Introduction: Lyme disease is a tick-borne illness that is most commonly caused by Borrelia burgdorferi and transmitted by the Ixodes tick. Common manifestations are fevers, headache, arthralgia, erythema migrans, and, if left untreated, can progress to neuropathy and carditis. Lyme carditis most commonly presents with high degree atrioventricular block, however, may present with other arrhythmias.

Case presentation: We present a case of a 70 year old male with past medical history of hypertension and polycythemia vera who presented to the hospital with lightheadedness and bradycardia following 2 weeks of fever. His electrocardiogram showed atrial fibrillation with regularized conduction at 40 bpm suggesting complete heat block and a junctional escape rhythm. Lyme antibody screen index and lyme IgM and IgG were positive. His echocardiogram showed a normal ejection fraction with moderate pulmonary hypertension. He was started on antibiotics and anticoagulation. Later, given the unresolved atrial fibrillation, the patient underwent cardioversion to sinus rhythm.

Discussion: Lyme carditis is a rare but potentially fatal complication of Lyme disease. It may rarely present with atrial fibrillation. In the appropriate clinical context, testing for Lyme disease may be indicated in the setting of atrial fibrillation and evidence of AV block. Further investigation is necessary to determine if patients who develop atrial fibrillation in the setting of Lyme carditis can stop their anticoagulation once the carditis has resolved and sinus rhythm is maintained.

Conclusion: Awareness of atypical cardiac presentations of Lyme carditis, such as atrial fibrillation, may help minimize misdiagnosis and facilitate early treatment.