A 36-year-old man comes to the emergency department after briefly losing consciousness while watching television half an hour ago. The patient had no preceding chest pain or shortness of breath, but he has been having recurrent palpitations over the past several days. Two weeks ago, he noticed a non-itchy rash on his thigh that he treated with an over-the-counter steroid cream. Medical history is otherwise unremarkable. There is no family history of heart disease or sudden cardiac death. The patient recently began working as a forest ranger in Pennsylvania. He does not use tobacco, alcohol, or illicit drugs. Blood pressure is 130/80 mm Hg and pulse is 46/min. The lungs are clear on auscultation and heart sounds are normal. The remainder of the examination shows no abnormalities. ECG reveals complete atrioventricular block. Echocardiography shows normal ventricular function with no major valvular disease. Which of the following is the most likely underlying cause of this patient's current condition?

Question

A 36-year-old man comes to the emergency department after briefly losing consciousness while watching television half an hour ago.  The patient had no preceding chest pain or shortness of breath, but he has been having recurrent palpitations over the past several days.  Two weeks ago, he noticed a non-itchy rash on his thigh that he treated with an over-the-counter steroid cream.  Medical history is otherwise unremarkable.  There is no family history of heart disease or sudden cardiac death.  The patient recently began working as a forest ranger in Pennsylvania.  He does not use tobacco, alcohol, or illicit drugs.  Blood pressure is 130/80 mm Hg and pulse is 46/min.  The lungs are clear on auscultation and heart sounds are normal.  The remainder of the examination shows no abnormalities.  ECG reveals complete atrioventricular block.  Echocardiography shows normal ventricular function with no major valvular disease.  Which of the following is the most likely underlying cause of this patient's current condition?

Quizplus · Accepted Answer

This patient most likely has early disseminated Lyme disease. Lyme disease is caused by the bacterial spirochete Borrelia burgdorferi, which is transmitted by the Ixodes tick that is endemic to the northeastern and parts of the midwestern United States. Most cases initially present with a non-itchy, bull's-eye-shaped rash, known as erythema migrans, at the site of infection. Left untreated for several weeks or more, a small percentage of those infected will develop cardiac involvement (Lyme carditis). Lyme carditis most commonly manifests with varying degrees of atrioventricular (AV) conduction block. Many patients remain asymptomatic; however, patients who develop complete AV block are likely to have symptoms of dyspnea, lightheadedness, or syncope. Treatment for symptomatic AV block due to Lyme carditis typically requires hospitalization with temporary pacing and the administration of intravenous antibiotics (ceftriaxone is the drug of choice). (Choice B) Granulomatous myocardial inflammation occurs in cardiac sarcoidosis, and AV conduction block is the most common disease manifestation. However, this patient's recent rash and residence in an endemic area make Lyme carditis more likely. (Choice C) A missense mutation affecting the cardiac sarcomere protein beta-myosin heavy chain is a common cause of hypertrophic cardiomyopathy. Myocardial disarray creates a propensity for ventricular arrhythmias, but AV conduction block is not typical. (Choice D) Congenital long QT syndrome results from a mutation affecting cardiomyocyte potassium channels. It can cause syncope or sudden cardiac death, typically via the triggering of torsades de pointes (polymorphic ventricular tachycardia). AV conduction block would be an unusual manifestation. (Choice E) Viral infection of the heart typically manifests as myocarditis. The most common symptomatic presentation is decompensated heart failure due to dilated cardiomyopathy. AV conduction block is not typical. Educational objective: Early disseminated Lyme disease can have cardiac involvement (Lyme carditis) that most commonly manifests with varying degrees of atrioventricular (AV) conduction block. Patients may be asymptomatic, but those with complete AV conduction block are likely to have dyspnea, lightheadedness, or syncope.

A 36-Year-Old Man Comes to the Emergency Department After Briefly