Erythema chronicum migrans is a cutaneous manifestation of Lyme disease, a spirochetal infection caused by Borrelia burgdorferi after a tick bite. It is most commonly described as a slowly expanding circular erythematous "target" lesion, although various atypical presentations are reported. For this reason, a detailed inspection of the skin and a complete patient history are most important parts of workup when it comes to identifying this skin lesion.
Lyme disease is a tick-borne infection caused by Borrelia burgdorferi, a spirochetal bacterial pathogen  . In approximately 80% of cases, cutaneous manifestations of this infection appear, predominantly in the form of erythema chronicum migrans (or erythema migrans), a pathognomonic finding of Lyme disease. It is described as an erythematous papule or macule at the site of the tick bite    . The incubation period is variable (3-30 days), but in the majority of cases, erythema chronicum migrans is seen after 1-2 weeks and can occur on virtually any site of the body   . However, the waist, the extremities, the groins, the back, and the head and neck in the pediatric population are most frequent sites . The progression of the erythematous macule, which may be asymptomatic, painful, or pruritic, eventually leads to a "target" or "bull's eye" lesion (as a central clearing around the center of the erythematous lesion is observed), with a diameter of > 5 cm, although a diameter as large as 60 cm has been reported  . Numerous reports have confirmed that up to 30% of lesions exhibit significant variations    . Firstly, multiple lesions can develop, the reason being the dissemination of the infection . Secondly, an increasing number of cases present only with profound erythema and the absence of central clearing, thus the typical "target" lesion is not a mandatory finding in Lyme disease  . Moreover, necrosis of the lesion, central hemorrhage, and formation of bullae are less common, but still possible variants of erythema chronicum migrans  . It must be noted that numerous symptoms of Lyme disease can accompany erythema chronicum migrans, such as neurological deficits, fever, neck stiffness (suggesting meningitis), cardiac conduction abnormalities and arthritis (a sign of disseminated disease)   .
The importance of recognizing erythema chronicum migrans lies in the fact that up to 60% of patients have negative serology tests for B. burgdorferi in the first several weeks of the infection . For this reason, a meticulous physical examination supported by data obtained during patient history is the pivotal step in confirming Lyme disease and observing its hallmark lesion . Physicians must inquire whether patients reside in or have recently visited at-risk areas, and note the signs and symptoms that possibly suggest an infectious etiology, while the course and progression of the skin lesion should also be discussed with the patient (if the patient had noticed the lesion etc.). The entire body should be examined in order to exclude multiple erythema chronicum migrans, and a close inspection, as well as palpation of the lesion, is vital for raising clinical suspicion of Lyme disease as a potential diagnosis     . Physicians must be aware of the incubation period of erythema chronicum migrans, its variable presentation, and the broad differential diagnosis . In all patients in whom this lesion is suspected, microbiological investigation to confirm Lyme disease is mandatory. As serologic testing may be initially negative, blood cultures, biopsy samples of the lesion and polymerase chain reaction (PCR) testing can be implemented    . But because of their limited use and sensitivity/specificity in the absence of erythema chronicum migrans, clinical assessment remains the most important part of Lyme disease workup.