What causes a rash?

Common Causes of Rashes

Rashes are most commonly caused by dermatological conditions, systemic diseases, infections, allergic reactions, and medication side effects, with proper diagnosis requiring careful evaluation of the rash's morphology, distribution, and associated symptoms.

Primary Dermatological Causes

Inflammatory Skin Conditions

• Eczema/atopic dermatitis: Characterized by chronic pruritus, erythema, xerotic scaling, lichenification, and hyperpigmentation 1
• Seborrheic dermatitis: Presents with greasy yellowish scaling and itching, particularly affecting the ears, scalp, and central face 1
• Psoriasis: Well-demarcated plaques with silvery scale 1
• Contact dermatitis: Can be either irritant or allergic, resulting from exposure to metals, chemicals, plastics, rubber, or drugs 1

Urticaria (Hives)

• Presents as raised, erythematous, pruritic wheals that can occur anywhere on the body
• May be acute (lasting <6 weeks) or chronic (≥6 weeks) 2
• Often idiopathic but can be triggered by:
  • Foods or food components
  • Medications (especially NSAIDs)
  • Infections
  • Physical stimuli (heat, cold, pressure, vibration) 2

Systemic Disease-Related Rashes

Hematological Causes

• Iron deficiency: Can cause generalized pruritus without visible rash 2
• Iron overload: Associated with generalized pruritus in hemochromatosis 2
• Polycythemia vera: Characterized by aquagenic pruritus (itching triggered by water contact) 2
• Lymphoma: Particularly Hodgkin lymphoma, can present with itching, weight loss, fevers, and night sweats 2

Other Systemic Conditions

• Uremic pruritus: Common in end-stage renal disease or chronic kidney disease 1
• Cholestatic liver disease: Associated with generalized pruritus 1
• Thyroid dysfunction: Both hyperthyroidism and hypothyroidism can cause pruritus 1
• Diabetes mellitus: Can cause regional pruritus affecting the trunk due to diabetic neuropathy 1

Infectious Causes

Viral Infections

• Roseola: Characterized by high fever followed by rash after fever resolution 3
• Erythema infectiosum (Fifth disease): Presents with "slapped cheek" facial rash following viral prodrome 3

Bacterial Infections

• Scarlet fever: Rash develops on upper trunk, then spreads throughout body, sparing palms and soles 3
• Impetigo: Superficial bacterial infection commonly affecting face and extremities 3
• Rocky Mountain Spotted Fever: Begins as small, blanching pink macules on extremities that evolve to maculopapules and potentially petechiae 2

Fungal Infections

• Tinea infections: Common fungal skin infections affecting scalp, body, groin, feet, hands, or nails 3

Allergic and Drug-Induced Rashes

• Drug reactions: Various medications can cause different types of rashes
• Allergic contact dermatitis: Reaction to allergens like metals, chemicals, plastics, rubber, or drugs 1
• Hydrocortisone-responsive conditions: Minor skin irritations, inflammation, and rashes due to various causes including poison ivy/oak/sumac, insect bites, detergents, jewelry, cosmetics, and soaps 4

Diagnostic Approach

Key Factors in Rash Evaluation

1. Morphology: Petechial/purpuric, erythematous, maculopapular, or vesiculobullous 5
2. Distribution: Location and pattern of spread
3. Associated symptoms: Presence of fever, pruritus, or systemic symptoms
4. Timing and progression: Acute vs. chronic, evolution of lesions

Laboratory Testing

• For generalized pruritus without rash: Check full blood count and ferritin levels 2
• For suspected systemic causes: Consider targeted testing based on clinical suspicion:
  • Thyroid function tests if thyroid disease is suspected
  • Tissue transglutaminase antibodies if celiac disease is suspected 1

Management Principles

• Identify and treat underlying cause when possible
• Symptomatic relief:
  • Topical treatments including emollients for dry skin
  • Topical anti-inflammatory agents for inflammatory conditions
  • Antihistamines for urticarial rashes 1
• Avoid aggravating factors: NSAIDs, alcohol, overheating, and stress can worsen certain rashes 1

Remember that accurate diagnosis is critical, as many different conditions can produce similar-appearing rashes, and a single condition can result in different rash appearances 6. When the diagnosis remains unclear, consider further diagnostic testing or dermatology referral, especially if the rash is severe, persistent, or associated with systemic symptoms.