What are the potential causes and treatments for a chronic rash lasting months in an adult?

Chronic Rash Lasting Months in Adults: Differential Diagnosis and Management

A rash persisting for months in an adult most likely represents chronic urticaria, chronic eczematous dermatitis, or a systemic condition requiring investigation—particularly if accompanied by nocturnal predominance, systemic symptoms, or lack of response to initial therapy. 1, 2

Primary Dermatologic Causes

Chronic Urticaria

• Chronic ordinary urticaria (COU) is defined by wheals lasting more than 6 weeks, with individual lesions typically present for 2-24 hours 1
• At least 30% of chronic urticaria cases have an autoimmune etiology, often remaining idiopathic after excluding allergic, infectious, physical, and drug-related triggers 1
• Prognosis varies significantly: 50% of patients with wheals alone clear by 6 months, but over 50% with wheals plus angioedema have active disease beyond 5 years 1
• If individual wheals last longer than 24 hours, urticarial vasculitis must be excluded via skin biopsy 1

Chronic Eczematous Conditions

• Contact dermatitis, atopic eczema, and drug eruptions commonly present as chronic generalized rashes 3
• These conditions typically respond to emollients and topical corticosteroids 4

Red Flag Systemic Causes Requiring Investigation

Hematologic Malignancies

• Nocturnal pruritus with or without visible rash strongly suggests Hodgkin lymphoma or polycythemia vera 2
• Hodgkin lymphoma should be suspected when nocturnal pruritus occurs with weight loss, fever, night sweats, or palpable adenopathies 2
• Polycythemia vera characteristically presents with aquagenic pruritus (water-induced itching) 2

Inflammatory Myopathies

• Dermatomyositis presents with a persistent photosensitive rash including heliotrope rash, Gottron papules, and poikiloderma over weeks to months 1
• The rash may precede muscle weakness and can involve face, neck, torso, fingers, and extensor surfaces 1

Adult-Onset Still's Disease

• Presents with an evanescent salmon-pink maculopapular rash (72.7% of cases) accompanying high-spiking fevers 1
• The rash is typically found on proximal limbs and trunk, often mildly pruritic and confused with drug allergy 1
• A Koebner phenomenon may be present 1

Essential Diagnostic Workup

Initial Laboratory Investigations

• Complete blood count with peripheral smear to detect polycythemia vera, lymphoma, and other hematological causes (account for ~2% of generalized pruritus) 2
• Serum ferritin: 25% of patients with systemic pruritus respond to iron replacement; ferritin below normal range warrants therapeutic trial 2
• LDH and ESR for hematological disease screening 2
• If polycythemia suspected: JAK2 V617F mutation test (positive in up to 97% of polycythemia vera cases) 2

Critical History Elements

• Nocturnal predominance of symptoms (suggests hematologic malignancy) 2
• Water-induced pruritus (highly suggestive of polycythemia vera) 2
• Systemic symptoms: fever, night sweats, weight loss (suggests lymphoma or Still's disease) 1, 2, 5
• Complete medication review: drug-induced rash is common and reversible 2, 5
• Duration of individual lesions: <1 hour (physical urticaria), 2-24 hours (ordinary urticaria), >24 hours (urticarial vasculitis) 1

Treatment Algorithm

For Chronic Urticaria

1. First-line: Second-generation H1 antihistamines (fexofenadine 180mg or loratadine 10mg daily) 6, 4

   • Doses may be increased above manufacturer recommendations when benefits outweigh risks 1
   • Avoid sedating antihistamines except short-term due to dementia risk with prolonged use 2, 6, 4

2. Second-line combinations: Add H2 antihistamines, sedating antihistamines at night, or antileukotrienes for resistant cases 1

   • Antileukotrienes more likely to benefit aspirin-sensitive and ASST-positive COU 1

3. Short-course corticosteroids: Prednisolone 50mg daily for 3 days for acute exacerbations 1

   • Long-term oral corticosteroids should NOT be used except in selected cases under specialist supervision 1

4. Immunomodulating therapy: Ciclosporin 4mg/kg daily for severe autoimmune urticaria unresponsive to antihistamines, effective in two-thirds of patients 1

Symptomatic Management While Awaiting Diagnosis

• Emollients as foundation of all treatment 6, 4
• Topical doxepin (limited to 8 days, 10% body surface area) 6
• Mild-potency corticosteroids (hydrocortisone 1%) for intertriginous areas; avoid high-potency in groin due to atrophy risk 4
• Non-sedating antihistamines (fexofenadine or loratadine) for symptomatic relief 2, 6

Specialist Referral Indications

Hematology Referral

• Suspicion of polycythemia vera or lymphoma based on laboratory findings 2
• Palpable adenopathies or masses requiring biopsy 2

Dermatology Referral

• Rash persisting despite primary care management 4
• Diagnostic uncertainty about possible cutaneous lymphoma 2
• Need for skin biopsy when wheals last >24 hours (urticarial vasculitis) 1
• Consideration of phototherapy (NB-UVB), which is highly effective for many chronic pruritus types 6

Critical Pitfalls to Avoid

• Do not dismiss nocturnal pruritus without investigation—it may herald hematologic malignancy 2
• Do not use long-term sedating antihistamines due to dementia risk 2, 6, 4
• Do not apply high-potency topical steroids to intertriginous areas (groin, axillae) due to atrophy risk 4
• Do not continue empiric treatment indefinitely without reassessment—if no improvement after 2 weeks, escalate workup or refer 4
• Remember that ferritin may be falsely normal in iron deficiency with concurrent inflammation; check serum iron and TIBC if clinical suspicion remains 2