Biomedical subjects
Publications and source records attributed to S Kwasniewska.
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.
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.
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.
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.
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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.
We describe a universal laser apparatus for multipurpose clinical tasks. The main unit consists essentially of a Haag-Streit slit lamp combined with a rigidly mounted Q-switched laser. The apparatus is designed basically for performing photodisruptive tasks. Radiation emitted by either a pseudocontinuous cw Nd:YAG laser (with a dynamic range of 10 W) or by an air-cooled 4-W argon ion laser energy source either may be transmitted via fiber optics into the main unit or, when detached from the main unit, may be used independently. Energy emitted by the modules and transported by specialized light cables and delivery systems can be used to perform transscleral Nd:YAG laser irradiation of the ciliary body or the choroid and retina. Probes are available for endocoagulation of the latter two by either cw Nd:YAG or argon laser light. Other probes are being developed for chorioretinotomy, internal sclerostomy, dissection of the sclera, or for other extrabulbar work. All functions are monitored and displayed by a central processor.
The morphological effects of trabeculectomy, performed as therapy for a case of chronic simple glaucoma, are described in the two eyes of a patient ten and thirteen months after treatment. Ten months after surgery, a transscleral channel, extending from the position formerly occupied by Schlemm's canal to the conjunctiva, is found. This channel is formed from a network of extended fibroblasts and loosely-packed collagenous tissue, and a large number of vessels, including both dilated blood capillaries and lymphatic vessels, develop within it. The sclera and subconjunctiva react differently to the presence of the nylon sutures. The findings support the hypothesis that the formation of an intrascleral outflow route stimulated by surgical intervention plays an important role in lowering IOP after trabeculectomy.
In a morphological study, the long-term effects of transscleral cyclophotocoagulation on rabbits using the free-running Nd:YAG laser with either a contact or a non-contact delivery system are described. A marked atrophy of the irradiated ciliary processes may be observed. Regeneration of the ciliary epithelial layers is incomplete at least up to 8 months after treatment. The vascular network is also atrophic, and shows a small regenerative capacity only.
Long-term changes in the vascular network of rabbit ciliary processes induced by Nd:YAG laser cyclophotocoagulation were examined both from a morphological viewpoint and with respect to three-dimensional organization (in vascular casts). Relative changes in intraocular pressure (IOP), monitored within eyes irradiated across one-quarter, one-half, three-quarters, or all of the ciliary body circumference, are discussed in relation to morphological changes. The clinical implications of these findings are considered.
The specific advantages of a number of integrated optical systems, consisting of optical probes of various shapes and materials, coupled to a number of optical fibers are described. The radiation fields of the various systems vary significantly in their performance and the clinical effects may be expected to vary correspondingly. This is a step forward in the theoretical evaluation of optimal laser microsurgical tools for specific applications.
Difficulties encountered when measuring the differential light sensitivity of the blind spot and its surroundings and the clinical interest of such measurements are reported. The determination of the differential light sensitivity across step gradients, as well as abrupt changes observed at and close to the blind spot, may pose severe problems, because both are strongly influenced by even slight fixation instability. However, other factors reported herein may be equally important. The insight gained here may be cautiously extrapolated to the difficulties (and to the efforts to overcome them) encountered when measuring other scotomata in the visual field having sensitivity distributions similar to those of the blind spot. The sensitivity distribution in the blind spot and the pericoecal region may be important in a number of disease entities, particularly in glaucoma and neurologic diseases.
The BSPOT evaluation program has been created to run on an IBM-PC type computer. It accepts Octopus SAPRO data that have been transferred to the PC and enables the user to display aspects of the data for evaluation purposes. Some applications are illustrated using the results of two blind spot examination series, one normal and the other pathologic, and the results of an angioscotoma examination. Although the program was designed for the display of blind spot data, any SAPRO examination data may be used.
The neodymium:YAG laser may be used to advantage in the thermal mode for iridectomy. A variety of other tasks which are traditionally performed using the argon laser can also be efficiently carried out using this mode of operation. In this article the authors describe in particular the method of transscleral cyclophotocoagulation with the neodymium laser in the treatment of glaucoma, which is becoming increasingly popular.
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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.