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 associated with underlying cardiovascular disorders, including coronary artery disease, hypertension, cardiomyopathies, and myocarditis, with coronary artery disease being particularly prevalent in elderly patients. 1

Primary Cardiovascular Causes

Structural Heart Disease

• Coronary artery disease (CAD) - A leading cause of LBBB, particularly in older patients 1, 2
• Cardiomyopathies:
  • Dilated cardiomyopathy
  • Hypertrophic cardiomyopathy
  • Infiltrative cardiomyopathies (e.g., amyloidosis, sarcoidosis)
• Hypertensive heart disease - Long-standing hypertension causing left ventricular hypertrophy 1, 2
• Valvular heart disease - Particularly aortic valve disease 2
• Myocarditis - Inflammatory conditions affecting the myocardium 1

Conduction System Degeneration

• Age-related degenerative changes - Progressive fibrosis of the conduction system
• Lenegre's disease (progressive cardiac conduction defect)
• Lev's disease (calcification of the cardiac skeleton)

Secondary/Non-Cardiac Causes

• Electrolyte abnormalities - Particularly hyperkalemia
• Drug toxicity - Certain antiarrhythmics, digitalis
• Infiltrative processes affecting the conduction system
• Trauma to the conduction system (e.g., during cardiac surgery)
• Coronary vasospasm - In rare instances 2

Clinical Significance and Evaluation

LBBB is found in less than 1 in 1,000 athletes but is common in patients with cardiomyopathy and ischemic heart disease 3. It represents an independent risk factor for cardiovascular mortality, requiring thorough evaluation in all cases 4.

LBBB causes:

• Immediate electrical and mechanical dyssynchrony of the left ventricle
• Gradual structural damage to Purkinje cells and myocardium
• Reduced left ventricular ejection fraction (to approximately 55% even in otherwise normal hearts) 5

Evaluation Approach

1. Echocardiography - Essential for all patients with newly discovered LBBB

   • The ACC/AHA guidelines recommend transthoracic echocardiography within 1-2 weeks for newly discovered asymptomatic LBBB 1
   • Evaluates for structural heart disease and left ventricular function

2. Advanced cardiac imaging - For selected cases

   • Cardiac MRI with perfusion study is recommended for athletes with complete LBBB 3
   • Disease-specific advanced imaging (transesophageal echocardiography, CT, cardiac MRI, or nuclear imaging) is reasonable if structural heart disease is suspected but not confirmed by other diagnostics 3

3. Coronary evaluation - Particularly in patients with risk factors for CAD

   • Patients with LBBB and perfusion defects extending beyond typical LBBB-related artifact areas have significantly worse prognosis (annual cardiac death rate ~6.4%) 1

Special Considerations

• Intermittent LBBB - An unusual phenomenon that may unmask underlying coronary artery disease 2
• LBBB-associated cardiomyopathy - A potentially reversible form of cardiomyopathy that may benefit from cardiac resynchronization therapy 6
• Painful LBBB syndrome - A rarer phenomenon that can occur in structurally normal hearts 4

Prognosis

The prognosis of LBBB depends on the underlying cause:

• Isolated LBBB without structural heart disease has a relatively benign course
• LBBB with structural heart disease carries a worse prognosis
• Annual progression to complete AV block is approximately 1-2% in patients with LBBB 1
• LBBB shortens median survival of patients with heart failure with moderately reduced ejection fraction by 5.5 years 5

LBBB should never be dismissed as a benign finding, as it often signals underlying cardiovascular pathology that requires thorough evaluation and appropriate management.