What could cause a rash on the arm a week after a steroid (corticosteroid) injection treatment?

Arm Rash One Week After Steroid Injection

A rash appearing one week after a corticosteroid injection most likely represents a delayed hypersensitivity reaction (Type IV) to the steroid itself, though this is uncommon. While corticosteroids are typically used to treat allergic conditions, paradoxically they can themselves trigger allergic reactions in rare cases 1, 2.

Understanding the Timeline and Mechanism

• Delayed hypersensitivity reactions to injectable corticosteroids typically manifest 24-96 hours after administration, with the dermatitis often peaking at 72 hours 1.
• The one-week timeframe in your case falls within the extended window for Type IV delayed hypersensitivity reactions, which are T-cell mediated rather than IgE-mediated 1, 3.
• These reactions are more common than immediate (Type I) reactions to corticosteroids, though both types remain relatively rare overall 1, 2.

Clinical Presentation to Assess

Key features to evaluate include:

• Distribution pattern: The rash may appear as an acute eczematous dermatitis, maculopapular eruption, or even urticarial lesions localized to the injection site or more widespread 1, 3.
• Associated symptoms: Look for pruritus, erythema, or infiltrated skin changes 1.
• Systemic involvement: Check for fever, lymphadenopathy, or mucosal involvement that would suggest more serious conditions like DRESS syndrome 4.
• Histologic findings: If biopsied, delayed reactions may show leukocytoclastic vasculitis with eczematiform epidermal changes 5.

Differential Diagnosis to Exclude

Before attributing the rash to steroid allergy, rule out:

• Infection: Bacterial, viral, or fungal causes, particularly if there are pustules, yellow crusts, or discharge 4.
• Other medications: Any concurrent drugs started around the same time 4.
• Underlying systemic disease: Conditions that could manifest with cutaneous findings 4.

Management Approach

For a localized, mild rash without systemic symptoms:

• Topical emollients and mild-to-moderate strength topical corticosteroids can be used paradoxically, as the reaction is to the specific injectable steroid formulation, not necessarily all topical preparations 4.
• Oral antihistamines for symptomatic relief of pruritus 4.
• Monitor for progression: If the rash worsens, covers >10% body surface area, or develops systemic symptoms, escalate care 4.

For more extensive or symptomatic rashes:

• Consider systemic corticosteroids (0.5-1 mg/kg prednisone) if the reaction is severe, though this requires careful consideration given the suspected allergen 4.
• Dermatology consultation is warranted for grade 2 or higher reactions, particularly if diagnostic uncertainty exists 4.

Diagnostic Confirmation

If steroid allergy is suspected:

• Intradermal testing performed 6 weeks to 6 months after complete resolution can confirm the diagnosis, with readings at 24-72 hours for delayed reactions 4, 1.
• Patch testing is an alternative, particularly useful for identifying cross-reactivity patterns among different corticosteroid classes 4, 5.
• Testing can identify which specific corticosteroids to avoid and which alternatives may be safely used, as cross-reactivity is not universal 2, 5.

Critical Pitfalls to Avoid

• Do not assume all corticosteroids are safe or unsafe: Cross-reactivity varies significantly between different steroid classes (e.g., betamethasone may be tolerated when prednisolone causes reactions) 5.
• Avoid rechallenge without testing: Repeat exposure can cause more severe reactions 6.
• Do not dismiss worsening symptoms: If the patient develops mucosal involvement, skin sloughing, or systemic symptoms, this could represent life-threatening conditions like Stevens-Johnson syndrome or toxic epidermal necrolysis requiring immediate hospitalization 4.

Alternative Etiologies

Less likely but possible causes include:

• Granulomatous foreign-body reaction to the injected corticosteroid material itself, which can occur but typically presents differently 7.
• Nonspecific immune-mediated rash if the patient recently received other immunologic interventions, though the timing and isolated arm involvement make this less probable 4.