Major ReviewMicropulsed Diode Laser Therapy: Evolution and Clinical Applications
Section snippets
I. Introduction
Therapeutic retinal photocoagulation has been practiced for more than 50 years. Since the initial experiments carried out by Meyer-Schwickerath in the late 1940s, laser treatment has gradually become more refined, effective, and safe. With conventional continuous-wave laser therapy, energy is absorbed mostly by the retinal pigment epithelium (RPE). However, heat is also conducted to neighboring structures, including the neural retina and the choroid, which can result in collateral thermal
II. Laser Parameters
Laser light has three important characteristics: it is non-divergent (collimated or parallel), monochromatic, and coherent (in phase). Because laser light is highly directional, it can be focused into a small spot size with high irradiance (laser power per unit area, W/cm2).41, 51 Increasing the spot size spreads power over a larger area and decreases irradiance intensity. The minimal spot size into which a laser beam can be focused is proportional to its wavelength and the focal length of the
III. Laser–Tissue Interaction
When the laser energy is emitted in the visible and near infrared spectrum, a rise in the initial temperature occurs where ocular chromophores absorb the laser energy and convert it into heat. These chromophores consist mainly of the melanin in the RPE cells and choroidal melanocytes. After approximately 1 msec, heat spreads toward adjacent locations where there is no light absorption.46, 47, 48 As the heat wave spreads, it releases energy and the temperature of the wave gradually decreases
IV. Mechanism of Action of Laser
Although the mechanism of action of laser photocoagulation remains poorly understood and subject to numerous theories, full-thickness retinal damage may not be needed to obtain beneficial effects.35 A recent hypothesis suggests that the benefits of laser photocoagulation may be derived from the up-and-down regulation of factors mediated by the biological reaction of RPE cells. This is more likely to occur in RPE cells that have been only sub-lethally injured by a lower thermal elevation than at
V. Complications Associated with the Use of Conventional Lasers
Undoubtedly, laser photocoagulation is the first line of treatment for many chorioretinal disorders, validated by many clinical studies.16, 19 Conventional laser treatment with an ophthalmoscopically visible endpoint, however, causes iatrogenic anatomic and functional chorioretinal damage.
VI. Limiting Retinal Damage
Although conventional photocoagulation is practiced as a destructive procedure, chorioretinal damage can be minimized by modifying laser parameters and clinical endpoints. Mainster summarized the optical and thermodynamic principles that can be applied to minimize retinal damage.49 We now discuss these principles in detail.
VII. Micropulse Operating Mode and Terminology
Dorin has provided a summary of the micropulse operating mode and terminology.17, 18 In continuous wave mode, the laser energy is delivered as a single pulse, with a “width” typically in the range of 0.1–0.5 sec that constitutes the exposure duration. In micropulse mode, the laser energy is delivered with a train of repetitive short pulses (typically 100–300 μsec in duration each) within an “envelope” whose width is typically in the range of 0.1–0.5 sec, and this envelope duration constitutes
VIII. Biophysical Basics and Mechanism of Action of Minimal Intensity Photocoagulation
Subthreshold minimal intensity photocoagulation (MIP) protocols are intended to minimize the laser-induced chorioretinal damage and spare the neurosensory retina.8 They are designed to avoid intra- and postoperatively visible burn endpoints. These protocols produce only sub-lethal thermal elevations, with effects that are invisible during treatment and remain so thereafter. The treatments adhere to the following biophysical criteria:17
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axial confinement of the thermal gradient (rise in
IX. Axial Confinement of the Thermal Gradient during Laser Exposure
In retinal photocoagulation, the temperature first increases at the RPE. From the RPE, it spreads by conduction to re-equilibrate with adjacent cooler tissues. In MIP, the inner retinal temperature must remain below the threshold of irreversible damage. Any increase in temperature at the RPE must be axially confined, which can be achieved by making the heat production time (the laser exposure duration) shorter than the thermal relaxation time for the space between the RPE and the neural retina.
X. Control of the Thermal Gradient in the Retinal Pigment Epithelium
Even when the rise in temperature is successfully confined around the RPE during laser exposure, unavoidable thermal equilibration could still damage the neurosensory retina.81 This can occur when the temperature at the RPE creates a heat wave that reaches the inner retina at lethal temperature levels. This post-operative inner retinal damage must be avoided by limiting the temperature rise at the RPE by adjusting the laser “irradiance” (power density in W/cm2). In order to deliver the needed
XI. Clinical Applications of Subthreshold Micropulse Diode-laser Photocoagulation
Subthreshold micropulse diode-laser (MPD) protocols using a micropulse 810 nm diode laser were originally pioneered by Friberg24 and Hamilton57 and are now gaining the interest of retinal surgeons worldwide. Emerging evidence indicates that subthreshold MIP protocols with a micropulse 810 nm laser can be as effective as the more destructive conventional photocoagulation with visible endpoint in several chorioretinal diseases.9
XII. Problems with Micropulse Diode-laser Laser Treatment
The greatest limitation of MPD laser procedures is the difficulty of titrating the treatment without the feedback of an ophthalmoscopically visible endpoint. Conversely, minimizing chorioretinal laser damage permits confluent therapy and re-treatment of the same areas, which may be needed in macular edema. Re-treatment is feasible after MPD because MPD does not produce chorioretinal scars that could expand or increase the risk of choroidal neovascularization.
The treatment protocol is not yet
XIII. Summary
The introduction of the infrared diode laser and its proven efficacy in treating diabetic macular edema has provided a valuable insight into the mechanism of action of retinal laser therapy. Direct closure of microvascular abnormalities with a relatively heavy burn is not necessary to achieve the desired clinical therapeutic endpoint. Micropulsed diode laser therapy has lent further weight to this concept, with an increasing body of clinical evidence suggesting that resolution of retinal
XIV. Method of Literature Search
Searches were done on Medline/Ovid/Embase in addition to a manual search from 1975 to the present. Search words used alone or in combination included micropulse, laser, diabetic macular edema, selective laser, diode laser, central serous chorioretinopathy, vein occlusions, glaucoma. Additional sources were articles cited in the reference lists of retrieved articles. All articles relevant to the field were considered for the review. Non-English literature was reviewed with the help of
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Giorgio Dorin is an employee of Iridex Corporation. The authors reported no proprietary or commercial interest in any product mentioned or concept discussed in this article.