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Biomedical subjects

F Fankhauser

Publications and source records attributed to F Fankhauser.

At least 19 recordsLinked to original sources
PubMed1992

Induction of vascular haemostasis by Nd:YAG laser light in melanin-rich and melanin-free tissue.

Haemostasis was effected in vessels of melanin-rich (MR: choroid) and melanin-free (MF: mesentery) rabbit tissue irradiated with a cw-Nd:YAG laser. The following parameters were employed: - pulse duration: 200 ms (MR) and 100 ms (MF); focal spot diameter: 200 microns (MR) and 80 microns (MF); pulse energies: 100-250 mJ (MR) and 0.5-1 J (MF); irradiances: 1.6-4.0 kW cm-2 (MR) and 1-2 x 10(2) kW cm-2 (MF). In melanin-rich tissue, laser energy is absorbed principally by melanin granules contained within the stromal melanocytes. The heat generated in these structures radiates into the surrounding tissue where it is dissipated. The damage thus incurred by the endothelium of blood vessels encompassed within this field triggers the haemostatic mechanism whereby blood flow is arrested. This effect is realized by the formation of an occluding plug of platelets, which is stabilized by the deposition of fibrin, particularly in capillaries, and to a lesser degree in larger vessels of the vascular lamina. In melanin-free tissue, haemoglobin serves as the primary site of energy absorption, which is thus shifted from the stroma to the vessel lumen. Irradiation of vessels in such tissue leads to thermocoagulation of plasma proteins and consequent stasis of blood flow.

Animals↗
PubMed1992

A comparison of five methods for estimating general glaucomatous visual field depression.

The separation of local and diffuse visual field loss is important for evaluating the nature and extent of glaucomatous visual field damage. Here, five automated methods for estimating diffuse loss in glaucomatous visual fields (as measured with the Octopus G1 program) are compared. Four are taken from the published literature, and one is introduced in this investigation. It is shown that the new index (here called diffuse loss) provides the best agreement with a value determined using a more empirical approach.

Glaucoma↗
PubMed1992

Problems related to diffuse versus localized loss in the perimetry of glaucomatous visual fields.

The separation of diffuse loss from more localized loss in the glaucomatous visual field is desirable for a number of reasons. In previous papers, the characteristics of several methods and algorithms for distinguishing among normal visual fields and those exhibiting diffuse and localized loss have been described and compared. In particular, a new diffuse loss index has been introduced that serves to decide whether or not a diffuse loss component is present and, if so, helps to estimate the amount. Here the approaches utilized for calculating empiric probability maps (Heijl) and the pattern deviation plots (STATPAC) are compared with results obtained with the diffuse loss index algorithm. Based on 197 Octopus G1 examinations of glaucomatous visual fields, it is shown that both methods produce large differences when compared with the diffuse loss index. For this and other reasons, it is argued that both empiric probability maps and pattern deviation plots may lead the inexperienced clinician to ignore valuable information concerning actual pathology.

Algorithms↗
PubMed1992

Effects of timolol on visual-field mean retinal sensitivity in normal subjects.

This single-blind, randomized study investigated the effects of unilateral instillation of 0.5% maleate timolol twice a day for 7 days on visual-field parameters of both eyes. Twelve normal, young subjects (30.6 +/- 4.2 years) participated in this study. Using program G1 of the Octopus 500 automated perimeter, the visual fields were measured one week and one day before therapy and a third time one week after beginning therapy. The paired T-test was used for statistical analysis, where a P-value < or = 0.05 indicated significance. Whereas nothing statistically significant was found in the treated eyes, the contralateral, untreated eyes showed a statistically significant decrease of mean sensitivity between the first visual fields and the third ones (P < 0.01) This diminution of mean sensitivity in the untreated contralateral eyes may be due to statistically significant lowering of blood pressure (P < 0.01) and slowing of cardiac frequency, which were not compensated by a local decrease of intraocular pressure. If confirmed, these findings could be of clinical importance in the management of glaucoma patients, since some authors have, in some cases, recommended a unilateral therapy.

Adult↗
PubMed1992

[Hemostatic effect of the Nd:YAG laser in cw function].

The Nd:YAG laser may be effectively employed to induce blood flow stasis and vessel closure in rabbits. The mechanism by which these effects are achieved depends upon the nature of the primary absorbing structures in the irradiated tissue, as well as upon the energy level of radiation. Vascular wall damage occasioned in consequence of energy absorption in melanin granules triggers blood flow stasis according to the classic clotting cascade, defects in the endothelial cell wall being closed by aggregated platelets. In tissue lacking melanin pigmentation, energy is absorbed primarily by the haemoglobin of erythrocytes, and blood flow is arrested by coagulation of plasma proteins, platelets being absent from the impact centre. The portent of these different haemostatic mechanisms upon repair of the endothelial cell wall and neovascularization have yet to be determined.

Animals↗
PubMed1992

Laser cyclocoagulation.

Since its introduction in 1973 by Beckman and Sugar, cycloablation, using Nd: YAG laser energy has gained increasing attention. Nd: YAG laser energy may both be delivered in the contact and non-contact mode and the energy source may both work in the pulsed or cw mode. The physical and clinical effects of the various design principles differ. A short survey of the present state of the art is presented.

Ciliary Body↗
PubMed1992

Devices for the control of laser transmission across the sclera during transscleral photocoagulation.

Improving the reproducibility of transscleral photocoagulation necessitates controlling the transmission of the laser beam through the sclera. Two factors make such control problematic: the locally increased transparency of the sclera resulting from the contact procedure and the occurrence of time-related relaxation phenomena. Two instruments have been devised to help control these factors. The first is a mechanical compensation unit comprised of magnets and a spring that allows the force exerted on the sclera to be adjusted to between 0.1 and 0.4 N, the pressure being determined by the outer diameter of the contact tip. The second instrument monitors the portion of the aiming beam reflected by the sclera as a means of determining the exact level of power actually transmitted through it. This information theoretically could be fed back to the treatment laser, allowing the level of power being delivered to be adjusted accordingly.

Ciliary Body↗
PubMed1992

A new instrument for controlling pressure exerted on the sclera during contact Nd:YAG laser cyclodestruction.

We tested the device described by Rol et al in their article in this issue, designed to define and precisely control the pressure applied to the sclera during contact cyclodestruction with a cw-Nd:YAG laser. Since scleral transparency, and hence transmission of laser energy, depends on the pressure applied, control of this parameter represents an important advance toward the goal of standardizing the cyclodestructive procedure. In eight of nine cadaver pig eyes, pressure was exerted on the sclera using the pressure-controlling device, with a setting of 0.25 N for two of these eyes, and one of 0.4 N for the other six. In the remaining eye, pressure was controlled intuitively by an experienced surgeon. Applying forces of 0.25 or 0.4 N yielded similar success rates, although these fluctuated greatly even under constant pressure conditions (from 17% to 50% at 0.25 N; from 27% to 42% at 0.4 N). The highest success rate (55%) was achieved by the intuitively controlled pressure irradiations of the experienced surgeon. The variation in coagulation intensity that occurred despite constant pressure suggests that additional factors contribute to the results achieved. Nonetheless, this pressure-controlling probe represents an important achievement. It will permit not only the novice but even experienced operators to adhere unambiguously to a protocol of quantitative, reproducible parameters, and will also help researchers to better understand the relationship between energy delivered and intraocular pressure reduction achieved in contact cyclophotocoagulation.

Animals↗
PubMed1992

Optical principles related to optimizing sclerostomy procedures.

A simple probe, consisting of a 200-micrometer uncladded silica optical fiber, advanced from a protecting 22-gauge hypodermic needle, was used to create sclerostomy fistulas both ab interno and ab externo by means of a combined radiation/mechanical effect. Perforation was achieved by exerting gentle forward pressure on the fiber in synchrony with the delivery of radiation pulses. An irradiation protocol suitable for producing such canals in cadaver porcine eyes was delineated using Ho:YAG, Nd:YAG, and diode laser energy sources. Despite significant differences in the pulse energy required for perforation (0.25, 7, and 8 J for the Ho:YAG, diode, and Nd:YAG lasers, respectively), the extent of collateral damage was comparable for each type of laser. The physical mechanisms underlying these findings are discussed.

Animals↗
PubMed1991

A comparison of unweighted and fluctuation-weighted indices (within the central 28 degrees of glaucomatous visual fields measured with the Octopus automated perimeter).

The main visual field indices of Flammer and the analogous ones of Heijl have been compared based on a population of 113 glaucomatous visual fields. It is shown that for the population studied, the differences between the two sets of indices are negligible for Octopus G1 program results and that they may thus be used interchangeably.

Glaucoma↗
PubMed1991

The effects of weighting the "mean defect" visual field index according to threshold variability in the central and midperipheral visual field.

Two visual field indices, the mean defect (MD) of Flammer and the mean deviation (MD) of Heijl, have found wide acceptance among perimetrists. We compared these indices in 169 visual fields from normal- and high-tension glaucomatous eyes. Visual field damage in these eyes varied from slight to severe. In computations of the mean deviation index, the threshold values are weighted by the threshold deviations obtained from normal eyes as a function of eccentricity. However, the present study shows that the differences between the two indices in the population studied are negligible. Thus, subsequent interpretation is not affected by the choice of index, and the two MD indices may be considered to be interchangeable for the types of visual fields used in this study and for program-G1 examinations carried out using Octopus automated perimeters. Since we found smaller increases in local intersubjective fluctuations as a function of eccentricity in 274 normal visual fields as compared with results published by others, caution is indicated for interpretation of the visual field using probability weighting.

Humans↗
PubMed1979

Morphological studies about the efficiency of laser beams upon the structures of the angle of the anterior chamber. Facts and concepts related to the treatment of the chronic simple glaucoma.

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 Schlemm's 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.

Animals↗
PubMed1979

[Reparative processes of the iris after irradiation with the argon-ion laser (author's transl)].

The reparative processes of the pigmented iris of the rabbit were analysed with ultrastructural methods. 1. Clearing of the damaged area by macrophages is the first step in the reparative processes. Clump cells are macrophages which are observed from the first day of the injury until the ninth week. 2. Repair of the anterior surface of the iris is largely finished after 32 days. 3. The repair of collagenous fibres reaches its maximum activity 32 days after irradiation. 4. The pigment epithelium has only an insignificant regeneration potential. 5. Irradiation of the iris by the argon-ion laser results in an atrophic, hyperpigmented scar. The rapid regeneration of a lesion induced by the argon-ion laser in the rabbit iris casts doubt as to whether this method could be applied to the human eye with equal success.

Animals↗
PubMed1979

The ultrastructural features of laser trabeculopuncture and cyclodialysis. Problems related to successful treatment of chronic simple glaucoma.

Microexplosions induced by a Q-switched Nd:glass laser working above optical breakdown power density levels are able to destroy the inner wall of Schlemm's 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 Schlemm's 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.

Animals↗
PubMed1979

[Morphology of the primary damage caused by the argon-ion laser to the iris of the pigmented rabbit (author's transl)].

The effects of the argon-ion laser upon the iris of the pigmented rabbit were analysed by ultrastructural methods. 1. Apart from the physical parameters of the energy source, the damage strongly depends on the concentration, location, and distribution of the iris pigment. 2. The irradiation of the iris results in the formation of a crater. Depending on the distance of the epicentre of the impact, various degrees of ultrastructural damage are observed. a. The region immediately adjacent to the crater lumen at a revealing distance of 25 microns consists of homogenous collagenous masses, revealing a vacuolar structure. As the only residuals of destroyed cells, melanin granules are observed within the homogenous masses. b. Destruction of the collagen fibrils and their disintegration into subfibrils with elimination of all cell compartments is found in an area ranging from 25 microns to 300 microns from the wall of the crater. c. Damage to the chromatin structure is visible up to a distance of 300 microns to 375 microns from the wall of the crater. 3. Characterised by an invasion of macrophages into the damaged area, the cleaning phase starts 24 h after irradiation. 4. The results of this experiment indicate that because of the great energy dose required for man with the inherent widespread tissue damage and low probability of a lasting iridectomy, the cw argon-ion laser appears to be an unsuitable energy source for clinical iridectomy.

Animals↗