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Biomedical subjects

F Fankhauser

Publications and source records attributed to F Fankhauser.

At least 55 records · Page 3Linked to original sources
PubMed1993

[Recollections of Hans Goldmann, 1899-1991].

Professor Hans Goldmann died in Bern on November 19th, 1991 at the age of 91 years. His outstanding intellectual capacity was discovered at an early age and demonstrated throughout his life. He was appointed as teaching assistant of the famous A. Cermak von Seysenegg, Chairman of the Institute of Physiology of the German Charles University in Prague. During his stay at the University of Prague, he was influenced by the famous people of his time, such as Einstein (physicist), Mach (physicist and psychophysicist), Lorenz (behavioral scientist), Popper (philosopher), Schlick (physicist and philosopher), Hering (physiologist), and others. Goldmann absorbed the essence of these disciplines to a very large extent. This, together with his remarkable intellect, enabled him to produce outstanding research work within a large spectrum of sciences more or less directly related to ophthalmology. Goldmann became known in particular for his exceptional and fundamental work on perimetry and glaucoma and he managed to cast his basic insight into practical, easy-to-operate, high-precision diagnostic instruments which, several decades after their invention, are still used by every ophthalmologist. He will enter history as one of the very great pioneers in ophthalmology.

History, 20th Century↗
PubMed1993

[Surgery with the laser scalpel. Physical principles and clinical effectiveness].

BACKGROUND: The conventional surgical method for cutting vascularized tissue with scalpel and scissors may now be improved by use of the laser scalpel. In particular, the frequent interruption necessitated and the poor visibility caused by bleeding may be mitigated owing to the haemostatic properties of laser light. This instrument may be of particular value for tasks involving extrabulbar structures, such as extraocular muscles and lacrimal tissue, as well as for facial surgery. MATERIALS AND METHODS: The mode of action and physical properties of a new laser scalpel are described and its characteristics compared with those of other instruments on the market. The probe consists of a clad, sculptured silica fibre with a core diameter of 0.6 mm and a conical 0.15 mm-diameter cutting tip. Radiation generated by a low cw-Nd:YAG laser module is fed into the probe, at the exit point of which a maximal power density of 57 kW/cm2 is attained. Radiated laser energy penetrates the tissue as an incision is made, thereby inducing an efficient blood flow stasis which is amplified by thermal energy diffusing from the immediate surroundings of the scalpel tip. RESULTS: In this report, the laser scalpel is implemented for the excision of a vascularized, amelanotic, facial naevus. Both the cutting and haemostatic effects were found to be excellent, occurrence of the latter phenomenon being supported by ultrastructural findings. The healing response was comparable to that observed after conventional surgery. CONCLUSION: The new laser scalpel represents an inexpensive and effective cutting and haemostatic tool powered by a standard cw Nd:YAG laser module, with a wide spectrum of potential applications.

Aged↗
PubMed1993

The role of laser cyclocoagulation in cyclodestructive glaucoma surgery.

As a last-resort, antiglaucomatous procedure, laser cyclodestruction continues to attract attention. Whether this approach will be applied at an earlier stage in patients with glaucoma remains to be seen, although this procedure is already used in patients who refuse or do not tolerate surgery. The success rate of this procedure will improve as our understanding of the underlying mechanisms that contribute to intraocular pressure reduction are more thoroughly realized, and as safety improves with increased sophistication of instrumentation.

Aluminum↗
PubMed1993

Versatility of the cw-Nd:YAG and diode lasers in ocular surgery.

The effectiveness of cw-Nd:YAG and diode laser light in performing transscleral chorioretinal coagulation is demonstrated in cadaver porcine eyes using a new contact probe that facilitates irradiation of these layers. Both radiation sources also may be viable alternatives to argon laser light in effecting chorioretinal and cycloendocoagulation.

Animals↗
PubMed1993

Evaluation of numerical displays of visual field data by human observers--the definition of scotomas and clusters by human interpreters.

Numerical printouts of 23 visual field examinations performed with glaucoma program G1 were analyzed by 3 perimetrists. The task consisted of defining the existence of clusters or scotomas. Considerable discrepancy with regard to the existence or the size of clusters and/or scotomas was found among the three perimetrists. The median difference in number of clusters as compared for each pair of interpreters was found to differ from zero at a significance level of 5% (Wilcoxon's signed-rank nonparametric test). These findings suggest that the interpretation of a visual field status by human observers will always remain subjective and to a large extent contradictory. This situation calls for the implementation of interpretative computer software to supplement the currently available algorithms.

Adult↗
PubMed1993

[Physical principles of sclerostomy].

Various sclerostomy procedures, performed ab interno and ab externo have found increasing interest for some time. With few exceptions, invasive procedures are described, in which laser energy is transmitted by means of specially designed fibers and probes ab interno and ab externo to the perforation site. Laser energy sources, ranging from the UV through the visible to the IR range have been described. Thus far, no detailed models quantifying the ablation mechanism of such sclerostomy procedures have been published. All such procedures have in common that the wavelength of the laser should be matched to water absorption bands. Now more sophisticated models describing the action mechanism of various lasers begin to emerge. A system described by us takes advantage of the high efficiency of rapid steam explosions occurring in a closed space, i.e. in the region delimited by the probe end and the fistula bottom. The high efficiency of such mechanisms has been verified in model experiments on porcine eyes in vitro. Electron microscopical results also show that the amount of thermal damage is negligible and that there is very little wavelength-dependence. This permits the adoption of "common" laser energy sources such as diode and Nd:YAG lasers. The clinical survival probability of fistulas as related to the amount of collateral damage is controversial.

Animals↗
PubMed1993

The efficacy of cw Nd:YAG laser trabeculoplasty.

The long-term (mean follow up, 51 weeks) efficacy of cw Nd:YAG laser trabeculoplasty in reducing intraocular pressure (IOP) was studied in 106 eyes of 75 patients. Fifty patients (73 eyes) of this group had primary open-angle glaucoma; 11 (17), mixed glaucoma; 10 (12), pseudoexfoliation syndrome; and 4 (4), secondary glaucoma of varied etiology. Fifty laser impacts were delivered over 180 degrees of the circumference of the chamber angle with an Nd:YAG laser working in the cw mode (spot diameter, 100 microns; exposure duration, from 200 to 300 milliseconds; constant power level, 9 W, resulting in 1.8 to 2.7 J/pulse and a total energy dose/session of 90 to 135 J). In a few cases, an exposure duration of 400 milliseconds was used, corresponding to a pulse energy of 3.6 J/pulse and a total energy dose/session of 180 J. Success was defined as a reduction in IOP to 22 mm Hg or less without any further laser or surgical intervention. The cumulative success rate was 97.2% at 1 month, 93.3% at 3 months, 83.3% at 6 months, 78.7% at 12 months, and 71.5% at 24 months. Thus, the efficacy of cw Nd:YAG laser trabeculoplasty is comparable to that reported for trabeculoplasty performed using the argon laser (ALT), the Nd:YAG laser working in the free-running mode, and the diode laser. Biomicroscopy failed to reveal any serious inflammatory reactions persisting longer than 1 week. We conclude that cw Nd:YAG laser trabeculoplasty is a safe and effective alternative to ALT.

Adult↗
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

Blood flow stasis induced by cw-Nd:YAG laser irradiation: comparative morphology of mesenteric and choroidal vessels in pigmented rabbits.

cw-Nd:YAG laser radiation was effective in producing blood flow stasis within arteries (35-40 microns in diameter) of pigmented rabbit mesentery (beam spot size: 80 microns; fluence: 2 x 10(4) J cm-2) and choroid (beam spot size: 200 microns; fluence: 6 x 10(2) J cm-2). However, the mechanism by which this was achieved differed in each case, and depended upon the energy absorbing structures present in the irradiated tissue. In non-pigmented tissue, such as the mesentery, haemoglobin represents the primary absorption centre, and the temperature attained within the vessel lumen (as inferred from morphological changes) is sufficient to denature plasma proteins, in particular fibrinogen, which consequently occlude the vessel lumen and arrest bleeding. In pigmented tissue, such as the choroid, melanocyte granules represent the primary absorption centre, which is thus shifted from the vessel lumen to the stroma. The temperature rise achieved within the vessel lumen is consequently lower, as evidenced by the absence of plasma protein denaturation. Blood flow stasis nonetheless occurs, but is triggered according to the normal haemostatic mechanism.

Animals↗