Urticarial lesions are usually intensely pruritic welts that can be generalised or localised. They normally last less than 24 hours in the one place, being migratory, and leave no residual marks on the skin. Angioedema lesions may be uncomfortable or sometimes painful and occur in the deeper dermis or mucosa and may take 72 hours to resolve.
Acute urticaria may be allergic, mediated by inappropriate IgE responses to food allergens. It usually occurs rapidly after exposure to the causative allergen: within 30-60 minutes, up to six hours and rarely eight hours.
The most common allergens are either ingested (food or oral drugs) or parenteral (bee or wasp stings or drugs, for example, penicillin). Aeroallergens are not usually the cause of allergic urticaria except when due to grains (in bakers) and latex. However, people who are allergic to grass pollen may develop localised urticaria on contact, for example, when sitting on the grass.
T-cell mediated contact dermatitis may sometimes mimic urticaria but usually elicits a burning or painful reaction, with lesions often lasting a few days to a week. This type of contact dermatitis responds poorly to antihistamines and topical steroids and really needs oral steroids for control.
Chronic spontaneous urticaria is defined as urticaria coming and going most days for more than six weeks. It is differentiated from urticaria that is only induced by physical factors such as dermographism, cold, solar, vibration, pressure or cholinergic elictors.
Persons with chronic spontaneous urticaria will usually notice physical exacerbators, however they will also get spontaneous lesions and angioedema. Very occasionally, chronic spontaneous urticaria will present as intractable itch without classical macroscopic welts, but these people usually have many of the other clinical characteristics of chronic urticaria.
Perhaps a third of people with chronic spontaneous urticaria improve with a bland diet in which food components such as salicylates, amines. MSG, colours and flavours are excluded for a four week period.
Pursuit of external factors has little clinical utility for the majority.
The evidence suggests about two-thirds of people with chronic spontaneous urticaria have autoantibodies to either the mast cell IgE receptor or autoantibodies to IgE that cross-link mast cell IgE receptors. This has been demonstrated by the autologous serum skin test as well as in vitro activation of basophils with serum factors from patients with chronic spontaneous urticaria. Unfortunately robust assays for these autoantibodies have not been developed for use by clinical laboratories and in clinical practice.
When urticarial lesions last longer or leave marks like bruises it suggests an urticarial vasculitis rather than the cause being food intolerance or IgE-mediated food allergy mechanisms. In others, very persistent lesions are due to cutaneous mastocytosis. Investigations should be guided by clinical features and history:
Non-allergic urticaria is common with infections or sometimes vaccines as a non-specific short-term autoimmune phenomenon in children. Urticaria in a child with infection usually requires no further investigation unless there is a history of immediate prior exposure to a possible allergen or a history of urticarial reactions possibly associated with allergen at other times.
If the history suggests an allergy, skin prick or in vitro specific IgE testing to the suspected allergen may be useful although concomitant dermographism may result in false positive skin prick tests.
Chronic urticaria is rarely due to allergy and skin testing is virtually never indicated unless there is a clear repeated association between ingestion of a food or drug and onset. Focused clinical assessment should exclude other exacerbating disorders, for example, autoimmune thyroid disease, connective tissue disorders, and should be initially supplemented with thyroid autoantibodies, TSH, FBC, ESR, CRP and basic biochemistry (electrolytes, liver and renal function). Tests for connective tissue diseases and vasculitis, for example, ANA, ANCA, checking serum tryptase and skin biopsies have limited clinical utility unless lesions are burning or painful, remain for more than 24 hours in the one place and leave bruises.
Chronic spontaneous urticaria treatment may include second-generation H1-blocking antihistamines (up to four a day) and H2-blocking antihistamines and if not controlled patients might be offered omalizumab, sometimes cyclosporin and sometimes disease-modifying anti-rheumatic drugs.
General Practice Pathology is a regular column each authored by an Australian expert pathologist on a topic of particular relevance and interest to practising GPs. The authors provide this editorial free of charge as part of an educational initiative developed and coordinated by Sonic Pathology.