastructural effects of the treatment are strongly dependent on the site of the laser burns. Whereas burns delivered to the anterior part of the meshwork are tightly sealed by a monocellular layer of atypical corneoendothelial cells, this mechanism is less marked when delivered more posteriorly.

Morphological studies about the efficiency of laser beams upon the structures of the angle of the anterior chamber

Working at a power density above optical breakdown threshold, irradiation effects upon the angle of the anterior chamber of the Macaca speciosa monkey by a Nd glass Q-switched laser, have been analysed with scanning electron microscopy.Two different damage effects can be identified: openings of Schlemms canal and the creation of a cyclodialysis, that is opening the uveoscleral outflow routes; the latter may be a more effective one than the former. Also a third mechanism, namely a structural alteration of the trabecular meshwork, at the molecular level by laser action, has been inferred. The physical effects leading to optically induced mechanical damage are discussed. Applicability of such effects upon the morphological findings described here, is limited and to a greater part speculative. A better definition of the physical parameters, required for optimal therapeutical damage effects in the treatment of the wide angle glaucoma may be obtained by trial and error methods, which are guided by the results of electron microscopical analyses of samples obtained from in vivo and in vitro experiments.

The Ultrastructural Features of Laser Trabeculopuncture and Cyclodialysis

Microexplosions induced by a Q-switched Nd:glass laser working above optical breakdown power density levels are able to destroy the inner wall of Schlemms canal and to tear open both the supraciliary and suprachoroidal space. Furthermore, the ciliary muscles bundles may be disrupted, becoming subsequently permeable to the aqueous and thus also acting as a new outflow route. However, whereas breaks of the inner wall of Schlemms canals are sealed in the monkey by the activity of endothelial cells forming a new Descemet membrane over the impact site, the supraciliary and suprachoroidal spaces remain open over an observation period of up to 480 days. Perfusion experiments with a HgS tracer substance have shown that these newly created, potential outflow routes may be considered as real additional filtration areas.

Morphology of the Trabecular Meshwork within Monkey (Macaca Speciosa) Eyes after Irradiation with the Free-running Nd:YAG Laser

The long-term tissue response of monkey eyes to trabeculoplasty produced at the chamber angle by neodymium:YAG (Nd:YAG) laser (thermal mode) was investigated. Four weeks after irradiation, a monolayer of corneal-like endothelial cells extended over the trabeculum in impact zones, with partial occlusion of intertrabecular spaces; this condition persisted at all subsequent stages of observation (viz, up to 13 weeks). Both primary and secondary degeneration of trabecular endothelial cells occurred predominantly at the posterior border of the juxtacanalicular tissue, and continued throughout the observation period. Lysis and degradation of collagen fibrils were also prominent in this region, and collagen-phagocytosing cells could be seen at all stages. Although activated fibroblasts were marked in this zone, regeneration processes were, in general, less marked than those of degeneration. The significance of these findings in relation to the mechanism by which trabeculoplasty works is discussed.

The effect of thermal mode Nd:YAG laser radiation on vessels and ocular tissues. Experimental and clinical findings.

Ultrastructural examination of mesenteric vessels of pigmented rabbits after irradiation with the thermal mode of Nd:YAG laser showed an excellent hemostatic effect. Clinical experience parallels those findings.