Rash

Basics

Description

Chief complaint in 2% of ED visits
Characteristics of the lesions:
- Diameter
- Flat or elevated
- Solid or fluid filled
- Color:
  - Hyperpigmented
  - Hypopigmented
  - Erythematous
- Shape:
  - Discoid
  - Oval
  - Annular
  - Arcuate
  - Targetoid
Morphology:
- Macule:
  - Nonpalpable areas of distinct coloration
- Papule:
  - Superficial, solid raised bump on the skin <5 mm
- Nodule:
  - Solid, raised lesion >5 mm seated in deeper layer of skin and tissue
- Vesiculobullous lesions:
  - Fluid-filled lesions due to disruption of epidermal/dermal integrity
  - Vesicles <5 mm filled with clear fluid
  - Bullae >5 mm filled with clear fluid
  - Pustules: Containing purulent fluid
- 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:
  - Annular or arcuate-shaped erythema
- Livedo racemosa:
  - Mottled or lace-like pattern of reddish–blue to violet discolorations on the skin
- Papulosquamous:
  - Papules with scaly desquamation
  - Lesions may be red and macular
  - Includes psoriasiform, pityriasiform, lichenoid, annular, and eczematous
- Granulomatous lesions:
  - Infiltrated papules and plaques without scales
  - “Apple jelly” appearance when pressed with glass slide
- Nodules:
  - Formed from prolonged inflammation, cysts, or infiltrative processes
Distribution:
- Peripheral vs central
- Symmetrical
- Discrete (separated from one another)
- Linear
- Dermatomal
- Serpiginous
- Reticular
- Grouped
- Photo distributed
Progression of the rash:
- Onset and duration
- Severity and flares
Red flags:
- Fever or hypotension
- Erythroderma
- Blistering/desquamation
- Purpura
- Severe pain
- Blackening of the skin
- Mucosal involvement

Etiology/Differential Diagnoses

Maculopapular rash:
- Viral exanthema
- COVID-19:
  - Chilblains
- Drug eruption
- Erythema multiforme
- Contact dermatitis
- Secondary syphilis
- Systemic lupus erythematosus (SLE)
Vesiculobullous lesions:
- Toxic epidermal necrolysis (TEN):
  - Mucosal and >30% body surface area involvement
- Stevens–Johnson syndrome (SJS):
  - Mucosal and ≤10% body surface area involvement
- Rhus dermatitis:
  - Poison ivy
  - Poison oak
  - Poison sumac
- Pemphigus vulgaris
- Bullous pemphigoid
- Dermatitis herpetiformis
- Disseminated herpes simplex
- Herpes zoster
- Varicella
- COVID-19
- Hand, foot, and mouth disease
- Smallpox
- Vaccinia
- Allergic contact dermatitis
- Epidermolysis bullosa
Purpura and petechiae (see “Purpura”):
- Meningococcemia
- Endocarditis
- Gonococcemia
- Purpura fulminans
- Disseminated intravascular coagulopathy
- Rocky Mountain Spotted Fever (RMSF):
- Ecthyma gangrenosum:
- Pseudomonas aeruginosa
- Hemorrhagic fevers:
  - Ebola virus disease
  - Marburg virus disease
  - Lassa fever
  - Crimean–Congo hemorrhagic fever
- Vasculitis:
  - Henoch–Schönlein purpura
  - SLE
Erythroderma:
- Toxic shock syndrome
- Drug reactions:
  - Anticonvulsants, antibiotics, NSAIDs
  - DRESS syndrome
- Psoriasis
- Atopic dermatitis
- Seborrheic dermatitis
- Cutaneous T-cell lymphoma:
  - Mycosis fungoides
  - Sézary syndrome
- Pityriasis rubra pilaris
- SLE
- Idiopathic
Erythematous rashes:
- Cellulitis
- Early necrotizing
- Urticaria:
  - Viral exanthema
  - Allergic reaction
  - Staphylococcal scalded skin syndrome (SSSS)
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 SJS
Livedo racemosa:
- Vasculitis
- COVID-19
- Sneddon syndrome
- SLE
- Cholesterol embolism 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