Key Teaching Points•Cardiac sarcoidosis is associated with a high risk of ventricular arrhythmias and worsening left ventricular ejection fraction. Prompt diagnosis (using 18FDG positron emission tomographic computed tomography scan/13NH3 imaging and endomyocardial biopsy) and treatment reduce disease progression and arrhythmia severity.•Lyme carditis is associated with varying degrees of atrioventricular (AV) conduction anomalies. These usually resolve with appropriate antibiotic therapy.•Several reports have associated Borrelia burgdorferi infection with sarcoidosis, based on detection of "Borrelia-like" organisms in biopsy samples and positive antibodies. Nonetheless, a causative role of B. burgdorferi in sarcoidosis has not been clearly established.•Cardiac sarcoidosis should be included in the differential diagnosis of AV conduction anomalies. •Cardiac sarcoidosis is associated with a high risk of ventricular arrhythmias and worsening left ventricular ejection fraction. Prompt diagnosis (using 18FDG positron emission tomographic computed tomography scan/13NH3 imaging and endomyocardial biopsy) and treatment reduce disease progression and arrhythmia severity.•Lyme carditis is associated with varying degrees of atrioventricular (AV) conduction anomalies. These usually resolve with appropriate antibiotic therapy.•Several reports have associated Borrelia burgdorferi infection with sarcoidosis, based on detection of "Borrelia-like" organisms in biopsy samples and positive antibodies. Nonetheless, a causative role of B. burgdorferi in sarcoidosis has not been clearly established.•Cardiac sarcoidosis should be included in the differential diagnosis of AV conduction anomalies.