Long-pulsed lasers, small spot size reduces lentigine treatment risks in Asian skin

Key Points

Results of a retrospective study comparing outcomes using four different pigment-specific lasers to treat freckles and lentigines in Asian patients are consistent with principles of cutaneous laser surgery. They also underscore the importance of considering both the pulse duration and laser spot size to optimize the benefit-to-risk ratio when treating these benign melanocytic lesions in darker skin, according to researchers at the 2010 annual conference of the American Society for Laser Medicine & Surgery.

The study identified 40 Chinese patients with Fitzpatrick skin types III or IV who were treated with either a 595 nm long-pulsed dye laser (LPDL; Vbeam Perfecta, Candela), a 755 nm LP alexandrite laser (GentleLASE, Candela), a 532 nm Q-switched (QS) Nd:YAG laser (MedLite C3, HOYA ConBio) or a 532 nm LP KTP laser (Gemini, Iridex). The patients were randomly selected from a large cohort of patients treated by Henry Chan, M.D., Ph.D., F.R.C.P., F.H.K.C.P., F.H.K.A.M., honorary professor, Li Ka Shing Faculty of Medicine, University of Hong Kong, HKSAR, and visiting scientist, Wellman Center for Photomedicine, Massachusetts General Hospital, Harvard Medical School, Boston.

Patients received between one and four treatments at intervals of four to six weeks. The number of treatments depended on lesion response, and the mean for the entire study population was 1.8. An independent dermatologist assessed pigmentation clearance and postinflammatory hyperpigmentation (PIH) through visual analogue scale grading of pre- and post-treatment photographs.

PIH, SIGNIFICANT RESULTS Statistically significant improvement of both global and focal facial pigmentation was achieved in patients treated with the 595 nm LPDL, 532 nm QS Nd:YAG and 532 nm LP KTP lasers. The 755 nm LP alexandrite laser failed to produce statistically significant improvement and was also associated with the highest incidence of PIH (38 percent). The risk of PIH was also relatively high following treatment with the 532 nm QS Nd:YAG laser (22 percent), was small in the 595 nm LPDL group (6 percent) and absent after treatment with the 532 nm LP KTP laser.

"As a retrospective review, this study has methodological limitations, the most important of which is potential selection bias, as patients with a poorer clinical response or who developed a complication may be more likely to return for follow-up care than those who had an excellent outcome," Dr. Chan says.

"Nevertheless, the findings are consistent with the results of a prospective, randomized, split-face study we published that showed a long-pulsed 532 nm Nd:YAG laser was gentler than a QS 532 nm Nd:YAG laser when treating skin of color (Dermatol Surg. 2000;26:743-749). However, spot size is also important, since with a larger spot size, there is a risk of laser energy absorption by both the targeted melanocytic lesion and darker-pigmented, normal surrounding skin," Dr. Chan says.

Stephanie G.Y. Ho, M.D., honorary clinical research associate, University of Hong Kong, presented the research at the meeting. She says, "There has been little research comparing different pigment-specific lasers for darker skin types. Our study highlights the importance of using long-pulsed lasers, smaller spot sizes and cooling strategies in order to minimize the risk of PIH during laser treatment for skin of color. In addition, the 595 nm LPDL and 532 nm LP KTP, which appeared to be safe and effective for lentigines in darker skin types, may also be used to treat other conditions, and therefore may be good cost- and space-saving solutions for clinicians."

PIH RISKS FOR EACH LASER Dr. Chan explains that while all of the lasers used are effective for treating lentigines and freckles, complications are a particular concern in Asian patients because of the higher epidermal melanin content of their darker skin. The risk of PIH in skin of color is higher when using a QS versus LP laser because the QS device, with its relatively short pulse duration (nanoseconds), causes photomechanical injury in addition to a therapeutic photothermal effect.

"With the nanosecond pulse duration, there is a rapid change in the thermal gradient of the target that generates an ultrasonic wave and subsequently photomechanical injury leading to inflammation," Dr. Chan says. "The millisecond pulse duration of the LP lasers more closely matches the thermal relaxation time of the epidermal basal layer. It is more gentle because it produces only a photothermal effect, without inducing rapid thermal expansion that can lead to an ultrasonic wave."

Despite its long pulse duration, the LP alexandrite had the highest risk of PIH among the four lasers included in the study. This association may be explained by the large, 10 mm spot size of the device used.

"Spot size is not a major issue if there is significant contrast in pigmentation between the target lesion and surrounding normal skin," Dr. Chan says. "However, in darker Asian skin, the difference in pigmentation is less. Therefore, laser energy is also absorbed by the surrounding normal skin, which can lead to injury and an increased risk of complications."

Both the 532 nm QS Nd:YAG and the 532 nm LP KTP lasers have only a 2 mm spot size so that they selectively target smaller freckles and lentigines. In addition, the 532 nm LP KTP laser has a cooling device that helps protect normal surrounding skin.

Although the 595 nm LPDL has a relatively large, 7 mm spot size, the device used also features a compression handpiece that empties blood from vessels in the underlying skin, reducing the risk of purpura and therefore PIH, as well.

"Even with its larger spot size, the 595 nm LPDL will only affect the target lesion if it is used to treat larger lentigos. However, hyperpigmentation can also develop through induction of purpura secondary to mechanical injury to blood vessels. If bruising occurs, there can be some hemosiderin deposition that appears as PIH," Dr. Chan says.