The Immune System and Immunologic Complications

Allergic Reactions and Anaphylaxis

The term allergy refers to an IgE-mediated (Type 1) hypersensitivity reaction; the antigens that result in an allergic reaction (atopy) are termed allergens. Common allergens include foods (eggs, peanuts, shellfish, many others), antibiotics, local anesthetics and other medications, insect stings (hymenoptera venom, fire ant bites), latex and hormones (insulin, methylprednisolone, progesterone).

The mildest form of a Type 1 hypersensitivity reaction is an allergic, or atopic, reaction. Patients are considered atopic when they have a predisposition towards hypersensitive responses. Atopy is a localized reaction that occurs after the host is exposed to an allergen to which it is sensitized. During the second exposure, histamine released from mast cells and basophiles results in localized increased capillary permeability, vasodilation, smooth muscle contraction and sensory nerve stimulation. When released locally these effects give rise to signs and symptoms such as localized edema, urticaria (hives), flushing, pruritus, cramping and abdominal pain. Localized vasodilation and capillary permeability in the respiratory tract produce symptoms including wheezing and complaints of mild difficulty breathing.

Anaphylaxis represents the most serious manifestation of a Type 1 hypersensitivity reaction. Though the true incidence of anaphylaxis is unknown, a recent study concluded that the lifetime prevalence of anaphylaxis is about 1%–2% of the U.S. population as a whole, and another found that 1%–15% of the U.S. population is at risk of experiencing an anaphylactic reaction.2,3 Anaphylaxis is characterized by an acute multiorgan system reaction to an allergen and results in a more global distribution of increased capillary permeability, vasodilation, smooth muscle contraction and sensory nerve stimulation compared to a localized allergic reaction. Common organ systems affected include the respiratory, cardiovascular, cutaneous, gastrointestinal and central nervous systems. Signs and symptoms include:

• Upper respiratory: nasal congestion and itching, rhinorrhea, sneezing, laryngeal edema resulting in stridor, dyspnea, hoarseness and sensation of a “tight throat.”
• Lower respiratory: Bronchospasm can result in dyspnea, chest tightness, wheezing, cough, tachypnea.
• Cardiovascular: Profound fluid volume shifts can result in circulatory collapse characterized by weakness, dizziness, near-syncope and syncope. Tachycardia, hypotension and shock can be present in severe anaphylaxis.
• Cutaneous (skin): urticaria, flushing, pruritis, angioedema, cyanosis.
• Gastrointestinal: cramping, abdominal pain, nausea, vomiting, diarrhea.
• Central nervous: anxiety, apprehension, confusion, headache (all occur as a result of cerebral hypoperfusion and subsequent hypoxia).

Management of Allergic Reactions and Anaphylaxis

A patient with a mild allergic reaction that is clearly localized can be treated as a BLS patient. If the resulting urticaria and pruritis prove to be uncomfortable for the patient, administering an antihistamine such as diphenhydramine via the oral or IM route can improve patient comfort. At the first sign of any clinical manifestation of anaphylaxis, the patient should be considered an ALS patient and immediately administered 0.3 mL of epinephrine 1:1000 IM. IM administration in the vastus lateralis (thigh) has been shown to result in a more rapid maximum plasma concentration of epinephrine than IM or SQ administration in the deltoid (arm) of asymptomatic patients and may be a preferred route.4 Ideally, the patient with known hypersensitivity to an allergen will have been prescribed an EpiPen and will have self-administered epinephrine prior to EMS’ arrival. In patients with laryngeal swelling, racemic epinephrine via small-volume nebulizer can be used to reduce swelling, but it should never take the place of more definitive airway control, such as endotracheal intubation, if airway compromise is imminent. Epinephrine should be used with caution in elderly patients and in patients with known heart disease or hypertension.