Rash

Basics

Description

Use appropriate terminology to categorize the unknown rash:
- Basic morphology:
  - Diameter
  - Flat or elevated
  - Solid or fluid filled
- Color:
  - Hyperpigmented
  - Hypopigmented
  - Erythematous
- Shape:
  - Discoid
  - Oval
  - Annular
  - Arcuate
  - Targetoid
- Distribution:
  - Peripheral vs. central
  - Symmetrical
  - Discrete (separated from one another)
  - Linear
  - Dermatomal
  - Serpiginous
  - Reticular
  - Grouped
  - Photodistributed
- Associated symptoms:
  - Itch
  - Pain
  - Tenderness
  - Burning
  - Systemic symptoms
- Evolution of the rash:
  - Onset and duration
  - Severity and flares
Morphology:
- Vesiculobullous lesions:
  - Fluid-filled swelling of the skin or sloughing due to disruption of epidermal/dermal integrity
- Purpura and petechiae:
  - Failure of normal vascular integrity/hemostatic mechanisms
  - Do not blanch on palpation
- Erythema:
  - Erythroderma when covering ≥90% of the skin surface
  - Vascular dilatation of the superficial vessels leading to red macular lesions
  - Blanches on palpation
- Figurate erythema:
  - Erythema classified by its particular annular or arcuate shape
- Papulosquamous:
  - Papules and scaly desquamation of the skin
  - Lesions may also be red and macular
  - Classified into psoriasiform, pityriasiform, lichenoid, annular, and eczematous
- Granulomatous lesions:
  - Infiltrated papules and plaques, lack of scales
  - “Apple jelly” appearance when pressed with glass slide
- Nodules:
  - Secondary to prolonged inflammatory response, cyst, or infiltrative process
Warning signs of a potential emergency condition:
- Erythroderma
- Blistering/desquamation
- Purpura
- Skin pain with systemic symptoms

Etiology/Differential Diagnoses

Vesiculobullous lesions:
- Toxic epidermal necrolysis (mucosal and >30% body surface area involvement)
- Stevens–Johnson syndrome (mucosal and ≤10% body surface area involvement)
- Pemphigus vulgaris
- Bullous pemphigoid
- Disseminated herpes simplex
- Herpes zoster
- Varicella
- Smallpox
- Vaccinia
- Allergic contact dermatitis
Purpura and petechiae:
- Meningococcemia
- Gonococcemia
- Purpura fulminans/disseminated intravascular coagulopathy (DIC)
- Rocky Mountain spotted fever (RMSF):
  - Pronounced prodrome of fever, headache, myalgia, rash, peripheral moves to palms/soles
- Ecthyma gangrenosum:
  - Pseudomonas infections in critically ill and immunocompromised patients
- Babesiosis: Similar to RMSF, rash less often, frequent coinfection with Lyme
- Vasculitis
- Multiple systemic illnesses (see “Purpura”)
Erythroderma:
- Toxic shock syndrome
- Drug-induced
- Psoriasis
- Seborrheic dermatitis
- Mycosis fungoides
- Lymphoma of the skin
Erythematous rashes:
- Localized:
  - Cellulitis
  - Early necrotizing fasciitis with concomitant skin pain
  - Urticaria
- Diffuse:
  - Staphylococcal scalded skin syndrome
  - Toxic shock syndrome
  - Drug-induced, including drug reaction with eosinophilia and systemic symptoms (DRESS)
  - Viral exanthema
  - Allergic reaction from drugs, food, infection, pressure, heat, or cold
Figurate erythema:
- Erythema chronicum migrans (large red ring that arises around a tick bite):
  - Lyme disease
- Erythema multiforme:
  - Mycoplasma pneumoniae
  - Herpes simplex
  - Drug reaction leading to Steven–Johnson syndrome
Papulosquamous:
- Psoriasiform:
  - Psoriasis
  - Seborrheic dermatitis
  - Drug-induced
- Pityriasiform:
  - Pityriasis rosea
  - Secondary syphilis
  - Tinea versicolor
- Lichenoid:
  - Lichen planus
  - Drug-induced
- Annular:
  - Tinea
  - Figurate erythema (see below)
- Eczematous:
  - Atopic dermatitis
  - Allergic contact dermatitis
  - Irritant dermatitis
Nodules:
- Granulomatous disease:
  - Sarcoid
  - Granuloma annulare
  - Infectious: Leprosy, tuberculosis, deep fungal infection
- Panniculitis:
  - Erythema nodosum
- Lymphoma of the skin
- Cysts
- Tumors and metastatic disease

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