Applied Laser Optics: Techniques For Retinal Laser Surgery
NELSON R. SABATES and FELIX N. SABATES
Table Of Contents
FACTORS INFLUENCING RETINAL PHOTOCOAGULATION
|Laser photocoagulation is the transfer of light energy into heat energy, which
then denatures proteins and produces tissue coagulation. Laser
radiation is directional and essentially coherent, allowing the beam
to be focused to a very small spot of high irradiance. In addition, its
monochromatic light allows the choice of different wavelengths to produce
desired effects in specific tissue layers. These wavelengths extend
from 400 nm, which is blue light, to 800 nm, which is the infrared
Laser application can affect tissue according to different interaction modes: thermal, photochemical, and ionizing. These applications require high irradiance in a small area to produce their main effects. Nearly all of the laser procedures used in the treatment of retinal disease are thermal processes with the exception of photodynamic therapy (PDT), which is a photochemical process. (Fig. 1).
The photocoagulation effect is the most commonly used in retinal laser surgery. Ocular photocoagulation uses heat produced through the absorption of light by ocular pigments. A rise in temperature of about 10°C to 20°C will cause coagulation of the ocular target tissue. The amount and rate of increase in temperature depend on the location of the target tissue as well as the degree of absorption of the specific wavelength that is used.
If the power is increased beyond the point of producing a minimal effect, the lesion size in the treated area will increase beyond the spot size. Associated with this is the important fact that it requires only a relatively small increase in temperature to change from a minimal tissue response to a heavy one. Obviously, it is necessary to make power increases in small increments in photocoagulation.
Laser energy applied to the retina has the highest absorption variance per unit in the pigment epithelium, owing to the high density of melanin granules in that tissue. In a small spot, the temperature rises in the pigment epithelium rapidly, reaching a constant value and equilibrium when the heat being introduced is equal to the heat being removed by conduction from the tissues. In larger spots, a higher temperature is obtained in the center of the burn, since heat removal from the center is less effective than from the edges. If the spot size is changed, an adjustment must be made in the power level. Decreasing a spot size requires a decrease in power level, whereas increasing a spot size requires a power increase.
Photocoagulation can be used to obliterate abnormal neovascular membranes, produce chorioretinal scars in retinal reattachment surgery, and treat single blood vessels. Photocoagulation is used in the treatment of many retinal diseases, including diabetic retinopathy, subretinal neovascularization, retinal vascular disease, sickle cell retinopathy, and peripheral retinal tears. It is also used in laser trabeculoplasty for the treatment of glaucoma. The most common laser used for these procedures is the argon green light source.
It is important to understand the concept of power density delivery of the coagulating beam applied to retinal tissue. Spot size, power settings, and exposure times are determined prior to treatment but can be, and often are, adjusted during the treatment. Different protocols have been established for different applications and interactions; however, these are only general guidelines. Flexibility is important and is based on training and experience with different settings and parameters.
The relationship between spot size, exposure time, and power, which are used to produce the correct power density, need to be closely monitored. If the spot size does not change, the power and exposure intervals should be kept constant until a good power density has been found. Any decrease or increase in spot size should be accompanied by a decrease or increase in input power until an appropriate response is obtained. Careful titration of the laser response is warranted. In practice, there are many factors that affect the size of the spot and power density, such as wavelength, media opacity, and absorption quality of the tissue to be treated.
Photovaporization develops when laser energy increases the temperature of the cellular and extracellular water to 100°C, producing steam and vaporization of the tissue. Usually, a wavelength with low penetration is used, so most of the laser energy is absorbed superficially and creates tissue incision by tissue vaporization. After the tissue is vaporized, further application of laser energy results in even higher temperatures, producing photocarbonization of tissue.
The carbon dioxide laser has been used in ophthalmology primarily for photovaporization and photocautery. A great advantage of photovaporization is that it produces an almost bloodless incision by sealing blood vessels and lymphatic tissue in its path.1 Intravitreal photocautery has been used in treating fibrovascular fronds.2
Hyperthermia is the process whereby the laser causes an elevated tissue temperature above the normal 37°C but still avoids coagulation or vaporization effects. This temperature generally ranges between 42°C and 52°C. At these temperatures, biomolecules undergo changes that may result in significant membrane alteration. If this type of hyperthermia is maintained for a period of time (20 seconds to 3 to 4 minutes), irreversible effects occur.
Hyperthermia procedures on the retina are represented by transpupillary thermotherapy (TTT). TTT has been used in the treatment of choroidal melanoma and is being investigated as a treatment for occult choroidal neovascular membranes.3 Indocyanine green may be injected prior to treatment of a choroidal neovascular membrane to enhance specific absorption at the irradiation wavelength. The usefulness of these treatments is still under investigation.
Extremely high irradiance delivered with a very short exposure in a small spot will result in electrons being energized from molecules in target tissue, producing a collection of free electrons and ions called plasma. Rapid expansion of the plasma creates acoustic and shock waves that, combined with latent tissue stress, incise the target tissue, producing photodisruption independent of tissue pigmentation or laser absorption. Transparent tissue can be incised in this way, obviating the need for conventional surgery. The most common laser source used is the Nd:YAG operating at the 1064-nm wavelength used in the anterior segment of the eye for discission of the posterior lens capsule, peripheral iridectomy, and synechotomy. In the posterior segment, it has been used successfully in the cutting of vitreous membranes and vitreoretinal bands. Because of the high amount of energy delivered at the focal point of disruption, it is important that the focus of energy be as far away from the retina as possible and at least 6 mm from the crystalline lens.4 Although there is intense heat at the point of plasma formation, the spot is so small that the heat dissipates rapidly.
A chemical reaction can be made to occur by intense laser energy if the energy of the photons is high enough. Energizing of photons increases with shorter wavelengths; therefore, ultraviolet light is more effective in producing photochemical reaction than is visible or infrared light. The photochemical effect of laser radiation has been used experimentally in photoradiation by red (630 nm) wavelength to produce cytotoxic free radicals in a tumor previously sensitized by a hematoporphyrin-derived uptake.5
The most common photochemical effect in use today is PDT, an example of laser treatment with a photosensitizing agent. This new treatment modality combines low-power diode lasers (689 nm) with an infusion of verteporfin to ablate subretinal neovascular membranes. The absorbed energy of the laser activates a series of photochemical events, beginning with the verteporfin and ending with the occlusion of vascular tissue in the subretinal membrane.6 The advantage of this therapy over conventional photocoagulation is that the lower energy levels result in much less damage to adjacent normal tissue.
Photoablation is produced by a photochemical reaction using an intense excimer (ultraviolet) laser. The most common ophthalmologic use of this therapy is in excimer lasers used in refractive surgery. At present, there are no uses for this therapy in the treatment of retinal diseases.
|FACTORS INFLUENCING RETINAL PHOTOCOAGULATION|
|Optical characteristics of ocular tissues play an important role in the
effectiveness of photocoagulation in the retina. Laser light traveling
through the eye toward the target tissue is affected by the degree of
reflection, scattering, and absorption of laser energy by ocular tissues
and by media opacities. Other important factors affecting laser photocoagulation
are the degree of absorption by ocular pigments, specific
wavelengths used, spot size, power applied, and the exposure time
used to produce the desired reaction in the retina (Fig. 2).|
REFLECTION, SCATTERING, AND MEDIA ABSORPTION
When a laser beam irradiates a biological tissue, this optical radiation usually undergoes a series of processes, which can ultimately affect the power of the laser beam at the target tissue.
Reflection of the beam usually occurs when an abrupt change in refractive index occurs, most commonly at the interface of different refractive media. The reflection of the beam depends on the texture of the reflective surface: the smoother the surface, the cleaner the reflection. The amount of reflection is usually not a major issue aside from its ability to decrease the useful power density of the laser.
Scattered light represents the loss of laser energy available to reach the target tissue. This loss is due to absorption by ocular tissues other than the target tissue and by reflection away from the target or back out of the eye. Increased scattering occurs with shorter wavelengths. The short blue wavelength scatters more than the longer red light, which explains why lesions produced in the retina with argon blue-green light are larger than those produced with the red wavelength using the same size spot at the corneal level. Importantly, media opacities also increase scattering and decrease the energy available to produce the retinal lesion. The power density needed to penetrate a media opacity to produce a satisfactory retinal lesion suddenly becomes too great when applied to another area through clear media, resulting in a severe reaction.
Starting in early adulthood and advancing with age, scattering in the ocular media increases, more with the shorter blue wavelength than with the green light and longer wavelengths. For example, three times more power is needed at the cornea when using blue light than green, and approximately two times more power is needed with green light than with the yellow to produce the same retinal lesion.
The presence of yellow pigment in the aging lens produces increased scattering and absorption at the level of the lens, most noticeably with the short blue wavelength. Scattering and absorption at the level of the lens may increase temperature, resulting in damage to the lens fibers and decreased energy reaching the retina. The difference between scattering and absorption is that scattering processes do not really alter the energy balance, while absorption transfers energy from radiation to matter. Absorption by ocular pigments has a great deal to do with the power density at which a laser beam affects a target tissue.
ABSORPTION BY OCULAR PIGMENTS
Laser light is mainly absorbed in ocular tissue by three pigments: hemoglobin, melanin, and xanthophyll. The reaction of retinal tissue to the irradiation with a given wavelength, time setting, and power density varies depending on the concentration of these three pigments (Fig. 3). Melanin strongly absorbs all ultraviolet and invisible wavelengths. However, this absorption by melanin decreases with increasing wavelengths. Hemoglobin has strong absorption in the violet (420 nm) and green (514 nm) wavelength. Deoxygenated hemoglobin absorbs red more strongly than does oxyhemoglobin. Xanthophyll, which is the pigment most densely distributed in the macular area, absorbs the blue wavelength (460 nm).
Several wavelengths are clinically available for photocoagulation (see Fig. 2). The argon laser with blue and green wavelengths was the laser used for many years in the treatment of choroidal retinal diseases. The majority of commercial argon lasers in the past used a light beam that was 70% blue (488 nm) and 30% green (514.5 nm). Blue light, however, has many disadvantages (Fig. 4). It is scattered more in the media than is green, yellow, or red, and the scattering increases with aging of the ocular media. Higher levels of energy at the corneal level are needed to obtain an appropriate retinal burn compared with longer wavelengths, especially in older persons.
Photochemical damage is higher for shorter wavelengths than for longer wavelengths, and scattered light could possibly produce damage in the retina adjacent to the area treated. This is most important when large amounts of irradiation are applied in the retina (such as in panretinal photocoagulation in the treatment of diabetic retinopathy).
Most importantly, blue light is absorbed by the xantbthophyll (yellow) pigment present in the inner and outer plexiform layer of the macular area, producing increased damage during photocoagulation of this area. This increases the risk of central scotomas following treatment with blue laser light. Figure 5 shows marked destruction of the inner layers of the retina in experimental studies when blue light is used for photocoagulation.7,8
The use of the blue wavelength has been discontinued for use in photocoagulation because of its destructive properties and in favor of other wavelengths, most commonly the green wavelength. The green argon laser light has a wavelength of 514 nm. It is the most widely available and popular laser light for retinal photocoagulation and can be found in lasers made entirely for pure green output and blue-green lasers with filters providing the pure green wavelength.
Green and yellow wavelengths have similar advantages (Figs. 6 and 7). Both wavelengths are absorbed by hemoglobin in blood vessels and are not absorbed by the xanthophyll pigment of the macular area. However, yellow is at least two times better absorbed by hemoglobin and is a more effective wavelength in the destruction of abnormal vascular structures of the eye.9 Much less energy is necessary to produce a retinal lesion with the yellow wavelength than with the green, and, as shown in animal experiments, less involvement of the inner layer of the retina is observed with either wavelength as compared with the blue (Fig. 8; see Fig. 5).
Green and yellow have some disadvantages, namely, absorption occurs at the vitreous level in cases of vitreous hemorrhage, resulting in possible tissue damage and a decrease in energy available to produce the desired lesion. Furthermore, if a layer of blood is present in the inner layer of the retina, increased energy uptake is produced in the inner layer of the retina, preventing treatment of deeper structures, such as a subretinal neovascular membrane (Fig. 9). The yellow laser wavelength is not frequently used because of the cost of instrumentation and equipment. It still remains, however, the best wavelength to treat vascular lesions because of the increased absorption by oxyhemoglobin, which reduces the amount of power required to obtain the tissue reaction needed to coagulate the vascularized tissue.
The red wavelength has been used in some retinal diseases, such as age-related macular degeneration. The red wavelength, such as that produced by red krypton, is very useful and has unique features (Fig. 10). It penetrates more deeply through ocular structures, producing its absorption in the melanin at the deeper retinal layer close to the choroidal vessels, the most likely source of the abnormal subretinal neovascular membrane in age-related macular degeneration.
Less energy is absorbed by the inner retina (see Fig. 8) with decrease of retinal fibrosis. Red light is not absorbed by yellow macular pigment (xanthophyll). Decreased absorption by hemoglobin is also useful when treating a deep lesion through a thin layer of blood in the inner layer of the retina (see Fig. 9). This is true clinically, although theoretically red light is reflected by the red hemorrhage. Less retinal vasculitis is observed when treating subretinal structures through or close to retinal blood vessels.
The main disadvantage of the red wavelength is that it is absorbed primarily by melanin in the choroid and pigment epithelium. Variations in pigmentation will result in unequal energy uptake with similar power settings, creating the risk of too much irradiance in a small area, which could result in chorioretinal bleeding. Additionally, the bleeding is difficult to control with the red laser since the wavelength is not absorbed by oxygenated hemoglobin (though it is absorbed somewhat by deoxygenated hemoglobin).
Coherent light with an orange wavelength has been obtained with the use of dye lasers.10 The effect in retinal tissue appears to be a combination of the damage produced by the yellow and red wavelengths, with much of the damage present at all levels of the retina. The orange wavelength has never really been substantially used in the treatment of retinal diseases.
The diode laser produces an infrared, long wavelength in the range of 700 to 820 nm. Because of decreased scattering and absorption, the infrared laser penetrates vitreous hemorrhage and nuclear sclerotic cataracts better than shorter wavelengths, such as green and yellow. The deeper penetration spares the inner sensory retina. The lack of xanthophyll absorption provides safety of delivery to the macula. The lack of hemoglobin absorption allows penetration through thin types of pre-retinal or subretinal hemorrhage without excessive laser energy uptake.
Some of the disadvantages of the diode laser include the inability to treat vascular abnormalities and to adequately treat subretinal neovascular membranes in light-colored fundi because of lower light absorption. Because of the higher energy that is usually needed with diode lasers, a stronger pain response may be elicited (Fig. 11).
The diode laser can be used in direct retinal photocoagulation, transscleral irradiation, and cycloablative procedures in glaucoma. Additionally, the diode laser is used for PDT in age-related macular degeneration and for TTT in occult subretinal membranes.
After a wavelength has been selected, the next question is which system to use to deliver the laser energy. Delivery systems include the traditional slit lamp system, endofiber optics for use intraoperatively, the indirect ophthalmoscope, and contact probes. All ophthalmologists are familiar with the slit lamp delivery system, which is the most commonly used. Endolaser photocoagulation is the method in which laser light is brought directly inside the eye through a fiber optic to apply treatment to the retina and is only used during vitreous surgery. This form of treatment is more commonly used than intraocular diathermy, external cryotherapy, or endodiathermy as a method to produce intraocular tissue reaction. Binocular indirect ophthalmoscopic laser photocoagulation has made it possible to treat peripheral retinal lesions that are unable to be treated by the commercial slit lamp delivery system. This procedure can be used to treat peripheral retinal tears, proliferative diabetic retinopathy, and retinopathy of prematurity, to name a few.
There are two modes of application of laser energy to tissue: continuous wave and pulsed. Continuous wave energy is the most commonly used mode for retinal coagulation because of absorption and the thermal tissue reaction it produces. Pulsed delivery results in further ablation of the tissue. The continuous wave laser is produced by excitation of a gas, which is in a vacuum tube or solid state. The vacuum tube system has been in use for many years, but is usually larger, less productive, and depends on a cooling system. Meanwhile, the solid-state system is smaller, is lightweight, and can be used with an electrical output. Although it has a limited range of wavelengths, its portability, ease of use, and size make the solid-state laser the choice of many ophthalmologists.
|Many factors are involved in the successful application of photocoagulation. In
all applications of retinal photocoagulation, it is important
to obtain an estimate of the clarity of the media. If opacities are present, it
is important to take into consideration the reflection, scattering, and
absorption of energy that may occur while the laser energy
is traveling toward the target tissue. Longer wavelengths have less
scattering and are more efficient in delivering the energy to the retina. Consideration
should also be given to the absorption characteristics
of the media and target tissue in relation to the type of wavelength.|
The goals in retinal photocoagulation vary with the clinical entity being treated. For example, in panphotocoagulation for diabetic retinopathy, the aim is the elimination of abnormal retinal blood vessels through direct treatment of the vessels or destruction of the ischemic areas of the retina. This kind of laser treatment entails marked destruction of retinal tissue. Conversely, application of laser energy in the macular area has to be modified owing to factors present in that area, such as the presence of xanthophyll pigment, the reduced thickness of the retina, the increased density of melanin in retinal pigment epithelium and the inner choroid, and the need to minimize retinal damage to avoid disabling scotomas in the central field of vision.
It is important to produce the desired effect in the lesion to be treated with a minimal amount of damage to the surrounding tissues. The availability of photocoagulation with green, yellow, red, infrared, wavelengths allows the operator to use the most effective type of irradiance in retinal tissue. Each wavelength has its own advantages and disadvantages, so it is unlikely that any single light will be satisfactory in all retinal applications, although the argon green wavelength has become the one most used by ophthalmologists. The degree of absorption by the tissue with the variations present throughout the retina due to different concentrations of pigment is an important factor to consider if we are to obtain the most consistent tissue response. The same dosage may produce either excellent therapeutic lesions or undesirable damage, depending on the amount of pigment in the fundus. This is even more noticeable in the use of red krypton wavelength, which is dependent on melanin absorption.
With so many variables in laser treatment of the retina, it is important to use the lowest power level at the beginning of the application, increasing power in small increments until the required lesion is obtained. Longer exposures of 0.2 to 0.5 seconds, although they may require the use of a retrobulbar anesthetic, allow the operator to titrate the response according to retinal tissue absorption. A longer exposure time will allow a slower coagulative effect and allow the operator to modify the amount of energy applied by releasing the foot switch sooner than the set time if necessary.
6. Bressler M: Photodynamic therapy of subfoveal choroidal neovascularization in age-related macular degeneration with verteporfin. Two-year results of two (2) randomized clinical trials. Arch Ophthalmol 119:198, 2001
9. Sabates F, Lee K, Ziemianski M, et al: Use of the krypton laser for ocular histoplasmosis and senile macular degeneration. In: Advances in Ophthalmic Laser Therapy, pp 101–119. Birmingham, AL, Aesculapius Publishers, 1983