Noninvasive Assessment of Melasma Pathological Features: Side-By-Side Comparison of Two-Photon Microscopy and Reflectance Confocal Microscopy

Abstract

Melasma, a refractory hyperpigmentation disorder, requires noninvasive tools for accurate pathological assessment. This study compared two-photon microscopy (TPM) and reflectance confocal microscopy (RCM) for the in vivo characterization of melasma pathology. In this cross-sectional study, TPM and RCM features and imaging clarity were evaluated in 130 melasma patients. Spearman correlation analyses were performed between TPM-quantified epidermal melanin, the melanin index (MI), and the individual typology angle (ITA). Features were also compared between active and stable disease stages. TPM and RCM showed substantial agreement in detecting increased epidermal melanin (κ = 0.651), activated melanocytes at the dermal-epidermal junction (DEJ) (κ = 0.711), and flattened rete ridges (κ = 0.691) (all p < 0.001). TPM excelled in visualizing intracellular melanin distribution, pendulous melanocytes under the DEJ, and solar elastosis, while RCM better identified activated melanocytes at the DEJ. TPM-quantified epidermal melanin content correlated positively with MI and negatively with ITA. RCM-detected dermal inflammatory cells were more prevalent in active than in stable melasma. In conclusion, TPM and RCM synergistically capture critical melasma features, with TPM assessing disease severity via epidermal melanin quantification, whereas RCM reflects disease activity by detecting inflammatory cells. This provides clinicians with tailored imaging tools for precision management.