How do you differentiate and treat anaphylaxis from gastroenteritis (stomach flu)?

Differentiating Anaphylaxis from Gastroenteritis

Anaphylaxis is distinguished from gastroenteritis by its acute onset (minutes to hours), multi-system involvement beyond isolated GI symptoms, and the presence of skin/mucosal changes or respiratory/cardiovascular compromise—when suspected, immediately administer intramuscular epinephrine 0.3-0.5 mg in adults (0.01 mg/kg in children) in the lateral thigh. 1

Key Distinguishing Features

Anaphylaxis Presentation

• Rapid onset: Symptoms develop within minutes to a few hours after allergen exposure 1
• Multi-system involvement: Typically affects 2 or more organ systems simultaneously 1
• Skin/mucosal findings (present in most cases): Generalized urticaria, itching, flushing, lip/tongue/uvula swelling, facial swelling 1
• Respiratory symptoms: Throat tightness, sensation of throat closing, stridor, hoarseness, difficulty breathing, wheezing, cyanosis 1
• Cardiovascular collapse: Hypotension, tachycardia, weak pulse, dizziness, syncope, pallor, altered consciousness 1
• GI symptoms occur WITH other systems: Nausea, crampy abdominal pain, persistent vomiting, diarrhea 1

Gastroenteritis Presentation

• Gradual onset: Symptoms typically develop over hours to days 1
• Isolated GI system: Predominantly nausea, vomiting, diarrhea, abdominal cramping without multi-system involvement 1
• No skin changes: Absence of urticaria, angioedema, or flushing 1
• No respiratory compromise: No throat tightness, stridor, or bronchospasm 1
• Stable vital signs: No hypotension or cardiovascular collapse (though dehydration may develop later) 1

Critical Diagnostic Algorithm

When Anaphylaxis is Highly Likely (Requires Immediate Epinephrine):

1. Acute onset with skin/mucosal involvement PLUS respiratory compromise OR hypotension 1

2. Two or more systems involved after allergen exposure: skin/mucosal changes, respiratory symptoms, hypotension, or persistent GI symptoms 1

3. Hypotension after known allergen exposure (systolic BP <90 mmHg or >30% decrease from baseline) 1

Common Pitfall to Avoid

Do not wait for "classic" multi-system presentation—anaphylaxis can present with predominantly GI symptoms initially, especially in children, but will rapidly progress to involve other systems. 1 One case report documented anaphylaxis presenting primarily as gastroenteritis with urticaria, hypotension, and syncope, confirmed by elevated serum tryptase. 2

Immediate Treatment Protocol

For Suspected Anaphylaxis (When in Doubt, Treat):

• First-line: Intramuscular epinephrine 0.3-0.5 mg (adults) or 0.01 mg/kg up to 0.3 mg (children) in the mid-outer thigh (vastus lateralis) 1, 3
• Activate emergency response system immediately 1
• Position patient supine with legs elevated (unless respiratory distress/vomiting present) 1
• Repeat epinephrine every 5-15 minutes if symptoms persist (7-18% of patients require second dose) 1
• Establish IV access and administer normal saline 1-2 L rapidly in adults (30 mL/kg in first hour for children) 1, 4
• Administer supplemental oxygen 1

Second-Line Adjunctive Therapy (Never Alone):

• H1-antihistamines: Diphenhydramine 25-50 mg IV/IM (1-2 mg/kg in children) 4
• H2-antihistamines: Ranitidine 50 mg IV (1 mg/kg in children) 4
• Bronchodilators: Albuterol 2.5-5 mg nebulized for refractory bronchospasm 4
• Corticosteroids: Consider for severe/prolonged reactions (not helpful acutely but may prevent biphasic reactions) 1, 4

Critical Clinical Pearls

Err on the side of epinephrine administration—it can be difficult to distinguish early anaphylaxis from other conditions, and delayed epinephrine is associated with increased hospitalization and death. 1, 5, 6 There are no absolute contraindications to epinephrine in anaphylaxis. 4

Never rely on antihistamines or corticosteroids alone—these are not substitutes for epinephrine and do not prevent fatal outcomes. 4, 5, 6

Observe for minimum 6 hours—biphasic reactions can occur, and there are no reliable predictors of which patients will experience protracted anaphylaxis. 1, 4

Provide epinephrine autoinjector and refer to allergist-immunologist after resolution for evaluation, trigger identification, and prevention strategies. 1, 4