Sub‐ultraviolet reflectance dermoscopy in longitudinal melanonychia

Longitudinal melanonychia represents a diagnostically challenging presentation in daily practice, particularly when clinicians must differentiate melanocytic activation from lentigo, nail matrix nevus and early subungual melanoma. These entities reflect distinct biologic processes yet may share overlapping clinical and dermoscopic features under conventional dermoscopy (CD). Accurate non-invasive assessment is therefore critical for guiding follow-up strategies and biopsy decisions.1, 2 Sub-ultraviolet reflectance dermoscopy (sUVRD) is an emerging imaging modality based on the differential absorption and scattering of violet–blue light by chromophores, generating high-contrast greyscale images that accentuate melanin distribution and pigment density.3 Recent studies suggest that sUVRD may reveal subtle pigment patterns that are inconspicuous under CD alone.4, 5 In this context, we evaluated the appearance of four distinct causes of longitudinal melanonychia (melanocytic activation, lentigo, nail matrix nevus and subungual melanoma) using paired CD and sUVRD imaging. CD images were obtained with a DermLite Foto II Pro dermoscope (3gen DermLite, CA, USA), and sUVRD images with a digital camera-integrated dermoscope equipped with a 405-nm sub-ultraviolet light source (DZ-D100, Casio Computer Co., Ltd., Tokyo). Dermoscopic images were evaluated by two dermatologists and final assessments were reached by consensus, without significant discrepancies. For each category, 10 representative cases were assessed. A summary of the quantitative sUVRD features across four causes of longitudinal melanonychia is provided in Table 1. In all cases of subungual melanoma, the diagnosis was confirmed by histopathological examination. In benign entities, biopsy was deliberately avoided to prevent unnecessary permanent nail unit damage. Lentiginous lesions included in the study showed isolated single-nail involvement and remained clinically and dermoscopically stable during follow-up. Lesions classified as melanocytic activation presented as longitudinal melanonychia involving multiple nails, consistent with a reactive process rather than focal proliferation. Nail matrix nevi in this series were observed in prepubertal children and showed typical clinical and dermoscopic features. Written informed consent was obtained from all adult participants and from the parents or legal guardians of paediatric patients, and the study was approved by the local ethics committee (I05-424-25). In melanocytic activation (hypermelanosis), melanocyte density remains within normal limits and visible pigmentation results from increased melanin synthesis in suprabasal melanocytes, particularly within the distal nail matrix. Accordingly, under sUVRD, these lesions display a faint, low-density grey signal, with relative sparing of the proximal nail matrix (Figure 1a,e). In lentigo, melanocyte density is increased; however, melanocytes remain singly distributed without nest formation. The lesion is characterized by a regular lentiginous proliferation with fine, scattered melanin granules. Associated acanthosis and increased keratin thickness contribute to a greyish appearance on CD. Under sUVRD, lentigo demonstrates a more conspicuous signal than melanocytic activation, while retaining a relatively low-to-moderate reflectance intensity (Figure 1b,f). In contrast, both nail matrix nevus and subungual melanoma are characterized by melanocytic hyperplasia with a tendency towards nest formation and increased melanocyte density. Subungual melanoma, in particular, shows a higher density of atypical melanocytes with architectural disorder, including irregular distribution, nest confluence, loss of symmetry, pagetoid spread and cytologic atypia. This structural organization, together with more compact melanin deposition, is reflected on sUVRD as a markedly darker grey-to-black signal (nail matrix nevus, Figure 1c,g; subungual melanoma, Figure 1d,h). Overall, these progressive changes in sub-ultraviolet reflectance appear to mirror known histopathological differences across the spectrum of longitudinal melanonychia, from increased melanin synthesis without proliferation to true melanocytic hyperplasia with nest formation.6 Taken together, our observations suggest that sUVRD may provide complementary optical information across the spectrum of longitudinal melanonychia by translating microscopic differences in melanocyte density, organization and melanin distribution into distinct greyscale reflectance patterns. Lesions characterized by increased melanin synthesis without melanocytic proliferation tend to display lighter, low-density sub-ultraviolet signals, whereas entities associated with melanocytic hyperplasia and nest formation exhibit progressively darker grey-to-black reflectance beginning at the proximal nail matrix. A limitation of this study is the absence of histopathological confirmation in benign lesions. Nail matrix biopsy is technically challenging and may result in permanent nail dystrophy. Moreover, histopathological evaluation of nail matrix melanocytic lesions can be challenging and occasionally inconclusive, especially in early stages. For these reasons, biopsy was not performed in lesions showing benign clinical and dermoscopic characteristics and remaining stable during follow-up. Nevertheless, the lack of histopathological confirmation may have introduced a degree of classification bias. The study population also showed age-related heterogeneity, as nail matrix nevi were predominantly represented by prepubertal patients. This was a deliberate choice, as subungual melanoma is exceedingly rare in children, allowing these cases to serve as representative examples of benign melanocytic proliferation. Nevertheless, nail matrix nevi are not confined to paediatric populations, and the predominance of prepubertal cases may represent a source of age-related selection bias. Biological differences between paediatric and adult melanocytic lesions should be considered when interpreting the findings. SUVRD has inherent technical limitations, including limited penetration depth and variability related to nail plate thickness and optical scattering. Given its novelty in longitudinal melanonychia, the findings should be interpreted with caution. In conclusion, the integration of sUVRD with CD may provide complementary structural and pigment-related information in the non-invasive evaluation of nail pigmentation. This combined approach may be particularly useful in clinically equivocal cases, where additional optical contrast could improve risk stratification and support more individualized clinical decision-making. Further studies are warranted to better define the role of sUVRD within diagnostic algorithms for longitudinal melanonychia. This study received no specific grant from any funding agency in the public, commercial or not-for-profit sectors. The authors declare that they have no potential conflict of interest regarding the investigation, authorship and/or publication of this article. The study was approved by the institutional ethics committee (I05-424-25). The patients in this manuscript have given written informed consent to publication of their case details. The data that support the findings of this study are available on request from the corresponding author.