**Abstract**

Anaphylaxis is a life threatening hypersensitivity reaction that can cause shock. Epidemiology studies show anaphylaxis and anaphylactic shock is relatively rare, but its incidence is increasing. A review of the pathophysiology of anaphylaxis can provide insight into clinical decisions. Diagnosing anaphylaxis can be difficult as symptoms and history are not always obvious. Diagnostic guidelines provide an objective tool to assess for anaphylaxis. Early intervention during anaphylaxis may prevent development of shock. Management is focused on circulation support with epinephrine and IV fluids, and airway maintenance. Following an acute anaphylactic reaction, patients should be provided with a referral for follow up and educated on avoidance of triggers and use of epinephrine autoinjectors.

**Keywords:** anaphylaxis, anaphylactic shock, distributive shock, management

### **1. Introduction**

In this chapter, we will learn the differences between anaphylaxis and anaphylactic shock. Epidemiology of anaphylactic shock will be reviewed. We will also discuss the biochemical markers and mediators most noted to triggering an anaphylactic reaction. Lastly, we will provide a review on clinical manifestations and management of an anaphylactic reaction in a nonclinical setting and in a clinical setting.

Throughout this chapter anaphylaxis will be defined according to the World Allergy Organization (WAO) definition *'a severe, life-threatening generalized or systemic hypersensitivity reaction*' [1]. This includes Gell-Combs Type I hypersensitivity, as well anaphylactoid reactions that are immunologically and nonimmunological mediated.

There are few studies on anaphylactic shock, and most recommendations for anaphylactic shock management come from major allergy organizations: WAO, AAAA/ACAAI, and EAACI. The recommendations of these groups for management of anaphylactic shock are presented in this chapter. Recent changes in anaphylaxis definitions require a review of the immunologic and nonimmunologic biochemical pathways of anaphylaxis.

#### **1.1 Definitions**

#### *1.1.1 Anaphylactoid reactions*

Reactions that occur via mechanisms other than IgE mediated mast cell degranulation have been referred to as *Anaphylactoid reactions*. However, the WAO, AAAA/ ACAAI, and EAACI have recommended stoppage of the term [2]. Throughout this chapter reactions that would be included in this term will be referred to as anaphylaxis.

#### *1.1.2 Anaphylaxis*

Anaphylaxis is typically taught as Gell-Combs classification type 1 hypersensitivity, that is IgE mediated. However, the World Allergy Organization (WAO) proposed a new expanded definition of anaphylaxis *'a severe, life-threatening generalized or systemic hypersensitivity reaction*' [3]. This new definition of anaphylaxis now includes reactions previously categorized as 'anaphylactoid reactions'.

#### *1.1.3 Anaphylactic shock*

End manifestation of anaphylaxis, occurs when there is inadequate tissue perfusion causing end organ damage.

## **2. Epidemiology**

Studies have estimated that the incidence of anaphylaxis is between 0.05 and 2.0% of the population [4], although the actual incidence is not clear. Issues previously identified with epidemiologic studies include variations in definitions, under reporting of anaphylaxis, and unclear use of incidence and prevalence of disease [5].

Although the actual incidence is not clear, there have been multiple studies showing that the incidence of anaphylaxis in the United States has increased in recent years [6–9], although the case fatality rate has decreased [8]. Similar findings are reported in other countries, with UK reporting increasing rates of anaphylaxis [10–12], but no increase in the incidence of fatal anaphylaxis [10]. In Australia the incidence of anaphylaxis [13–15] and fatal anaphylaxis has increased as well [16]. One study on the incidence of anaphylaxis with circulatory symptoms found a rate of approximately 8–9 cases per 100,000 persons per year [17]. Severe anaphylaxis, including respiratory or circulatory symptoms, occurs more frequently at a rate of about 1–3 per 10,000 people [18].

Factors that may affect the incidence of anaphylaxis include geography, seasonal variations, age, and gender [19]. Demographic factors associated with higher incidence include living in northern areas of US [7, 20].
