Systematic review of the clinical characteristics and natural history of solar urticaria.

To the Editor: Solar urticaria (SU) is a rare, idiopathic photodermatosis characterized by cutaneous mast cell degranulation following exposure to specific wavelengths of solar electromagnetic radiation. Its clinical manifestations comprise recurrent wheals and/or angioedema and are typically treated with photoprotection and antihistamines.1Botto N.C. Warshaw E.M. Solar urticaria.J Am Acad Dermatol. 2008; 59: 909-920https://doi.org/10.1016/j.jaad.2008.08.020Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar To date, the presentation of SU has been described predominantly in single-center observational studies, which limits global understanding of its clinical features and natural history.2Du-Thanh A. Debu A. Lalheve P. Guillot B. Dereure O. Peyron J.L. Solar urticaria: a time-extended retrospective series of 61 patients and review of literature.Eur J Dermatol. 2013; 23: 202-207https://doi.org/10.1684/ejd.2013.1933Crossref PubMed Scopus (38) Google Scholar Therefore, this systematic review was undertaken to summarize the clinical and photobiologic features of SU, as well as its natural history. A systematic search of literature databases (MEDLINE, Embase, CENTRAL, and Web of Science) was performed on November 7, 2021, with support from a medical librarian (CRD42020181948). Article screening and data extraction were performed in duplicate, in accordance with recognized guidelines.3Higgins J. Thomas J. Chandler J. et al.Cochrane Handbook for Systematic Reviews of Interventions Version 6.2. Cochrane, 2021Google Scholar Full-text observational studies published in English that described SU clinical features (including photoprovocation results) in ≥3 patients were included. Qualitative and quantitative data syntheses were performed. Twenty-three observational case series were included (Supplementary Table I, available via Mendeley at https://data.mendeley.com/datasets/6wyjs7d6pz/1), comprising 854 SU cases (Supplementary Table II, available via Mendeley at https://data.mendeley.com/datasets/6wyjs7d6pz/1). Cases were derived from 12 countries, of which 6 were European, although reported cases incorporated all skin phototypes. There were no reports of familial disease. The mean age of onset was 30.6 years (standard deviation 19.8) and there was a female preponderance (63.7%). Symptom onset was rapid and 94.8% developed symptoms <15 minutes after sun exposure. Most patients experienced pruritus or burning (87.4%), as well as erythema or wheals (81.4%), and presentations such as erythema alone (10.8%) or angioedema (2.4%) were infrequent. Symptom resolution varied more substantially, resolving in ≤1 hour in 64.1% and persisting up to 24 hours in 33.5%. Systemic symptoms were uncommon (4.4%), as was anaphylaxis (0.9%), and there were no deaths. SU was associated with atopic disorders in 31.6% and amongst those with available data (n = 183), serum IgE levels were raised (>100 IU/mL) in 39.3%. Consistently reported treatments included H1-antihistamines (74.0%), phototherapy (35.7%), and omalizumab (2.1%). There were limited data on SU natural history and available study estimates could not be directly compared. However, analysis of 371 cases demonstrated that 35.3% experienced complete disease resolution 5 years following disease onset. Photoprovocation was performed in 97.2% and most centers used combinations of monochromator/broadband electromagnetic sources (50.1%) or broadband sources alone (43.6%). There was little standardization of photoprovocation protocols. Symptoms were not elicited in 14.0%. Amongst those with positive photoprovocation, the most common triggering wavebands were visible light only (29.3%) or combinations of UVA and visible light (24.9%) (Fig 1). Within the limits of the data available, this systematic review indicates that SU is sporadic, occurs worldwide, and can affect all skin phototypes. Symptoms predominantly comprise pruritus and erythema or wheals, but these cannot be provoked in many using available phototesting protocols. This may be attributable to observed variations in phototesting practices, but also implies that more complicated interactions can occur between skin and radiant sunlight in SU. This and other features of SU could be better understood via prospective data collection in an international registry and standardization of international phototesting practices. McSweeny is a sub-investigator and Tziotzios is a principal and (national) chief investigator on the Pfizer-funded ALLEGRO clinical trial in alopecia areata. Dr Tziotzios provides consulting services to Pfizer and has received speaker fees from Leo Pharma. Drs Kloczko, Chadha, Sarkany, Fassihi, and McGrath have no conflicts of interest to declare.