Diagnosis of Erythema MigransDiagnosis of Erythema Migrans

To the Editor: In their Rational Clinical Examination article, Drs Tibbles and Edlow¹ reviewed the accuracy of history and physical examination findings for the diagnosis of erythema migrans. There are 2 issues that are important to consider.

First, Table 2 states that erythema migrans is accompanied by mild pain or itch. However, in most cases erythema migrans is entirely asymptomatic.² Pruritus, when present, may be due to a hypersensitivity reaction to the tick bite, irrespective of the presence of borrelial pathogens.³ This pruritus lasts about a week, resolving around the time that most cases of erythema migrans begin to appear.² Moreover, a study conducted in Rhode Island suggests that patients with significant pruritus after a tick bite are less likely to develop Lyme disease than those who experience mild or no itch.³ An explanation offered is that individuals with prominent itch at the bite site are more likely to be aware of the affected area, leading to early removal of the tick, thus preventing transmission of the pathogen. Also, the presence of an itchy hypersensitivity response indicates prior exposure to tick bites. Individuals with multiple previous tick bites may have acquired a protective immune response (perhaps to tick salivary antigens) that thwarts disease transmission.³

Second, the entry for granuloma annulare in Table 2 of the study may not be correct. This condition is almost never scaly; indeed, the presence of scale generally excludes the diagnosis of granuloma annulare.⁴ Thus, granuloma annular is even more similar to erythema migrans than the table suggests.