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

Most Likely Cause: Mitral Valve Prolapse

In an asymptomatic young man with a pansystolic murmur radiating to the axilla discovered on routine examination, mitral valve prolapse (MVP) with mitral regurgitation is the most likely diagnosis. 1, 2

Clinical Reasoning

Key Diagnostic Features Present

• Pansystolic murmur radiating to the axilla is pathognomonic for mitral regurgitation, specifically indicating anterior leaflet pathology according to the American College of Cardiology 3, 1

• The asymptomatic presentation in a young adult undergoing routine pre-employment screening strongly favors MVP as the underlying etiology 3, 4

• MVP is the most common valvular disorder diagnosed in the United States, occurring in 1-2.5% of the population using modern echocardiographic criteria 3

Why Other Options Are Less Likely

Ischemic mitral regurgitation (Option B) is unlikely because:

• This occurs in patients with coronary artery disease and prior myocardial infarction causing papillary muscle dysfunction 3
• The patient is young and asymptomatic with no history suggesting ischemic heart disease 3

Functional mitral regurgitation (Option C) is unlikely because:

• Functional MR produces a midsystolic murmur, not a pansystolic murmur, according to ACC/AHA guidelines 2
• This occurs secondary to left ventricular dilation from cardiomyopathy or heart failure, which would produce symptoms 3

Rheumatic mitral regurgitation (Option D) is less likely because:

• Rheumatic heart disease has markedly decreased prevalence in industrialized countries 3
• Rheumatic MR typically presents with mixed valvular disease and would likely have a symptomatic history 3
• The patient's young age and asymptomatic status make this less probable in a modern pre-employment screening context 3

Natural History and Prognosis

• MVP has a benign prognosis in most patients, with age-adjusted survival similar to the general population 3

• The complication rate is approximately 2% per year, with most patients remaining asymptomatic throughout life 4, 5

• When MVP progresses to significant regurgitation, it can present with either a late systolic murmur or a pansystolic murmur depending on severity 1

• Approximately 5% of affected men and 1.5% of affected women ultimately require valve surgery, with complications concentrated disproportionately in older men 5

Recommended Next Steps

• Echocardiography should be performed to confirm MVP, assess the severity of mitral regurgitation, evaluate left ventricular size and function, and measure mitral annular dimensions 2

• Dynamic auscultation can support the diagnosis: the murmur typically becomes louder with Valsalva maneuver or standing and softer with squatting 1, 2

• Look for associated findings including midsystolic clicks (which may have been subtle or missed on initial examination), thoracic skeletal abnormalities such as pectus excavatum or straight thoracic spine 3, 5

Clinical Pitfalls to Avoid

• The click-murmur of MVP may be misdiagnosed as a benign flow murmur if the ejection click is not recognized 2

• In patients with thickened mitral valve leaflets (≥5 mm), there is increased risk of complications including endocarditis, progressive MR requiring surgery, and arrhythmias 3

• Antibiotic prophylaxis for endocarditis should be considered if echocardiography demonstrates thickened redundant leaflets or significant mitral regurgitation 4, 5