Lasers and Energy-Based Devices for Treatment of Pigmented Lesions With Histologic and Ultrastructural Imaging Correlation: A Systematic Review

Understanding histologic and ultrastructural changes in pigmented lesions after laser/energy-based device (EBD) treatment is essential for elucidating treatment mechanisms, optimizing outcomes, and preventing complications. To summarize histologic and imaging findings after laser/EBD for pigmented lesions. A systematic review using PubMed identified 39 studies examining laser/EBD treatment, correlating light microscopy (LM), electron microscopy (EM), reflectance confocal microscopy (RCM), and optical coherence tomography findings. Light microscopy and EM of lentigines revealed that picosecond lasers created smaller vacuoles with minimal surrounding damage, possibly explaining reported lower rates of postinflammatory hyperpigmentation compared with nanosecond lasers. Reflectance confocal microscopy showed that smooth-bordered café-au-lait macules have denser papillae and more elongated rete ridges, possibly hindering laser penetration to the lesion base and contributing to poorer outcomes. Electron microscopy after laser toning for melasma demonstrated sub-cellular selective photothermolysis, removing melanosomes without destroying melanocytes. However, cumulative toxicity induced melanocyte malfunction, which could explain confetti hypomelanosis after aggressive treatments. Picosecond lasers confined tattoo particles within lysosomal structures, whereas nanosecond lasers dispersed them into surrounding tissues, which may help explain superior efficacy and safety reported with picosecond lasers. Histologic and imaging analyses can enhance the understanding of mechanisms of treating pigmented lesions with laser/EBD, providing insights regarding reported efficacy and safety.