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Acute rash

Medicine > Infection

Differential Diagnosis Schema 🧠

Infectious Causes

  • Viral Exanthems: Typically present in children; consider measles (preceded by Koplik spots), rubella (postauricular lymphadenopathy), roseola (high fever followed by rash).
  • Bacterial Infections: Scarlet fever (sandpaper rash, strawberry tongue), impetigo (honey-colored crusts), toxic shock syndrome (diffuse rash with desquamation).
  • Fungal Infections: Tinea corporis (ringworm; annular lesions with central clearing), candidiasis (moist, erythematous plaques).
  • Parasitic Infections: Scabies (burrows, intense pruritus, especially at night), cutaneous larva migrans (serpiginous track).

Inflammatory and Autoimmune Causes

  • Atopic Dermatitis: Chronic, relapsing condition with eczematous plaques, typically in flexural areas.
  • Psoriasis: Well-demarcated, erythematous plaques with silvery scales, often on extensor surfaces.
  • Systemic Lupus Erythematosus (SLE): Malar rash (butterfly distribution), photosensitivity, may accompany systemic symptoms.
  • Erythema Multiforme: Target lesions, often following infections or medications, may be associated with mucosal involvement.
  • Vasculitis: Palpable purpura, especially in Henoch-Schönlein purpura (associated with abdominal pain, haematuria).

Drug Reactions

  • Drug-Induced Exanthems: Widespread erythematous maculopapular rash, often 1-2 weeks after starting a new medication.
  • Steven-Johnson Syndrome (SJS) / Toxic Epidermal Necrolysis (TEN): Severe mucocutaneous reactions with widespread necrosis and detachment of the epidermis, usually triggered by drugs.
  • Fixed Drug Eruption: Localised, sharply demarcated red plaques that recur at the same site with re-exposure to the offending drug.

Vascular Causes

  • Petechiae and Purpura: Non-blanching rashes; consider meningococcal septicaemia, vasculitis, thrombocytopenia.
  • Erythema Nodosum: Tender, erythematous nodules usually on shins; associated with infections, sarcoidosis, inflammatory bowel disease.
  • Livedo Reticularis: Net-like pattern; consider vasculitis, antiphospholipid syndrome, cholesterol emboli.

Oncological Causes

  • Leukemia: May present with petechiae, purpura, ecchymoses, and non-specific erythematous rashes.
  • Mycosis Fungoides: The most common form of cutaneous T-cell lymphoma; presents with patchy, erythematous, scaly plaques, which may evolve into tumors.
  • Kaposi’s Sarcoma: Purple or dark brown plaques or nodules, often in patients with HIV/AIDS.

Key Points in History 🥼

Onset and Duration

  • Sudden onset: Suggestive of infectious or drug-related causes.
  • Chronic course: Could indicate autoimmune or inflammatory conditions.

Character of the Rash

  • Maculopapular: Often seen in viral exanthems and drug reactions.
  • Vesicular: Suggestive of herpesvirus infections, dermatitis herpetiformis.
  • Pustular: May indicate bacterial infections like impetigo or acne, or drug-induced.
  • Scaling: Often seen in psoriasis, seborrheic dermatitis, and fungal infections.
  • Purpuric: Suggests vasculitis, meningococcal septicaemia, thrombocytopenia.

Associated Symptoms

  • Fever: Common in infectious causes like viral exanthems, toxic shock syndrome.
  • Joint Pain: Consider vasculitis, systemic lupus erythematosus, parvovirus B19 infection.
  • Pruritus: Common in atopic dermatitis, scabies, urticaria.
  • Mucosal Involvement: Look for erythema multiforme, Stevens-Johnson syndrome, or viral infections.
  • Systemic Symptoms: Weight loss, night sweats, lymphadenopathy could suggest malignancy.

Background

  • Past Medical History: A history of autoimmune diseases might suggest a flare of lupus or psoriasis.
  • Drug History: Recent introduction of medications could indicate a drug reaction.
  • Family History: Atopy, psoriasis, or autoimmune conditions can have familial links.
  • Social History: Travel history may suggest tropical infections; occupation might point to contact dermatitis.

Possible Investigations 🌡️

Blood Tests

  • Full Blood Count (FBC): To assess for infection, anaemia, or thrombocytopenia.
  • C-Reactive Protein (CRP) / Erythrocyte Sedimentation Rate (ESR): Indicators of inflammation, possible infection.
  • Autoimmune Screen: ANA, ANCA, and other antibodies if an autoimmune cause is suspected.
  • Liver and Renal Function Tests: To check for systemic involvement or drug-induced injury.

Microbiological Tests

  • Swabs: Bacterial or viral culture if infection is suspected (e.g., swab from vesicles, pustules).
  • Serology: For viral infections such as EBV, CMV, HIV.
  • Skin Scraping: For fungal infections (e.g., KOH preparation for tinea).

Imaging

  • Chest X-ray: Useful if systemic involvement is suspected (e.g., sarcoidosis, tuberculosis).
  • Ultrasound: To assess for deep-seated infections, abscesses, or lymphadenopathy.

Skin Biopsy

  • Indicated when the diagnosis is unclear; can help differentiate between inflammatory, infectious, and neoplastic causes.

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Infection