4.2.1.1 Vascular Lesions
With the advances in theory and technology, new devices evolved for conditions that had typically been resistant to old technology. As systems began to utilize increasingly selective wavelengths, and improved safety measures, it became possible to selectively target a specific chromophore with minimal damage to the surrounding tissue. This was and is the key to the treatment of smaller facial vessels, which range from <0.1 to 1 mm. In this case, an optimal wavelength and pulse duration are combined with safety measures to target and treat these small vessels. This brought the pulsed dye laser (PDL) into the spotlight of the laser market.
When developing light-based technology, in order to treat facial vessels (telangiecta – sias), there are three key factors; pulse duration, fluence, and wavelength. The target chro – mophore in these treatments is oxyhemoglobin, which has an absorption peak around 577 nm (Fig. 4.1), so the PDL uses filters to emit light at and around this wavelength. The light is sent into the vessel, selectively absorbed by the oxyhemoglobin, converted to heat, and then diffused to the endothelial lining of the vessel. The endothelium is composed
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Figure 4.1 Absorption spectrum of major skin chromophores—melanin, oxyhemoglobin, and water—against wavelength emissions. |
primarily of proteins, so thermal denaturation occurs, causing the vessel to collapse. The theories behind this mechanism of treatment are called the theory of selective photothermolysis, and the extended theory of selective photothermolysis. Literally, photothermolysis means light-based thermal change; the change in this case is the rise in temperature and the subsequent denaturation of the interior vessel linings. The PDL was able to produce optimal parameters to produce this effect.
For wavelength choice, the key principle, as stated earlier, is oxyhemoglobin absorption. This type of absorption has three peaks: 418, 542, and 577 nm. While the highest absorption coefficient is available at 418 nm, there is a very high level of melanin absorption and very short wavelength for sufficient penetration. The presence of a competing chromophore causes energy absorption in areas other than the target, decreasing the amount of energy delivered to the target, and thus decreasing treatment results and increasing the risk of side effects. The 577 nm peak represents the best of both worlds; a high absorption coefficient with a sufficient amount of energy penetration. Most leading PDLs are filtered between 585 and 600 nm, and obtain good results. They are used to treat all forms of facial telangiectasia. (Figure 4.2, showing treatment with Palomar Medical Technology, Inc.’s Lux G pulsed light hand piece, demonstrates the type of vessels being discussed.)
Correct pulse duration depends mainly on the thermal relaxation time of the target, which is an estimation of how long it takes for heat to dissipate from the target. The thermal relaxation time can be used to allow an operator to precisely tune his or her pulse duration to each treatment. For most treatments of small facial vessels, the correct pulse duration with PDL will be between 0.45 and 40 ms, depending on vessel diameter. This pulse duration will be coupled with fluence high enough to cause sufficient injury to the targeted vessel. Using short pulse duration and a high fluence creates a high peak power that is necessary for the treatment of small vessels. The fluence must be high enough to raise the oxyhemoglobin inside the vessel to above 70°C, which is the temperature required for the endothelial denaturation, causing vessel collapse. In large clusters of lesions such as port wine stains, multiple treatments are necessary; it is not uncommon to see lightening of the lesion even after 20 treatments and purpura is a common side effect. Purpura is a dark-red/purple colored reaction posttreatment that comes from aggressively treating smaller vessels. These
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Before After Figure 4.2 Facial vessels before (left) and after (right) three treatments with the Palomar Lux G hand piece. |
vessels explode and the blood shows up in this type of reaction, not unlike that of a common hematoma (bruise). Purpura will occur when high levels of fluence are used at short pulse durations, or too much pulse overlapping occurs. This purpura is not uncommon in PDL treatments, and usually resolves itself with topical ointments and bandages in 7-10 days.