Abstract: 【Abstract】 Objective To evaluate the clinical efficacy of Q-switched Nd:YAG laser in the treatment of melasma, and to explore factors influencing the efficacy. Methods Forty-four female patients with melasma were enrolled into this study, who received treatment with Q-switched Nd:YAG laser once every 2 - 3 weeks for a total of 10 sessions in Department of Dermatology, Hangzhou Third Hospital between August 2017 and October 2017. Skin lesions were classified into epidermal type and mixed type by reflectance confocal microscopy, and a VISIA skin imaging detector was used to evaluate if subclinical melasma existed in patients. According to the modified melasma area and severity index （mMASI）, the severity of skin lesions before and after the treatment, as well as the efficacy, was evaluated by dermatologists. Statistical analysis was carried out by using paired t test, chi-square test and multivariate unconditional Logistic regression. Results Of the 44 patients, 5 were cured, 15 received marked improvement, 12 received improvement, 12 showed no response, and the total response rate was 45.4%. The mMASI score significantly decreased after the treatment （3.2 ± 2.1） compared with that before the treatment （6.7 ± 2.9, t = 8.955, P < 0.001）. The mMASI score changed along with the treatment sessions. Specifically speaking, the mMASI score started to be significantly lower after the 3rd treatment than after the previous treatment （t = 3.780, P < 0.01）, and gradually decreased along with the increase in treatment sessions till the 8th treatment session, and no significant difference was observed between the 8th and 7th treatment sessions （t = 1.735, P > 0.01）. The response rate was significantly higher in the patients with stable melasma than in those with progressive melasma （χ2 = 16.454, P < 0.05）, in patients without dendritic melanocytes in skin lesions than in those with dendritic melanocytes in skin lesions （χ2 = 9.582, P < 0.05）, and in patients without subclinical melasma than in those with subclinical melasma（χ2 = 6.188, P < 0.05）, while there was no significant difference in the response rate between the patients with epidermal-type melasma and those with mixed-type melasma （χ2 = 2.463, P = 0.120）. Logistic regression analysis showed that clinical stages （OR ［95% CI］ = 0.180 ［0.046 - 0.746］, P = 0.018）, dendritic melanocytes （OR ［95% CI］ = 0.231 ［0.059 - 0.769］, P = 0.018） and subclinical melasma （OR ［95% CI］ = 0.158 ［0.063 - 0.854］, P = 0.011） were associated with the efficacy. Conclusion Q-switched Nd:YAG laser is effective and safe for the treatment of melasma, especially for patients with stable melasma, without dendritic melanocytes in skin lesions, and those without subclinical melasma.

Key words: Chloasma, Laser therapy, Treatment outcome, Factor analysis, Clinical stage, Dendritic melanocytes, Subclinical melasma