Solar urticaria (SU)

Snapshot

Other common terms: SU

ICD-10 classification: L56.3

Prevalence: Very rare; 3.1:100,000 individuals globally. Solar urticaria is more likely to affect women

Causes: Exposure of skin to light. Exact allergen is unknown.

Symptoms: Systemic: anaphylaxis, breathing difficulty, nausea and headaches. Immediate localised reactions on skin: characteristic ‘wheal’ formation, erupting flares on exposed skin sites and to swelling of soft tissues.

Treatments/cures: No known cure. Anti-histamines and topical steroids may be useful in some cases. Immunosuppressants and plasmaphoresis in extreme cases.

Differential diagnosis: Polymorphous light eruption, drug induced photosensitivity, other allergies/physical urticarias, cutaneous lupus

Solar Urticaria reaction on skin
Solar Urticaria reaction on skin

Solar urticaria (SU) is a rare, sunlight or UV induced hypersensitivity (allergic) reaction that causes wheals (raised red skin welts) very soon after or during sun or light exposure. In solar urticaria, the reaction is triggered by exposure to UV or visible light. It may be severely disabling and can even be life threatening.

Incidence

Solar urticaria is rare but occurs worldwide. 3.1 per 100,000 people are affected and females are more likely to be affected than males.

Symptoms

Following limited exposure to sunlight, sufferers may develop an itchy or burning redness on exposed skin. Initial presentation has also been reported after first solarium use.

Symptoms usually develop within five minutes of sun/UV exposure and often develop from an unpleasant sensation to itching, redness and swelling, followed by localized or widespread development of wheals (an urticarial flare). Gradual resolution then follows over 1-2 hours.

Rarely, a more prolonged exposure may be required for symptoms to develop, or the onset of symptoms may be delayed for several hours. With extensive whealing some patients also experience headache, nausea, bronchospasm (asthma-type respiratory symptoms) and syncope (dizziness) which may become life threatening (although this is rare). Conversely, in some people with mild disease, or in those who quickly recognize their onset and avoid further exposure, whealing may not be reported.

Sun-exposed areas are most commonly affected, although occasionally reactions are seen in dermal areas that are not exposed to the sun. Rarely sun-exposed sites are spared suggesting that tolerance may occur.

Causes

Solar urticaria may be primary or secondary.

Primary solar urticaria is an immediate hypersensitivity response (IgE mediated allergic reaction) towards an allergen (a compound produced in the body when UV light is absorbed by a cellular pre-cursor) induced in the body following sunlight exposure. The responsible allergen is not know but is termed a photoallergen. The allergic reaction that follows prolonged sunlight exposure leads to a widespread inflammatory response.

Mast cell degranulation and histamine release are important factors in SU but many other inflammatory cells, particularly neutrophils and eosinophils are involved in amplifying the whealing response.

Many wavelengths light may trigger the production of different photoallergens but SU is most commonly caused by UVA or visible light. There appears to be no genetic basis for this condition.

Very rarely, secondary solar urticaria occurs in association with other photosensitivity disorders, such as cutaneous porphyria or lupus, or with certain medications known to cause photosensitivity.

Diagnosis

Complete potential action spectrum for Solar Urticaria
Complete potential action spectrum for Solar Urticaria

Phototesting confirms the diagnosis and reveals the wavelengths responsible for inducing an urticarial response. Phototesting may be performed with a monochromator (single wavelengths of light selected at a time), broad spectrum source or natural sunlight to estimate the minimal urticarial dose (MUD) of light required to induce symptoms. Screening tests to exclude lupus (ANA,eNA) and cutaneous porphyria (porphyrin studies) must be done to exclude these conditions. Medications must also be considered as a possible cause.

Differential diagnosis

  • Cutaneous lupus
  • Polymorphic light eruption (PLE)
  • Photo-exacerbated dermatoses i.e. eczema
  • Other physical urticarias

Treatment

Treatment of solar urticaria is usually directed towards relief of symptoms and avoidance of their onset through behavioural change. High doses of H1-antihistamines taken an hour before sun exposure are very effective in one third of patients and give another third partial relief.

Avoidance of sunlight, photoprotective clothing and broad-spectrum sunscreens are necessary to prevent symptoms; however this may not always be useful in cases where visible light is responsible for the solar urticaria.

Desensitisation with phototherapy may be useful for some patients however therapy generally needs to be continued to maintain its benefit and so consequently carries a risk of long-term risks such as skin cancers. In severely affected individuals, this treatment also carries the risk of disease onset and anaphylaxis (severe, often life threatening allergic reaction) and so should be undertaken with extreme caution.

In extreme cases these patients need to be hospitalized to undergo plasmaphoresis (a procedure similar to dialysis where the plasma in their blood is removed and the blood cells are returned to the patient). Immunosuppressant medications such as Cyclosporin and intravenous immunoglobulins may also need to be considered in the most severe cases.

Prognosis

For the majority of patients symptoms will persist indefinitely. For a small proportion of patients solar urticaria will deteriorate however some do experience improvement with an estimated 26% chance of resolution at 10 years.

References

  • Dice, J P. (2004). 'Physical Urticaria', Immunology and Allergy Clinics of North America, 24, pp225-246.
  • Roelandts, R (2003). 'Diagnosis and treatment of solar urticaria" Dermatologic Therapy, pp52-56.
  • Beattie, P E, Dawe, R S, Ibbotson, S H, & Ferguson, J. (2003) 'Characteristics and prognosis of idiopathic solar urticaria: A cohort of 87 cases', Archives of Dermatology. 139 (9), pp1149-1154.
  • Buxton, P K, (2003). ABC of Dermatology. London: BMJ Publishing Group Ltd.
  • Ng, J H C, Foley, P A, Crouch, R B, & Baker, C S. (2002). 'Changes of Photosensitivity and Action Spectrum with Time in Solar Urticaria', Photodermatology, Photoimmunology & Photomedicine, 18, pp191-195.

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