Anaphyaxis

Pathophysiology
Allergic anaphylaxis occurs when a patient’s immune system responds inappropriately to a food, drug, or other protein and is mediated by IgE (antibodies); like all allergic reactions, it requires pre-exposure. Common allergens are penicillin or other beta-lactam antibiotics, tree nuts, Hymenoptera venom, and shellfish. But…non-IgE mediated reactions can also occur: Originally known as an anaphylactoid reaction or idiopathic anaphylaxis, triggers act directly on both mast cells and basophils; they are not IgE mediated and do not require pre-exposure. Triggers for non-IgE mediated anaphylaxis include: vancomycin, opiates, aspirin, NSAIDs, temperature, and exercise. Signs, symptoms, and treatment for both IgE mediated and non-IgE mediated anaphylactic reactions is the same. Patient history while useful to avoid triggers, remains somewhat unreliable for predicting future events, especially initial reactions. And, anaphylaxis is on the rise. While epinephrine remains the mainstay of treatment, it is not always successful and fatalities are possible even with treatment. In most cases, anaphylaxis is a mono-phasic reaction: What you see is what you get. That said, roughly 20-30 percent of all anaphylactic reactions are biphasic with the original signs and symptoms returning 3-10+ hours later (rebound). H1 and H2 blockers (antihistamines) and corticosteroids are used to help prevent biphasic reactions, but are not always successful; the available data is inconclusive. Some outdoor programs with long evacuation times carry oral prednisone to be administered under direct physician control—via cell or satellite phone—or by written standing orders

Assessment

• No local swelling occurs with most anaphylactic reactions: In very rare cases (<10%), a severe local allergic reaction may progress to anaphylaxis.
  
• Onset of S/Sx typically occurs within 5-30 minutes with 90+ percent of S/Sx presenting with an hour; in rare cases, usually with ingested proteins, onset of S/Sx can be delayed 3-6 hours.
• Anaphylaxis is rare with contact allergens.
  
• Suspect and treat for anaphylaxis if a patient presents with a positive MOI and S/Sx, regardless of their history.
• Given the potentially fatal consequences of delaying epinephrine administration and the general safety of epinephrine at the normal adult or child dose, consider treating otherwise healthy patients with epinephrine who have a history of life-threatening anaphylaxis and a positive MOI, but no S/Sx. If possible, get a written prescription (standing orders) from the patient's primary care physician prior to the trip.

S/Sx

• Skin: hives (groin, armpits, flanks, back), flushing, itching, airway obstruction due to subcutaneous swelling (angioedema)
• Respiratory: difficulty breathing, wheezing, stuffy or runny nose, respiratory arrest
• Gastrointestinal: nausea, vomiting, diarrhea, cramping, abdominal pain
• Circulatory: increased pulse, low blood pressure, chest pain, cardiac arrest
• Nervous: headache, dizziness, seizures, sense of impending doom
• Other: metallic taste, loss of bladder control

Treatment

• Adult Dose: 0.3cc intramuscular injection of 1:1000 epinephrine in the mid-lateral thigh. Child Dose: < 75 pounds 0.15 intramuscular injection of 1:1000 epinephrine in the mid-lateral thigh. There are no contraindications for epinephrine with anaphylaxis or suspected anaphylaxis; fatalities are associated with delayed epinephrine use.
• Give a second dose of epinephrine (maximum of three doses) if S/Sx do not resolve within 5-15 minutes.
• Oral antihistamine (diphenhydramine at OTC dose or other antihistamine as directed by a physician). Keep current for 24-72 hours or as directed by a physician.
• Oral prednisone 10-50 mg as directed by a physician (typically administered for a maximum of two doses). Some patients may be allergic to prednisone, others, especially children and teens, may have adverse side-effects. For those who tolerate it, it is good insurance if the evacuation to definitive care is greater than 8 hours.
• A 24-72 recovery and observation period may be required for patient's treated for moderate to severe anaphylaxis. Follow your local protocols.

Evacuation

• Begin a Level 3 evacuation to a hospital for physician follow-up if S/Sx resolve with initial treatment (required for most outdoor programs).
• Begin a Level 1 evacuation if S/Sx do not resolve with treatment or rebound occurs.

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