Solar urticaria (SU) is an uncommon idiopathic photodermatosis that accounts for 7% of all photodermatoses.1 It is also classified as a chronic inducible (physical) urticaria,2 and in this context, it accounts for between 0.08%1 and 0.4%3 of all urticarias. It primarily affects women in the third decade of life and is characterized by the appearance of pruritus, erythema, or wheals in sun-exposed areas generally within an hour of exposure to sun or artificial light. Manifestations disappear within a maximum of 24hours of cessation of exposure.4

Most cases of SU follow a benign course, but some patients develop systemic symptoms, such as headache, nausea, mucosal involvement, and even anaphylaxis. Whether these symptoms are present or not, SU can negatively impact the personal and even professional life of those affected.5

A patient's clinical history is usually sufficient to reach a diagnosis of SU. Recent guidelines, however, recommend additional evaluation by a photobiology unit,2 as a photobiological study can confirm diagnosis, identify the wavelength or wavelengths responsible for inducing SU, and help to select the most appropriate treatment depending on the severity of the condition.

Different treatment modalities have been used depending on the intensity of the accompanying symptoms, and include sunscreens, oral antihistamines,6 ciclosporin,7 desensitization with various forms of phototherapy,8,9 omalizumab,10 plasmapheresis,11 intravenous immunoglobulin therapy,12 and afamelanotide.13 Although treatment recommendations have been made for chronic urticaria, and more recently for chronic inducible urticaria, consensus- and evidence-based treatment guidelines are lacking for SU.2

Numerous case reports and series published over the years1,14–22 and recently summarized in a review article22 have helped to improve our understanding of the characteristics of SU.

In this study we present the largest series of SU published to date. The series includes 20 cases already reported on in a preliminary study.20 We describe the epidemiological, clinical, laboratory, and photobiological characteristics of SU with the aim of better defining the phenotypic characteristics of this photodermatosis.

In total, 224 patients from 9 photobiology units were included in the study. The main results are summarized in Table 2. The number of cases pertaining to each center varied considerably depending on the activity of the unit and the length of the data collection period (Table 1).

Incidence

Analysis of SU incidence (number of new cases diagnosed yearly) in the 2 centers with the longest follow-up times did not reveal a tendency towards an increase in annual incidence over time (Figure 1).

Figure 1.

Annual incidence of solar urticaria between 1998 and 2014 at Hospital La Santa Creu i Sant Pau in Barcelona (43 cases) and Hospital de Cruces en Baracaldo (34 cases). The mean number of cases was approximately 3 cases per year with a slight trend towards a reduction in incidence, as shown by the trend curve.

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Probability of Resolution

The maximum probability of resolution after 135 months (11.5 years) of follow-up was 70%. Half of the cases had resolved after 4 to 5 years of follow-up (Figure 2).

Figure 2.

Graph showing a maximum probability of resolution of 70% after 135 months of follow-up.

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We have presented the epidemiological, clinical, laboratory, and photobiological characteristics of SU based on a chart review of 224 patients evaluated by 9 photobiology units at 9 hospitals in different parts of Spain. The heads of these units are all members of the GEF.

Ten series describing 370 patients with SU from different countries have been published to date,1,14–22 and 1 of these reported on 20 patients included in the current series.20

SU is a rare photodermatitis with an unknown etiology and pathogenesis. The most recent classification system proposed by Leenutaphong et al.24 is based on in vitro activation studies and divides SU into 2 types: type I, caused by an abnormal chromophore present only in patients with SU, and type II, caused by abnormal circulating IgE antibodies directed against a normal chromophore.

Some recent publications have reported a trend that suggests an increase in the incidence of SU (although the possible reasons remain unidentified),22,25 together with a reduction in the incidence of actinic reticuloid.25 We cannot confirm this trend based on our data, as the temporal distribution of SU incidence in the 2 hospitals with the longest follow-up periods showed a slight trend towards a reduction in incidence (Figure 1).

The sex and age distribution of the patients in the current series is similar to that already described in the literature. The only series to report a predominance of male sex is a series from Singapore.1 In all the studies published to date, the third and fourth decades of life were the most common periods for onset of SU. The incidence of SU in patients younger than 18 years was 7.8% in our series, contrasting with reports of 5% to 15% for patients under 15 years14,16 and 21% to 27% for those under 20 years.14,16,17 We agree that the main phototypes involved in SU will vary according to geographic area.1,22 In our series, the most common type was phototype III, followed by phototype II; this is consistent with skin types described for the healthy population in Spain.23

The incidence of atopy in patients with SU varies from one series to the next, with rates ranging from 0% to 48%.14,15 However, none of the series to date have specified the types of atopy involved. We observed a history of atopy in 26.7% of the patients in our series, and this is consistent with reports from other Mediterranean countries.18 Rhinitis and allergic asthma were the most common manifestations in our series, and there were very few cases of atopic dermatitis. The incidence of concomitant chronic urticarias (16% in our series) has only been reported in 3 series to date, with rates ranging between 5% and 34%.1,17,22

The low incidence of concomitant photodermatosis in our series coincides with most reports to date.14,21,22 Just 3 patients in our series had polymorphous light eruption in association with SU. The series described by Beattie et al.16 is the only one to date to report a considerable incidence of both conditions (23% of patients with SU had polymorphous light eruption). The association between SU and connective tissue disorders and lymphoma detected in our series appears to be incidental and highlights the importance of thorough history taking and follow-up in patients with SU.

The clinical manifestations of SU are easy to recognize. In all the series described to date, most patients reported the development of erythema or wheals with pruritus in sun-exposed areas within minutes of exposure. The mean time from onset of symptoms to evaluation at a photobiology unit was 1 to 3 years in our series. This implies that there was an equivalent delay in determining the characteristics of the patients’ SU and consequently prescribing adequate treatment. Some patients took over 18 years to consult about their condition, and similar diagnostic delays have been reported.14,15,18,20,22

The most common wavelength implicated in SU in our series was visible light, either alone or associated with UV-A. Again this coincides with the majority of previous reports.14,16–18,20,21 The high proportion of negative phototests in our series (21%) is noteworthy. Although negative results have been reported in the literature, the rates have been lower (0%-3.5%).1,14–17,20 There are several possible explanations for the high rates of negative phototests observed in our series. First, not all data were available for all patients due to the retrospective nature of the study. Second, a substantial proportion of patients came for evaluation at a time of the year when they were free of symptoms (which occur only in certain seasons). Third, the doses required to achieve a positive result may have been higher than those used in the photobiology units. It is also possible that certain patients may have had SU induced by the simultaneous action of several spectral bands and would therefore only have reacted to exposure to sunlight and not to individual bands emitted by the test devices.

Our findings also show altered plasma IgE levels. Although IgE was measured in just 50% of patients, we believe that our findings are relevant as approximately 40% of the patients tested had above-normal levels; this rate is similar to that reported for other types of chronic urticaria (36%).26 We do not know whether the younger mean age of the subset of male patients with high IgE levels is relevant or not. However, we do believe that regular IgE measurements are important as SU is an IgE-mediated disease.

The enormous variability in treatment options identified in our series can be explained by the absence of standardized treatment guidelines and the fact that choices were determined by the experience of the different clinicians. Our results suggest that oral H1 antihistamines are the treatment of choice in most hospitals. The patients’ records did not specify whether or not the doses had been escalated up to 4 times the recommended dose as H1 antihistamine dose escalation in such settings is a very recent recommendation.27,28 Perhaps though, the time has come to consider treating SU as if it were another form of chronic urticaria. The latest recommendations for the management of chronic inducible urticaria support this idea,2 and based on a systematic review of the evidence, show that the use of nonsedating H1 antihistamines is supported by level A evidence. The use of desensitization phototherapy or afamelanotide is supported by level B evidence, while that of omalizumab, ciclosporin, or intravenous immunoglobulin therapy is supported by level C evidence.

Omalizumab was used to treat 4 patients in our series, and 3 of these responded favorably with the doses currently recommended for chronic urticaria. The literature shows varying results for the effectiveness of omalizumab in the treatment of SU.10,29 In our opinion, controlled multicenter studies are necessary to draw more definitive conclusions regarding the use of this drug in SU.

None of the factors studied in the multivariate analysis had a significant influence on the resolution or nonresolution of SU. We only saw that patients who reacted exclusively to visible light or natural light were significantly more likely to achieve complete resolution compared with those who reacted to other light spectrum regions. Complete resolution was achieved by 25.9% of patients in our series. The equivalent rates reported by Beattie et al.16 and Monfreccola et al.17 were 25% and 74%, respectively. The maximum probability of resolution in our series was 70% after 135 months (11 years). Beattie et al. reported resolution rates of 12%, 26%, and 36% for 5, 10, and 15 years, respectively.

In conclusion, the epidemiological, clinical, and photobiological characteristics of SU in our series of 224 patients, the largest published to date, are similar to most of the findings reported in the literature. One particularly noteworthy finding is that we did not detect a trend towards an increase in annual incidence. Rhinitis and asthma were the most common atopic manifestations and chronic urticaria in association with SU was not uncommon. The high rate of negative phototests compared with other series is interesting. Patients who reacted exclusively to visible light or natural light were more likely to achieve resolution of SU than those who reacted to other portions of the spectrum. H1 antihistamines were the most widely used treatment in our series, followed by phototherapy. Nevertheless, due to a lack of clear evidence-based recommendations for the treatment of SU, it would be interesting to conduct multicenter studies to help identify an adequate treatment protocol for this photodermatosis.

The authors declare that they have no conflicts of interest.