What is the likely diagnosis and emergency management for a patient with sudden facial erythema, dyspnea, vomiting, diarrhea, and leg mottling without any other rash or swelling?

Anaphylaxis: Immediate Life-Threatening Emergency

This patient is experiencing anaphylaxis and requires immediate intramuscular epinephrine 0.3-0.5 mg in the lateral thigh and emergency medical services activation without delay. 1, 2

Clinical Diagnosis

This presentation meets established diagnostic criteria for anaphylaxis with:

• Acute onset involving multiple organ systems including skin (facial erythema), respiratory (dyspnea), gastrointestinal (vomiting, diarrhea), and cardiovascular (leg mottling suggesting poor perfusion) 3

• Leg mottling is a critical sign of cardiovascular compromise indicating reduced perfusion and impending hemodynamic collapse, which can occur rapidly even without classic urticaria or angioedema 3

• The absence of urticaria or swelling does not exclude anaphylaxis—10-20% of anaphylactic reactions, including fatal cases, present without cutaneous manifestations beyond erythema 3

• Gastrointestinal symptoms (vomiting, diarrhea) occur in up to 40% of anaphylaxis cases and combined with respiratory and cardiovascular involvement confirms the diagnosis 3

Immediate Emergency Management

Epinephrine is the only first-line treatment and must not be delayed:

• Administer intramuscular epinephrine 0.3-0.5 mg (adults) or 0.01 mg/kg (children, max 0.3 mg) in the mid-outer thigh (vastus lateralis) immediately 2, 4

• Activate emergency medical services (911/EMS) simultaneously—do not wait to see if symptoms improve 1, 2

• Repeat epinephrine every 5-15 minutes if symptoms persist or worsen, as 7-18% of patients require multiple doses 2

• Position patient supine with legs elevated to improve venous return, given the mottling suggests cardiovascular compromise 3

Adjunctive Treatments (After Epinephrine)

• Administer supplemental high-flow oxygen for respiratory symptoms 2

• Establish IV access and give aggressive fluid resuscitation (1-2 liters normal saline rapidly in adults) for hypotension and poor perfusion indicated by leg mottling 3, 2

• Consider inhaled albuterol if bronchospasm persists after epinephrine 2

• H1 antihistamines may be given for cutaneous symptoms only—they are not primary treatment and should never delay epinephrine 2

Critical Pitfalls to Avoid

Delayed epinephrine administration is associated with increased mortality:

• Never administer antihistamines or corticosteroids before or instead of epinephrine—this is a common fatal error 2

• Never use subcutaneous epinephrine or inject in the arm—intramuscular lateral thigh administration is essential for rapid absorption 2

• Never wait to "see if symptoms improve" before giving epinephrine—anaphylaxis can progress to cardiovascular collapse within minutes, with up to 50% of intravascular fluid shifting to extravascular space in 10 minutes 3

• Do not be falsely reassured by absence of urticaria—hemodynamic collapse can occur rapidly with minimal cutaneous manifestations 3

Post-Emergency Care

• All patients require emergency department transfer by EMS with minimum 4-6 hour observation after symptom resolution due to biphasic reaction risk (occurring 8-12 hours later) 3, 2

• Extended observation or admission is required given the severity indicated by multi-system involvement and cardiovascular compromise 2

• Prescribe two epinephrine autoinjectors with hands-on training and provide written anaphylaxis emergency action plan 2

• Refer to allergist for trigger identification, as the history suggests possible bowel movement-related trigger (though this is unusual and requires investigation) 2