Melasma is a common acquired hyperpigmented skin disease, with an incidence of about 40% among women in Southeast Asian countries. The pathogenesis of melasma is complex and the real cause of the disease is still unclear. It is currently believed that it is mainly related to the following factors: sun exposure, endocrine changes, oral contraceptives, genetic factors, etc. There are many treatment methods for melasma, including systemic and local treatment, but there is not yet a real method with sure efficacy. In recent years, with the continuous development of laser technology, its role in the treatment of melasma has attracted more and more attention from researchers. In the absence of breakthroughs in the current therapeutic drugs, laser treatment for melasma has been given a new hope. The principle of laser treatment for melasma is based on the following points: melanin has a wide absorption spectrum, so there can be multiple wavelengths of laser light acting on melanin; the thermal relaxation time of melanin particles is relatively short, about 50-500 nanoseconds; the longer the wavelength, the stronger the penetrating ability of laser light, but the less effective it is absorbed by melanin. The basic principle of laser melanin removal is based on selective photothermolysis, that is, a certain wavelength of laser, after irradiating the skin of the lesion for a certain period of time, is selectively absorbed by the melanin particles of the lesion, and the melanin particles swell and rupture rapidly after absorbing the light, forming small fragments, which are then swallowed by phagocytes in the body and discharged out of the body, while the normal tissue is not damaged. However, due to the complex pathogenesis, melasma responds differently to different types of lasers or photons, the following will introduce the therapeutic effects of various lasers and photon technologies on melasma one by one. Q-switched laser 1. Ruby laser (Q-switched ruby laser): It can release 694nm wavelength laser, which is the earliest reported exploration of laser for melasma treatment. One of the studies compared the ruby laser for refractory melasma and hyperpigmentation with other treatments, and none of the patients showed improvement, and some of them even showed aggravation of lesions. Pathological results before and after the treatment showed a significant increase in extracellular melanin immediately after the laser exposure. Even after several months of laser treatment, the pigmentation in the epidermis returned to the pre-treatment level, but the melanophages in the dermis remained actively proliferating. Similar results have been reported in other reports, where instead of improvement, some of the melasma patients treated with the ruby laser became more severe. Therefore, the Q-switched ruby laser is not recommended for the treatment of melasma at this time. 2. Emerald laser (Q-switched alexandrite laser): Emerald laser emits a longer wavelength (755nm) than ruby laser, so it can penetrate deeper into the skin. One scholar applied the alexandrite laser for the treatment of refractory melasma in yellow ethnic groups. Six patients showed no significant improvement, three of them developed post-inflammatory hyperpigmentation after the laser, which lasted from several weeks to three months, and one patient showed hypopigmentation, which recovered after six months. The authors concluded that when applying the alexandrite laser for melasma, it is important to fully evaluate the post-treatment side effects in order to weigh the need for treatment. Because there were so few cases in this study, the sample size needs to be expanded to further evaluate the efficiency and safety of the alexandrite laser for melasma treatment, but at least for now, this treatment is not recommended. Nd:YAG laser (Q-switched Nd:YAG laser): In the past, Q-switched laser treatment for melasma used small spot, large energy and fewer scanning passes, which usually caused damage to the surrounding skin tissues and basement membrane, and was prone to the side effect of pigmentation aggravation. Polnikorn firstly reported 2 cases of refractory dermal melasma with combination of topical arbutin and sunscreen, and obtained satisfactory results with no hyperpigmentation. Cho et al. treated 25 Korean melasma patients with Q-switched Nd:YAG laser with a 6-mm spot, an energy density of 2.5 J/cm2, a 2-week treatment interval, and a total of 5 to 15 sessions. 7 patients (28%) had complete regression of melasma lesions, 11 patients (44%) had significant regression, 2 patients were ineffective, and only 2 patients had post-inflammatory hyperpigmentation after treatment. Post-inflammatory hyperpigmentation occurred in only 2 patients after treatment. In a recent study, 27 female patients with melasma were treated with microdermabrasion technology combined with Nd:YAG laser, with 5-6 mm spot, energy density of 1.6-2.0 J/cm2, treatment interval of 4 weeks, and treatment course of 1-4 times. Most patients had more than 50% improvement within one month after one treatment, and only 4 patients (16%) had mild recurrence at 6-month follow-up. In recent years, the treatment mode of dual-mode Nd:YAG laser (Cosjet TR laser, black face doll) proposed by Korean scholars, with pulse time of 6ns in Q-switched mode and pulse width of 300us in long pulse width mode, the principle of dual-mode Nd:YAG laser for melasma treatment may be: through Q-switched Nd:YAG laser blasting the carbon powder left on the skin surface and in the pores and achieving When the Q-switched Nd:YAG laser acts directly on melasma lesions, the nanosecond ultrashort pulse of the Q-switched laser is much shorter than the thermal relaxation time of melanin particles and melanocytes and can be selectively absorbed by them. When the laser acts on the melanin particles and melanocytes, it can instantly produce strong photomechanical effects, which will instantly destroy them and disintegrate them, and the damaged tissues will be eliminated from the body through the skin surface shedding and phagocytosis. The long pulse width 1064nm laser (pulse width 300us) works deeper than Q-switched mode, which heats deeper tissues, including fibroblasts and melanocytes, and promotes the production of collagen by fibroblasts and regulates overactive melanocytes, so it plays the role of skin rejuvenation and melasma improvement. Moreover, the double pulse width Nd:YAG laser energy output peak is in flat-top hat mode, so it is more gentle and more evenly distributed in melasma treatment, which greatly reduces the occurrence of adverse reactions. However, due to the lack of clinical studies with large samples, its effectiveness needs to be further verified. Exfoliative laser 1. Erbium laser (Er:YAG laser): Erbium laser releases 2940 nm laser beam and water is its target chromophore, so it is often used as an exfoliative rejuvenation technique. manaloto et al [11] applied erbium laser to treat 10 cases of refractory melasma, Fitzpatrick skin type II to V, and there was significant improvement immediately after treatment, although all patients were oral steroids for 5 days after treatment, all developed post-inflammatory hyperpigmentation that persisted for 3 to 6 weeks. Therefore, the short and limited efficacy is insignificant compared to the severe postoperative adverse effects, and erbium laser is not recommended for melasma treatment at present. 2.Carbon dioxide laser: The target color group of pulsed carbon dioxide laser is also water, which is mostly used to remove epidermal pigment. The pulsed CO2 laser is first used to remove the epidermis, followed by the emerald laser immediately afterwards, which facilitates the penetration of the emerald laser into the dermis to remove the dermal pigment. Theoretically, the pulsed CO2 laser minimizes downward heat transfer, thus reducing the occurrence of post-inflammatory hyperpigmentation. However, relevant studies have shown that this method still has a high incidence of hyperpigmentation and is therefore also not recommended for melasma treatment. Fractional laser 1. Erbium glass fractional laser (Er:glass fractional laser): Emitting laser beam with wavelength of 1550nm, it is a non-exfoliating fractional laser, and the exfoliating fractional laser (ultra-pulsed carbon dioxide fractional laser, 2940nm erbium fractional laser, etc.) can produce exfoliating damage and easily produce post-inflammatory hyperpigmentation, so it is rarely used for melasma treatment. treatment. Non-ablative fractional laser can produce an array-like arrangement of tiny thermal damage areas while the surrounding skin is intact, thus allowing rapid skin recovery and reducing the incidence of hyperpigmentation. The initial indication for fractional laser is skin rejuvenation, but it has been found to be clinically effective in some pigmented diseases, but the exact mechanism is unknown. Some scholars speculate that the fractional laser may initiate the removal of melanin targeting the dermal epidermal junction via damaged epidermal shedding, and in addition, the mechanism of action for dermal melasma may be that the fractional laser causes the melanocytes in the superficial dermis to disintegrate, promoting the release of The mechanism of action for dermal melasma may be that the fractional laser disintegrates melanocytes in the superficial dermis and promotes their release of melanin vesicles into the dermis, resulting in cosmetic improvement. The results of recent studies have shown some efficacy of 1550 nm fractional laser for melasma treatment, but the number of reported cases is small and most of them lack a control group. A recent study of a large sample of 51 patients with melasma, Fitzpatrick skin types II to III, in which the 1550 nm fractional laser was applied to a control group and both groups used sunscreen, found no statistically significant difference in effectiveness between the two groups. Although the non-ablative 1550nm fractional laser was approved by the FDA in 2005 for the indication of melasma and is the only laser approved for the treatment of melasma, current studies are not sufficient to support its effectiveness in the treatment of melasma, especially for the Asian population, and more studies are needed to confirm its effectiveness. 2. Ruby Fractional Laser (694-nm Q-Switched Ruby Fractional Laser): The Q-switched ruby laser in fractional mode is an attempt to treat pigmented diseases in the past year, and the effectiveness of this laser for melasma treatment has been confirmed in preliminary studies. Korean scholars reported the application of ruby fractional laser to treat 15 Korean female melasma patients with energy density of 2 to 3 J/cm2, coverage rate of 27.7%, 3 scans, and treatment interval of 2 weeks. 16 weeks after completing 6 treatments, the mean MASI score decreased from 15.1±3.3 before treatment to 10.6±3.9, and none of the patients showed hypopigmentation or depigmentation. The mechanism for the effective treatment of melasma by this method is thought to be probably the combination of the advantages of fractional laser and low-energy, large spot, multiple-scan treatment methods, and the fact that the 694-nm laser is more readily absorbed by melanin particles compared to the 1064-nm Q-switched Nd:YAG laser. However, the number of reported cases for this method is too small and will be evaluated when more cases are accumulated. Intense pulsed light Intense pulsed light emits ordinary light from 515 to 1200 nm, in which melanin particles have a wide range of absorption wavelengths. A scholar from Taiwan, China, compared the effect of Intense pulsed light with topical hydroquinone in the treatment of melasma. The Intense pulsed light group achieved an average of 39.8% improvement in pigment index after 4 treatments with 4 weeks interval each time, while the hydroquinone group only had 11.6% after 16 weeks, but 24 weeks after the end of the treatment, the pigment index of the Intense pulsed light group rebounded and most patients relapsed. China Medical University reported 89 patients who received 4 treatments with 3-week interval using intense pulsed light technology with more uniform output pulses to observe the improvement of facial pigmentation and capillary dilation, and the results showed that 77.5% of patients obtained 51% to 100% improvement, and the epidermal type had better effect than the mixed type with no significant side effects. Although the pulse width of emitted light is milliseconds, which is larger than the thermal relaxation time of melanin particles, the powerful pulse energy generated instantly can still destroy the epidermis in the melanin aggregation area and then discharge the melanin through the metabolism of damaged epidermis. Therefore, most patients will have a small thin scab after IPL treatment, which will last for 1 to 2 weeks and then fall off. In addition, some scholars believe that intense pulsed light works by destroying the erythema or capillaries associated with the development of melasma, but this speculation needs to be proved by pathophysiological studies. Since there are few reports of intense pulsed light treatment for melasma and the risk of post-inflammatory hyperpigmentation or pigmentation, it should be applied with caution. Copper bromide laser can emit 511nm and 578nm wavelengths alone or sequentially, with the former targeting melanin and the latter mainly targeting capillaries. In 10 patients with melasma treated with copper bromide laser in Korea, the mean MASI score decreased from 12.3 to 9.5 after 4 treatments with an interval of 2 weeks, and none of them had postoperative pigmentation abnormalities 1 month after the treatment. The pathology showed significant reduction of capillaries, reduction of melanin granules in the basal layer of the epidermis, and reduction of melanin vesicles in melanocytes. This method can provide some improvement in melasma with few side effects, but a larger sample of studies or own half-face controlled studies are needed to prove its effectiveness. In conclusion: Due to the specificity of melasma which is pathophysiologically manifested by abnormal melanocyte activity, laser or intense pulsed light should minimize the stimulation of melanocytes in the treatment of melasma to avoid the aggravation of melasma. Therefore, in the energy selection, only the melanin particles within or between melanocytes are selectively photoblasted, and the function of melanocytes is inactivated or inhibited by low energy and multiple photoblasts, while the multiple photoblasts of melanin particles can make the melanin particles more miniaturized and more conducive to be phagocytosed and discharged. Fractional laser is the most promising method for effective treatment of melasma, especially with the recent rapid development of laser technology, more wavelengths of fractional mode laser have appeared one after another, such as 694nm fractional laser, 755nm fractional laser and 1064nm fractional laser, but in the process of exploration and research, attention should be paid to starting from low energy, slowly increasing energy density, and scanning should be uniform to avoid pigment uneven side effects. For the Asian population, Nd:YAG laser with large spot, low energy, and multiple scanning modes is also recommended, but the adverse effects of “black and white” pigmentation and hypopigmentation are not uncommon with long-term repeated treatments, and the author’s experience is that it is better to increase the number of scans per treatment than to easily increase the energy density. The author’s experience is that it is better to increase the number of scans per treatment than to increase the energy density. Although intense pulsed light treatment for melasma has certain effect, the risk of aggravation after treatment often discourages many laser aesthetic practitioners, especially for dark-skinned people, who should be treated with caution. Finally, the recurrence of melasma is an important issue that all the above treatment techniques have to face, and unfortunately, there is no laser or photon technology that can stop the recurrence of melasma. Therefore, laser treatment combined with other methods, including oral medication, topical medication and sun protection, is a more feasible treatment option at present.