Differential Diagnosis for RBBB with LAFB vs LPFB

When differentiating between a Right Bundle Branch Block (RBBB) with Left Anterior Fascicular Block (LAFB) and a Left Posterior Fascicular Block (LPFB), it's crucial to consider the electrocardiographic (ECG) findings and clinical context. Here's a structured approach to the differential diagnosis:

• Single Most Likely Diagnosis

  • RBBB with LAFB: This is often considered the single most likely diagnosis when the ECG shows a typical RBBB pattern (wide QRS complex with a broad R wave in lead V1 and a deep S wave in lead V6) combined with left axis deviation (LAD), which is indicative of LAFB. The presence of LAD in the context of RBBB strongly suggests LAFB.

• Other Likely Diagnoses

  • LPFB: This could be considered if there's a suspicion of a fascicular block but the axis deviation is not typical for LAFB. LPFB is less common and might present with a right axis deviation, but it's essential to differentiate it from other causes of right axis deviation.
  • Bilateral Bundle Branch Block: Although less common, if there are signs suggestive of both right and left bundle branch blocks (e.g., alternating bundle branch block), this diagnosis should be considered, especially in the context of cardiac disease.

• Do Not Miss Diagnoses

  • Cardiac Sarcoidosis: This condition can cause various conduction abnormalities, including bundle branch blocks and fascicular blocks. It's crucial to consider, especially if there are other systemic symptoms or if the ECG findings are atypical.
  • Lyme Carditis: This can cause various degrees of AV block and may also affect the bundle branches. Although less common, it's a diagnosis that should not be missed due to its treatable nature.
  • Cardiac Amyloidosis: This condition can infiltrate the heart, leading to conduction system abnormalities. It might present with a combination of bundle branch blocks and fascicular blocks, among other findings.

• Rare Diagnoses

  • Lenegre-Lev Disease: A rare condition characterized by progressive conduction system disease, which can present with bundle branch blocks and fascicular blocks.
  • Myotonic Dystrophy: A genetic disorder that can affect the heart, leading to conduction abnormalities, including bundle branch blocks and fascicular blocks. It's rare but should be considered in patients with known muscular dystrophy.

Each of these diagnoses has distinct clinical and electrocardiographic features. A thorough evaluation, including clinical history, physical examination, ECG analysis, and potentially further diagnostic tests (e.g., echocardiography, cardiac MRI), is necessary to accurately differentiate between them.