Mechanism of Action for Lasers and Intense Pulsed Light

Lasers and IPL are pulsed so that they act within the thermal relaxation times of blood vessels to produce specific destruction of vessels of various diameters based on the pulse duration (Table 54-1). Lasers of various wavelengths and broad-spectrum IPL are used to selectively treat blood vessels by taking advantage of the difference between the light absorption of the components in a blood vessel (oxygenated hemoglobin, deoxygenated hemoglobin, and methemoglobin) and the overlying epidermis and surrounding dermis to selectively thermocoagulate blood vessels. Deoxygenated hemoglobin has distinct optical properties, with two absorption spectrum peaks at approximately 545 and 580 nm, and a broader peak beyond 650 nm (Fig. 54-1). The main feature to note in the curve is the strong absorption at wavelengths below 600 nm, with less absorption at longer wavelengths. This is because the absorption coefficient in blood is higher than that of surrounding tissue for wavelengths between 600 and 1064 nm.

TABLE 54-1 Thermal Relaxation Times of Blood Vessels

Figure 54-1 Oxygenated and deoxygenated hemoglobin. Water and melanin absorption curves are given as a function of wavelength.

(Adapted from Boulnois JL: Photophysical processes in recent medical laser developments: A review. Lasers Med Sci 1:47–66, 1986. Reproduced with permission from Goldman MP, Bergan JB, Guex JJ: Treatment of leg telangiectasia with laser and high-intensity pulsed light. In Sclerotherapy: Treatment of Varicose and Telangiectatic Leg Veins, 4th ed. London, Mosby, 2006, p 361, Fig. 13.29.)

The goal is to deliver sufficient energy to thermocoagulate the target vessel, while the overlying epidermis and perivascular tissue remains unharmed. To accomplish this selective preservation of tissue, some form of epidermal cooling is also required. A number of different laser and IPL systems have been developed toward this end.

An understanding of the appropriate target vessel for each laser or IPL device is important so that treatment is tailored to the appropriate target. As detailed in sclerotherapy textbooks, most telangiectases arise from reticular veins. Therefore, the single most important concept to keep in mind is that feeding reticular veins must be treated completely before treating telangiectasia. This minimizes adverse sequelae and enhances therapeutic results. Failure to treat feeding reticular veins and short follow-up periods after the use of lasers may give inflated values to the success rates of laser treatment.

Histology of Leg Telangiectasia

The choice of proper wavelength(s), degree of energy fluence, and pulse duration of light exposure are all related to the type and size of target vessel treated. Deeper vessels necessitate a longer wavelength to allow penetration. Large-diameter vessels necessitate a longer pulse duration to effectively thermocoagulate the entire vessel wall, allowing sufficient time for thermal energy to diffuse evenly throughout the vessel lumen. The correct choice of treatment parameters is aided by an understanding of the histology of the target telangiectasia.

Venules in the upper and middle dermis typically maintain a horizontal orientation. The diameter of the postcapillary venule ranges from 12 to 35 µm. Collecting venules range from 40 to 60 µm in the upper and middle dermis and enlarge to 100 to 400 µm in diameter in the deeper tissues. Histologic examination of simple telangiectases demonstrates dilated blood channels in a normal dermal stroma with a single endothelial cell lining, limited muscularis, and adventitial layers. Most leg telangiectases measure from 26 to 225 µm in diameter. They are found 175 to 382 µm below the stratum granulosum. The thickened vessel walls are composed of endothelial cells covered with collagen, elastic, and muscle fibers.

Review of Available Lasers

Patients seek treatment for leg veins mostly for cosmetic reasons. Any effective treatment should be relatively free of adverse sequelae.

Krypton Triphosphate and Frequency-Doubled Nd-YAG (532 nm)

Modulated krypton triphosphate lasers have been reported to be effective at removing leg telangiectases using pulse durations between 1 and 50 msec. The 532-nm wavelength is one of the hemoglobin absorption peaks. Although this wavelength does not penetrate deeply into the dermis (about 0.75 mm), relatively specific damage (compared with argon laser) can occur in the vascular target by selection of an optimal pulse duration, enlargement of the spot size, and addition of epidermal cooling (Fig. 54-2).

Figure 54-2 Leg telangiectasia. A, Before treatment. B, After treatment.

(Courtesy of David Vasily, MD, Bethlehem, Penn.)

Effective results have been achieved by tracing vessels with a 1-mm projected spot. Typically the laser is moved between adjacent 1-mm spots following the vessels at 5 to 10 mm/sec. Immediately after laser exposure, the epidermis is blanched. Lengthening of the pulse duration to match the diameter of the vessel is attempted to optimize treatment.

We and others have found the long-pulse 532-nm laser (frequency-doubled Nd:YAG) to be effective in treating leg veins less than 1 mm in diameter that are not directly connected to a feeding reticular vein. When used with a 4° C chilled tip, a fluence of 12 to 15 J/cm² is delivered as a train of pulses in a 3- to 4-mm diameter spot size to trace the vessel until spasm or thrombosis occurs. Some overlying epidermal scabbing is noted, and hypopigmentation is not uncommon in dark-skinned patients. Although individual physicians report considerable variation in results, usually more than one treatment is necessary for maximum vessel improvement, with only rare reports of 100% resolution of the leg vein. Efficacy is technique dependent, with the potential for achieving excellent results. Patients need to be informed of the possibility of prolonged pigmentation at an incidence similar to that with sclerotherapy as well as temporary blistering and hypopigmentation that is predominantly caused by epidermal damage in pigmented skin (type III or above).

Pulsed-Dye Laser (585 or 595 nm)

The pulsed-dye laser (PDL) has been demonstrated to be highly effective in treating cutaneous vascular lesions consisting of very small vessels, including port wine stains (PWSs), hemangiomas, and facial telangiectasia. The depth of vascular damage is estimated to be 1.5 mm at 585 nm and 15 to 20 µm deeper at 595 nm. Consequently, penetration to the typical depth of superficial leg telangiectasia may be achieved. However, telangiectases over the lower extremities have not responded as well, with less lightening and more post-treatment hyperpigmentation. This may be due to the larger diameter of leg telangiectases compared with dermal vessels in PWSs and larger-diameter feeding reticular veins, as described previously.

Vessels that should respond optimally to PDL treatment are red telangiectases less than 0.2 mm in diameter, particularly those vessels arising as a function of telangiectatic matting after sclerotherapy (Fig. 54-3). This is based on the time of thermocoagulation produced by this relatively short-pulse laser system (see Table 54-1).

Figure 54-3 Red telangiectasia. A, Before treatment. B, After treatment.

(Courtesy of Khalil A. Khatri, MD, Skin and Laser Surgery Center of New England, Nashua, NH, and Palomar Medical Technologies, Inc., Burlington, Mass.)

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