Laser Thermal Ablation: Current Data

Historical Background

Endovenous laser ablation (EVLA) therapy arose from a background of managing refluxing truncal veins using open surgical techniques, such as stripping with high ligation. The technology was first introduced in 1999, and following US Food and Drug Administration approval for commercial use in 2002, has exhibited exponential growth in use. This is, in part, because of the ability to perform this endothermal ablation technique in an ambulatory setting using tumescent anesthesia. For the most part, the increased use of EVLA for the treatment of refluxing truncal veins, as well as symptomatic refluxing perforators, is in large part because of an outstanding safety, efficacy, and durability profile.

Increased mitigation of postprocedural symptoms remains the final frontier in improving EVLA treatments. Bruising, transient pain, and induration of the thigh are common adverse events after endovenous laser therapy (EVLT) and are most likely caused by laser-induced perforation of the vein wall, with extravasation of blood into surrounding tissue. It is known that conversion of an incompetent vein into a fibrous cord, with subsequent sonographic disappearance, generally leads to permanent occlusion. At the onset of EVLT, little was known about the mechanism of action and durability of treatment after intervention with these devices. Studies have indicated that heat-related damage to the inner vein wall leads to thrombotic occlusion of the treated vein, and this has led to iterative improvements in technology and technique.

EVLA can be classified into hemoglobin-specific laser wavelengths (HSLWs) and water-specific laser wavelengths (WSLWs) ( Fig. 7.1 ). Wavelengths of 808, 810, 940, 980, 1064, 1319, 1320, 1470, 1510, and 1920 nm have been successfully used for great saphenous vein (GSV) ablation and for other superficial axial and perforating veins. Hemoglobin and, to a lesser extent, myoglobin in venous smooth muscle cells are the dominant chromophores at the shorter end of this range, whereas in the longer wavelengths, water dominates as the energy-absorbing molecule.

Published reports suggest that delivery of higher energy is required to effect secure vein closure; however, with increased energy delivery, pain and bruising after treatment are encountered more frequently. After EVLA, studies have demonstrated that 70% of limbs experience some degree of pain, and 50% require analgesics for pain management. Kabnick reported an average pain score of 2.6 on a scale of 0 to 5 after EVLA. There is increasing focus on reducing perioperative pain and bruising in the field of EVL saphenous ablation. As an example, the 1470-nm laser requires less energy delivery for closure, with concomitant reports of less pain and bruising postprocedure. One report by Shutze et al., compared the 1470-nm laser (295 procedures) with the 810-nm laser (1144 procedures). This study demonstrated that pain, bruising, and quality of life scores were all improved in the 1470-nm cohort. The WSLWs were developed to target the interstitial water in the vein wall and minimize perforations. Trends in the literature suggest that these longer wavelength lasers may produce fewer side effects than HSLW lasers at comparable linear endovenous energy density (LEED). Two comparisons of different wavelengths with similar delivered laser energy have been performed. One study compared 940-nm and 1320-nm wavelengths in a retrospective analysis, and another compared 810-nm and 980-nm wavelengths in a randomized prospective study. The two studies demonstrated equivalent safety and efficacy at similar energy dosing. However, EVLA performed at comparable LEED with either the 940-nm (HSLW) or the 1320-nm (WSLW) lasers showed a reduction in postoperative pain and bruising with the 1320-nm device. Also use of less power (5 W) demonstrated a lower rate of side effects than did 8 W, with a laser operating at a wavelength of 1320 nm. A low rate of pain and bruising was reported after GSV treatment for GSV reflux with the 1470-nm wavelength at 5 W, 30 J per cm. To further the concept, Kabnick and Sadek performed a three-way comparison of the 810-nm, 980-nm, and 1470-nm lasers. Essentially, there was a nearly dose-dependent improvement in pain and bruising scores, which correlated directly with increasing wavelength. Efficacy remained equal.

In addition to laser wavelength, fiber type may play an even more pronounced role in the development of postprocedural symptoms. The underlying principle behind this, is the thought that direct contact between the laser fiber and the vein wall may contribute to vein wall perforations and subsequent pain and bruising. Consequently, jacket-tip fibers were developed to minimize this contact. A variety of jacket-tip fibers exist, including ceramic and metallic types, and some are even configured to disperse the emitted energy. In the same three-way comparison of the 810-nm, 980-nm, and 1470-nm laser fibers, concomitant evaluations of the bare versus jacket-tip fibers demonstrated that the fiber type contributed dominantly to whether or not postprocedural bruising/pain would develop. Moreover, this was corroborated by in vitro analysis, which demonstrated decreased thermal injury depths in the jacket-tip fibers as compared with the bare-tip fibers.

Etiology and Natural History of Disease

There remains controversy over the mechanism of action of EVL, and most of the investigations were performed with HSLWs. Proebstle found in vitro with an HSLW, that the extensive heat damage of the endothelium and the intima came from steam-bubble formation and induced full-length thrombotic occlusion of the vein. Bush et al., in a histologic study, found that the mechanism of injury with HSLWs is secondary to steam bubbles caused by water evaporation of the blood followed by transmitted heat injury to the tissue. Acutely, there is complete loss of the endothelium and early thrombus formation, followed by the injury response with inflammatory cellular infiltration into the subintimal layers. Eventually, fibroblasts deposit collagen and represent the predominant histologic finding at 4 months postoperatively.

Opposed to the steam-bubble theory, Fan and Rox-Anderson studied existing histologic reports from studies with excellent closure rates using tumescent anesthesia and found that HSLWs produce a transmural, vein wall injury, typically associated with perforations and carbonization. The pattern of injury was eccentrically distributed, with maximum injury occurring along the path of laser contact. The authors concluded that steam production during EVLA accounted for only 2% of applied energy dose (i.e., EVLA causes permanent vein closure through a high-temperature photothermolytic process at the point of contact between the vein and the laser). Precise mechanism-of-action studies with ample histologic information are lacking with WSLWs. Kabnick and Sadek did perform an adjunctive evaluation using a porcine collagen matrix molecule, which demonstrated that thermal injury depths were less in the 1470-nm WSLW as compared with the 810-nm HSLW laser.

Although it is not known exactly how much damage to the individual layers of the vein wall is required, it seems that at a minimum, intimal and medial coagulation are necessary for long-term closure. Technically, the depth of penetration of a 940-nm laser beam into blood is limited to approximately 0.3 mm. Qualitative analysis with optical coherence tomography in an ex vivo model matched with histologic cross-sections showed a symmetric, complete, circular disintegration of intima and media structures, without any transmural tissue defects after radiofrequency ablation (RFA). However, with an HSLW laser, pronounced semicircular tissue ablations and complete vessel wall perforations were detected at 35 J per cm LEED. The quantitative analysis demonstrated a significant ( P < .0001) increase in intima-media thickness after RFA (38% to 67%) and EVLA (11% to 46%), and a significant ( P < .0001) reduction in vessel lumen diameter (36% to 42%) after RFA. No linear correlation could be identified between laser energy level and effects on tissue such as ablation/perforation, media thickening, or vein lumen diameter.

Weiss examined the gross tissue effects and tissue temperatures generated during EVLA with an 810-nm diode laser in an in vivo goat model. Using thermal sensors mounted adjacent to the laser optical fiber, they determined the mean temperature at the firing tip was 729°C (peak 1334°C). The intense, thermal-heating zone appeared to be focally situated around the laser tip; the mean temperature decreased to 231°C and 307°C, 2 mm proximal and distal to the fiber tip, respectively. At 4-mm distal from the fiber tip, the mean temperature decreased further to 93°C. Recently, Disselhoff et al., with intravascular temperature measurements in an in vitro system, found that despite the intense heat at the laser tip, the thermal heating zone is predominantly contained within the venous lumen. Zimmet and Min demonstrated in a swine model that during EVLA with an 810-nm diode laser, ear vein outer wall temperatures ranged from 40°C to 49°C. In hind extremity veins, these investigators showed that with tumescent anesthesia, the external vein wall temperatures never exceeded 40°C.

These findings were corroborated in humans by Beale et al., when he inserted thermocouples percutaneously, positioned at 3 mm, 5 mm, and 10 mm from a small saphenous vein (SSV), after administration of tumescent anesthesia. He recorded temperatures during EVL with an 810-nm diode laser, using 1-second pulse application at 12 W, and found peak temperatures of 43°C, 42°C, and 36°C at 3 mm, 5 mm, and 10 mm, respectively, in perivenous tissues.

Pearls and Pitfalls