Perhaps more than any other recent advance in medical science, the advent of laser technology has produced a major effect on clinical ophthalmology. The effect of solar radiation was well known in ancient times; the first description of a central scotoma after a solar burn of the retina was reported in the mid-17th century. Although Albert Einstein described the concept of stimulated emission of light in 1917, a doctoral student, Gordon Gould, in 1957 figured out a way to make light waves march in unison, and Bell Labs produced the first useable laser in 1960.
Medical applications of this new energy were explored at an early stage, but the first experiments were poorly controlled and commonly produced unsatisfactory results. Some of these early investigators used effects of the sun or the carbon arc to produce a lesion in the retina. In 1927 Maggiore focused sunlight on two eyes that were later enucleated. These eyes showed a significant reaction in the retina as a result of this focused energy. In 1949 Dr. Meyer-Schwickerath focused light from the sun through a rudimentary optical instrument and successfully photocoagulated the human retina with a retinal hole.
In 1960 the first optical laser ( l ight a mplification by s timulated e mission of r adiation), as it was eventually called, was produced, which provided the ophthalmologist with an intense, pure beam of light that could produce extremely small burns of varying intensities. Since the first clinical use of Ruby laser in the 1960s, new sources and applications have been introduced and argon, krypton, dye, neodymium yttrium aluminum garnet (Nd:YAG), diode, excimer, and Ti:Sapphire laser have been tested and applied to treatment of vitreoretinal diseases, glaucoma, posterior capsulotomy and more recently refractive, corneal, and cataract surgery.
Laser theory
A laser is a source of extremely intense monochromatic coherent light. The electromagnetic spectrum is composed of radiant energy that ranges from short cosmic waves (10 nm) (nanometers; a nanometer is 1 millionth of a meter) to the longest radio waves (1000 m). Laser light is basically the same as light from the sun or a household light bulb. It is formed by photons and, like any other form of light, is propagated in an electromagnetic wave form. In the visible portion of the electromagnetic spectrum, the radiation with the shortest wavelength is in the violet region. These wavelengths are in the region of 400 nm. The visible radiation with the longest wavelength is red light. Red light waves may be up to 700 nm.
Lasers can produce light energy with wavelengths shorter than the visible spectrum (ultraviolet) or longer than the visible spectrum (infrared), each one causing different forms of tissue destruction in the eye ( Fig. 35.1 ).
Three basic conditions must be met for most lasers to operate ( Fig. 35.2 ):
- 1.
There must be an active medium, that is, a material, such as a gas or solid in which the atoms, molecules, or ions emit optical radiation when properly stimulated.
- 2.
There must be a suitable energy source that can pump the atoms, molecules, or ions in the active medium, producing the emission of photons of radiation.
- 3.
There must be some form of optical feedback or gain, which usually is provided by mirrors or other reflecting surfaces in the laser’s optical cavity.
Argon, krypton, carbon dioxide, helium-neon, various liquids (dyes) and solids, such as neodymium supported by YAG, as well as many other types of semiconductors, are all in use as lasing media. The active medium in the Nd:YAG laser, for instance, consists of an insulating crystal fabricated from yttrium, aluminum, and garnet and doped with the rare earth neodymium (Nd) ion. The energy source used to pump or excite the neodymium ions is typically a quartz body or flashlamp for pulsed applications or a direct current arc lamp when continuous laser output is desired. Optical gain is provided by placing mirrors at each end of the Nd:YAG rod in this type of laser to reflect the light back and forth through the crystal. Alternatively, the ends of the laser rod can be coated with reflective material and thereby serve as the laser mirrors.
Pumping and spontaneous emission
When photons of light from the pumping lamp collide with the active lasing medium, they often impart enough energy to raise some of their orbiting electrons to higher-than-usual energy levels. This is referred to as optical pumping . These electrons remain at the higher energy levels for varying periods of time, dropping back to lower energy levels randomly and spontaneously. When they drop from a higher to a lower level, they in turn emit energy in the form of photons of radiation. This phenomenon is called spontaneous emission . The wavelength of the radiation emitted depends on the difference in the potential energy of the two levels.
Stimulated emission
If a photon strikes an electron that is in a high (or pumped) energy level, the electron instantaneously drops back to a lower energy level only if the triggering photon is of the same wavelength or frequency as the one that is emitted when the electron falls to the lower energy level. When an electron is stimulated to give off a photon of radiation, the photon emitted travels in exactly the same direction as the photon that triggered it. Therefore the laser light is reflected back and forth along the axis. In a short period, many identical photons form in a standing wave (all of them in phase), producing coherent radiation. This standing wave, which has now become a beam of laser radiation, continues to be amplified as it passes back and forth through the laser rod cavity between the two mirrors. This process is referred to as stimulated emission .
Spontaneous emission therefore occurs randomly without any need for external intervention, whereas stimulated emission occurs when an ion, in its excited state, interacts with a photon of the proper wavelength. To achieve release of the stimulated emission, one of the mirrors is made fully reflective and the other only partially reflective. The portion of the light wave striking the second mirror leaves the cavity as the emitted laser beam, and the reflectivity of the mirror is selected to satisfy the requirements for efficient application in a particular system of reflecting mirrors, which are then fitted to either a slit lamp or another delivery system.
Types of lasers and their clinical use
Each lasing medium produces a different wavelength with a selective absorption effect in tissue. Tunable dye lasers are capable of providing a broad range of wavelengths and a wide range of tissue responses. The most powerful lasers are generally used in industrial applications. Although such lasers may generate many kilowatts of energy, those used in medicine require much less, generally no more than 100 watts.
Many factors other than power levels determine a laser’s effectiveness as a medical instrument. The characteristics of both the target tissue and the laser source determine the biologic consequences of laser radiation. Laser tissue interaction is directly influenced by the irradiance and decreases with depth because of absorption and scattering. Absorption is one of the most important factor and is related to the presence of several chromophores in the retina, such as melanin, hemoglobin, and xanthophil and each of these pigments present variation of absorption by different laser wavelengths and should be taken into consideration when treating different retinal diseases. For example, when treating close to the fovea, we should avoid blue wavelengths that are absorbed highly by xanthophylls in inner retina and could cause undesired damage of retinal tissue. When treating retinopathy of prematurity or patients with severe hemorrhages, longer wavelengths, such as red or infrared should be used with limited or no absorption from hemoglobin. Interactions of light with tissue can be photochemical , which is a nonthermal light induced reaction used mainly in the photodynamic therapy; photothermal reaction , either by photocoagulation or photothermal stimulation with nonlethal thermal stimulation of the retinal pigment epithelium (RPE); photoablation used in refractive surgery and produced by high power ultraviolet lasers that break the covalent bond of the corneal protein and allowing submicron corneal layer removal without opacification of the adjacent tissue; photodisruption , widely used in capsulotomy and more recently in intrastromal ablation for corneal surgery and cataract surgery.
Thermal mechanism
The human eye transmits light between wavelengths of 380 and 1400 nm. In principle, light throughout this range may be used to treat intraocular structures by delivery through the pupil. At wavelengths shorter than 380 nm, the ultraviolet-absorbing properties of the lens and cornea limit the exposure to the retina. At wavelengths longer than 1400 nm, water absorption sharply limits this transmission. Because laser light is monochromatic, highly columnated and intense, and because the eye is an optically open system, laser irradiation is well suited to produce thermal effects resulting in photocoagulation. When absorbed by tissue, laser light is transformed to heat energy, causing a thermal response.
Absorption of laser light is related to wavelength and absorption characteristics of the tissue. When light strikes a tissue surface, part of this light is reflected, part is absorbed by various cells or cell layers, and part is transmitted inward until the energy is depleted. The absorption of laser light depends on the chromophore content of the tissues. Tissue chromophores include hemoglobin (present in blood vessels), melanin (present in the RPE, iris pigment, epithelium, uvea, and trabecular meshwork), and xanthophyll (present in the inner and outer plexiform layers of the retina in the macula) ( Fig. 35.3 ). When these tissues absorb light, the light is transformed into heat energy, a thermal reaction occurs, and photocoagulation results with surrounding tissue destruction. Argon blue-green (composed primarily of the 488 and 514.5 nm emission lines), argon monochromatic green (514.5 nm), and krypton red (647 nm) have been used in the past, however, currently, the most commonly used lasers are the frequency doubled Nd:YAG (532 nm) laser and diode (semiconductor) laser, especially with yellow wavelength (577 nm).
