What is the most likely cause of a pansystolic murmur radiating to the axilla in an asymptomatic patient?

Mitral Valve Prolapse

In an asymptomatic patient presenting with a pansystolic murmur radiating to the axilla on routine pre-employment examination, mitral valve prolapse is the most likely diagnosis.

Pathophysiology and Clinical Reasoning

A pansystolic (holosystolic) murmur indicates continuous flow between chambers with widely different pressures throughout systole, most commonly representing mitral regurgitation, tricuspid regurgitation, or ventricular septal defect 1. The key distinguishing feature here is the radiation to the axilla, which is pathognomonic for mitral regurgitation rather than other causes of pansystolic murmurs 2.

Why Mitral Valve Prolapse is Most Likely

Mitral valve prolapse (MVP) is the most common cause of mitral regurgitation in asymptomatic young adults, particularly those discovered incidentally on routine examination 3. The clinical context strongly favors MVP:

• Asymptomatic presentation: MVP is frequently asymptomatic and discovered incidentally, most commonly becoming clinically manifest in the third and fourth decades of life 4, 3
• Pre-employment screening context: This suggests a younger, otherwise healthy individual without prior cardiac history
• Primary MVP is inherited and represents the most common primary valvular abnormality in this demographic 3

Distinguishing from Other Causes

Ischemic mitral regurgitation would be unlikely in an asymptomatic patient without risk factors or prior myocardial infarction. Ischemic MR typically occurs with coronary artery disease and is associated with inferior wall asynergy, particularly severe right coronary disease 5. This patient lacks the clinical context for ischemic heart disease.

Functional mitral regurgitation produces a midsystolic murmur, not a pansystolic murmur 1. The ACC/AHA guidelines explicitly state that functional MR causes midsystolic murmurs and requires echocardiography to distinguish from other causes 1.

Rheumatic mitral regurgitation would be extremely unlikely in a modern pre-employment screening context without a history of rheumatic fever. Rheumatic heart disease has become rare in developed countries and typically presents with a known history of acute rheumatic fever.

Clinical Pearls and Diagnostic Considerations

Auscultatory Features of MVP

• Late systolic murmurs with midsystolic clicks are classic for MVP, though the murmur can become holosystolic (pansystolic) when regurgitation is more severe 1, 6
• The murmur lengthens and intensifies with standing, unlike most other murmurs which diminish 1
• The murmur softens with squatting, which increases preload and reduces prolapse 1

Common Diagnostic Pitfall

The click-murmur of MVP may be misdiagnosed as a benign flow murmur if the ejection click is not recognized 1. In this case, the pansystolic nature and axillary radiation make the diagnosis more apparent, but clinicians should actively listen for the characteristic midsystolic click.

Natural History and Risk Stratification

While this patient is currently asymptomatic, understanding progression risk is important:

• Most MVP patients with mild MR remain stable, but approximately 50% of those with moderate MR progress to severe MR over 4-5 years 7
• Complications are concentrated in older men, with about 5% of affected men and 1.5% of affected women ultimately requiring valve surgery 3
• Severe MR typically develops after age 50, with patients remaining asymptomatic for an average of 25 years before symptoms appear 4

Independent Risk Factors for Complications

The presence of these factors warrants closer follow-up 3:

• Presence of mitral regurgitation (already present in this patient)
• Male gender
• Age over 45 years

Immediate Management

Echocardiography should be performed to confirm MVP, assess the severity of mitral regurgitation, evaluate left ventricular size and function, and measure mitral annular dimensions 6, 7. A mitral annulus diameter >39.6 mm predicts progression from moderate to severe MR with high accuracy 7.

Endocarditis prophylaxis should be considered based on the severity of regurgitation and individual risk factors, as complications including endocarditis occur at a rate of approximately 1,100 cases per year in the United States among MVP patients 3.