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Erythema Migrans

David L. Simel; Carrie D. Tibbles; Jonathan A. Edlow

Make the Diagnosis: Erythema Migrans

Prior Probability

Erythema migrans is a macular or popular erythematous skin lesion that evolves over days or weeks and expands to form a large, red lesion. The US Centers for Disease Control and Prevention (CDC) has defined additional size criteria that are to be used for surveillance monitoring (when solitary, the lesion must be at least 5 cm in diameter).1 The European case definition is similar, but it does not specify a particular size. The lesion is described as having a bluish-red hue with a distinct advancing edge that is more intensely colored. Because the clinical evaluation itself defines the presence of erythema migrans, the prior probability is not known. Complicating these considerations is that erythema migrans differs geographically.

Population for Whom Erythema Migrans Should Be Considered

While a rash consistent with erythema migrans is a clinical observation, there are many mimics of the lesion (see Table 61-1). The factors that should be considered are the location (tends to be in skin creases, under clothing straps, or on the trunk), evolution (evolves over days or weeks and increases 2-3 cm per day), seasonal tendency (summer months), and associated symptoms. When a patient reports a tick bite, inquire about the type of tick (see Figure 61-1).

Table 61-1Some Diagnostic Mimics of Erythema Migransa
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Table 61-1 Some Diagnostic Mimics of Erythema Migransa
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Diagnosis Morphology Location Size Evolution Seasonal Tendency Associated Symptomsb
Erythema migrans See text Skin creases, clothing straps, central Large, 5-70 cm (median, 16 cm) Evolves over days to weeks; 2-3 cm/d Yes Mild systemic symptoms, mild pain or itch
Tinea (ringworm) Ringlike, may have satellite lesion, scaling, especially peripheral is common Variable Variable Variable progression over days to weeks No Prominent itch
Nummular eczema Coin-shaped, initially weepy with later tendency to scale and crust, sharply demarcated

Symmetric, usually acral lesions

Can involve trunk

 Typically 2-10 cm Often red, weepy, evolves to ring shape No Usually itchy, sometimes burning
Granuloma annulare

Tend to scale peripherally

Can have central clearing

 Acral, especially dorsal hand and feet 1-5 cm Fixed (or slow increase) over weeks to months No No
Hypersensitivity to insect or tick bite Can be uniform erythema, often with tick still attached Variable at site of bite Small, usually several cm Develops rapidly after bite and usually does not evolve much over days Yes Often itchy, may be history of insect bite
Bacterial cellulitis Usually a homogeneous erythema, tender and warm to touch Typically acral at site of skin trauma Variable but rarely large except on legs Typically grows more rapidly than erythema migrans No Systemic symptoms, painful
Contact dermatitis Shape often suggests the diagnosis, often a prominent vesicular or bullous component Variable Variable Slow progression No Very itchy
Urticaria Raised, multiple, often serpiginous circumference Variable

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