Laser treatment of pigmented lesions.

In 1958, Schawlow and Townes, working with microwaves, first proposed a technique for the generation of monochromatic radiation by stimulated emission. They produced monochromatic radiation in the infrared optical region of the electromagnetic spectrum with an alkali vapor used as the active medium. Maiman, using a ruby crystal in 1960, developed stimulated emission of a red-light beam with a wavelength of 694 nm. This was the first working laser, and it is from this prototype that today's lasers are derived. Since 1960, research and technical advances have adapted lasers to dermatology. Leon Goldman, the father of dermatologic laser surgery, published preliminary results on the effects of a ruby laser on skin diseases. Early work with the ruby laser consisted of ablation techniques. Little bleeding was noted after nonspecific damage to the superficial dermal layers, and small areas of skin could be treated with high-intensity radiation with few sequelae. Not surprisingly, the ruby laser was found to treat pigmented lesions well owing to its ability to target melanin. Today, there are numerous lasers that can specifically target pigmented lesions, including other red light lasers (ie, alexandrite, neodymium:yttrium-aluminum-garnet [Nd:YAG]) and green light lasers (ie, 510-nm pulsed dye, 532-nm frequency-doubled Nd:YAG). The wide range of lasers that can be used to treat pigment is a result of the broad absorption spectrum of melanin. Even so, other less pigment-specific lasers have been used to treat pigmented lesions, including the CO 2 and argon lasers. The CO 2 laser exerts its effect on tissue by simple vaporization of water-containing cells. Textural skin changes and scarring may result from this nonselective destruction. A very low wattage CO 2 laser appears to reduce the risk of scarring and has been used effectively to treat superficial epidermal pigmented lesions, such as solar lentigines. The green and blue light (488 and 514 nm, respectively) of the argon laser is specifically absorbed by melanin. The problem with the system is that it functions as a continuous wave laser. Thus, although this laser selectively targets the melanin chromophore, the heat produced dissipates from the absorbing melanosomes, causing thermal damage to surrounding tissue with resultant hypopigmentation and scarring. Lasers that produce pulses of light shorter than the thermal relaxation time of melanosomes are now used to selectively destroy targeted melanin. The process of removing pigmented lesions using sufficiently shorter laser pulses is called selective photothermolysis. The targeted melanosome selectively absorbs the laser light and the resultant increase in temperature induces thermal damage of the melanosome. Because this damage occurs over a time period shorter than the melanosome's thermal relaxation time, the absorbed energy is confined to the targeted melanin-containing melanosomes within melanocytes and keratinocytes. Selective destruction of melanosomes results without damage to surrounding tissue structures. The pigmented skin of guinea pigs and human volunteers has been shown to respond to short laser pulses over a wide range of visible light wavelengths. Lasers that target pigment include the pulsed dye (510 nm), copper vapor (511 nm), krypton (520–530 nm), frequency-doubled Q-switched Nd:YAG (532 nm), Q-switched ruby (694 nm), Q-switched alexandrite (755 nm), and Q-switched Nd:YAG (1064 nm). All these wavelengths can effectively treat pigmented lesions without disrupting the normal surrounding tissue. Pigment-specific lasers can be divided into three categories: (1) green, (2) red, and (3) near-infrared. Green light lasers are further subdivided into both pulsed and nonpulsed systems. The red and near-infrared lasers currently available are pulsed (or Q-switched) systems. Green light lasers do not penetrate as deeply into the skin as do the red and near-infrared lasers owing to their shorter wavelengths. Green light lasers are therefore effective only in the treatment of epidermal pigmented lesions.