Anaphylaxis

Etiology

Common inciting sources may include exposure to certain medications, foods, or insect stings. Immunotherapy injections directed at improving overall allergic response can induce a hyper-acute reaction. Latex hypersensitivity is occurring in increased prevalence, and as with any hypersensitivity, anaphylaxis is an inherent risk. Occasionally, the offending agent is not identified; these reactions are idiopathic anaphylaxis.[4][5][6] The most common causes include bee stings, fire ant bites, latex, and foods (peanuts, tree nuts, fish, shellfish, milk, eggs, wheat, spelt, rye, barley, soy, red meat, and sesame). Alpha-gal anaphylaxis: This involves an IgE antibody response to mammalian galactose alpha-1,3-galactose.

Epidemiology

The world population is estimated to have a lifetime prevalence of 1% to 3%, though the prevalence is increasing. While reactions can occur in any age group, they are most commonly noted in the younger population and developed countries. Unfortunately, anaphylaxis is often misdiagnosed or not diagnosed at all. The consequences of missed or delayed diagnosis result in increased morbidity and mortality.[7][8][9]

Pathophysiology

Anaphylaxis is typically an IgE-mediated (type 1) hypersensitivity reaction that involves the release of numerous chemical mediators from the degranulation of basophils and mast cells after re-exposure to a specific antigen. IgE crosslinking and resultant aggregation of high-affinity receptors induce the rapid release of stored chemical mediators. These chemical mediators include histamine, tryptase, carboxypeptidase A, and proteoglycans. Via activation of phospholipase A, cyclooxygenases, and lipoxygenases, they then form arachidonic acid metabolites, including leukotrienes, prostaglandins, and platelet-activating factors.[10][11] The inflammatory response is then mediated by TNF-alpha (tumor necrosis factor) as a preformed and late-phase reactant. The detailed physiology of these chemical mediators is as follows:

• Histamine increases vascular permeability and vasodilation, leading to tissue hypoperfusion. The body responds to these changes by increasing heart rate and cardiac contraction.

• Prostaglandin D functions as a bronchoconstrictor, simultaneously constricting cardiac and pulmonary arteries. It also potentiates peripheral vasodilation, contributing to the hypoperfusion of vital organs.

• Leukotrienes add to bronchoconstriction and vascular permeability and induce airway remodeling.

• The platelet activation factor also acts as a bronchoconstrictor and increases vascular permeability.

• TNF-alpha activates neutrophils (as part of stress response leukocytosis) and increases chemokine synthesis.

Histopathology

Rapid mast cell and basophil degranulation are attributed to the causation of the overactive immune response that culminates in the clinical picture of anaphylaxis.

History and Physical

Clinical presentation often begins as a mild allergic reaction. The primary symptoms depend on the mode of exposure to the causative antigen. While cutaneous flushing with pruritus and urticaria are common, they may not develop until after respiratory symptoms occur, which is common in oral exposures. Fullness or a "lump in the throat," persistent clearing of the throat, or difficulty breathing are all concerning symptoms of anaphylaxis and should be treated aggressively. Other respiratory symptoms include hoarseness, wheezing, and stridor. Rapid treatment should be initiated with intramuscular epinephrine if any of these symptoms are present. Anaphylaxis is a rapidly evolving presentation, usually within 1 hour of exposure. Roughly half of the anaphylactic-related fatalities occur within this first hour; therefore, the first hour after the initial symptom onset is the most crucial for treatment. It is important to note that the more rapid the onset and progression of symptoms, the more severe the disease process. Morbidity and mortality are most often related to loss of airway and distributive shock. Early recognition and aggressive treatment greatly reduce the risk of adverse outcomes.

The first hour is not the only time of concern, however, as anaphylactic reactions can also occur biphasically in up to 20% of cases. Even after successful management of the initial presenting symptoms, there can be a recurrence of symptoms peaking 8 to 11 hours after the initial reaction. While the biphasic response is only clinically significant in 4 to 5% of patients with diagnosed anaphylaxis, the potentially fatal second reaction should not be dismissed after treatment of the initial reaction. While most of the focus is on the above symptoms of respiratory, cutaneous, and hypotension, it is important to understand and evaluate for any other signs of end-organ damage from hypo-perfusion. This may include abdominal pain and cramps, vomiting, hypotonia, syncope, or incontinence. Gastrointestinal (GI) symptoms occur in 25 to 30% of patients. While the loss of airway control due to laryngeal edema is the primary concern, one must not fail to recognize anaphylaxis in a patient who does not endorse upper airway edema or respiratory complaints.

Evaluation

Diagnosing anaphylaxis is clinical; thus, laboratory studies or other diagnostics are unnecessary. Most anaphylactic deaths occur within the first hour after antigen exposure. Rapid recognition and action are imperative.[2][12][13] Consideration for anaphylaxis is appropriate in the presence of 2 or more involved systems, even in the absence of airway involvement or hypotension (see above). A consensus criterion has been constructed to improve clinical recognition to prevent delayed treatment, as this poses a great risk to patients. See Image. Anaphylaxis.

Clinical Criteria for Anaphylaxis (1 of the following with onset within minutes to hours)*

Unknown exposure to an antigen yet rapidly developing urticaria or other skin/mucosal layer symptoms associated with any 1 of the following:

1. Respiratory symptoms (dyspnea, wheezing, stridor, hypoxemia, inability to maintain patency; persistent cough or throat clearing can be heralding symptoms) 
2. Hypotension (systolic less than 90 mm Hg or a decrease of greater than 30% from baseline)
3. Signs or symptoms of end-organ dysfunction, for example, hypotonia, syncope, incontinence

Likely exposure to an antigen and symptoms involving any 2 of the following body systems:

1. Integumentary symptoms: Skin or mucosal layer (rash, pruritus, erythema, hives, swelling of the face, lips, tongue, or uvula). See Image. Anaphylaxis, Hives.
2. Respiratory symptoms: Dyspnea, wheeze, stridor, hypoxemia, inability to maintain patency; persistent cough or throat clearing can be heralding symptom
3. Hypotension: Systolic less than 90 mm Hg or a decrease of greater than 30% from baseline
4. Gastrointestinal symptoms: Persistent painful cramps or vomiting

Known exposure to an antigen and hypotension (systolic less than 90 mm Hg or a decrease of greater than 30% from baseline). It is not required to meet these criteria to treat; it is only to serve as a guide for diagnosis.

Angioedema can also mimic these symptoms. A key differentiation between angioedema and anaphylaxis is urticaria; the oral symptoms and need for airway control can otherwise mimic each other. When in doubt, treat aggressively. Laboratory testing is of little to no use, as there is no accurate testing for diagnosis or confirmation. Serum histamine is of no use due to transient elevation and late presentation. Serum tryptase can be considered for confirmation of an anaphylactic episode as it remains elevated for several hours; however, as a diagnostic modality, this has low sensitivity. Kounis syndrome (allergic angina): This is myocardial infarction or ischemia that can occur in the setting of anaphylaxis.[14] 

Treatment / Management

Triage

Triage any allergic reaction with urgency as they are at risk for rapid deterioration with the development of anaphylaxis, if not already anaphylactic.[15][16][17](B3)

Airway

Airway management is paramount. Thoroughly examine the patient for airway patency or any indications of an impending loss of airway. Perioral edema, stridor, and angioedema are very high risk, and obtaining a definitive airway is imperative. Delay may reduce the chances of successful intubation as continued swelling occurs, increasing the risk for a surgical airway.

Decontamination

After the airway is secured, decontaminating offending agents (if known) is the next priority to prevent continued exposure and clinical worsening. Remove any stingers, if present. Do not attempt gastric lavage in cases of ingestion, as this may not be effective and delays treatment.

Epinephrine

Epinephrine is given through intramuscular injection and at a dose of 0.3 to 0.5 mL of 1:1,000 concentration of epinephrine. Pediatric dosing is 0.01 mg/kg or 0.15 mg intramuscularly (epinephrine injection for pediatric dosage). Intramuscular delivery has proven to provide more rapid delivery and produce better outcomes than subcutaneous or intravascular. Note if intravenous epinephrine is to be given, the concentration required is 1:10,000; see the next paragraph. The thigh is preferred to the deltoid when possible. Repeat studies have shown that providers often wait too long before giving epinephrine; it is the treatment of choice, and the rapid benefit much outweighs the risks of withholding treatment. While most patients require only a single dose, repeat doses may be given every 5 to 10 minutes as needed until symptoms improve. If patients require multiple doses, a continuous infusion of epinephrine may be considered; start an initial intravenous infusion of 0.1 mg of 1:10,000, given over 5 to 10 minutes. If more is required, begin infusion at 1 microgram per minute and titrate to effect. Stop intravenous infusion if arrhythmia or chest pain develops. The risk of cardiovascular complications is much greater for intravenous epinephrine. For beta-blocked patients, close blood pressure monitoring is suggested due to the risk of unopposed alpha-adrenergic effects from epinephrine.

IV Fluid Resuscitation

Anaphylaxis induces a distributive shock that typically is responsive to fluid resuscitation and the above epinephrine. One to 2 L or 10 to 20 mL/kg isotonic crystalloid bolus should be given for observed hypotension. Albumin or hypertonic solutions are not indicated.

Adjunctive Therapies

When anaphylaxis is diagnosed, co-treatment is often initiated with steroids, antihistamines, inhaled bronchodilators, and vasopressors. Glucagon can also be used if indicated. These agents can assist in refractory initial anaphylaxis or prevent recurrence and biphasic reactions.

Corticosteroids

Corticosteroids are given to reduce the length or biphasic response of anaphylaxis. There is minimal literature to support this use, specifically in anaphylaxis, but it has been proven effective in reactive airway diseases. Therefore, the use, dosages, and proposed mechanism of action mimic those of airway management protocols. 

Methylprednisolone (80 to 125 mg IV) or hydrocortisone (250 to 500 mg IV) are the accepted treatments during the acute phase, after which oral treatment of prednisone (40 to 60 mg daily or divided twice per day) is continued for 3 to 5 days. Again, if the source is unknown or there is a concern for a prolonged time before physician follow-up, a steroid taper of up to 2 weeks may be provided. Mineralocorticoid activity is responsible for fluid retention; dexamethasone and methylprednisolone are the preferred agents for those at risk as they induce the least mineralocorticoid effect.

Antihistamines

Antihistamines are often routinely used; the most common is H­1 blocker administration of diphenhydramine 25 to 50 mg IV/IM. While the clinical benefit is unproven in anaphylaxis, its utility is evident in more minor allergic processes. In severe cases, H1 blockers such as famotidine 20 mg intravenous (given over 2 minutes) or cimetidine (300 mg IV) may also be used in conjunction with H-­blocker as there is evidence suggesting histamine has crossover selectivity of receptors. Note that cimetidine has multiple precautions in at-risk populations such as renal or hepatic impaired patients or those taking beta-blockers. While intravenous is the initial route during stabilization, once the patient is stabilized, they may be switched to oral if continued therapy is desired.

Bronchodilators

Bronchodilators are useful adjuncts in patients with bronchospasm. Patients with previous histories of respiratory disease, most notably asthma, are at the highest risk. Treated with inhaled beta-agonists is the first-line treatment for wheezing; albuterol alone or as ipratropium bromide/albuterol. If there is refractory wheezing, intravenous magnesium is appropriate with dosage and treatment similar to severe asthma exacerbations.

Vasopressors

Vasopressors may be substituted when a patient requires more doses of epinephrine but has unacceptable side effects from the epinephrine intravenous infusion (arrhythmia or chest pain). If no clear second-line pressor is identified, treatment guidelines would follow those of any other patient in hypotensive shock.

Glucagon

Glucagon is the reversal agent for beta-blockers and can be used as such if needed in cases where anaphylaxis is resistant to treatment in patients with beta-blockade. Known side effects include nausea, vomiting, hypokalemia, dizziness, and hyperglycemia.

A therapeutic agent under research:

Sirtuin 6

A NAD-dependent deacetylase suppresses protein tyrosine phosphatase, receptor type C transcription. This, in turn, leads to negative regulation of the FcERI signaling cascade in mast cells. Hence, activation of Sirt6 can be tried as a therapeutic strategy for anaphylaxis.[18] 

Differential Diagnosis

The differential diagnoses for anaphylaxis includes:

• Angioedema
• Anxiety
• Arrhythmias
• Asthma
• Carcinoid syndrome
• Epiglottitis
• Foreign-body airway obstruction
• Gastroenteritis
• Mastocytosis
• Myocardial ischemia/infarction
• Seizure
• Vasovagal episode
• Vocal cord dysfunction

Prognosis

With rapid and adequate treatment and monitoring, the risk of morbidity and mortality is low. Rapid access to medical care and identification of the disease process are essential to patient prognosis. As stated above, the first hour after symptoms exposure is responsible for half of the related fatalities.[15][19][10] Hospital admission is required in 4% or less of acute allergic reactions diagnosed in the emergency department. If epinephrine is required, as in cases of anaphylaxis, complete resolution is noted. Emergency department observation for 4 hours is recommended. If no further intervention is required, the patient can be discharged home with a prescription for an epinephrine autoinjector, and follow-up is appropriate. If the patient requires airway intervention, is refractory to treatment, or is deemed unstable, admission to the intensive care unit for close monitoring is advised. Patients with a history of biphasic reactions, severe reactions, beta-blocker use, older, those who live alone, or those with poor access to health care or deemed at risk should be monitored longer.

Along with the prescription for an epinephrine pen, antihistamines and corticosteroids are appropriate for 3 to 5 days. If the inciting source is unknown and the patient has a prolonged time before follow-up, consider corticosteroid use for 1 to 2 weeks with an appropriate taper. Also, consider writing for multiple epinephrine autoinjectors to ensure they are kept in various locations (home, school, work, and vehicle). Educate and document the need for 24-hour access to the epinephrine autoinjector if symptoms begin to recur. Patients with severe allergic reactions and anaphylaxis who take beta-blockers are at greater risk for prolonged or more severe symptoms; consider other classes of medication if possible. Patients may also consider obtaining medical alert bracelets or the like for assistance in the future.

Complications

The main complications which can occur following an anaphylactic reaction include the following:

• Wheeze
• Stridor
• Hypoxemia
• Hypotension
• End-organ dysfunction and 
• Death