What are the initial treatment recommendations for an itchy rash with various differential diagnoses?

Initial Treatment of Itchy Rash

For most itchy rashes, start with emollients applied at least once daily plus topical corticosteroids matched to severity and location, combined with oral antihistamines for symptomatic relief. 1

Immediate First-Line Management (All Patients)

• Apply emollients at least once daily regardless of the underlying cause, as this prevents xerosis which commonly triggers pruritus 1
• Avoid physical triggers including wool clothing, harsh soaps, and hot showers 1, 2
• Keep skin dry after bathing using clean, separate towels 1

Topical Corticosteroid Selection by Location and Severity

Mild Rash (<10% Body Surface Area)

• Apply Class I topical corticosteroids (clobetasol propionate, halobetasol propionate, or betamethasone dipropionate) to the body 2
• Use Class V/VI corticosteroids (hydrocortisone 2.5%, desonide, or aclometasone) on the face to avoid skin atrophy 2
• Apply no more than 3-4 times daily 3
• Continue immune checkpoint inhibitors or other causative medications if applicable 2

Moderate Rash (10-30% Body Surface Area)

• Use the same topical corticosteroid potency as mild rash (Class I for body, Class V/VI for face) 2
• Add oral antihistamines: cetirizine or loratadine 10 mg daily (non-sedating), or hydroxyzine 10-25 mg four times daily or at bedtime 2
• Consider non-urgent dermatology referral 2
• Continue causative medications with close monitoring 2

Severe Rash (>30% Body Surface Area)

• Hold any immune checkpoint inhibitors or causative medications 2
• Start systemic corticosteroids: prednisone 0.5-1 mg/kg/day (or equivalent methylprednisolone) until rash resolves to grade 1 or less 2
• Arrange same-day dermatology consultation 2
• Rule out systemic hypersensitivity with CBC with differential and comprehensive metabolic panel 2
• Continue topical corticosteroids and oral antihistamines as above 2

Oral Antihistamine Options

• Non-sedating options: cetirizine 10 mg daily or loratadine 10 mg daily 2
• Sedating option for nighttime pruritus: hydroxyzine 10-25 mg four times daily or at bedtime 2
• Large doses may be required in children 2

Application Frequency and Duration

• Apply topical corticosteroids once daily for potent preparations, as this is equally effective as twice daily application 4
• Maximum application frequency is 3-4 times daily for less potent preparations 3
• Limit treatment to 7 days maximum for high-potency topical corticosteroids 2
• For topical doxepin (if used for generalized pruritus of unknown origin), limit to 8 days maximum, 10% body surface area maximum, and 12 grams daily due to contact dermatitis and toxicity risks 1

Specific Differential Considerations

Atopic Dermatitis/Eczema

• Use moderate to high-potency topical corticosteroids as first-line treatment 1, 5
• Avoid systemic corticosteroids except for severe, refractory cases, as they can cause rebound phenomenon with marked worsening after cessation 2, 6
• Consider adding oral antibiotics (flucloxacillin or erythromycin) if secondary bacterial infection with Staphylococcus aureus is suspected 2

Contact Dermatitis/Drug Reactions

• For EGFR inhibitor-induced rash: continue medication if mild, add topical antibiotics (clindamycin 2% or metronidazole 0.75%) plus topical corticosteroids 2
• For moderate severity: add oral doxycycline 100 mg twice daily or minocycline 100 mg twice daily for at least 2 weeks 2

Pruritus Without Visible Rash

• Apply emollients with urea or polidocanol-containing lotions 2, 1
• Topical doxepin is the most evidence-based topical agent but must be limited as noted above 1
• Avoid crotamiton cream, topical capsaicin, and calamine lotion, as they lack efficacy 1

Fungal Infections (Tinea)

• Apply terbinafine 1% cream once daily for 1 week (94% mycological cure rate) 7
• Alternative: butenafine twice daily for 2 weeks or clotrimazole twice weekly for 4 weeks 7
• Completely dry affected areas after bathing and use separate clean towels 7

Critical Reassessment Point

Reevaluate after 2 weeks if no improvement or worsening occurs 2, 1. At this point:

• Advance to next treatment step (higher potency corticosteroids or systemic therapy) 2
• Investigate for underlying systemic disease 1
• Refer to dermatology if not already done 2

Common Pitfalls to Avoid

• Do not apply topical corticosteroids to sites of immediate-type allergy skin tests, as this provides no relief of itching 8
• Do not use systemic corticosteroids routinely for atopic dermatitis, as rebound phenomenon with severe exacerbation can occur after cessation 6
• Do not exceed 2 mg/kg/day methylprednisolone for severe cases, as higher doses provide no additional benefit 2
• Do not use high-potency topical corticosteroids on the face, as this causes skin atrophy 2
• Avoid hot showers and excessive soap use, which worsen xerosis 2, 1