What causes a rash to appear only when scratching the skin?

Dermographism (Dermatographic Urticaria)

A rash that appears only when you scratch the skin is dermographism, a physical urticaria caused by mechanical pressure and shearing forces that trigger mast cell degranulation and histamine release, producing linear wheals along the scratched area. 1, 2

Pathophysiology

• Mechanical stimulation of the skin activates mast cells, which release histamine and pro-inflammatory mediators, forming wheals along the area of friction or scratching 1
• The condition results from a combination of pressure and shearing forces applied to the skin 3
• This is distinct from spontaneous chronic urticaria, where lesions appear without external triggers 4

Clinical Characteristics

• Individual wheals typically last 2-24 hours before resolving without scarring 5
• The wheals appear as linear, raised, red marks that follow the pattern of scratching or friction 1
• Patients may experience itching with or without angioedema 1
• Dermographism is the most common form of chronic inducible urticaria and accounts for approximately 20-30% of all chronic urticaria cases 6, 2

Diagnosis

• The diagnosis is primarily clinical, based on medical history and a provocation test (dermatographic test) where firm stroking of the skin reproduces the characteristic linear wheals 1, 5
• No laboratory investigations are required for typical cases 5
• The Urticaria Control Test (UCT), Chronic Urticaria Quality of Life questionnaire (CU-Q2oL), and Dermatology Life Quality Index (DLQI) can evaluate disease control 1

Treatment Approach

First-line treatment is non-sedating second-generation H1-antihistamines, which have proven effective and well-tolerated in all studies 2

Treatment Algorithm:

1. Start with standard-dose second-generation H1-antihistamines (e.g., cetirizine, loratadine, fexofenadine) 2

2. If inadequate response, increase the dose up to 4-fold the standard dose, extrapolating from chronic spontaneous urticaria evidence 2

3. Consider adding an H2-antihistamine to the H1-antihistamine regimen if symptoms persist, as combination therapy increases efficacy 2

4. For refractory cases, add omalizumab (off-label use), which has shown improvement in dermographism 1, 2

5. Avoid triggers by minimizing scratching, tight clothing, and mechanical friction 1

Important Caveats:

• First-generation H1-antihistamines show variable efficacy and significant side effects (sedation), making them less desirable 2
• H2-antihistamine monotherapy is not effective and should only be used in combination with H1-antihistamines 2
• There is currently no evidence supporting updosing of second-generation H1-antihistamines specifically for dermographism, but this approach is reasonable based on chronic spontaneous urticaria data 2
• Avoid topical steroids, salves, creams, or ointments on active lesions, as these are unnecessary and potentially problematic 7