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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

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↗
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↗
PubMed1990

Evaluating human and automated interpretation of visual field data in perimetry.

The complexity of visual field result evaluation has increased with the advent of automated perimetry. This in turn makes software assistance such as provided by the new Octosmart program interesting and desirable. Its accuracy and reliability in visual field evaluation must be tested, though, and the testing methodology itself must be developed. For this purpose, 27 visual fields were evaluated by three human interpreters and the Octosmart program. Based on a newly developed compound index, the performance of the three interpreters and Octosmart is described and a high degree of consistency is demonstrated.

Diagnosis, Computer-Assisted↗
PubMed1990

Transpupillary irradiation of the rabbit retina with the cw-Nd:YAG laser. I. Acute morphologic effects produced using two different pulse forms.

The retinas of six pigmented rabbits (12 eyes) were irradiated with a cw-Nd:YAG laser using two different pulse forms, in conjunction with a constant pulse duration of 200 msec and a retinal focus diameter of 200 microns. The first pulse form (a) was characterized by an initial transient, lasting 15-20 msec, with a peak power twice that of the ensuing plateau phase of 180-185 msec. The second pulse form (b) was almost rectangular in shape. Although the average energies (and powers) for each pulse form were identical, almost 20% of this entity was delivered within the first 20 msec for the first pulse form. Impact lesions were examined at the biomicroscopic, light, and electron microscopic levels, and the characteristic morphologic features described for the cw-mode in general. Comparison of the effects produced using each pulse form revealed a number of differences. At low pulse energies (less than 150 mJ) the rectangular pulse form produced a photocoagulation reaction within the outer sensory retina, but when using the pulse form with the initial transient, this effect was superimposed by a degree of mechanical disruption. Ruptures produced within the elastic component of Bruch's membrane first appeared at lower pulse energies (ie, at 75 mJ, as opposed to 150 mJ for the rectangular pulse form) and were more frequent, and choroidal macrohemorrhaging (observed at pulse energies greater than or equal to 200 mJ) occurred only, when this latter pulse form was employed. Each of these phenomena may be attributed to the higher power and energy densities attained during the initial transient, which has disruptive properties. In order to avoid the risk of severe bleeding, these irradiation parameters should be maintained below approximately 6.4 X 10(3) W/cm2 and approximately 0.13 X 10(3) J/cm2.

Animals↗