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Ophthalmology was the first medical specialty to utilize laser energy in patient treatment, and it still accounts for more laser operations than any other specialty.
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The main use of ophthalmic lasers was to treat various intraocular conditions. The transparency of the optical media allows laser light to be focused upon the intraocular structures without the need for endoscopy. Lasers are now used in many other areas of ophthalmic practice, including refractive surgery, cosmetic eyelid surgery, and diagnostic imaging of ocular structures.
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Because laser surgery irreversibly changes tissue, ocular laser surgery should be performed only by ophthalmologists with laser experience.
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The word “laser” is an acronym for light amplification by stimulated emission of radiation. Most sources of visible light radiate energy at different wavelengths (ie, different colors) and at random time intervals (noncoherent). The unique properties of laser energy are monochromaticity (single wavelength), spatial coherence, and high density of electrons. These allow focusing of laser beams to extremely small spots with very high-energy densities.
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A laser consists of a transparent crystal rod (solid-state laser), or a gas- or liquid-filled cavity (gas or fluid laser) constructed with a fully reflective mirror at one end and a partially reflective mirror at the other. Surrounding the rod or cavity is an optical or electrical source of energy that will raise the energy level of the atoms within the rod or cavity to a high and unstable level, a process known as population inversion. When the excited atoms spontaneously decay back to a lower-energy level, their excess energy is released in the form of light. This light can be emitted in any direction. In a laser cavity, however, light emitted along the long axis of the cavity can bounce back and forth between the mirrors, setting up a standing wave that stimulates the remaining excited atoms to release their energy into the standing wave, producing an intense beam of light that exits the cavity through the partially reflective mirror. All of the light produced has the same wavelength (monochromatic) and phase (coherent), with little tendency to spread out (low divergence). The laser light energy can be emitted continuously or in pulses, which may have pulse durations of nanoseconds or less.
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Mechanisms of Laser Effects
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The principal lasers used in ophthalmic therapy are the thermal lasers, in which tissue pigments absorb the light and convert it into heat, thus raising the target tissue temperature high enough to coagulate and denature the cellular components.
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These lasers are used for retinal photocoagulation; for treatment of diabetic retinopathy (Figure 23–1), retinal vein occlusions, and retinopathy of prematurity; for sealing of retinal holes; for photocoagulation of the trabecular meshwork, iris, and ciliary body in the treatment of glaucoma; and for the treatment of both benign (eg, choroidal hemangioma) and malignant (eg, ...