Erythema Migrans vs. Allergic Reaction: A Diagnostic Dilemma
Case at a Glance
A 22-year-old female with a history of chronic migraines presented with concerns about a evolving skin lesion on her lower extremity, questioning whether it represented erythema migrans indicative of Lyme disease.
Patient's Story
The patient noticed a small, intensely red spot (few millimeters) on her leg approximately 8-10 days prior to presentation. The lesion was initially surrounded by a pink halo, with the entire area measuring approximately 3 cm in diameter. Over 1-2 days, the lesion evolved as the central red coloration appeared to expand into the previously pink peripheral area while maintaining similar overall dimensions. The affected area felt indurated compared to surrounding healthy skin and was associated with significant pruritus, which extended to the surrounding normal-appearing skin. After 4 days, the lesion began to fade in a centripetal pattern - the original central red spot remained unchanged while the peripheral area that was initially pink began to lighten from the center outward. At presentation, the small central red spot remained visible with the surrounding area appearing pinkish-brown, and the pruritus had resolved. The patient denied any constitutional symptoms including fever, fatigue, or flu-like illness. She had self-treated with topical hydrocortisone cream.
Initial Assessment
The patient appeared well with no acute distress. Vital signs were stable. Physical examination revealed a resolving skin lesion on the lower extremity with central erythema and peripheral hyperpigmentation consistent with her description. No lymphadenopathy was noted. Neurological examination was unremarkable. The patient's medical history was significant for chronic migraines managed with pregabalin.
The Diagnostic Journey
The clinical presentation raised suspicion for erythema migrans, the characteristic rash of early Lyme disease, given the initial target-like appearance and evolution pattern. However, the relatively small size, rapid resolution, and absence of systemic symptoms also supported the possibility of an allergic reaction to an arthropod bite. The patient's concern about potential neurological manifestations of Lyme disease prompted consideration of diagnostic testing.
Final Diagnosis
Early localized Lyme disease with erythema migrans could not be definitively ruled out based on clinical presentation alone. Differential diagnosis included allergic reaction to insect bite versus early Lyme disease.
Treatment Plan
Given the clinical uncertainty and patient concern, serologic testing for Lyme disease was recommended. The consulting physician explained that neurological manifestations of Lyme disease typically occur months after initial infection, not weeks. If Lyme disease were confirmed, treatment would consist of oral doxycycline for 10-21 days, which is highly effective for early localized disease.
Outcome and Follow-up
The patient was advised to monitor for any new symptoms and to return if the lesion worsened or if systemic symptoms developed. Follow-up was planned to discuss serologic results and determine if antibiotic therapy was indicated. Patient education was provided regarding the timeline of Lyme disease progression and the importance of early treatment if diagnosed.