Make the Diagnosis: Erythema Migrans
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 |Favorite Table|Download (.pdf) Table 61-1 Some Diagnostic Mimics of Erythema Migransa
|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 ||Individual ...|
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