Ophthalmic lasers are used to treat a variety of anterior segment disorders. The anterior segment tissues that can undergo laser treatment include the cornea, iris, trabecular meshwork (TM), lens capsule, and ciliary processes. The more commonly performed anterior segment techniques include:
- Laser trabeculoplasty
- Laser iridotomy
- Laser iridoplasty
- Laser pupilloplasty
- Cyclophotocoagulation
- Photokeratectomy
Other anterior segment techniques that will not be addressed in detail in this tutorial include laser sclerostomy, laser suturelysis, laser repair of filtering blebs, and goniophotocoagulation.
Most medical lasers are solid-state, gas, semiconductor, or dye lasers. The laser is usually named for the active medium (i.e., the component that emits the coherent radiation). Medical lasers omit light in the ultraviolet, visible, or infrared range. The specific effect on a particular tissue is a function of the wavelength of the laser light and the energy or power delivered, as well as the intrinsic molecular properties of an individual tissue. The most commonly used lasers for anterior segment applications include the argon, Nd:YAG, diode, and excimer lasers.
Laser Types
Solid-state lasers use a solid material, such as crystal or glass, as the active medium. The most common crystal laser employs elemental neodymium in a crystal of yttrium- aluminum-garnet and is referred to as an Nd:YAG laser. This laser emits light in the infrared range with high power.
Gas lasers use a gas or gas mixture as the active medium. Some common gas lasers include argon, krypton (both producing visible light), and carbon dioxide (producing infrared light). A variation of the gas laser is the excimer laser that uses noble gases such as argon or xenon combined with a halide such as fluorine as the active medium to produce high power ultraviolet radiation. The gas-halogen combination is referred to as an excited "dimer," giving rise to the term "excimer."
Semiconductor lasers are diode lasers. Some common semiconductor materials include gallium and arsenic with the active medium being the junction between two layers that differ in electrical properties. Diode lasers produce infrared light.
Dye lasers employ a dissolved dye in a liquid as the active medium and have the advantage of being tunable. Therefore, a dye laser can be adjusted to produce various wavelengths of monochromatic light.
Ophthalmic lasers deliver laser energy either as a continuous beam or in a pulsed fashion. Continuous beam lasers indicate the power (watts) of the laser output, whereas pulsed lasers indicate the energy (joules) of each pulse. Because the duration time of each laser application can be varied in a continuous beam laser, the total energy delivered will be a function of the time of the application and the power. Pulsed lasers deliver a total power that is a function of the fixed pulse duration and energy. Although pulsed lasers may deliver much less total energy than continuous lasers, the total peak power is much higher because the individual pulse duration is much shorter (remembering that power is energy per unit time). A pulsed laser such as a Q-switched Nd:YAG laser delivers only 2 mJ per pulse compared to 100 mJ for an argon laser pulse. However, the peak energy is several orders of magnitude higher since the pulse duration is in the nanosecond range.
Tissue Interactions
Laser light is transmitted, absorbed, or scattered by biological tissues. These properties are a function of the molecular composition of a tissue and the wavelength of the laser light. Most biological molecules are radio-opaque to ultraviolet light due to the presence of conjugated chemical bonds that tend to absorb ultraviolet energy. Conversely, many biological molecules are transparent in most of the visible range. Therefore, clear ocular media, such as the cornea, aqueous humor, and lens, transmit visible light but absorb ultraviolet light. Pigmented tissues absorb visible and infrared light.
At the molecular level, photons of laser light interact with electrons of molecules at the target site leading to a variety of effects. These effects can be broadly categorized as thermal, photochemical, or ionizing.
Thermal effects result from laser energy inducing a temperature rise by increasing the kinetic energy of component molecules in the target tissue. This will, in turn, cause macromolecular denaturation by breaking weak intramolecular forces such as van der Waals and electrostatic interactions, but chemical bonds will remain intact. Longer wavelengths of visible or infrared light at lower power will induce thermal effects. Infrared light also has the effect of directly transferring energy to molecules by causing molecular vibration.
Photochemical changes result from photons of laser light being absorbed by outer electrons leading to excited electron states and subsequent chemical reactions. This is usually the result of shorter wavelength lasers of moderate power.
Ionizing effects are caused by high power, short duration treatments. At sufficiently high power levels, electrons can be separated from molecules resulting in plasma formation. /P>
Because, in many cases, there can be overlap between these effects it is more practical to describe specific processes that occur within tissues as a result of laser treatment. These processes include photoradiation, photoablation, photocoagulation, photovaporization, and photodisruption.
- Photoradiation. A photosensitive molecule can be selectively absorbed and retained in a specific tissue. When the photosensitive agent is irradiated by laser light, the resulting excited state induces photochemical reactions in adjacent molecules, such as free radical formation. This will cause cytotoxic effects in the tissue. Photodynamic therapy with verteporfin to treat macular degeneration is an example of photoradiation using red laser light.
- Photoablation. Laser energy of sufficient power can cause photochemical reactions with precise uncoupling of intermolecular bonds. This process is achieved with high power, ultraviolet lasers such as the excimer laser. The molecules are fragmented leading to volatilization of the tissue with minimal collateral damage to adjacent tissue. Photoablation of corneal tissue for keratorefractive surgery is the primary anterior segment application of the excimer laser.
- Photocoagulation. Lower power gas lasers, as well as the diode and continuous wave Nd:YAG, cause photocoagulation of tissue. Thermal disruption of structural molecules, especially protein denaturation, results in the tissue changes that are observed such as contraction, condensation, and separation. Formation of amorphous gels as well as clot formation can occur also. Higher temperatures result in non-selective coagulation necrosis of the target tissue and will lead to the burning of tissue. Examples of photocoagulation include laser trabeculoplasty treatment with the argon laser and cyclophotocoagulation with the diode laser.
- Photovaporization. When the tissue temperature increases rapidly from thermal laser energy, water is converted from the liquid to vapor state leading to tissue disruption before denaturation occurs. The tissue temperature will stabilize at the water phase change point until all the water is vaporized. During this time, incisional effects are observed. In some cases, such as with the CO2 laser, photovaporization occurs with photocoagulation so that tissues can be incised and cauterized simultaneously.
- Photodisruption. High power laser energy, achieved with short-pulse lasers, causes disruption of tissue. Extremely high peak laser power can cause ionization leading to a mixture of electrons and ions referred to as plasma. Tissue disintegration occurs in a small area and adjacent tissue is disrupted by mechanical destruction as the plasma wave expands. This effect is useful to treat transparent tissues that would otherwise not absorb much laser energy. The more a tissue is already under some mechanical stress, such as the posterior lens capsule before laser capsulotomy, the greater the effect that is observed.
Instrumentation
Argon Laser
This laser is the most commonly used visible light laser for anterior segment treatment. The specific wavelengths are argon blue,
488 nanometers (nm) and argon green, 514.5 nm. The laser is usually mounted on a slit lamp. The focusing beam is coaxial with the
treatment beam and cannot be intentionally misaligned. The focusing beam will also indicate the diameter of the laser treatment
beam to be delivered. Aiming is achieved by moving the focusing beam directly or by moving the slit lamp. The operator can
adjust most of the laser parameters. The laser beam can vary in size from 50 µm to 500 µm. Laser power can be varied from 0
to 2,000 milliwatts (mW) and the duration ranges from 0.01 to 2 seconds. A corneal contact lens is usually placed to provide
direct or indirect visualization and magnification of the treatment zone. A foot pedal activates the beam. Argon lasers can be
used for laser trabeculoplasty, laser iridotomy, laser iridoplasty and pupilloplasty, cyclophotocoagulation, laser suturelysis,
goniophotocoagulation, and repair of bleb leaks.
Nd:YAG Laser
The Nd:YAG lasers available for ophthalmic use deliver either fixed pulse or continuous laser energy. All Nd:YAG ophthalmic
lasers deliver infrared light of 1,064 nm but vary in the total energy and power delivered. To obtain high peak power, the laser
energy is delivered over a short duration. The two methods of achieving this with the Nd:YAG laser are with mode-locking and
Q-switching. A mode-locked Nd:YAG laser delivers 2 mJ of energy from 30 to 200 picoseconds. If delivered in 30 picoseconds,
the peak power is 6.7 x 106 watts. A Q-switched Nd:YAG laser delivers 2 mJ in 5 to 30 nanoseconds giving a peak power of 1.3 x
105 watts. The result of these high peak powers is tissue disruption. The continuous wave, long pulse Nd:YAG laser delivers
more total energy but the delivery time is also correspondingly longer so that the peak power is much lower. For example, the
long pulsed Nd:YAG will deliver 1,000 mJ of total energy in 0.1 to 10 milliseconds such that the peak power is only 100 watts.
Therefore, the primary effect is thermal.
When used for anterior segment applications, the Nd:YAG lasers either are slit lamp guided or use direct contact delivery systems. For slit lamp systems, an aiming beam is used to direct the laser at the target point. The aiming beams are separate, off-axis, helium/neon beams (HeNe) and may be single or multiple. In the case of multiple aiming beams, focusing is achieved by beam overlap. Because the aiming beams and treatment beam are not coaxial, they can be offset to allow more posterior laser treatment as in the case of laser capsulotomy or cyclophotocoagulation. Nd:YAG lasers are used for laser iridotomy, laser capsulotomy, cyclophotocoagulation, and laser sclerostomy.
Diode Laser
Semiconductor diode lasers are compact units that may be slit lamp mounted or portable with a separate contact probe.
The laser emits near infrared light of 810 nm, which is a shorter wavelength than emitted by the Nd:YAG laser. With noncontact
lasers, the laser power can be varied from 0 to 2,000 mW and delivered as a continuous wave up to 2 seconds' duration. The beam
size varies up to 500 µm. A laser with a contact probe has a fixed beam size but the power and duration can be varied. The diode
laser is used for cyclophotocoagulation.
Excimer Laser
The excimer laser delivers fixed pulse laser light for corneal ablation. The major anterior application is for photokeratectomy
including phototherapeutic keratectomy and photorefractive keratectomy. All excimer lasers currently in use for photokeratectomy
are argon-fluoride (ArF) lasers emitting high power ultraviolet light of 193 nm. The term "cool laser" is often used to describe
this laser to indicate that little or no thermal effect occurs.
Corneal remodeling is accomplished by removal of corneal tissue. Each laser pulse will ablate approximately 0.24 µm of corneal tissue at the impact site.
Specific changes in corneal topography are achieved by varying the size, shape, or position of the laser beam that impacts the corneal surface. Therefore, the lasers vary primarily in the laser beam delivery systems. The beam of broad beam lasers is circular and varies in size and shape because a diaphragmatic opening expands. The beam shape can be varied further by a slit like diaphragm or an ablatable disc. "Flying-spot" lasers use smaller, fixed diameter beams, typically 1 mm to 2 mm, with the beam constantly redirected at the corneal surface. These lasers use tracking systems to monitor eye movements to guide the laser beam.
Clinical applications
Laser Iridotomy
Laser iridotomy is indicated for the treatment of acute and chronic angle-closure glaucoma, anatomical narrow angle
configuration with a risk for angle-closure, and various forms of pupillary block. The iridotomy is created peripherally
in the superior iris. Iris crypts are the preferred sites because of the thinner tissue. Laser iridotomies can be created
with argon lasers by thermal effects or with Nd:YAG lasers by photodisruption. Penetration and perforation is achieved
more easily in lightly pigmented irides with the Nd:YAG laser and in darker tissues with the argon laser.
A corneal contact lens, such as an Abraham iridotomy lens, is used to facilitate beam focusing as well as tissue magnification. The laser source is guided by the slit lamp. When using an argon laser, the initial penetration treatment is followed by cleanup treatment. The parameters for stromal penetration include a 50-µm spot size and 1,000 mW of power for 50 to 500 milliseconds depending on the degree of tissue pigmentation. Clusters of several burns are applied until penetration is achieved.
Residual pigment epithelial tissue can be removed with laser spots of lower power. Iridotomies with the Nd:YAG laser require fewer pulses and less total energy. A Q-switched Nd:YAG is used with a spot size and duration fixed for each instrument. Typical laser settings are 3 mJ to 10 mJ with single bursts of one to two pulses per burst. The morphology of the resulting iridotomy is also a function of the laser being used. The Nd:YAG laser produces small, irregular shaped iridotomies, whereas the argon laser creates larger, more regular openings.
Treatment complications include hemorrhage and hyphema, rise in intraocular pressure (IOP), inflammation, closure of the iridotomy, damage to the lens capsule, and malignant glaucoma.
Laser Trabeculoplasty
Laser trabeculoplasy (LTP) is indicated for the treatment of open-angle glaucomas and is particularly effective in the treatment
of pigmentary and pseudoexfoliation glaucomas. LTP causes alteration of the TM by photocoagulation and a greater effect is seen
with more heavily pigmented TM. The precise mechanism of action is unknown but it relates to the change in conformation of the
TM by collagen shrinkage leading to the opening of meshwork in adjacent, nontreated regions.
A slit lamp-mounted argon laser is usually used with the laser beam focused at the outflow angle with a contact lens such as a Goldmann three-mirror lens. The laser settings are typically 800 mW to 1200 mW for 0.1 second and a 50-µm spot size. The laser is focused on the TM and the power adjusted to cause a slight focal bubble or blanching of the TM. The TM is treated for 180º to 360º with a total of 50 to 100 spots. Because treatment is performed with the argon laser, it is sometimes referred to as argon laser trabeculoplasty (ALT).
Treatment complications include elevation of IOP, inflammation, inadvertent treatment of the cornea or ciliary body, hemorrhage, and pain.
Laser Iridoplasty and Pupilloplasty
Laser iridoplasty is indicated in plateau iris syndrome and some forms of angle-closure glaucoma with peripheral anterior
synechiae, and as a pretreatment for laser iridotomy or trabeculoplasty. Laser pupilloplasty is indicated for miotic pupils
that do not respond to pharmacological dilation. In both cases, laser energy causes thermal contraction of the tissue.
Laser iridoplasty is performed with the slit lamp-mounted argon laser using a three-mirror lens focusing the laser on the peripheral iris. Contraction burns are placed in a circumferential fashion for 360º. A broad beam of 500 µm is used with a low power of 150 mW to 200 mW for 0.5 seconds. The iris contracts after each pulse pulling the tissue centrally.
Similar laser settings are used for pupilloplasty with the laser being focused through the center of the three-mirror lens. The laser is applied as a ring of two to three radially placed burns. The pupil enlarges as the iris tissue contracts.
Complications from iridoplasty and pupilloplasty include elevation of IOP, inflammation, hemorrhage, and pain.
Cyclophotocoagulation
Cyclophotocoagulation (CPC) is a type of cyclodestructive procedure indicated for refractory, difficult-to-treat glaucoma.
The technique can be performed by transscleral, transpupillary, or intraocular approaches. The general mechanism of action
is the decrease in aqueous production that results from thermal destruction of the ciliary body epithelium.
Transscleral cyclophotocoagulation (TSCPC) is performed as either a noncontact or contact procedure with either the Nd:YAG
or diode laser. Most patients undergoing laser CPC receive a retrobulbar injection of anesthesia. For noncontact procedures,
the patient is positioned at the slit lamp and a focusing contact lens is used.
For noncontact CPC with the Nd:YAG laser, a pulsed laser system is used. The laser is set at multiple mode preset at 20 milliseconds and an energy of 4 J to 8 J. The laser is set to a maximum offset to allow the greatest separation between the aiming beam and treatment beam. The laser is applied approximately 1.5 mm from the limbus with 30 to 50 treatments applied 270º to 360°. The 3 o'clock and 9 o'clock positions are avoided to prevent damage to the long posterior ciliary arteries.
Contact TSCPC is performed with a fiberoptic probe placed directly on the conjunctiva. The continuous wave Nd:YAG laser is set at 4 to 9 watts with exposure times of 0.5 to 2 seconds. The probe is positioned 0.5 mm to 1.5 mm from the limbus and 30 to 40 treatments are applied for 270º to 360°, sparing the 3 o'clock and 9 o'clock positions.
The diode laser, when used for contact TSCPC, uses a G-probe specifically designed to align the fiberoptic over the ciliary body when the probe footplate is placed at the limbus. The laser is set at 1 to 2 watts for 2 seconds. The superior, nasal, and inferior quadrants are treated with six to eight exposures per quadrant.
Transpupillary CPC can be performed if the ciliary process can be visualized as in cases where a large sector iridectomy is present. The argon laser is focused on the ciliary processes using a three-mirror lens and all visible ciliary processes are treated. Typical settings include 100-µm to 200-µm spot sizes, 700 mW to 1,000 mW, for 0.1 to 0.2 seconds. Three to five laser burns are applied to all regions of a visible process until whitening and shrinkage is observed.
Endoscopic CPC can be performed with the diode laser using ocular endoscopy to visualize the ciliary processes directly. The laser endoscope probe is a combined unit with three fiberoptics - one each for the image guide, the light guide, and the laser guide. Probes are introduced through a limbal or pars plana approach and the image visualized on a monitor. Treatments are performed at 180° to 360° with powers ranging from 200 mW to 1,500 mW for 1 to 2 seconds and the spot size is 200 µm. As with transpupillary treatment, the endpoint is whitening and shrinkage of the ciliary processes.
Complications of all types of CPC include pain, inflammation, increased IOP, hemorrhage, or hypotony from overtreatment.
Laser Capsulotomy
Laser capsulotomy is indicated for visually significant posterior capsule opacification in pseudophakic eyes. The Nd:YAG laser is
used to create a central opening in the posterior capsule by photodisruption of the tissue. The patient is positioned at the slit
lamp and a focusing contact lens is usually used. The pupil is usually dilated for treatment but the visual axis is first identified
through the undilated pupil. HeNe focusing beams are brought to focus on the posterior lens capsule; the treatment beam is offset
slightly to prevent damage to the intraocular lens. Pulsed laser light is used to deliver 1 mJ to 3 mJ of energy. For Q-switched
lasers, each pulse is 5 to 30 nanoseconds and for mode-locked lasers, each pulse is 30 to 200 picoseconds. In lasers where the
pulse duration is fixed, the power varies in this range. The laser is applied in a series of linear pulses in a crossed fashion,
observing the impact of each treatment and refocusing between treatments if necessary. On average, six to 30 applications are
required to open the posterior capsule.
Treatment complications include transient pressure rise, inflammation, vitreous prolapse and pupillary block, retinal detachment, and impact marks on the intraocular lens.
Bibliography
Fankhauser F, et al. Clinical studies on the efficiency of high power laser radiation upon some structures of the anterior segment of the eye. Int Ophthalmol. 1981;3:129.
Hennis HL, Stewart WC. Semiconductor diode laser transscleral cyclophotocoagulation in patients with glaucoma. Am J Ophthalmol. 1992;113:81.
Hillenkamp F. Interaction between laser radiation and biological systems. In: Hillenkamp F, Pratesi R, Sacchi CA, eds. Lasers in Biology and Medicine. New York: Plenum; 1980.
L'Esperance FA Jr. Ophthalmic Lasers: Photocoagulation, Photoradiation, and Surgery. St. Louis, Mo: Mosby; 1989.
Mainster MA, Ho PC, Mainster KJ. Argon and Krypton laser photocoagulators. Ophthalmology. 1983;90:48.
Robin AL, Pollack IP. A comparison of neodymium:YAG and argon laser iridotomies. Ophthalmology. 1984;91:1011.
Schuman JS, et al. Contact transscleral continuous wave neodymium:YAG laser cyclophotocoagulation. Ophthalmology. 1990;97:571.
Steinert RF, Puliafito CA. The Nd:YAG Laser in Ophthalmology: Principles and clinical applications of photodisruption. Philadelphia, Pa: WB Saunders; 1985.
Trokel SL, Srinivasan R, Braren B. Excimer laser surgery of the cornea. Am J Ophthalmol. 1983;96:710.
Wise JB. Long-term control of adult open angle glaucoma by argon laser treatment. Ophthalmology. 1981;88:197.