Principles of Laser Surgery for Glaucoma

The introduction of laser (i.e., light amplification by stimulated emission of radiation) therapy was a significant advance in the surgical treatment of glaucoma during the second half of the 20th century. The concept of using light energy to alter the structure of intraocular tissues, however, actually preceded the development of laser technology. Meyer-Schwickerath (1), beginning in the late 1940s, pioneered this field of ocular surgery, first using focused sunlight and later the xenon-arc photocoagulator. Although the latter technique was useful for certain retinal disorders, xenon-arc photocoagulation for the treatment of glaucoma never gained clinical acceptance.

In 1960, Maiman (2) described the first laser that used a ruby crystal stimulated by a flash lamp to emit red laser light at a wavelength of 694 nm. It was the development of the continuous-wave argon laser, near the end of that decade, that brought on a virtual explosion of laser applications for ocular diseases. Since the first report of argon laser use for ocular disease in the late 1960s, numerous wavelengths arising from different energy-emitting sources have been tried. Lasers are now used to treat various forms of glaucoma, and today it is the most commonly used mode of glaucoma surgery (3–6).

This chapter briefly reviews the physical and biologic aspects of laser therapy. The application of these principles to the treatment of specific forms of glaucoma is considered in subsequent chapters.

BASIC PRINCIPLES OF LASERS

When light is shined on a metal surface in a vacuum, it may free electrons from that surface. These electrons can be detected as a current flowing in the vacuum to an electrode. Only certain wavelengths can cause photoemission of electrons. In 1917, Albert Einstein wrote “Zur Quantum Theorie der Strahlung” (the quantum theory of radiation), in which he speculated that light consists of photons, each with discrete quantum of energy proportional to its wavelength. For an electron to be freed from the metal surface, it would need a photon with enough energy to overcome the energy that bound it to the atom. His theory formed the basis of laser technology.

When atoms absorb energy, called “pumping,” they are “excited” from a lower to a higher energy level. When a substance (e.g., gas, liquid, or a semiconducting material) is excited by energy, it emits light in all directions. The sources of energy used to excite the lasing medium typically include electricity from a power supply or flash lamps, or the energy from another laser. If more atoms are in the excited state than in the unexcited state, population inversion is said to exist. Under such circumstances, photons with energy equal to the difference between the two levels of excitation have an enhanced probability of stimulating the atoms to decay back to their lower energy level by emitting photons, a process called stimulated emission. The emitted photons stimulate the emission of more photons, leading to a chain reaction.

If this system is enclosed between two mirrors, the photons bounce back and forth, creating multiple stimulated emissions of light, or light amplification. The mirrors form an optical cavity, which, in addition to amplifying the light, creates a parallel beam and acts as a resonator to limit the number of wavelengths. When the light amplification is sufficient, some photons are allowed to leave the cavity in the form of a laser beam through a partially permeable mirror (Fig. 35.1).

The laser beam can be delivered as a continuous wave or in a pulsed mode. In the latter situation, the energy is concentrated and delivered in a very short period of time, which can be accomplished in one of two ways. With one technique, called Q-switching, light is not allowed to travel back and forth in the cavity until maximum population inversion is reached. This is accomplished with an electronic shutter or misalignment of the mirrors. When the shutter is opened or the mirrors are aligned, stimulated emission and light amplification occur suddenly, and the energy is released in a pulse of a few to tens of nanoseconds. In the other form of pulsed delivery, called mode-locking, the energy is also released after achieving maximum population inversion, but different modes of light are synchronized, creating peaks of energy, which are emitted in tens of nanoseconds as a chain of pulses, each of which lasts a few tens of picoseconds. To provide some appreciation for the brevity of these exposures, it has been noted that the ratio between the duration of a Q-switched laser pulse and a conventional continuous-wave argon laser exposure is roughly the same as the ratio between the argon exposure and a human lifetime (6).

PROPERTIES OF LASER ENERGY

Light emitted by a laser differs from normal “white” light in several ways.

Coherence

Unlike the photons in a light bulb, which are emitted randomly, the resonator effect of the laser cavity causes the photons to be synchronized or coherent—that is, in phase with each other in time and space.

Collimation (Directionality)

Because light amplification occurs only for photons that are aligned with the mirrors, a nearly parallel beam, in which all the waves travel in the same direction, is produced, as opposed to the diverging beam of an incandescent lamp. Although limited divergence occurs with all laser beams, it is minimal enough that a small focal spot can be created when the light is delivered through an optical system.

Monochromacy

Because the photons are emitted through the release of energy between two defined levels of the atom, the resulting light has only one discrete wavelength. In contrast, ordinary white light is a combination of many different wavelengths.

High Intensity

The light amplification of a laser can produce a beam with significantly more intensity than that of the sun.

LASER-INDUCED TISSUE INTERACTIONS

The tissue effects produced by laser surgery are of three types: thermal, ionizing, and photochemical (7).

Thermal Effects

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