Two-photon imaging tracks hydroquinone’s effect on melanocytes without a biopsy.
Two-photon microscopy can noninvasively detect key histopathologic features of melasma and monitor cellular-level treatment responses, researchers say.
A single-centre observational study from the China-Japan Friendship Hospital looked at 60 adults with clinically diagnosed melasma.
TPM 3D imaging identified seven significant lesion characteristics compared to perilesional skin, including increased melanin throughout the epidermis, reduced viable epidermal thickness, flattened rete ridges, more activated and pendulous melanocytes at the dermal–epidermal junction, and more severe solar elastosis (all P<.001).
Of the 53 patients completing 12 weeks of twice-daily 2% hydroquinone, TPM detected significant reductions in epidermal melanin and activated melanocyte numbers, consistent with clinical improvements.
Mean mMASI scores fell 29.8% and dermoscopic scores dropped 36.2%. Pendulous melanocytes and solar elastosis remained unchanged.
Results of the study have been published this month in JAMA Dermatology.
The researchers say TPM could become a valuable adjunct for in vivo diagnosis and therapeutic monitoring in pigmentary disorders, offering histology-like detail without biopsy.
“Melasma is currently challenging to manage, and the available therapeutic approaches have limitations including insufficient efficacy, risk of recurrence, and potential adverse events,” they wrote.
“Conventional skin biopsies are invasive, time-consuming, and carry scarring risks, making it difficult to obtain histopathologic data, which is of great importance in exploring pathogenesis, making a diagnosis, and monitoring the development of melasma.”
TPM is a three-dimensional, subcellular-resolution imaging technology capable of visualising melanin granules, melanocytes, and skin microarchitecture in vivo.
While established non-invasive methods such as Wood lamp, dermoscopy, reflectance confocal microscopy, colourimetry, and spectrophotometry each have clinical utility, TPM’s ability to combine structural and pigmentary information in a single high-resolution scan has made it an attractive candidate for pathologic examination of pigmentary diseases, the researchers said.
Its use, however, is currently confined to research institutions and select major medical centres, partly due to the need for specialist training, as well as technical limitations including a small imaging field of view (≤250 × 250 μm), limited penetration depth (<200 μm), and relatively slow scan speeds.
“Future technological advancements addressing these limitations could be crucial for promoting the widespread use of TPM in dermatology,” the authors wrote.
Noting that hydroquinone cream was considered the first-line topical drug option for melasma, the researchers followed up on the therapeutic effect of 2% hydroquinone cream in melasma using TPM and found a reduction in activated melanocytes and epidermal melanin after treatment.
“This effect can be explained by the known mechanisms of hydroquinone: competitively inhibiting tyrosinase to block the conversion of L-3,4-dihydroxyphenylalanine (L-DOPA) to melanin, thus reducing melanin formation and melanisation; inducing melanosome degradation via self-oxidation–generated free radicals; and causing melanocyte necrosis by inducing melanocyte membranous structure degeneration,” they wrote.
“However, no improvement in pendulous melanocytes or solar elastosis was observed under TPM.
“Pendulous melanocytes, possibly resulting from UV-induced basement membrane damage and subsequent melanocyte migration, may be associated with refractory and recurrent melasma. The persistence of pendulous melanocytes and solar elastosis might be because they are not the therapeutic targets of hydroquinone.”
Dermal inflammation and increased vascularisation have been implicated in the pathogenesis of melasma, however TPM seems to be suboptimal in visualising these features. The researchers explained that blood vessels emit almost no fluorescence signal, and inflammatory cells lack distinctive signatures for reliable identification and classification.
“Consequently, the inability to assess dermal inflammation in melasma constituted a limitation of this study,” they wrote.
“As an alternative approach, we used dermoscopy to assess the vascular elements, revealing that hydroquinone cream failed to ameliorate melasma-associated telangiectasia.
“This underscores the need for future multimodal imaging that integrates TPM, RCM, and dermoscopy to comprehensively characterise pathologic changes of melasma, particularly inflammatory information.
“Moreover, these results highlight the necessity of adopting therapeutic strategies targeting multifactorial pathologic mechanisms of melasma simultaneously.”
They also noted that UV radiation played a key role in melasma pathogenesis by modulating multiple signalling pathways that enhance tyrosinase-mediated melanocyte activation and melanogenesis, while also driving dermal inflammation and solar elastosis.
“Regular sunscreen application can help avoid the exacerbation of melasma and improve clinical outcomes,” they wrote.
“In the current study, participants used sunscreen uniformly to standardize UV exposure, reducing variable interference. However, this may have enhanced hydroquinone efficacy.”
While TPM showed promise for non-invasive treatment monitoring in melasma, providing not only efficacy evaluation but also insights into cellular mechanism of drug action, further studies were needed to determine the clinical value of TPM in the diagnosis and monitoring of various pigmented dermatoses, the authors concluded.
