15.1 Anaphylaxis: Recognition & Emergency Management

ABIM Internal Medicine Exam

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15 Chapter 14: Geriatric Syndromes (3% of Exam)

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16 Chapter 15: Allergy & Immunology (2% of Exam)

15.1 Anaphylaxis: Recognition & Emergency Management 15.2 Urticaria & Angioedema 15.3 Drug Allergy, Desensitization & Adult Immunodeficiency 15.4 Food Allergy, Insect Venom Hypersensitivity, Serum Sickness & Contrast Reactions 15.5 Rhinitis, Conjunctivitis, Eosinophilic & Mast Cell Disorders

17 Chapter 16: Preventive Medicine, Immunization & Clinical Epidemiology

16.1 The USPSTF Framework & Evidence-Based Cancer Screening 16.2 Cardiovascular & Metabolic Screening and Primary Prevention 16.3 Adult Immunization & Vaccines in Special Populations 16.4 Clinical Epidemiology, Biostatistics & Study Design

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19 Chapter 18: Perioperative Medicine & Medical Consultation

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20 Chapter 19: Ophthalmology (1% of Exam)

19.1 The Acute Red Eye & Ophthalmic Emergencies 19.2 Painless Vision Loss & Chronic Ophthalmic Disease 19.3 Uveitis, Scleritis & Orbital Disease

21 Chapter 20: Otolaryngology & Dental Medicine (1% of Exam)

20.1 Dizziness, Vertigo & Epistaxis 20.2 Ear Disorders, Hearing Loss & Upper Respiratory Infections 20.3 Oral Cavity, Salivary Gland, Laryngeal & Taste and Smell Disorders

15.1 Anaphylaxis: Recognition & Emergency Management

Key Takeaways Explain This Topic AI

General anaphylaxis, aspirin idiosyncrasy, stinging insect hypersensitivity, desensitization therapy and serum sickness are enumerated under the anaphylaxis blueprint subsection.

Intramuscular epinephrine into the anterolateral thigh is first-line and must not be delayed for antihistamines or corticosteroids.

Anaphylaxis can be diagnosed without hypotension when acute skin or mucosal involvement occurs with respiratory compromise or gastrointestinal symptoms.

Patients taking beta-blockers may have epinephrine-refractory anaphylaxis and require glucagon.

Biphasic reactions occur in a minority of cases hours after apparent resolution, warranting a period of observation and an epinephrine autoinjector prescription.

Last updated: August 2026

Allergy, angioedema, and immunodeficiency encompass a broad spectrum of acute medical emergencies and chronic multisystem diseases. Mastery of immunoglobulin-mediated pathways, mast cell and bradykinin biology, drug desensitization protocols, and primary/secondary immunodeficiency syndromes is critical for the internal medicine physician.

1. Anaphylaxis: Recognition & Emergency Resuscitation

Anaphylaxis is an acute, life-threatening, systemic hypersensitivity reaction resulting from the sudden, massive release of inflammatory mediators (histamine, tryptase, leukotrienes, prostaglandins, and platelet-activating factor) from tissue mast cells and circulating basophils.

Clinical Diagnostic Criteria (NIAID / FAAN Consensus)

Anaphylaxis is highly likely when ANY ONE of the following 3 clinical criteria is fulfilled:

Criterion 1 (Acute Skin/Mucosa + Respiratory or Cardiovascular Compromise):

Acute onset (minutes to several hours) of symptoms involving the skin, mucosal tissue, or both (e.g., generalized hives, intense pruritus, flushing, swollen lips/tongue/uvula),

PLUS AT LEAST ONE of the following:

Respiratory compromise: Dyspnea, wheeze-bronchospasm, stridor, hypoxemia, reduced peak expiratory flow (PEF).

Reduced Blood Pressure or End-Organ Dysfunction: Hypotension (systolic BP < 90 mmHg or > 30% decline from baseline), syncope, hypotonia, loss of consciousness, incontinence.

Criterion 2 (Two or More Systems After Exposure to a LIKELY Allergen):

Rapid onset (minutes to hours) of >= 2 of the following after exposure to a likely allergen for that patient:

(a) Cutaneous/mucosal involvement (hives, pruritus, angioedema).

(b) Respiratory compromise (dyspnea, wheezing, stridor).

(d) Persistent gastrointestinal signs/symptoms (severe cramping abdominal pain, repetitive vomiting).

Criterion 3 (Isolated Hypotension After Exposure to a KNOWN Allergen):

Reduced blood pressure rapidly after exposure to a known allergen for that specific patient (systolic BP < 90 mmHg or > 30% decrease from baseline in adults).

Common Allergenic Triggers

Foods: Peanuts, tree nuts (walnuts, almonds, cashews), shellfish, finned fish, cow's milk, eggs, soy, wheat, sesame.

Insect Venoms (Hymenoptera): Yellow jackets, honeybees, hornets, wasps, fire ants.

Medications: Beta-lactam antibiotics (penicillins, cephalosporins), NSAIDs/Aspirin, neuromuscular blocking agents (succinylcholine, rocuronium), monoclonal antibodies/biologics, platinum-based chemotherapeutics.

Other Triggers: Natural rubber latex, seminal fluid, radiocontrast media (non-IgE anaphylactoid), idiopathic.

Emergency Pharmacotherapy Algorithm [ PATIENT IN ANAPHYLAXIS ]
│
▼
┌───────────────────────────────────────────────┐
│ FIRST-LINE & ONLY LIFE-SAVING INTERVENTION: │
│ INTRAMUSCULAR (IM) EPINEPHRINE │
│ • Adult Dose: 0.3 to 0.5 mg (1:1,000) │
│ • Site: ANTEROLATERAL THIGH (vastus lateralis)│
│ • Repeat every 5 to 15 minutes as needed │
│ • NO ABSOLUTE CONTRAINDICATIONS │
└───────────────────────┬───────────────────────┘
│
┌───────────────────┴───────────────────┐
▼ ▼
[ Hemodynamically Unstable / Shock ] [ Secondary Adjunctive Therapies ]
Rapid IV Crystalloid Bolus (1-2 L) (NEVER DELAY EPINEPHRINE)
Elevate lower extremities (Trendelenburg) • IV H1 Blocker: Diphenhydramine 25-50 mg
Continuous IV Epinephrine Infusion • IV H2 Blocker: Famotidine 20 mg
Refractory on Beta-Blockers: • Inhaled Albuterol 2.5-5 mg (bronchospasm)
➜ IV GLUCAGON (1-5 mg over 5 min) • Systemic Steroid: Methylprednisolone 1-2 mg/kg
Detailed Pharmacotherapy Guidance

1. Intramuscular (IM) Epinephrine (First-Line Standard of Care):

Dose & Formulation: 0.3 to 0.5 mg (0.3 to 0.5 mL) of 1:1,000 (1 mg/mL) aqueous solution administered intramuscularly.

Anatomical Site: Anterolateral mid-thigh (vastus lateralis muscle) . Pharmacokinetic studies show that IM injection into the vastus lateralis achieves significantly higher and faster peak plasma concentrations (within 8 minutes) compared to subcutaneous or deltoid intramuscular administration.

Frequency: Repeat every 5 to 15 minutes if symptoms fail to improve or progress.

Safety: There are NO absolute contraindications to epinephrine administration during a life-threatening anaphylactic episode (even in elderly patients or those with coronary artery disease).

2. Aggressive Intravenous Fluid Resuscitation:

Rapid IV infusion of 1 to 2 Liters of isotonic crystalloid (0.9% Normal Saline or Plasmalyte) via large-bore peripheral IV lines for distributive/hypovolemic shock resulting from massive vascular permeability and third-spacing (up to 35% of intravascular volume can extravasate within 10 minutes).

3. Secondary Adjunctive Medications (Administer ONLY AFTER Epinephrine):

H1-Antihistamines: IV Diphenhydramine (25–50 mg) or IV Cetirizine (reduces urticaria and pruritus; does not relieve airway edema, bronchospasm, or hypotension).

H2-Antihistamines: IV Famotidine (20 mg; combined H1 + H2 blockade provides superior cutaneous symptom relief compared to H1 blockade alone).

Inhaled Beta-2 Agonists: Inhaled Albuterol (2.5–5.0 mg nebulized) for persistent wheezing and bronchospasm refractory to epinephrine.

Systemic Corticosteroids: IV Methylprednisolone (1–2 mg/kg) or oral Prednisone (40–60 mg). Corticosteroids have a delayed onset of action (4–6 hours) and do not treat acute shock or laryngeal edema; their primary traditional role is to decrease the severity of biphasic reactions.

4. Refractory Anaphylaxis in Patients on Beta-Blockers:

Patients taking chronic beta-adrenergic blockers (e.g., Metoprolol, Atenolol, Propranolol) are prone to severe, refractory anaphylaxis characterized by profound bradycardia, severe hypotension, and resistance to epinephrine (beta-blockade blunts cardiac inotropy/chronotropy and allows unopposed alpha-adrenergic vasoconstriction, increasing afterload and precipitating cardiovascular collapse).

Specific Antidote: Intravenous Glucagon (1 to 5 mg IV administered over 5 minutes, followed by a continuous infusion of 5 to 15 mcg/min) .

Mechanism: Glucagon binds to specific non-adrenergic G-protein-coupled glucagon receptors, stimulating intracellular adenylate cyclase and increasing cyclic AMP (cAMP) independently of beta-adrenergic receptors , restoring myocardial contractility, heart rate, and vascular tone.

5. Biphasic Anaphylaxis & Post-Discharge Care:

Biphasic Anaphylaxis: A recurrence of anaphylaxis symptoms occurring without re-exposure to the allergen in 10% to 20% of patients , typically arising within 1 to 72 hours (median 8–10 hours) after complete resolution of the initial attack.

Observation Period: Minimum 4 to 8 hours of continuous medical observation for mild, rapidly responsive anaphylaxis; 12 to 24 hours for patients with severe initial presentation (hypotension, severe airway edema, or requiring > 1 dose of epinephrine).

Discharge Plan: Prescribe two Epinephrine Auto-Injectors (0.3 mg) with hands-on demonstration; provide a written Emergency Anaphylaxis Action Plan; refer to an Allergist/Immunologist for confirmatory testing and potential Venom Immunotherapy (VIT) (which reduces recurrent severe venom anaphylaxis risk from > 60% to < 5%).