What is the treatment for a delayed hypersensitivity skin reaction after a subcutaneous injection?

Treatment of Delayed Hypersensitivity Skin Reactions After Subcutaneous Injection

Topical corticosteroids are the first-line treatment for delayed hypersensitivity skin reactions after subcutaneous injections, supplemented with oral antihistamines for symptom relief. 1

Clinical Presentation and Mechanism

Delayed hypersensitivity reactions to subcutaneous injections:

• Typically appear 5-6 hours after injection
• Peak at 48-72 hours
• Characterized by induration, erythema, and sometimes vesiculation or necrosis
• Represent T-cell mediated (Type IV) hypersensitivity reactions 1

Treatment Algorithm

First-Line Management:

1. Clean the area

   • Wash with soap and water to prevent secondary infection 2

2. Topical treatments

   • Apply medium to high-potency topical corticosteroids to reduce inflammation
   • Apply cold compresses to reduce local pain and swelling 2

3. Systemic medications

   • Administer oral H1-antihistamines for symptom relief (particularly pruritus) 2
   • Use over-the-counter acetaminophen or NSAIDs for pain relief 2

For Moderate to Severe Reactions:

• Consider short course of oral corticosteroids for extensive reactions
• Topical lidocaine may help relieve local pain if skin is intact 2

For Infected Reactions:

• Antibiotics if signs of secondary infection develop (increasing erythema, warmth, purulence, lymphangitis)
• First-line options include:
  • Cephalexin 250 mg four times daily
  • Clindamycin 300-400 mg three times daily (for penicillin-allergic patients)
  • Amoxicillin-clavulanate 875/125 mg twice daily 2

Special Considerations

Arthus-Type Reactions

• Severe local reactions starting 2-8 hours after injection
• Often associated with fever and malaise
• May occur in persons who have received multiple previous injections
• These patients should not receive booster doses more frequently than every 10 years 1

Monitoring and Follow-up

• Monitor the injection site for 48-72 hours for signs of infection or worsening reaction 2
• Most delayed reactions resolve spontaneously within a few days 1

Prevention of Future Reactions

• For patients with history of severe reactions, consider:
  • Premedication with antihistamines before future injections
  • Referral to an allergist for evaluation if reactions are recurrent or severe 2
  • Skin testing to identify specific allergens if appropriate

Important Caveats

• Delayed hypersensitivity reactions are generally not severe and typically resolve without complications 1
• Distinguish from immediate hypersensitivity reactions (occurring within minutes to hours), which may require different management and could indicate anaphylaxis
• Biphasic reactions can occur, with symptoms recurring 2-24 hours after initial resolution, though these are typically less severe than the initial reaction 1
• Patients on beta-blockers may experience more severe and treatment-resistant reactions 1
• For patients with recurring severe delayed reactions, consider alternative medications or desensitization protocols under specialist supervision 1

When to Refer

• History of severe reactions
• Reactions that don't respond to standard treatment
• Systemic symptoms accompanying local reaction
• Uncertainty about whether reaction represents delayed hypersensitivity or another process