Cause

The signs and symptoms of urticaria are caused by mast cell degranulation, with release of multiple vasoactive substances, including histamine (Figure 8.1). The mechanisms underlying this may be different but the end result, increased capillary permeability leading to transient leakage of fluid into the surrounding tissue and development of a wheal, is the same (Figure 8.1). For example, in autoimmune COU circulating antibodies are directed against the high affinity immunoglobulin E receptor (FcIgE) or receptor bound IgE on mast cells whereas the reaction in others in this group may be caused by immediate IgE-mediated hypersensitivity (see Figure 2.14), direct degranulation by a chemical or trauma, or complement activation.

Classification

The various types of urticaria are listed in Table 8.1. They can often be identified by a careful history; laboratory tests are less useful. The duration of the wheals is an important pointer. Contact and physical urticarias, with the exception of delayed pressure urticaria, start shortly after the stimulus and go within an hour. Individual wheals of other forms resolve within 24 hours. If they persist for longer, then urticarial vasculitis should be considered (see Cutaneous small vessel vasculitis).

Cold urticaria

Solar urticaria

Heat urticaria

Cholinergic urticaria

Anxiety, heat, sexual excitement or strenuous exercise elicits this characteristic response. The vessels overreact to acetylcholine liberated from sympathetic nerves in the skin. Transient 2–5 mm follicular macules or papules (Figure 8.2) resemble a blush or viral exanthem. Some patients get blotchy patches on their necks.

Dermographism (Figure 8.3)

This is the most common type of physical urticaria, the skin mast cells releasing extra histamine after rubbing or scratching. The linear wheals are therefore an exaggerated triple response of Lewis. They can readily be reproduced by rubbing the skin of the back lightly at different pressures, or by scratching the back with a fingernail or blunt object.

Delayed pressure urticaria

Presentation

Most types of urticaria share the sudden appearance of pink itchy wheals, which can come up anywhere on the skin surface (Figures 8.4 and 8.5). Each lasts for less than a day, and most disappear within a few hours. Lesions may enlarge rapidly and some resolve centrally to take up an annular shape. In an acute anaphylactic reaction, wheals may cover most of the skin surface. In contrast, in chronic urticaria only a few wheals may develop each day.

Angioedema is a variant of urticaria that primarily affects the subcutaneous tissues, so that the swelling is less demarcated and less red than an urticarial wheal. Angioedema most commonly occurs at junctions between skin and mucous membranes (e.g. peri-orbital, peri-oral and genital; Figure 8.6). It may be associated with swelling of the tongue and laryngeal mucosa. It sometimes accompanies chronic urticaria and its causes may be the same. Angioedema without wheals may be the presenting feature of hereditary angioedema (see Hereditary angioedema) or more commonly as a result of angiotensin-converting enzyme (ACE) inhibitor consumption (thought to result from the inhibition of kinin degradation).

Course

Complications

Differential diagnosis

There are two aspects to the differential diagnosis of urticaria. The first is to tell urticaria from other eruptions that are not urticaria at all. The second is to define the type of urticaria, according to Table 8.1. Insect bites or stings (Figure 8.7) and infestations commonly elicit urticarial responses, but these may have a central punctum and individual lesions may last longer than 24 hours. Erythema multiforme can mimic an annular urticaria. A form of vasculitis (urticarial vasculitis; p. 109) may resemble urticaria, but individual lesions last for longer than 24 hours, are typically associated with a more burning pain, blanch incompletely and leave bruising in their wake. Some bullous diseases, such as dermatitis herpetiformis, bullous pemphigoid and pemphigoid gestationis, begin as urticarial papules or plaques but, later, bullae make the diagnosis obvious. In these patients, individual lesions last longer than 24 hours, providing an additional tip-off for further investigation (see Chapter 9). On the face, erysipelas can be distinguished from angioedema by its sharp margin, redder colour and accompanying pyrexia. Hereditary angioedema (see Hereditary angioedema) must be distinguished from the angioedema accompanying urticaria as their treatments are completely different.

Investigations

Almost invariably, more is learned from the history than from the laboratory. The history should include details of the events surrounding the onset of the eruption. A review of systems may uncover evidence of an underlying disease. Careful attention should be paid to drugs, remembering that self-prescribed ones can also cause urticaria. Over-the-counter medications (such as aspirin and herbal remedies) and medications given by other routes (Table 8.2) can produce wheals.

If a patient has ordinary urticaria and its cause is not obvious, investigations are often deferred until it has persisted for a few weeks or months, and are based on the history. If no clues are found in the history, investigations can be confined to a complete blood count and erythrocyte sedimentation rate (ESR). An eosinophilia should lead to consideration of bullous or parasitic disease, and a raised ESR might suggest urticarial vasculitis, or a systemic cause. If the urticaria continues for 2–3 months, the patient may be referred to a dermatologist for further evaluation. In general, the focus of such investigations will be on internal disorders associated with urticaria (Table 8.3) and on external allergens (Table 8.4

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