Therapeutic application of lasers in ophthalmology (Do any two)
Misiuk-Hojlo M¹., Krzyzanowska P.¹, Hill-Bator A.¹
¹ Department of Ophthalmology, Wroclaw Medical University, Poland
Laser effects in biological tissues can be divided into three general categories:
photochemical, thermal, and ionizing.
With the improvement of laser technology, the techniques with using different types of lasers (ruby, neodymium, neodymium: yttrium-aluminum-garnet, erbium, and argon) allowed to utilize lasers in the treatment and diagnostics of many eye disorders.
Photoradiation takes place when photosensitized tumor tissues are exposed to laser light for the purpose of producing photochemical damage. During photoablation, tissue is removed in some way by light, such as when intermolecular bands of biological tissues are broken, disintegrating target tissues, and the disintegrated molecules are volatilized. This can
be effected with, for example, excimer laser.
Photocoagulation causes denaturation of biomolecules when temperatures are
sufficiently high, about 600C or more. Temperature rise in tissues is proportional to the amount of light absorbed by that tissue. Absorption of certain light frequencies is high in pigmented trabecular meshwork, iris, ciliary body, and retinal pigment epithelium (owing to melanin), and in the blood vessels (owing to hemoglobin). Lasers commonly used
photocoagulation are argon, krypton, or diode Nd:YAG lasers.
Photovaporization occurs when the tissue temperature quickly reaches the boiling point of water, causing disruption (evaporation) before denaturation (photocoagulation).
Examples of clinical uses of these lasers are holmium: YAG or erbium: YAG laser sclerostomy.
In photodisruption, short-pulsed, high-power lasers disrupt tissues by delivering enormous irradiance to tissue targets. The high level of irradiance ionizes molecules in a small volume of space at the focal point of the laser beam, disintegrating into collections of ions and
electrons called plasma. This plasma expands rapidly, producing shock and acoustic waves that mechanically disrupt tissues adjacent to the region of laser focus.
Examples of photodisrupter lasers are the Q-switched and pulsed Nd:YAG laser.
Glaucoma laser treatment is often recommended when medical therapy alone is insufficient in controlling intraocular pressure, for those patients who have contraindications to glaucoma medications or, for any reason, are unable to use eye drops.
The most common glaucoma laser procedure is laser peripheral iridotomy (PI). A laser iridotomy is performed for patients with narrow angles, acute angle closure glaucoma, in the fellow eye of a patient with acute or chronic primary angle closure, or pupillary-block
glaucoma.
Laser peripheral iridotomy involves creating a tiny opening in the peripheral iris, allowing aqueous fluid to flow from behind the iris directly to the anterior chamber of the eye. This typically results in resolution of the forwardly bowed iris and thereby an opening up
of the angle of the eye.
There are two types of lasers in use today - Nd:YAG Q-switched
laser (2 – 8 mJ) or argon laser (800 – 1000 mW). Argon laser began to replace surgical iridectomy as a safer, non-invasive method of making an iridotomy in the late 1970s. It was demonstrated to be safe and effective, but required melanin for tissue absorption of the
energy, making it less easy to penetrate lightly pigmented blue irides.
The Nd:YAG laser replaced argon as the most common means of performing LPI in the late 1980s. The Q -switched mode of the Nd:YAG laser causes photodisruption of tissues by the formation of energy ionic plasma at the location of focus of very intense energy. It has the advantage
of not requiring the presence of melanin pigment for iris absorption.
Complications of laser iridotomy include: irritation, blurred vision, iritis, iris hemorrhage, elevated intraocular pressure, corneal injury or retinal burns.
Argon laser trabeculoplasty (ALT) is a procedure which has been proven to be efficacious for different types of open angle glaucoma: primary open angle glaucoma, pseudoexfoliation glaucoma and pigment dispersion glaucoma. Patients with poor medical compliance can benefit from ALT before other surgical intervention is considered.
In the ALT procedure, the eye surgeon directs a laser beam into the trabecular meshwork, which is the primary aqueous (fluid) drainage region of the eye. In most cases, 180 up to 360 degrees of the trabecular meshwork is treated with laser spots, which typically requires about 40 to 80 laser applications. The effect of the procedure is increased drainage of aqueous fluid out of the eye and intraocular pressure reduction to 20 – 25%. Efficacy of the ALT procedure lasts for about 5 years.
Modification of this procedure is selective laser trabeculoplasty (SLT) performed with a Q-switched 532 Nd:YAG laser. SLT works by using a specific wavelength to irradiate and target only the melanin-containing cells in the trabecular meshwork, without incurring collateral thermal damage to adjacent non-pigmented trabecular meshwork cells and
underlying trabecular beams. The laser beam bypasses surrounding tissue leaving it undamaged by light. This is why, unlike ALT, SLT is repeatable several times. Indications for this procedure and complications (intermittent intraocular pressure elevation, iritis or
heamorrhage) are similar like in ALT.
Another glaucoma laser procedure is argon laser peripheral iridoplasty (ALPI). ALPI is a method of opening an appositionally closed angle in situations in which laser iridotomy either cannot be performed or does not eliminate appositional angle-closure because mechanisms other than pupillary block are present. The procedure consists of placing
contraction burns of low power, long duration, and large spot size in the extreme iris periphery to contract the iris stroma between the site of the burn and the angle, physically pulling open the angle. ALPI is recommended in plateau iris syndrome, or angle closure
glaucoma. The argon laser is set to produce contraction burns (500 µm spot size, 0.5 to 0.7 second duration, and, 200-400 mW power).
Cyclodestructive procedures in glaucoma lower the intraocular pressure (IOP) by reducing aqueous inflow as a result of distruction ciliary processes. The use of light energy to ablate the ciliary body was first proposed by Weekers and co-workers in 1961 using xenon arc photocoagulation In 1972 Beckman and Waeltermann performed the first
transscleral cyclophotocoagulation (TSCPC) procedure with the ruby laser. Since then, the neodymium (Nd):YAG and diode lasers have been used for transscleral cyclophotodestruction. Due to the high complications rate, and the unpredictability of the amount of IOP reduction, these procedures are usually reserved for the following conditions:
eyes with glaucoma refractory to other forms of surgical or medicinal therapy, eyes with poor visual potential, neovascular glaucoma, traumatic glaucoma, aphakic and pseudophakic glaucoma, chronic partial or total angle-closure glaucoma, aniridia or iridocorneal endothelial
syndrome.
Light coagulation and laser treatment of the retina were introduced to ophthalmology around middle of the last century. They are widely used for the treatment of diabetic retinopathy and other ischemic retinopathies. Retinal laser photocoagulation improves inner retinal oxygenation, which affects retinopathy through the relief of hypoxia and consequent
change in growth factor production and hemodynamics.
Diabetic retinopathy is a leading cause of visual loss in industrialized countries. Its classification includes preclinical, nonproliferative (mild, moderate, and severe or preproliferative diabetic retinopathy) and proliferative stages (low risk, high risk, and advanced). Diabetic maculopathy (exudative, edematous, or ischemic) may be associated with
either nonproliferative or proliferative retinopathy. Prevention requires the tightest possible control of both blood glucose and blood pressure. Laser photocoagulation remains the only procedure recommended for severe nonproliferative or proliferative retinopathy and maculopathy. (14) The Diabetic Retinopathy Study (DRS) showed that the rate of severe
visual loss in high- risk proliferative diabetic retinopathy could be reduced by as much as 60% following the timely application of panretinal laser photocoagulation therapy.
Results from the Early Treatment Diabetic Retinopathy Study (ETDRS) demonstrated that focal laser photocoagulation treatment to the macula region could substantially reduce the risk of visual acuity loss in patients with clinically significant diabetic macular edema.
Retinal vein occlusion (RVO) is a common retinal vascular disorder that frequently is associated with severe visual loss. There are two forms of retinal vein occlusion, branch retinal vein occlusion (BRVO) and central retinal vein occlusion (CRVO). A branch retinal vein occlusion is essen tially a blockage of the portion of the circulation that drains the retina
of blood. Central retinal vein occlusion is closure of the final retinal vein (located at the optic nerve) which collects all of the blood after it passes through the capillaries. There is presently no effective treatment available to prevent or restore the visual loss from acute CRVO.
Following a vein occlusion, the primary concern is to treat the secondary complications: macular edema, macular ischemia (non -perfusion) and neovacularization (growth of new abnormal blood vessels). Argon or diode l aser treatment may be useful in managing these complications. One type of laser treatment, focal laser, can be used to close off areas of
leakage from the blood vessels that cause macular edema. Another type of laser treatment, panretinal photocoagulation, can cause neovascularization to regress by making the retina less starved for oxygen.
Nowadays, laser treatment is also available in the age-related macular degeneration (AMD), a disease of our civilization. Macular degeneration is a progressive eye condition affecting the central vision and causing irreversible blindness in people over the age of 50. AMD has two basic forms: dry and exsudative. Dry AMD accounts for about 90% of cases, is
the milder form of the disorder. Exsudative AMD is the much more visually debilitating form of macular degeneration, often accompanied by choroidal neovascular membranes, which are the leaky vascular structures under the retina. There are two basic forms of laser treatment for exsudative AMD: conventional argon or diode laser therapy and the recently approved photodynamic therapy (PDT). Conventional laser burns the abnormal blood vessels and thus stops the leakage. However, since it also damages the normal retina structures, it may itself lead to decreased vision. Hence, it is suitable only in selected cases where the new vessels are
not very close to the central macular area.
The concept of the new treatment for exsudative AMD is the closure of subretinal choroidal neovascularization (CNV) without significant
damage to the surrounding tissues, such as photoreceptors or retinal pigment epithelium (RPE).
In PDT, a photosensitizer, Verteporfin is administered intravenously and allowed to perfuse the CNV, as well as the remainder of the body. Fifteen minutes after the start of intravenous infusion, the verteporfin is activated by a red laser of a specific wavelength (689nm). The non -thermal laser light activates the verteporfin producing the singlet oxygen
that both coagulates and reduces the growth of abnormal blood vessels. This, in turn, inhibits the leakage of fluid from the CNV.
The first attempts to treat intraocular tumors by means of photocoagulation were carried out in the late 1950s by G. Meyer-Schwickerath with the xenon arc photocoagulator. (13) Nowadays, lasers are an irreplaceable tool in the management of malignant and benign
intraocular lesions. Transpupillary thermotherapy (TTT) using an 810nm i nfrared laser has become one of the most popular treatments for small melanomas. Lasers can be also used as an adjunctive tool in combination with other treatment modalities in therapy regimens for
medium or even large melanomas. The main advantages of laser treatment compared to other modalities like irradiation are the broad availability, the relatively easy performance and thus reproducibility, the high precision during the treatment, and the safety for the adjacent tissues.
Corneal laser surgery with the modern excimer laser is known to be the most frequently applied laser procedure in medicine. The interaction between corneal tissue and the excimer laser was first investigated in 1981 by Taboada, who studied the response of the epithelium to the argon fluoride (AF) and krypton fluoride (KrF) excimer laser. (19) The use
of lasers to reshape the anterior corneal curvature to correct refractive errors has become an established clinical procedure.
Surgical techniques such as photorefractive keratectomy (PRK) and laser in-situ keratomileusis (LASIK) are used to correct optical aberrations of the eye, such as myopia or hyperopia, as well as astigmatism. During PRK for the correction of myopia, direct flattening is achieved by the removal of a convex-concave lenticule of tissue from the outer surface of the central cornea. Clinical studies determined refractive success
rates of between 80 and 95% for corrections up to –6 D of myopia. (18) A modification of this technique involves the microkeratome to make a lamellar flap (average thickness, 120-160 mm) of anterior corneal stroma, followed by refractive ablation of the exposed stromal bed.
This flap is then repositioned on the exposed stroma, and good adhesion is usually obtained without the need for sutures. This procedure was particularly investiga ted in eyes needing high myopic corrections if more than – 6 D.
In the recent years, a new method has been developed and used. This is called laser subepithelial keratectomy (LASEK). (3) In this procedure, the epithelial layer is completely removed, but about 900
of the circumference is allowed to remain as a short of hinge. After
the laser treatment, which is equivalent to PRK, the epithelium is replaced.
Laser photocoagulation is also mainly used in the retina abnormalities such as: tears, breaks, holes, lattice degeneration or retinoschisis, which predispose to a rhegmatogenous retinal detachment. With argon laser photocoagulation a thermal burn is created to surround the lesion and any subretinal fluid associated with it. The burn becomes an adhesion between
the retina and retinal pigment epithelium, and this limits potential flow of fluid from the vitreous cavity through a break.
Visual loss occurring secondary to opacification of the posterior capsule after extracapsular cataract extraction is the major indication for laser capsulotomy. Posterior capsulotomy for creating openings in an opacified posterior capsule can be performed with the argon laser of the pulsed Neodymium-YAG laser.
It is impossible to imagine ophthalmology today without lasers, ubiquitously and thoroughly do they dominate the field. A rapid explosion of argon laser techniques occurred in the late 1970s and early 1980s. In the 1990s, another explosion occurred in the treatment of posterior segment disorders, including macular degeneration and intraocular tumors.
The development of lasers for plastic surgery, cataract extraction, and ocular imaging is progressing rapidly and is expected to find much greater use and usefulness in the coming years.
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