EDITORIAL COMMENTSolar urticaria
Authors: P.M. Farr
Journal: British Journal of Dermatology (2000)

Patients with solar urticaria, despite their rarity, are frequently memorable. This is largely because the disorder can be severely disabling, and is often difficult to treat. Progress in understanding the pathogenesis of solar urticaria and its treatment has largely come from individual case reports or from a small series of patients. In this issue, however, Uetsu and colleagues report their experience in investigating and managing 40 patients with solar urticaria.1 In 1998, Ryckaert and Roelandts similarly reported a series of 25 cases.2 Are we any nearer to being able to understand this disorder and develop a rational approach to treatment? Phototesting is useful as the diagnosis can usually be established with certainty, the relevant wavebands identified, and some estimation made of disease severity. The action spectrum varies greatly between patients, and may change with time if individual patients are tested repeatedly.3,4 Urticaria triggered by UVA or visible wavelengths (mainly green or blue) is found most frequently. A few patients are sensitive only to UVB (± shorter wavelength UVA)—sunscreens are potentially very helpful in this group. In general, patients with sensitivity to visible light have the most severe clinical problem. Japanese investigators, in particular, have shown that in most patients an 'inhibition spectrum' can be identified.1,5 If skin is first exposed to wavelengths known to induce urticaria, and immediately afterwards (or possibly concurrently) exposed to radiation within the inhibition spectrum, then the urticarial response is abolished or reduced in intensity. The inhibition spectrum is usually of longer wavelength than the action spectrum for inducing urticaria. As preirradiation with inhibiting wavelengths is not usually effective, and any inhibition achieved is temporary, it is difficult to see how this observation could be adapted to treat the disease. It may, however, explain why, in occasional patients in whom urticaria is readily provoked by daylight, no reaction can be induced using artificial light sources.2 In these cases it seems likely that the spectral emission of the test source contains sufficient inhibiting radiation to prevent the urticarial reaction. Solar urticaria often has a sudden and dramatic onset. Little information is available concerning disease duration and eventual outcome. The impression is that the problem persists with very few cases resolving spontaneously, at least among the group of patients referred to specialist centres.3,6 In the 25 patients investigated by Ryckaert and Roelandts,2 the range of disease duration before consultation was from 4 to 11 years. H1 receptor antagonists have an effect in most patients with solar urticaria, often achieving a 'protection factor' of 10 or greater.7 How clinically useful this is depends on the disease severity—a patient who develops urticaria after 30 s of sun exposure may not benefit greatly by a protection factor of 10, whereas useful protection would be achieved in a patient who, untreated, requires 10 min of exposure to trigger urticaria. Interestingly, as in the other physical urticarias, antihistamines block the weal and flare response, but do not inhibit the abnormal localized erythemal response.7,8 Thus, when patients are given a trial of antihistamines, it needs to be explained that, although their skin may still go red after sun exposure, the itching response may be helped. It has been suggested that higher than conventional doses of antihistamines are required for maximum effect.2,9 This observation requires confirmation and further study. Repeated exposure to radiation of wavelength within a particular patient's action spectrum can result in the development of tolerance.10 The mechanism is not known, but mast cell degranulation or depletion is not required.11 Unfortunately, the tolerance is of short duration and repeated exposure, for example on a daily basis, is required to maintain the response. Reports of single cases have demonstrated relatively long-lasting tolerance with broad-band UVB12 and narrow-band UVB.13 PUVA can achieve disease improvement or remission lasting several months, and, on the evidence presently available, is probably the treatment of choice for patients not helped sufficiently by antihistamines. PUVA treatment can be difficult in those patients who are exquisitely sensitive to UVA, particularly during the first few exposures. Close medical supervision is required. Roelandts14 has described a protocol of exposure to UVA alone in order to induce temporary tolerance before PUVA is commenced. An alternative is to fractionate the PUVA treatment, giving small doses of UVA (0·1–0·25 J/cm2, for example) every 15 min, beginning 1 hour after psoralen ingestion. In this way, eight or so exposures can be given, centred around the expected 2-hour peak of plasma psoralen concentration. This results in a therapeutically useful cumulative dose, yet individual exposures remain below the threshold required to trigger urticaria.15 Plasmapheresis, often in combination with other treatment, has been reported to be effective in a few cases.15-17 Solar urticaria remains a difficult disease. Although readily provoked in the phototesting laboratory, the cause remains largely elusive. Even with these recent reports of a further 65 cases,1,2 a number of important issues are unresolved. Can the action spectrum results in an individual case be used to predict disease duration or response to treatment? What proportion of patients respond adequately to antihistamines alone, and in what proportion is PUVA a successful therapy in the long term?