Background

LD is caused by members of the Borrelia burgdorferi sensu lato complex transmitted by Ixodes scapularis and I pacificus in the U.S. The estimated annual incidence in the U.S. is 300,000. 95% of reported cases come from 12 Northeastern and 2 North Midwestern states [1]. LC is a rarer manifestation of LD [2]. LC may present with or without other manifestations of LD and could involve all layers of the heart: myocardium [3,4,5,6,7], pericardium [8, 9], endocardium [10,11,12,13], or pancarditis [4, 14, 15]. Myocarditis could be focal [6, 16] or diffuse [14]. Any part of the conducting system could be affected, and conduction blocks of varying degrees (Table 1) are the most common. The incidence of LC varies from 0.2–10% in different reports [2, 17,18,19,20]. The actual incidence may be higher. A pediatric study of patients with Lyme disease found ECG changes in 29% [21]. LC could result in acute heart failure [22, 23], cardiogenic shock [23, 24], cardiac arrest [25,26,27], or sudden cardiac death [15, 28, 29]. We report a case of exercise induced progression of first-degree heart block to high degree AV block, with complete resolution on cessation of exercise, which could be reproduced. The patient made a complete recovery after treatment with intravenous ceftriaxone and doxycycline.

Table 1 ECG changes reported in Lyme carditis
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Case presentation

A 37-year-old male with past medical history significant for asthma, anxiety, and former tobacco use, presented to our emergency department after experiencing two episodes of syncope while at work. He was employed outdoors in a heavy manual labor industry. He and his co-workers have been frequently bitten by ticks at work in the past. Initial vital signs on admission were significant for bradycardia, with a heart rate of 57 bpm, and the ECG showed sinus bradycardia with first degree AV block, with a PR interval of 480 ms (NL 120–200 ms) (Fig. 1). Physical exam was unremarkable, except for hypopigmentation of fingers. Serum ALT level was elevated 115 (NL 12–78). Other labs on admission were all within normal including serum troponin. Further workup included a normal CT scan of head, a vascular study of the carotid vessels that showed minor right sided carotid stenosis of < 50%, and an echocardiogram that was unremarkable,except for mildly increased LV wall thickness with an EF of 60%. An exercise stress test done by the cardiologist, was terminated early. The patient developed dyspnea, and his ECG demonstrated progression of first-degree AV block to high degree AV block (Fig. 2). Once back at rest, the patient’s high degree AV block reverted to first degree AV block. He had a similar episode while walking in the hallway wearing a Holter monitor, on day 5, also reversible with rest. He was transferred to the critical care unit for close monitoring and treated with ceftriaxone 2G iv once daily and doxycycline 100 mg orally twice daily. The first-degree AV block improved with a gradual decrease in the PR interval (Table 2). His Lyme serology (Western Blot) was strongly positive (Table 3).

Fig. 1
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Admission ECG showing first degree heart block (PR = 480 ms)

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Fig. 2
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High degree A-V block during exercise

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Table 2 PR interval decrease with treatment
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Table 3 Lyme serology results
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The heart block significantly improved to 270 ms by day 7 of treatment. He was discharged and continued outpatient IV Ceftriaxone for 3 weeks. After completing treatment, the patient had a normal ECG with PR interval of 178 (Fig. 3) on day 16 and an uneventful exercise stress test. He returned towork without limitations, doing manual labor. He has been symptom free for 2 years. Now he uses tick-repellents at work.

Fig. 3
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ECG after treatment showing normal PR interval (PR = 178 ms)

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Discussion and conclusions

Cardiac involvement has been reported with B.burgdorferi ss (US cases), B garinii, B afzelli [12] and B bissettii [10]. Patients with CHB (Complete heart block) due to LC may have erythema migrans [32, 36, 37, 47, 48, 58] or other manifestations such as joint involvement [8]. Basic investigations include the history of possible tick exposure, laboratory testing, a 12-channel electrocardiogram, 24-h Holter monitor, chest x-ray, and echocardiography [2]. The two-tier antibody-based test recommended by the CDC is highly specific but has poor sensitivity [59]. Supplementary studies that have been used in the diagnosis include echocardiography [7, 57, 60], gadolinium enhanced cardiac MRI [16, 27], endo-myocardial biopsy [6, 7, 22, 61, 62] with special staining [3, 4, 8, 40], culture [3] or electron microscopy [7, 61,62,63], Ga67 Scan [64,65,66], and Indium 111 labelled anti-myosin antibody scintigraphy [4]. Histology of affected cardiac tissue shows infiltration predominantly with lymphocytes and plasma cells [15, 22, 29, 40].

Progression of first-degree heart block to second degree and then to complete heart block without treatment has been reported. Regression of complete heart block to second degree, then first degree followed by complete resolution with treatment is also known [32]. Inducible heart block in Lyme disease has not been reported. We present the first report of inducible heart block in a patient whose initial and sole manifestation of LD was HB. First-degree heart block worsened to high degree AV block on exercise, which was reversible with rest and reproducible during the hospitalization.

As illustrated in this case, exercise and stress testing should not be carried out in LD patients until complete recovery from heart block. HB secondary to Lyme disease rarely requires a permanent pacemaker. However, patients with a PR interval greater than 300 ms should be monitored in an intensive care setting, as they may rapidly progress to complete HB [30]. A temporary pacer is sometimes required; however, most patients respond to treatment within two to three weeks. The antibiotics used in LC include amoxicillin [12], ceftriaxone [8, 9, 32, 36, 38, 41, 53], doxycycline [49] and ceftriaxone and doxycycline [13, 37]. Duration of treatment varied from 2 weeks to one month.

LC may present with HB as the sole [53] or initial presentation [45, 66] with or without other manifestations of LD [30, 32, 36, 37, 48] or with multiple electrocardiographic (Table 1) and clinical presentations. First degree HB is the most common manifestation, which could rapidly progress to CHB [32]. It is important to keep a high degree of suspicion for Lyme disease in endemic areas in patients with cardiac symptoms with or without other manifestations of LD, particularly younger individuals with no other etiology evident. Some patients with complete heart block may need a temporary pacemaker [26, 39, 47, 55, 66, 67]. The majority of AV blocks in LC are reversible with antibiotic treatment. Exercise is contraindicated until resolution of LC.