Urticaria is a common condition which can present to many different physicians including dermatologists, immunologists and GPs.

Urticaria is known as ‘nettle rash’, itchy hives or wheals. It is a temporary eruption of erythema and oedema with swelling of the dermis and is usually itchy. Urticaria and angio-oedema (deeper dermal and subcutaneous swellings) may occur together.

Classification

Urticaria can be classified as ordinary urticaria consisting of acute or chronic urticaria. Urticaria is defined as chronic if lasting >6 weeks. A cause is less likely to be found than for acute urticaria. Other types include physical and cholinergic, contact urticaria and immune complex urticaria such as urticarial vasculitis.

Histology and Pathophysiology

Histology of ordinary urticaria wheals shows oedema and peri-vascular mixed cellular (eosinophils, lymphocytes, neutrophils) dermal infiltrate with vascular and lymphatic dilatation (Figure 32.1). Electron microscopy may show dermal mast cell degranulation.

Pathophysiology of urticaria includes increased capillary and venous permeability. Cutaneous mast cell activation releases mediators including histamine leading to activation of H1 receptors which induces itching, erythema and whealing.

Other histamine releasing factors involved include tryptase and neuropeptides (substance P). Plasma mediators (bradykinin) and complement may play a part in angio-oedema with complement activation leading to immune complex urticaria and urticarial vasculitis.

Clinical Features

Urticaria presents as itchy erythematous macules and wheals with pink swollen raised areas with a surrounding flare (Figure 32.2). The sites affected are variable and can include the palms and soles. The number, shape and size of the lesions vary with bizarre shapes including annular patterns (Figure 32.3). In ordinary urticaria the wheals resolve within 24 hours and may last only a few hours. They leave no skin change.

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