A Study on Applying Slide-free Label-free Harmonic Generation Microscopy for Noninvasive Assessment of Melasma Treatments with Histopathological Parameters

Melasma, which is thought to be associated with hyperactivation of melanocytes, is a common hyperpigmentary skin disorder. To treat this skin disorder, dermatologists can benefit from in vivo information of cellular morphometrics to evaluate pathologies of melasma and effectiveness of treatments. To acquire this useful information, we applied the in vivo slide-free label-free harmonic generation microscopy (HGM) to retrieve real-time HGM images and subsequent critical histopathological parameters. These in vivo quantitative parameters included melanin mass density (MMD), melanocyte dendricity score (MDS), melanophage density (MPD), and thickness of dermal papilla zone (DPZ). The statistical results from 33 recruited Asian female melasma patients showed that MMD, MDS, and MPD in melasma lesions were significantly higher than those in the surrounding normal skin; by contrast, the DPZ was lower. After treatment, the MMD, MPD, and DPZ restored toward the normal level; however, we observed no significant change of MDS. Our study indicates that these parameters are able to serve as clinical indices to evaluate the histopathological characteristics of melasma patients and the effectiveness of treatments.