Hypopigmentation Following Picosecond Laser Treatment for Melasma: A Case Series

Picosecond lasers (PSL) are increasingly used for treating melasma, with fewer adverse effects reported compared to Q-switched lasers (QSL). However, the incidence of hypopigmentation following PSL treatment remains unexplored in detail. This case series aims to explore outcomes of hypopigmentation following PSL therapy in patients with melasma, and explore potential contributing factors.

A retrospective chart review identified four patients with hypopigmentation following PSL treatment for melasma, including one referral from another clinic. Across the cohort, 796 patients underwent 3096 sessions between 2021 and 2025. Treatments used 755 and/or 1064-nm wavelengths at low fluences, with intervals of 4-12 weeks.

Three in-clinic patients (0.38%, 95% CI 0.13%-1.10%) and one referral developed hypopigmentation. Changes occurred across both wavelengths and beam profiles. Histology demonstrated preserved melanocyte density with reduced melanin pigment and melanosome content. None of the cases showed meaningful re-pigmentation at 6-month follow-up. No cases of post-inflammatory hyperpigmentation (PIH) were observed.

While hypopigmentation is rare with PSL treatment, it can occur even with conservative low-fluence settings and adequate intervals between sessions. One possible mechanism is thermal beam- stacking, where slow hand movement during treatment may lead to repeated pulses on the same area. This can result in localised thermal accumulation, potentially causing subcellular disruption of melanosomes without overt melanocyte loss. These findings suggest the need for clinician vigilance in monitoring for hypopigmentation, as re-pigmentation may not be achievable. Larger, controlled studies are needed to clarify risk factors and guide safer practice.