What is the possible diagnosis and treatment for itchy rash, facial swelling, and tearing eyes after experiencing a sting while swimming in the ocean?

Marine Envenomation with Delayed Allergic Reaction

This presentation is most consistent with a marine envenomation (likely jellyfish sting) with a delayed allergic reaction manifesting as facial angioedema and urticaria, requiring immediate antihistamine therapy and close monitoring for progression to anaphylaxis.

Immediate Assessment and Red Flags

Call emergency services immediately if the patient develops:

• Difficulty breathing or throat tightness
• Signs of shock (dizziness, rapid pulse, confusion)
• Severe facial swelling progressing to involve lips, tongue, or throat 1

The "heavy feeling" on the left side of the face suggests significant angioedema that could progress to airway compromise 1.

Most Likely Diagnosis

Marine envenomation (jellyfish sting) with systemic allergic reaction is the primary diagnosis based on:

• Ocean swimming exposure with immediate sting sensation on right arm 1
• Delayed onset of facial symptoms (itching, heat, heaviness) and rash on arms 1
• Tearing eyes suggesting histamine-mediated allergic response 2
• Timeline consistent with IgE-mediated reaction developing hours after initial exposure 1

Key differentiating feature: The progression from localized sting to facial symptoms indicates systemic venom absorption and allergic response, not just local envenomation 1.

Alternative Diagnoses to Consider

Swimmers' itch/cercarial dermatitis is less likely because:

• It typically presents as itchy maculopapular rash only at water contact sites 1
• Facial involvement and systemic symptoms are uncommon 1
• Incubation period is hours, but facial swelling is not characteristic 1

Insect sting hypersensitivity (from the evidence provided) does not fit because:

• The patient was swimming in ocean, not exposed to terrestrial insects 1
• The clinical pattern matches marine rather than insect envenomation 1

Immediate Treatment Protocol

First-Line Therapy

Administer oral antihistamines immediately:

• Fexofenadine 180 mg orally (preferred due to non-sedating properties) 2
• Alternative: Cetirizine 10 mg orally (may cause mild drowsiness but effective) 2
• Rationale: Second-generation antihistamines block histamine-mediated symptoms including facial swelling, itching, and tearing 2

Additional Symptomatic Treatment

For the arm sting site:

• Apply hot water immersion (40°C/104°F) or hot compress to the sting area for pain relief 1
• Remove any visible tentacle material by lifting/pulling without direct hand contact 1
• Rinse with seawater if tentacles remain 1

For facial symptoms:

• Cold compresses to face to reduce swelling and discomfort 3, 4
• Elevate head of bed to minimize facial edema 3

When to Escalate Treatment

Add oral corticosteroids if:

• Facial swelling is severe or rapidly progressive 3, 4
• Symptoms do not improve within 2-4 hours of antihistamine therapy 3
• Prednisone 40-60 mg orally as single dose 3

Prescribe epinephrine auto-injector if:

• Any signs of systemic reaction beyond localized swelling develop 1
• Patient has history of severe allergic reactions 1

Critical Pitfalls to Avoid

Do NOT mistake allergic swelling for bacterial cellulitis:

• The facial swelling and arm rash are IgE-mediated allergic inflammation, not infection 3
• Antibiotics are NOT indicated unless purulent drainage, fever, or progressive worsening beyond 48-72 hours occurs 3
• Common error: Prescribing antibiotics for allergic swelling leads to unnecessary medication exposure and delays appropriate anti-inflammatory treatment 3

Do NOT apply vinegar or other topical remedies to jellyfish stings:

• Hot water is the most effective first aid treatment 1
• Vinegar, baking soda, and meat tenderizer lack strong evidence 1

Observation Period and Follow-Up

Observe for 4-6 hours after initial treatment:

• Most severe systemic reactions occur soon after envenomation 1
• Monitor for progression of facial swelling, development of respiratory symptoms, or signs of anaphylaxis 1

Expected clinical course:

• Symptoms should peak within 24-48 hours and gradually resolve over 1 week 1
• Facial swelling typically improves within 24-48 hours with antihistamine therapy 3
• Arm rash may persist for several days but should not worsen 1, 3

Return immediately if:

• Facial swelling worsens or involves lips/tongue
• New difficulty breathing or swallowing develops
• Rash spreads extensively or becomes painful
• Fever develops (suggests secondary infection) 3

Long-Term Considerations

Allergy evaluation is NOT typically needed for marine envenomation unless:

• Severe systemic reaction occurred requiring epinephrine 1
• Patient has frequent ocean exposure and wants risk assessment 1

Prevention for future ocean swimming:

• Wear protective clothing in jellyfish-prone waters 1
• Avoid swimming during jellyfish season in affected areas 1
• Carry oral antihistamines when traveling to coastal areas 2