Psoriasis is a common skin disease displaying a broad spectrum of clinical presentations that can sometimes pose diagnostic challenges. Given its incidence across all age groups and its manifestation in cosmetically sensitive areas, the disease imposes a significant psychological and socioeconomic burden on affected individuals worldwide. Dermatoscopy serves as a non-invasive auxiliary diagnostic tool that aids in the diagnostic process of common dermatoses, thereby improving diagnostic accuracy and clinician’s confidence [ 1 – 3 ] . Ultraviolet-induced fluorescence dermatoscopy (UVFD) is a novel dermatoscopic mode applicable to neoplastic, inflammatory, infectious, and parasitic conditions [ 4 – 7 ] . This imaging method utilizes the Stokes shift. In this phenomenon, ultraviolet (UV) light emitted by the diodes induces fluorescence in the fluorophore (emission of light characterized by a longer wavelength and lower energy than the excitation light) [ 4 ] . A small portion of reflected UV light is filtered out by the dermatoscope UV filters, which allow only the visible light spectrum to pass through. A distinctive peripapillary red fluorescence exhibited under UVFD in certain psoriatic lesions is attributed to the accumulation of protoporphyrin IX in the stratum corneum, particularly in more severe cases [ 8 , 9 ] . The understanding of red fluorescence in psoriasis in regard to clinical-morphological features, site-dependency, geographic variability, and bleaching with sun exposure remains very limited. Moreover, it has been speculated that some fluorophores may either be washed away with water or diminish following sun exposure [ 4 , 9 , 10 ] .

The study objective was to explore the patterns of UVFD in relation to demographic factors, psoriasis subtype, anatomical distribution of the lesions, Psoriasis Area and Severity Index (PASI) score, geographic region, seasonal variations, and bathing practices on the day of examination.

In this retrospective international cohort study, performed between January 2023 and March 2024, we explored the patterns of UVFD in relation to the demographic factors, psoriasis subtype, anatomical distribution of the lesions, Psoriasis Area and Severity Index (PASI) score, erythema, induration, and desquamation scores of particular lesions (0–3), geographic region (Central Europe: Poland, Ukraine; Southern Europe: Italy, North Macedonia), seasonal variations (winter, spring, summer, autumn), and bathing practices on the day of examination (yes/no). In the study group, we included patients diagnosed clinically and/or histologically with psoriasis who had received no targeted treatment for psoriasis in the six weeks before their examination and who did not have any overlapping dermatoses. A hybrid (conventional/UVFD) DL5 dermatoscope (DermLite; 3Gen, San Juan Capistrano, CA, USA) paired with a smartphone camera was used for dermatoscopic image acquisition. Clinical pictures were obtained with smartphone cameras. Captured images were not proceeded/manipulated in any aspect.

A total of 111 patients (74 males, 37 females; mean age 49.75 years) contributed 356 photographs (mean, 3.21 photos per patient). The results are presented in Table 1 . In our study group, we found no correlation between red fluorescence and age ( P =0.603) or sex ( P =0.247). Psoriatic subtypes exhibiting red fluorescence more frequently included inverse, follicular, guttate, and plaque psoriasis ( P =0.006), with a notable reduction in frequency in palmoplantar variants ( Figure 1A–I ). Red fluorescence under UVFD was associated with higher PASI scores ( P =0.005) and correlated with erythema ( P =0.008) and induration ( P =0.034), but not desquamation ( P =0.752) intensity in photographed psoriatic lesions. A significant geographic variability in the red fluorescence of psoriatic lesions was observed between Central and Southern Europe ( P =0.022), with the winter and spring months showing a higher frequency of this phenomenon ( P =0.004). Red fluorescence in psoriasis was predominantly observed on thighs, buttocks, armpits, groin, intergluteal folds, trunk, and genital areas. Bathing practices on the day of examination in psoriasis did not affect the UVFD pattern significantly ( P =0.605) ( Figure 1J–M ).

We have confirmed that red fluorescence under UVFD was associated with higher PASI scores [ 8 ] , which is in line with a previously reported correlation between excited fluorescence under Wood’s lamp and disease severity [ 11 ] . In our study, we have demonstrated that erythema and induration are the key contributing factors. We hypothesize that vasodilation, inflammation, and papillomatosis may play crucial roles in the deposition of protoporphyrin IX within the stratum corneum and may influence UVFD findings. We believe that UVFD can be used as an auxiliary tool in erythematous psoriatic plaques without scales to rule out clinical differential diagnoses. It is still unclear whether the treatment modalities reducing induration or erythema could affect the UVFD fluorescence patterns. We posit that geographic, seasonal, and anatomic variabilities of excited red fluorescence can be explained by the influence of sunlight on reducing protoporphyrin IX fluorescence in areas exposed to sunlight, particularly during sunny months and in warmer geographic regions (lighter clothing and outdoor activities). Speculatively, the lack of red fluorescence on the scalp observed in our study group may be related to the presence of Malassezia spp., given that the fungus exhibits enzymatic activity that suppresses porphyrin production in various clinical contexts, leading to the formation of “blackout areas” observed in pityriasis versicolor and seborrheic dermatitis [ 4 , 5 ] . In patients assessed with UVFD, taking a detailed history concerning bathing and application of drugs/cosmetics may hugely affect the interpretation of the observed fluorescence patterns [ 4 ] . Conversely to Corynebacterium-related red fluorescence of porphyrins in erythrasma and pitted keratolysis [ 4 , 10 ] , patterns of excited red fluorescence in psoriatic plaques in our study were not significantly affected by bathing practices. The observed differences may be related to spatial distribution of the fluorophores. In erythrasma and pitted keratolysis, porphyrins are present on the surface of the epidermis and thus can be easily removed with bathing, whereas in psoriasis they can be detected in the stratum corneum [ 12 ] .

In this study we have demonstrated a correlation between sun exposure and the photo-bleaching of red fluorescence in psoriatic lesions, particularly noting that lesions in sun-protected areas more frequently display red fluorescence patterns under UVFD. The observed seasonal and geographic variability further substantiates this relationship. Our findings confirm that red fluorescence pairs with disease severity, as evidenced by higher erythema and induration scores of the assessed lesions. Moreover, we have shown that bathing practices do not influence the fluorescence patterns observed in psoriasis. We believe that our research contributes to a deeper understanding of the patterns of red fluorescence under UVFD in psoriasis.