Erythema Chronicum Migrans


Erythema Chronicum Migrans, Erythema Migrans

  • See Also
  • Definition
  1. Pathognomonic bullseye rash of early Lyme Disease
  • Epidemiology
  1. Now thought to occur in 90% of Lyme Disease cases
    1. Previously thought to be seen in only 50-60%
    2. Lower detection due to ECM rash that is missed
    3. Reinforces need for very careful skin exam
  2. Appears in 86-100% of symptomatic Lyme Disease cases
  • Timing
  1. Occurs 3-30 days (median 7 days) after deer Tick Bite
  • Distribution
  1. Localized rash at deer Tick Bite site
  • Appearance
  1. Initial
    1. Small red painless Papule at Tick Bite site
    2. Rash does not itch
    3. Expands centrifugally over days or weeks
  2. Later: Annular Lesion
    1. Circular, oval or triangular
    2. Central clearing or darkening with induration (bullseye or target appearance)
      1. Occurs in only 19-33% of cases
    3. Macular, papular or vesicular
    4. Rapid and prolonged expansion (unique to Erythema Migrans)
    5. Expands to >5 cm diameter
      1. Typically 20-30 cm (up to 70 cm) diameter
      2. Contrast with Insect Bite reaction
  3. Resolution
    1. Rash resolves in weeks if not treated
  4. Variations
    1. Multiple lesions (hematogenous spread of infection) in 10-20% of cases
    2. Central vessicle or Pustules (5% of cases)
    3. May appear purpuric on legs
  • Symptoms
  • Differential Diagnosis
  1. See Annular Lesion
  2. Localized hypersensivity reaction to Insect Bite
    1. Typically <2.5 to 5 cm in size
    2. Course
      1. Onset while tick still attached
      2. Starts to resolve within 48 hours
    3. Distinguish from Erythema Migrans
      1. Mark margins
      2. Observe for 48 hours off antibiotics
      3. Improvement suggests not a Lyme Disease rash
  3. Contact Dermatitis
    1. Pruritus (does not occur in Erythema Migrans)
  4. Cellulitis
    1. Distribution may help distinguish from EM rash
    2. Cellulitis uncommon in typical eyrthema migrans sites: Axilla, popliteal fossa, Abdomen, back and groin
  5. Erythema Multiforme
    1. Multiple target lesions each typically <2 cm diameter
  6. Granuloma Annulare
    1. Scaling erythematous lesions with central clearing (esp. hands and feet)
  7. Methicillin Resistant Staphylococcus Aureus (MRSA)
    1. Lesions with necrotic central eschar
  8. Tinea Corporis
  9. Urticaria
  • Interpretation
  • Classic Erythema Migrans rash
  1. Erythema Migrans is diagnostic for Lyme Disease
    1. Allows diagnosis without confirmatory testing
    2. Erythema Migrans is the only CDC-supported non-laboratory finding for definitive Lyme Disease diagnosis