Anaphylaxis

The exact incidence of anaphylaxis is unknown [1]. Perhaps the best insight into incidence is obtained from assessing prescriptions for automatic epinephrine injectors. Using such prescription data, Simons [2] found an overall incidence of approximately 1% of the population of Manitoba, Canada. Regardless of the exact incidence, the incidence of anaphylactic episodes seems to be increasing based on the assessment of admissions to the emergency room in the United Kingdom [3]. Factors affecting incidence are listed in Box 1: Atopy is a risk factor in all general series of anaphylactic events [1]. This is particularly true for agents administered by the mucosal route (eg, food). This is not surprising because atopy is a mucosally expressed and usually mucosally sensitized disease. Atopy, however, does not seem to be a risk factor for agents administered parenterally (eg, penicillin, insulin). It is interesting to note, however, that atopy is even a risk factor for episodes normally, considered anaphylactoid (not IgE mediated). This includes anaphylactoid reactions to radiocontrast material and exercise. The reasons for this have not been established, but are thought to be caused by the cytokine milieu with increased production of interleukin-4, -5, and -13 in atopics as compared with nonatopic individuals. These cytokines not only enhance the releasability of mast cells and basophils, but also sensitize the target organs to mediators, such as histamine [4]. Sex clearly exerts an effect on the incidence of anaphylaxis. Males under the age of 16 experience anaphylaxis more frequently than females that age [2], whereas after the age of 30, the incidence is higher in females [5]. The female predominance after puberty may be related to hormonal differences in that progesterone enhances susceptibility to anaphylaxis in animal models and progesterone-related (catamenial) anaphylactic events have been described in humans [6]. As a rule, anaphylactic events seem to be more common in adults than children, probably because of increased use of drugs in the older population [1]. In some series, however, children predominate [2]. In atopic individuals the route of administration is important in that the mucosal route is more sensitizing. The constancy of administration is also important. For example, insulin allergy is more likely to occur after recurrent administrations of insulin with interruptions between each administration as often occur in gestational diabetes [1]. The time since the previous reaction is important in that the longer the duration since the last administration of antigen, the less the likelihood of a recurrence. An unusual observation, but one that has been confirmed, is that there seems to be an increased incidence of anaphylaxis in individuals of higher socioeconomic status. This cannot be related to access to medical care [7]. Anaphylactic episodes show a seasonal predisposition because of the seasonality of insect sting reactions. Anaphylactic reactions are more common in the summer and early fall. Race, geographic location, and chronobiology seem to play no role. As opposed to other atopic conditions, which seem to worsen at night (eg, allergic asthma), there seems to be no increased incidence of anaphylactic episodes at this time [5].