Chapter 102 – Laser Photocoagulation
MICHAEL S. IP
CARMEN A. PULIAFITO
• LASER: Light amplification by stimulated emission of radiation.
• Lasers named for their active medium.
• Choice of optimal wavelength depends on absorption spectrum of target tissue.
• Indicated for treatment of retinal and choroidal abnormalities.
• Recent applications exploit the subthreshold effects of laser.
The history of retinal photocoagulation dates to 400 bc, when Plato described the dangers of direct sun gazing during an eclipse. Czerny and Deutschmann, in 1867 and 1882, respectively, focused sunlight through the dilated pupils of rabbits and created thermal burns in the animals’ retinas. Meyer-Schwickerath  undertook the study of retinal photocoagulation in humans in 1946 using the xenon arc lamp. Xenon arc lamps, commercially available in 1956, rapidly became popular for retinal photocoagulation because of their strong visible and near infrared emission.
The first functioning laser was demonstrated by Maiman in 1960. The active laser material was a ruby—a crystalline sapphire that contained a small percentage of chromium oxide. The chromium ions absorb radiation in the green–blue part of the spectrum and emit radiation of 649?nm (red light). The ruby crystal is pulsed with a xenon flash lamp.
The first clinical ophthalmic use of a laser in humans was reported by Campbell et al. in 1963 and Zweng et al. in 1964. They found that laser photocoagulation was efficient and effective and did not require anesthesia or akinesia. The ruby laser they employed operated in a pulsed mode because the thermal characteristics of the ruby crystal prohibited continuous operation at the power levels required for retinal photocoagulation. Use of the pulsed laser often led to the formation of retinal hemorrhages. In addition, the ruby laser was poorly absorbed by hemoglobin.
The argon laser, developed in 1964, provides an emission spectrum that is absorbed well by hemoglobin when the laser is used in a continuous mode. L’Esperance conducted the first human photocoagulation trial for ophthalmic disease using the argon laser in 1968; he also introduced the frequency-doubled neodymium:yttrium-aluminum-garnet (Nd:YAG) and krypton lasers in 1971 and 1972, respectively. The use of the Q-switched and mode-locked Nd:YAG lasers in 1980 and 1981, respectively, allowed transparent membranes (e.g., posterior capsule, vitreous) to be cut using extremely short bursts of laser energy. The tunable dye laser was introduced in 1981 and provided the theoretical advantage of a variable output wavelength to match the absorption spectra of specific ocular tissue.
The semiconductor infrared diode laser was developed in 1962. Since then, the diode laser has been employed in multiple delivery modes—transpupillary slit lamp, transpupillary laser indirect, trans-scleral, and endophotocoagulation. It has been used to treat choroidal neovascularization (CNVM), proliferative retinopathy, retinopathy of prematurity, macular edema, and choroidal melanoma. 
Laser is an acronym for light amplification by stimulated emission of radiation. The basic laser cavity consists of an active medium in a resonant cavity with two mirrors placed at opposite ends. One of the mirrors allows partial transmission of laser light out of the laser cavity, toward the target tissue. A pump source introduces energy into the active medium and excites a number of atoms. In this manner, amplified, coherent, and collimated light energy is released as laser energy through the mirror that partially transmits. The various lasers differ mainly in the characteristics of the active medium and the way this active medium is pumped.
The properties of laser light that make it useful to ophthalmologists and allow laser energy to be directed at specific target tissue in a controlled manner are monochromaticity, spatial coherence, temporal coherence, collimation, ability to be concentrated in a short time interval, and ability to produce nonlinear tissue effects.
Lasers are named for their active medium. Solid-state lasers include the ruby laser and the Nd:YAG laser. The organic dye laser contains a liquid laser medium that consists of a fluorescent organic compound dissolved in a liquid solvent. As a result, the dye laser can produce multiple wavelengths, because dyes are made up of large molecules that have various structures and complex spectra. Gas lasers include the ion laser and carbon dioxide laser. Ion lasers contain an ionized rare gas, such as argon or krypton, as the active medium. In a carbon dioxide laser, nitrogen and helium are typically present in the gas mixture also. Diode lasers have semiconductor materials as the laser medium. Different semiconductor materials are available that provide a range of wavelengths, such as gallium arsenide (660–900?nm) or indium phosphide (1300–1550?nm).
In gas or Nd:YAG lasers, only 2% of the energy used to generate the laser emission is converted into laser light. The remaining 98% of energy becomes heat, which is removed by extensive, bulky cooling mechanisms. In contrast, approximately 20% of the input power is converted to laser energy in an infrared diode
laser. The infrared wavelength enables increased transmission through lens opacity, hemorrhage, and macular xanthophyll pigment. Additionally, the sclera is transparent to infrared wavelengths, so the diode laser may be used for trans-scleral applications (e.g., cyclodestructive procedures, trans-scleral retinopexy).
Clinical laser delivery systems consist of the laser medium, the fiber-optic cable or a mirror arm to take laser light to the delivery system, and the delivery system to direct the treatment beam to the target tissue.
In the slit-lamp biomicroscope, the most common method, the delivery is transcorneal, with or without the aid of contact lenses. The indirect ophthalmoscope with a condensing lens also may be used transcorneally to photocoagulate the posterior segment. Other methods include endolaser and exolaser probes, in which treatment is delivered by fiber-optic probes used within the eye or trans-sclerally, respectively.
PARAMETERS AND TECHNIQUES
The choice of optimal wavelength depends on the absorption spectrum of the target tissue. Blue light is absorbed by macular xanthophyll and is a poor choice for macular photocoagulation. Light scatter also affects wavelength selection. Blue light, with its shorter wavelength, is scattered more than is light of longer wavelengths and thus has a greater potential to produce photochemical retinal damage in nearby untreated retina. Blue light also is scattered by the senescent crystalline lens, requiring more irradiation than longer-wavelength light for the clinical end point of retinal photocoagulation to be achieved.
Green light is absorbed well by melanin and hemoglobin. Red light also is absorbed well by melanin but is absorbed poorly by hemoglobin. Since the radiation of krypton red has a longer wavelength (647?nm) than that of argon green, it is absorbed more deeply; therefore, treatment using krypton red may be more uncomfortable. Hemorrhage is difficult to treat using krypton red, because hemoglobin poorly absorbs radiation of this wavelength. However, krypton red radiation penetrates through hazy media better than does radiation of shorter wavelengths. In general, the irradiation required to achieve an ophthalmoscopically visible lesion is most dependent on fundus pigmentation, where the bulk of the absorption occurs. The principal wavelengths of some common photocoagulation lasers are listed in Table 102-1 .
Retinal lesion size is strongly dependent on laser power. Most photocoagulation laser systems are controlled by changes in
TABLE 102-1 — PRINCIPAL WAVELENGTHS OF COMMONLY USED LASERS
Frequency doubled Nd:YAG
Variable (most 570–630?nm), depending on dye
Variable (most 780–850?nm), depending on diode
power level, whereas most photodisruptive lasers (Nd:YAG) are controlled by changes in energy level.
Short exposures may lead to photodisruptive effects, whereas exposures of longer duration lead to photocoagulation or photochemical effects. However, within the realm of photocoagulation, the power used affects lesion size to a greater extent than exposure time does.
Focal laser treatment is optimized by using spots of small size (50–100?µm diameter), and panretinal photocoagulation, which requires coverage of large areas of the retina, is facilitated by the use of larger spots (200–500?µm diameter). Small spots may result in complications such as choroidal rupture and secondary CNVM when high irradiance levels are used.
Several commercially available contact lens systems may be used to deliver laser energy to the posterior segment. To choose the correct lens and spot size for various clinical problems, it is important to understand the effect of different contact lens systems on spot size. For the Goldmann lens, spot size correlates closely with the actual size of the retinal lesion produced. However, for the Mainster lens, Rodenstock panfundoscope lens, and Krieger lens, the retinal spot size is actually larger than the set spot size by approximately 35–50%. This disparity between set spot size and actual spot size must be kept in mind when using these lenses.
The indications for laser photocoagulation in the treatment of ophthalmic disease are myriad and include diabetic retinopathy, branch vein occlusion, and CNVM.   
Other indications for laser photocoagulation include retinopexy of retinal tears for the prevention of retinal detachment. In most cases, slit-lamp delivery systems provide adequate access to all but the most peripheral retinal tears. Far peripheral tears are treated more effectively using laser indirect ophthalmoscopy, a method likewise preferred for the treatment of peripheral neovascularization. Laser photocoagulation using the indirect ophthalmoscope also is effective for the treatment of certain small ocular tumors. Additionally, argon or diode laser photocoagulation has proved effective in the treatment of threshold retinopathy of prematurity.
As with all procedures, patients must be given appropriate preoperative education about the potential risks of laser photocoagulation. Inadvertent photocoagulation of the fovea, cornea, iris, or lens can be minimized using careful technique and appropriate spot size. Choroidal effusions are seen most often after extensive panretinal photocoagulation, a complication that may be minimized if the treatments are spread over multiple sessions. Secondary CNVM is thought to result from damage to Bruch’s membrane caused by heavy laser treatment. Decreases in intensity and duration, along with the avoidance of smaller spot sizes (50?µm), may help minimize this complication.
Retinal pigment epithelium rips have been noted, particularly with the use of the krypton red laser in the treatment of CNVM. Sudden contraction of the neovascular membrane as a result of the thermal effects of the laser may produce a shearing force that causes a rip in the retinal pigment epithelium.
NEW DEVELOPMENTS IN LASER THERAPY
The most recent clinical applications of laser therapy have exploited the nonphotocoagulative and subthreshold effects of
laser. Photodynamic therapy utilizes a low-intensity laser of appropriate wavelength to activate an exogenous photosensitizing agent. The interaction between the laser light and the photosensitizing agent produces a photochemical reaction that results in cellular damage and vascular thrombosis of target tissue such as CNVMs. Thermal damage to adjacent retinal tissue is lessened, because the laser energy used in this technique is insufficient to result in coagulative damage. Subthreshold effects of laser therapy are currently under investigation for the treatment of CNVM (transpupillary thermotherapy) and prophylaxis of CNVM in patients with nonexudative age-related macular degeneration. 
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