What are the causes of Left Bundle Branch Block (LBBB)?

Causes of Left Bundle Branch Block (LBBB)

Left Bundle Branch Block (LBBB) is most commonly caused by structural heart disease, including cardiomyopathy, valvular heart disease, congenital anomalies, ischemic heart disease, and infiltrative processes, though it can also result from conduction system degeneration. 1, 2

Primary Causes of LBBB

Cardiac Structural Abnormalities

• Cardiomyopathy: LBBB is strongly associated with underlying cardiomyopathies
  • Studies show LBBB confers nearly a 4-fold increased likelihood of left ventricular systolic dysfunction 1
  • Can be both a cause and consequence of cardiomyopathy (LBBB-associated cardiomyopathy) 3
• Valvular heart disease: Particularly aortic valve disease 4
• Ischemic heart disease: Often associated with coronary artery disease and myocardial infarction
• Hypertension: Long-standing hypertension leading to left ventricular hypertrophy

Infiltrative and Inflammatory Conditions

• Cardiac sarcoidosis: Cardiac MRI can detect this in patients with LBBB and normal echocardiograms 1
• Connective tissue diseases: MRI identified significant abnormalities in 42% of patients with connective tissue disease and new onset LBBB despite normal echocardiograms 1
• Myocarditis: Can cause inflammation affecting the conduction system 1

Degenerative Processes

• Age-related degeneration: Progressive fibrosis of the conduction system
• Lenegre's disease: Progressive fibrosis of the cardiac conduction system
• Lev's disease: Age-related sclerosis of the cardiac skeleton

Neuromuscular Disorders

• Kearns-Sayre syndrome: Associated with conduction disorders requiring permanent pacing 1
• Anderson-Fabry disease: With QRS prolongation >110 ms may require pacing 1
• Other neuromuscular diseases with unpredictable progression of AV conduction disease 2

Iatrogenic and Functional Causes

• Cardiac procedures: Can develop following cardiac procedures including valve surgery 4
• Rate-dependent LBBB: Occurs only at certain heart rates, has been reported as a cause of non-ischemic chest pain 1
• Drug-induced: Certain antiarrhythmic medications and other cardiotoxic therapies

Clinical Significance and Prognosis

• Exercise-induced LBBB is associated with increased risk of death and cardiac events 1
• LBBB may be the first manifestation of a more diffuse myocardial disease 5
• LBBB can induce abnormalities in left ventricular performance due to asynchronous contraction patterns 5
• Even in patients with normal left ventricular function by echocardiography, cardiac MRI may detect subclinical cardiomyopathy in one-third of patients 1
• LBBB shortens median survival of patients with HFmrEF by 5.5 years 6

Diagnostic Approach

When LBBB is identified, further evaluation is warranted to identify underlying causes:

1. Echocardiography: Reasonable in patients with LBBB to assess for structural heart disease 1
2. Cardiac MRI: Consider in selected patients with LBBB and normal left ventricular function if sarcoidosis, connective tissue disease, myocarditis, or other cardiomyopathies are suspected 1
3. Electrophysiologic study (EPS): May provide diagnostic information in patients with syncope and bundle branch block 1
4. Ambulatory monitoring: Can detect clinically significant arrhythmias in patients with conduction system disease 1

Important Clinical Considerations

• LBBB masks signs of myocardial infarction, making diagnosis challenging if chest pain develops 2
• The diagnosis of left ventricular hypertrophy should not be attempted when LBBB is present 2
• Asymptomatic LBBB may progress to symptomatic conduction disease requiring intervention, though only 1-2% of patients with asymptomatic BBB progress to AV block per year 2
• LBBB-associated cardiomyopathy represents a potentially reversible form of cardiomyopathy with favorable response to cardiac resynchronization therapy 3

Understanding the underlying cause of LBBB is crucial for appropriate management and prognostication, as LBBB is not merely an ECG finding but often reflects significant underlying cardiac pathology.