By Sivisanker Bakthavachalam, MD

History
The development of lasers for use in medicine and surgery has revolutionized patient care over the past several decades. In the field of otolaryngology, lasers have been applied to treat a vast number of conditions in every sub-specialty including laryngology, facial plastics, pediatric otolaryngology, head and neck oncology, otology, and rhinology and sinus. The purpose of this discussion is to first provide a historical perspective on the laser's development, followed by a brief scientific explanation of how lasers work, and finally reviewing the various types of lasers and their past and current applications to specific areas in the field of otolaryngology.

The first concept of the laser was initiated by Albert Einstein in 1917. His "theory of stimulated emission" refers to the idea that a photon of electromagnetic energy can stimulate the emission of another photon from excited state atoms. The amplification of this process leads to the energy that generates a laser beam. In 1958, Schawlow and Townes developed a technique for generating monochromatic radiation by stimulated emission. Maiman in 1960 developed the first working laser from a synthetic ruby crystal surrounded by a flashlamp. Goldman subsequently pioneered the ruby laser to use on benign skin lesions. Jako in 1969 was the first to use the CO2 laser on the canine larynx.  Bredemeier then developed the micromanipulator which permitted greater accuracy for using the CO2 laser. Strong, Polyani, and Jako in 1971 were the first to use the CO2 laser on the human larynx. Anderson in 1983 developed the notion of "selective photothermolysis" which became the basis for treating cutaneous vascular lesions.

 "Laser" stands for Light amplification of stimulated emission of radiation. As alluded to earlier stimulated emission refers to the emission of two photons from an excited atom that has been stimulated by another photon of particular wavelength. This is the opposite of absorption which requires stimulation for an electron to go from a stable, unexcited state to an unstable, excited state. Spontaneous emission is the spontaneous release of a photon from an excited state atom without and external stimulation (Figure 1). Most atoms exist in an unexcited state, however, for stimulated emission to occur to generate a laser beam, more excited state atoms need to exist. This state is called a "population inversion." In order to attain this "inverted" state an external source of energy is required to provide a number of excited state atoms in a system. This introduction of energy is called "pumping." A laser is comprised of three elements: a pumping system, the medium, and the optical cavity (Figure 2). The pumping system provides the energy for population inversion, while the medium is the source of radiation and determines the wavelength of the laser beam. Different lasers get their distinguishing properties from the medium (e.g. gas, CO2 or solid, Nd:YAG). The optical cavity consists of two almost parallel mirrors that allow for the amplification of light energy. The one slightly non-parallel mirror reflects the amplified light energy and thus releases it in what we see as the laser beam.

Properties
The three properties of a laser are collimation, coherence, and monochromicity. Collimation refers to the beam traveling in a parallel, unidirectional array. Coherence means the laser energy is in phase temporally and spatially. Monochromicity means each laser beam of energy is of a single wavelength determined by the medium in which it is generated. A laser beam can be absorbed, transmitted, scattered, or reflected. The depth of penetration in tissue is determined by absorption and scatter. Three important measurements of laser energy are power, fluence, and irradiance. Fluence is a measure of energy density measured in joules per surface area (cm2). Irradiance is watts/cm2, or the amount of power delivered per area of tissue, and represents the most important value in measuring laser effect. Lasers have photothermal, photomechanical, or photochemical effects on tissue. Photothermal effects are the most visible form of tissue injury and leads to coagulation necrosis. A chromophore in the tissue absorbs laser energy of a specific wavelength and if the temperature of the tissue is greater than 100 degrees Celsius, then the tissue is vaporized. Heat is lost to surrounding tissue through conduction. The time for target tissue to cool to half the temperature to which it was heated to is called the thermal relaxation time (TRT). Photomechanical effects refers to damage done to surrounding tissue from acoustic waves generated from the point of tissue contact. The photothermal and photomechanical effects of the laser represent the most familiar type of tissue damage that we visualize after laser to tissue contact. Photochemical effects refer to the photodynamic therapy where a chemical sensitizer is administered, absorbed by target cells, exposed to laser light, and causing a chemical reaction that damages the target cells. This process is currently being developed to target tumor cells.

 The theory of selective photothermolysis, popularized by Anderson in 1983, is the basis for pulsed dye laser use in dermatologic lesions. The most selective damage occurs when energy is deposited at a rate faster than the rate of cooling. Laser exposure duration ought to be just shorter than the thermal relaxation time to limit adjacent tissue damage.

Types
The three types of lasers are continuous wave (e.g.-- CO2, Argon), quasi-continuous wave (copper wave), and pulsed laser (flashlamp pulsed dye). Argon laser emits blue-green light in the visible spectrum at wavelengths of 488 nm and 514 nm. This laser is strongly absorbed by hemoglobin and can be utilized to treat cutaneous vascular lesions and for stapedotomies. The Nd:YAG (Neodymium:Yttrium-aluminum-garnet) laser has a wavelength of 1064 nm with a 4 mm depth of penetration. It has poor precision but optimal coagulation and can be used with a fiberoptic scope. This laser is ideal for obstructive lesions in the tracheobronchial tree. The CO2 laser is the most versatile laser in otolaryngology. This laser has a wavelength of 10,400 nm, is strongly absorbed by water, and can be used for glottic, subglottic, tracheal lesions, oral and glottic cancer, the pediatric airway, and in laser facial resurfacing. The disadvantage is that it is incompatible with a fiberoptic scope, preventing its use in distal airway lesions. The potassium-titanyl-phosphate (KTP) laser has a wavelength of 532 nm and is strongly absorbed by hemoglobin. This laser is fiberoptic compatible and can be used for vascular lesions, stapes and middle ear surgery, sinus surgery, and pediatric airway surgery. The holmium:YAG laser has a wavelength of 2100 nm and is absorbed by water. The holmium laser has fiberoptic compatibility making it ideal for distal airway lesions. This laser provides good hemostasis and bone ablation. The flashlamp pulsed dye laser (585 nm) is ideal for cutaneous vascular lesions (e.g. port wine stain, hemangiomas) and minimizes damage to surrounding tissue. The laser is absorbed by oxyhemoglobin.

Treatments and Procedures
Hemangiomas are most common in the head and neck and are usually not present at birth. They are most susceptible to laser treatment during the early proliferative phase. Full treatment usually involves 4-6 treatments at 3-6 week intervals (Figure 3). Port wine stains, the most common cutaneous vascular malformation, is also susceptible to pulsed dye laser treatment. Telangiectasias, as seen on the nasal septum in Osler-Weber-Rendu syndrome, can also be treated with the pulsed dye laser. The most common complication with the pulsed dye laser is purpura, which normally lasts 7-14 days (Figure 4).

CO2  laser resurfacing is a common procedure in the field of facial plastic surgery. Resurfacing is indicated in photoaged skin and in acne scarring. Laser ablation removes the epidermis and part of the dermis and causes collagen shrinkage, thus eliminating wrinkles.

 Photodynamic therapy, as discussed earlier, is currently still being studied for use in treating head and neck cancer patients. Following a photosensitizer administration, the target is exposed to laser radiation, producing a chemical reaction, which leads to cell death (Figure 5). This technology has been reported to successfully treat soft palate SCCA, recurrent nasopharyngeal SCCA, and esophageal SCCA.

 Lasers have also been reported for use in endoscopic sinus surgery. The ideal sinus surgery laser needs to able to precisely ablate bone in thin layers, but at the same time ablate large volumes of tissue while achieving adequate hemostasis. Metson (1996), reported the used of the Holmium laser for endoscopic sinus surgery in 32 patients and found similar results compared to using conventional instrumentation. There is less blood loss with the laser, but greater operating room time. 

 Lasers have an important role in the field of otology. A stapedotomy, for treatment of otosclerosis, can be performed with a microdrill or one of many lasers (e.g.-CO2, Argon. KTP) to make a fenestra in the stapes footplate. The ideal laser for this procedure would need to have a precise spot size, be able to vaporize bone or collagen, not heat the perilymph, and not penetrate the perilymph. The CO2 laser is known to provide better absorption of bone and collagen, while the KTP and Argon lasers provide better precision than absorption. Past studies have not demonstrated a difference in stapedotomy efficacy among the various lasers and between the laser and the microdrill. Lasers have also been used in the past for myringotomies and to lyse middle ear adhesions.

 Transoral microsurgery for SCCA is an innovative method of resecting oropharynx and early glottic cancer lesions different from conventional techniques. Using the laser through an oropharyngoscope or a laryngoscope, cancer can be resected in piecemeal fashion as opposed to an en-bloc manner. Any positive margins can be repeatedly re-resected. Use of the laser leads to less post-operative pain, decreased hospital stay and reduces the chance of cancer spread. In addition, there are fewer fistulae, fewer required tracheotomies, and less financial costs. The disadvantages are poor exposure, more time consuming, and it is technically difficult. Steiner pioneered the use of the laser for cancer resection in Germany. He had enrolled hundreds of patients to resect glottic, supraglottic and oropharyngeal cancers with fair results. In one of his studies for base of tongue cancer, 48 patients were enrolled and 43/48 had delayed selective neck dissections in addition to the primary tumor resection. There was a 27% overall treatment failure with a 13% local recurrence rate. None of the patients require tracheotomies and 3 patients required G-tubes. Other studies have documented the success of the CO2 laser for T1/T2 glottic SCCA resection. Pradhan (2003) and Maurizio et. al (2005) report their experiences in treating early laryngeal SCCA with the laser. Pradhan found that patients treated with the laser versus an open resection had a better rate of salvageability and required fewer total laryngectomies in the future.

 The CO2 laser has been utilized in the pediatric airway since the early 1970's. This laser has the advantage of causing superficial damage to tissue while minimizing edema. The versatile laser is commonly employed to treat recurrent respiratory papillomatosis, glottic, subglottic and tracheal stenosis, hemangiomas, vallecular cysts, and choanal atresia. The Holmium:YAG laser, with it's fiberoptic capabilities, is currently being employed to treat distal tracheal lesions. Studies have shown that the CO2 laser provides successful results for early airway stenosis, but higher grades of stenosis fare better with open resection.

Safety
No laser discussion would be complete without mentioning safety precautions. While in the operating room, one must wear the proper safety eyewear with adequate skin protection. A suction device must be in place for smoke evacuation and constant communication with the anesthesiologist is necessary to ensure the anesthesia and ventilation are administered safely. Airway fires are always a concern when utilizing a laser in close approximation to high concentrations of oxygen. Superimposed supraglottic high frequency jet ventilation is a commonly used ventilatory technique for laser laryngeal surgery. Two streams of air are injected through the laryngoscope in coordination with inspiration. Reports of this technique reveal a  low incidence of airway fires due to the lack of a combustable endotracheal tube. One hundred percent O2 can be used in the airway, although the risk of airway fire increases. The patient ought to be maintained using the lowest possible oxygen saturation. At the end of inhalation, the concentration of pure oxygen is highest in the supraglottis and oropharynx because the diluted room air is not drawn into the airway with the pure oxygen. As a result, the chance for combustion between the concentrated oxygen and any objects in the field (e.g.-surgeon gloves) is higher. Using wet towels to cover the eyes and face will reduce the chance of combustion in case there is contact between the laser and skin in the presence of highly concentrated oxygen.

 Lasers in otolaryngology have definitely revolutionized the field in many ways. Future applications of lasers include the pulsed dye laser for glottic lesions, which may be able to be done in the office. Additionally, the pulsed dye laser may be able to treat tumors by targeting microvasculature. Finally, the role of photodynamic therapy will only increase as more testing is done in its efforts to treat head and neck cancer.

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