Anaphylaxis

British Medical Journal, May 9, 1998 by Pamela W. Ewan

Anaphylaxis and anaphylactic death are becoming more common and particularly affect children and young adults. Anaphylaxis can be frightening to deal with because of its rapid onset and severity. Doctors in many fields, but particularly those working in general practice and in accident and emergency departments, need to know how to treat it.

Definition

Anaphylaxis means a severe systemic allergic reaction. No universally accepted definition exists because anaphylaxis comprises a constellation of features, and the argument arises over which features are essential features. A good working definition is that it involves one or both of two severe features: respiratory difficulty (which may be due to laryngeal oedema or asthma) and hypotension (which can present as fainting, collapse, or loss of consciousness). Other features are usually present.

The confusion arises because systemic allergic reactions can be mild, moderate, or severe. For example, generalised urticaria, angio-oedema, and rhinitis would not be described as anaphylaxis, as neither respiratory difficulty nor hypotension--the potentially life threatening features--is present.

Mechanism

An allergic reaction results from the interaction of an allergen with specific IgE antibodies, bound to Fc receptors for IgE on mast cells and basophils. This leads to activation of the mast cell and release of preformed mediators stored in granules (including histamine), as well as of newly formed mediators, which are synthesised rapidly. These mediators are responsible for the clinical features. Rapid systemic release of large quantities of mediators will cause capillary leakage and mucosal oedema, resulting in shock and asphyxia.

Anaphylactoid reactions are caused by activation of mast cells and release of the same mediators, but without the involvement of IgE antibodies. For example, certain drugs act directly on mast cells. For practical purposes (management) it is not necessary to distinguish an anaphylactic from an anaphylactoid reaction. This difference is relevant only when investigations are being considered.

Incidence

Hardly any data exist on the overall incidence of anaphylaxis. One recent study examining cases of anaphylaxis presenting to an accident and emergency department in Cambridge (to which all cases from a defined area would be brought) found that 1 in 1500 patients attending the department had anaphylaxis with loss of consciousness or collapse (equivalent to 1 in 10 000 a year in the population) and that the rate almost trebled when systemic allergic reactions with respiratory difficulty were included. Most other data relate to specific causes--for example, anaphylaxis due to allergy to penicillin or to anaesthetic drugs--and are quite variable.

Aetiology

Foods are the commonest cause of anaphylaxis, and evidence suggests that this is an increasing problem, now documented for allergies to peanuts and other nuts. Insect venom is the next most common cause of anaphylaxis. A rapidly increasing problem is allergy to latex rubber. This is probably related to the enormous increase in the use of latex rubber gloves by medical and paramedical staff, as well as to the increase in atopy. Rare causes include exercise, vaccines, and semen. Allergen immunotherapy (desensitisation) may induce anaphylaxis.

Clinical features

It is important to recognise that the picture will vary with the cause. When an allergen is injected systemically (insect stings, intravenous drugs) cardiovascular problems, especially hypotension and shock, predominate. This is especially true when large boluses are given intravenously, as at induction of anaesthesia. Foods that are absorbed transmucosally (from the oral mucosa down) seem especially to cause lip, facial, and laryngeal oedema. Respiratory difficulty therefore predominates. With severe reactions onset occurs soon after exposure (within minutes), and progression is rapid.

Latex rubber anaphylaxis--unusually--develops more slowly (30 minutes or longer from the time of exposure), as the allergen has to be absorbed through the skin or mucosa (for example, during abdominal or gynaecological surgery, vaginal examination, dental work, or simply contact with, or wearing, rubber gloves). Healthcare workers are especially at risk.

Investigations

The only immediate test that is useful at the time of reaction is mast cell tryptase. Tryptase is released from mast cells in both anaphylactic and anaphylactoid reactions. It is an indicator of mast cell activation but does not distinguish mechanisms or throw light on causes. It is usually but not always raised in severe reactions but may not be in less severe systemic reactions. As mast cell tryptase is only raised transiently, blood should be taken when it peaks at about an hour after the onset of the reaction. This test remains to be fully evaluated.

Management

Adrenaline (epinephrine) is the most important drug for anaphylaxis and should be given intramuscularly. It is almost always effective.

This should be followed by chlorpheniramine and hydrocortisone (intramuscular or slow intravenous). This is usually all that is required, provided that treatment is started early. Treatment failure is more likely if administration of adrenaline is delayed. Biphasic reactions have been described but are probably rare; administration of hydrocortisone should minimise the risk of late relapse.