Lyme carditis atrial fibrillation represents an unusual initial presentation that can mislead clinicians. A 23-year-old man’s case demonstrates why atrial fibrillation in young patients from endemic areas deserves immediate Lyme disease consideration — even though classic Lyme carditis typically presents with heart block, not supraventricular arrhythmias.

Initial Presentation: Atrial Fibrillation in a Young Adult

A case study published in the British Medical Journal features a 23-year-old man with a history of degenerative joint disease who presented with a sudden onset of palpitations. His echocardiogram (ECG) revealed atrial fibrillation (AF) with a mildly dilated left atrium.

Atrial fibrillation in a 23-year-old is highly unusual. Young adults rarely develop AFib without structural heart disease or other predisposing factors. This demographic mismatch alone should have raised diagnostic red flags.

Why Lyme Carditis Was Initially Dismissed

The patient did not recall a tick bite or a rash. And, “Although Lyme carditis was on the differential diagnoses list, it was not considered high enough due to the initial rhythm being AF and not [atrioventricular] AV block,” writes Shabbir and colleagues.

This reflects a common diagnostic trap: clinicians expect Lyme carditis to present with heart block, so when they see atrial fibrillation instead, they deprioritize Lyme disease on the differential.

The literature supports the assumption that AV block is the classic presentation. Studies show that 90% of Lyme carditis cases involve conduction abnormalities. But this case demonstrates that the remaining 10% — presentations like atrial fibrillation — still represent Lyme carditis and require recognition.

Initial Treatment and Discharge

The man was treated with metoprolol (a beta-blocker for rate control) and released from the hospital after his heart spontaneously reverted back to normal sinus rhythm.

From a cardiology perspective, this approach seems reasonable: young patient with new-onset AFib, converts to normal rhythm, send home on medication. But the underlying infection remained untreated.

Four Days Later: The Classic Presentation Emerges

However, 4 days later the patient returned to the hospital.

“ECG now exhibited atrioventricular (AV) Mobitz-II block alternating with intermittent complete heart block (CHB) on telemetry confirmed with ECG,” writes Shabbir.

Now the presentation looked like classic Lyme carditis. The progression from atrial fibrillation to high-grade AV block over four days demonstrates how Lyme carditis can evolve rapidly.

Empiric Treatment and Recovery

He was tested for Lyme disease and treated empirically with intravenous ceftriaxone. Within 48 hours, his symptoms began to improve.

Lyme disease tests came back positive. And 1 month later, after antibiotic therapy, his heart rhythm had returned to normal.

The rapid improvement — symptoms improving within 48 hours of antibiotics — is characteristic of Lyme carditis response to treatment. The spirochetes causing cardiac inflammation are killed, inflammation resolves, and normal cardiac function returns.

What If Treatment Had Started Earlier?

Consider the timeline:

• Day 1: Presented with atrial fibrillation, Lyme disease considered but deprioritized, sent home on metoprolol
• Day 5: Returned with high-grade heart block, empiric Lyme treatment started
• Day 7: Symptoms improving
• 1 month later: Complete recovery

If Lyme carditis had been treated empirically at the first presentation when AFib was discovered, the progression to complete heart block might have been prevented entirely. The patient could have avoided a second hospitalization, the development of life-threatening arrhythmias, and four days of worsening cardiac involvement.

Key Learning Points from the Authors

The authors’ key learning points include:

1. Consider the unusual initial presentation of Lyme disease as atrial fibrillation.

While heart block is most common, Lyme carditis can present with other arrhythmias including atrial fibrillation and atrial flutter. Don’t exclude Lyme disease just because the rhythm isn’t AV block.

2. Keep Lyme carditis in the differential diagnoses when someone from a Lyme-endemic area presents with supraventricular arrhythmia (atrial fibrillation/flutter).

Geographic context matters enormously. A 23-year-old from an endemic area with new-onset AFib deserves Lyme testing regardless of tick bite or rash history. The absence of classic markers shouldn’t lower diagnostic suspicion when the demographic and geographic factors align.

3. Initiate appropriate antibiotic therapy empirically without waiting for the Lyme serologies if the clinical suspicion is high.

This is the most important lesson. When Lyme carditis is a reasonable possibility in a young patient from an endemic area with new cardiac arrhythmia, start antibiotics empirically. Don’t wait for test results. As this case demonstrates, waiting can allow progression to life-threatening heart block.

Atrial Fibrillation as a Cardiac Manifestation of Lyme Disease

The literature documents various cardiac manifestations beyond AV block:

• Atrial fibrillation (this case)
• Atrial flutter
• Myocarditis
• Pericarditis
• Ventricular arrhythmias
• Bundle branch blocks

A study of 189 children with Lyme carditis found 2% had atrial fibrillation/flutter. While less common than AV block, it’s a recognized manifestation that clinicians must consider.

Clinical Perspective

This case illustrates how diagnostic assumptions can delay recognition even when Lyme disease is on the differential. The clinicians considered Lyme carditis initially but dismissed it because the presentation didn’t match their expectations.

The assumption that Lyme carditis equals AV block is understandable — it’s what the textbooks emphasize, it’s what the CDC surveillance definition focuses on, and it’s what most case reports describe. But clinging to that assumption cost this patient four additional days of untreated infection and progression to complete heart block.

In my practice, when I see any unexplained cardiac arrhythmia in a young patient from an endemic area, Lyme disease moves to the top of my differential regardless of the specific rhythm. AFib in a 23-year-old demands explanation. The lack of structural heart disease, the endemic area residence, and the young age all point toward Lyme carditis even without tick bite or rash recall.

The empiric treatment question is critical. Some clinicians worry about “overtreating” with antibiotics before diagnostic confirmation. But the downside of empiric doxycycline or ceftriaxone is minimal compared to the downside of progression to complete heart block or cardiac arrest.

This case also demonstrates why patients need to advocate for themselves. If you’re a young person with new cardiac symptoms in an endemic area and your doctors aren’t considering Lyme disease, bring it up. Mention tick exposure even if you don’t recall a specific bite. Ask for testing. Request empiric treatment if clinical suspicion is reasonable.

Frequently Asked Questions

Can Lyme carditis cause atrial fibrillation?

Yes. While less common than AV block, Lyme carditis can present with atrial fibrillation or atrial flutter. Studies document AFib in approximately 2% of Lyme carditis cases. Any supraventricular arrhythmia in a young patient from an endemic area warrants Lyme disease consideration.

Why don’t doctors think of Lyme disease when they see atrial fibrillation?

Clinicians expect Lyme carditis to present with heart block, not atrial fibrillation. Textbooks and CDC definitions emphasize AV block, creating diagnostic blind spots for other presentations. This case shows why maintaining broader suspicion matters.

Can atrial fibrillation from Lyme disease progress to heart block?

Yes. This patient presented with AFib initially, then progressed to Mobitz-II and complete heart block four days later. Lyme carditis can evolve rapidly, with arrhythmias changing as spirochetal infiltration affects different parts of the cardiac conduction system.

Should young people with new atrial fibrillation be tested for Lyme disease?

Yes, if they live in or have traveled to endemic areas. AFib in patients under 40 without structural heart disease or other clear causes is unusual enough to warrant Lyme testing. Young adults are at high risk for Lyme carditis.

How quickly does Lyme carditis atrial fibrillation respond to antibiotics?

This patient’s symptoms improved within 48 hours of starting IV ceftriaxone, with complete recovery after one month of treatment. Lyme carditis typically responds rapidly to appropriate antibiotic therapy when treatment begins promptly.

Should antibiotics be started before Lyme test results come back?

Yes, when clinical suspicion is high. This patient was treated empirically and improved while waiting for test confirmation. The authors specifically recommend initiating antibiotics without waiting for serology when Lyme carditis is suspected. Delayed treatment risks progression to life-threatening arrhythmias.

Can you have Lyme carditis without a tick bite or rash?

Yes. This patient recalled neither tick bite nor rash, yet had confirmed Lyme carditis. Most Lyme carditis patients lack classic markers, yet clinicians often won’t consider Lyme disease without them, leading to dangerous diagnostic delays.