Erythema Migrans: Characteristic Skin Lesion of Lyme Disease
Erythema migrans (EM) is the pathognomonic skin lesion of early Lyme disease, appearing as an expanding erythematous lesion that develops at the site of a Borrelia burgdorferi-infected tick bite and should be at least 5 cm in diameter for a secure diagnosis. 1
Definition and Clinical Appearance
Erythema migrans is a round or oval, expanding erythematous skin lesion that develops at the site where B. burgdorferi is deposited by an Ixodes species tick. Key characteristics include:
- Typically appears 7-14 days (range: 3-30 days) after tick detachment 1
- Must reach ≥5 cm in diameter for definitive diagnosis 1
- Gradually expands over days to weeks 1
- Variable appearance:
Distinguishing Features
- Not pruritic: Unlike contact dermatitis, EM lesions with vesicles are not significantly itchy 1
- Not scaly: Unless long-standing and fading, or if topical corticosteroids have been applied 1
- Unusual locations: Often occurs at sites like axilla, popliteal fossa, and abdomen - locations unusual for bacterial cellulitis 1
- Size correlation with duration: EM size increases with duration, peaking at approximately 14 days 3, 4
- Gender differences: Males typically have larger lesions (average 2.18 cm larger than females) and are 65% less likely to have red (vs. blue/red) coloration 3
Differentiating from Tick Bite Hypersensitivity
An erythematous skin lesion present while a tick is still attached or appearing within 48 hours of detachment is more likely a tick bite hypersensitivity reaction rather than EM. Hypersensitivity reactions:
- Are usually <5 cm in diameter
- May have urticarial appearance
- Typically begin to disappear within 24-48 hours
- Do not expand over time (unlike EM) 1
To differentiate between the two, mark the borders of the skin lesion with ink and observe for 1-2 days without antibiotic therapy. EM will continue to expand. 1
Primary vs. Secondary Lesions
- Primary EM: Occurs at the site of the tick bite
- Secondary EM: Arises from hematogenous dissemination from the primary site
Associated Symptoms
Most patients with EM have systemic symptoms, including:
- Fatigue (54%)
- Arthralgia/myalgia (44%)
- Headache (42%)
- Fever and/or chills (39%)
- Stiff neck (35%)
- Anorexia (26%) 4, 5
Respiratory and gastrointestinal complaints are infrequent. Symptoms may begin before, during, or after resolution of the rash. 5
Diagnostic Approach
EM is primarily a clinical diagnosis:
- Visual inspection is the preferred diagnostic method 1, 6
- Serologic testing is too insensitive in the acute phase (first 2 weeks) to be helpful 1
- Patients should be treated based on clinical findings 1
- Laboratory abnormalities may include elevated liver function tests (37% of patients) 4
Clinical Pitfalls
Misdiagnosis: EM can be confused with other skin conditions including:
Atypical presentations: Vesicular forms of EM can be particularly challenging to diagnose and may lead to treatment delays 2
Central clearing misconception: Although the "bull's eye" appearance with central clearing is considered classic, it is often absent in U.S. patients 5, 2
Age-related variations: For every 10-year increase in age, the odds of central clearing decrease by 25% 3
Recall of tick bite: Only about 25% of patients recall a tick bite at the site of the primary lesion 4
EM remains the most reliable clinical marker for early Lyme disease, occurring in 60-80% of patients 1. Prompt recognition and appropriate treatment are essential to prevent progression to later stages of Lyme disease with potential neurologic, cardiac, and rheumatologic complications.