Anaphylaxis — Adult Recognition & Treatment

Key Points

• IM epinephrine is the first and most important treatment for anaphylaxis.
• Do not delay epinephrine while giving antihistamines, steroids, nebulizers, or IV access.
• Skin findings can be absent in severe anaphylaxis.
• Repeat IM epinephrine when airway, breathing, or circulation abnormalities persist.
• Large-volume isotonic crystalloid may be required because distributive shock causes profound intravascular depletion.
• Refractory shock should transition to a monitored epinephrine infusion with expert support.
• Antihistamines treat cutaneous symptoms but do not prevent death from anaphylaxis.
• Every discharged patient needs an epinephrine access and recurrence plan.

Algorithm

1. Recognize the clinical syndrome and remove or stop the trigger when feasible.
2. Administer IM epinephrine immediately in the anterolateral thigh.
3. Position the patient safely, provide oxygen, monitor continuously, and obtain IV access.
4. Give rapid isotonic crystalloid for hypotension and repeat IM epinephrine according to response.
5. Escalate refractory shock to a titrated IV epinephrine infusion and prepare for advanced airway management when edema progresses.
6. Use inhaled beta agonist for persistent bronchospasm and glucagon when beta blockade contributes to refractory shock.
7. Observe according to reaction severity and recurrence risk.
8. Discharge with epinephrine, education, trigger avoidance guidance, and allergy follow-up.

Clinical Synopsis & Reasoning

Anaphylaxis is diagnosed clinically. Typical presentations involve acute skin or mucosal findings plus respiratory compromise or hypotension, or acute involvement of multiple organ systems after a likely allergen. Skin findings may be absent, particularly in severe shock. Respiratory features include stridor, hoarseness, throat tightness, wheeze, and hypoxemia. Cardiovascular involvement includes hypotension, syncope, collapse, or shock. Gastrointestinal cramping and repetitive vomiting can be prominent after food exposure. The diagnosis should be made from the pattern and tempo rather than waiting for laboratory confirmation.

Common triggers include foods, medications, insect venom, biologic agents, and latex, although some episodes remain idiopathic. Consider mimics such as severe asthma, vasovagal syncope, panic, hereditary angioedema, scombroid poisoning, pulmonary embolism, and distributive shock from other causes. Bradycardia with pallor and rapid recovery when supine favors vasovagal syncope, while anaphylaxis more often produces persistent multisystem findings and may cause tachycardia. Angioedema from bradykinin pathways typically lacks urticaria and does not respond reliably to epinephrine, antihistamines, or corticosteroids.

Serum tryptase can support the diagnosis when obtained in an appropriate window but must not delay treatment and can be normal in true anaphylaxis, especially food-triggered reactions. The immediate question is not whether the allergen can be proven, but whether airway, breathing, or circulation are threatened. Patients taking beta blockers may have more refractory hypotension and can require glucagon in addition to epinephrine and fluids.

Treatment Strategy & Disposition

Give IM epinephrine into the mid-anterolateral thigh immediately. Current practice guidance uses 0.01 mg/kg of 1 mg/mL epinephrine up to a typical adult maximum of 0.5 mg per dose, repeated every 5 to 15 minutes according to clinical response. Do not substitute subcutaneous administration because absorption is less reliable. Place hypotensive patients supine with legs elevated when tolerated, provide high-flow oxygen, obtain IV access, and give rapid isotonic crystalloid boluses for distributive shock.

If severe hypotension, airway compromise, or bronchospasm persists despite repeated IM epinephrine and adequate fluids, escalate to monitored IV epinephrine infusion with experienced critical-care support rather than repeated unmonitored IV boluses. Nebulized beta-agonist treats persistent bronchospasm but does not replace epinephrine. Nebulized epinephrine may temporarily reduce upper-airway edema while definitive airway planning proceeds. Glucagon can be considered for refractory hypotension or bradycardia in a patient taking beta blockers. Prepare early for a difficult airway when progressive tongue, pharyngeal, or laryngeal edema is present.

H1 antihistamines improve itching and urticaria but do not reverse shock or airway obstruction. Corticosteroids have delayed onset and are not reliable prevention for biphasic reactions, so they are adjunctive at most. Observation duration is individualized. Longer observation is warranted after severe reactions, repeated epinephrine, hypotension, airway compromise, delayed epinephrine, unknown trigger, poor access to emergency care, or prior biphasic reaction. Before discharge, prescribe or dispense epinephrine autoinjectors, teach their use, provide an action plan, and arrange allergy follow-up.

Epidemiology / Risk Factors

• Food, medications, and insect venom are common triggers, with patterns varying by age and population.
• Patients with asthma, mast-cell disorders, or prior severe anaphylaxis can have greater risk from future reactions.
• Delayed epinephrine administration is repeatedly associated with worse outcomes.
• Biphasic reactions occur in a minority of patients and are more likely after severe initial reactions or multiple epinephrine doses.
• Anaphylaxis can occur without a previously known allergy.

Investigations

Key Medications

Pharmacology

Prognosis / Complications

• Most patients recover rapidly when epinephrine is given early and adequately.
• Fatality is more likely with severe airway obstruction, refractory shock, delayed epinephrine, and significant cardiopulmonary comorbidity.
• Recurrence can occur after initial resolution, particularly after severe reactions.
• The strongest long-term prevention strategy is trigger identification plus ready access to epinephrine.
• Allergy evaluation can clarify avoidable triggers and determine whether immunotherapy or additional preventive strategies are indicated.

Patient Education / Counseling

• Carry prescribed epinephrine at all times and know exactly how to use the device.
• Use epinephrine promptly for a recurrence with breathing difficulty, throat symptoms, faintness, or rapid multisystem symptoms, then call emergency services.
• Do not depend on diphenhydramine or another antihistamine as the rescue medication for anaphylaxis.
• Read labels carefully and discuss hidden or cross-contact exposures when food allergy is the trigger.
• Wear medical identification when appropriate and ensure family, school, or workplace contacts know the action plan.
• Arrange allergy follow-up for trigger assessment, autoinjector renewal, and prevention planning.

Notes

The practical definition of successful initial therapy is normalization or clear improvement of airway, breathing, and circulation, not disappearance of every rash lesion.

Discharge education is a treatment intervention. Epinephrine access, device competence, trigger planning, and allergy follow-up materially affect the risk of a future fatal event.

References

1. AAAAI Anaphylaxis Practice Resources — Link
2. 2023 Anaphylaxis Practice Parameter Update — Link