What causes a heart murmur in an adult with no prior medical history?

Causes of Heart Murmurs in Adults

Heart murmurs in adults with no prior medical history are caused by three main mechanisms: high blood flow through normal or abnormal orifices, forward flow through narrowed or irregular orifices, or backward regurgitant flow through incompetent valves. 1

Primary Pathophysiological Mechanisms

The production of murmurs fundamentally results from: 1

• High blood flow rate through normal or abnormal orifices (e.g., pregnancy, thyrotoxicosis, anemia, arteriovenous fistula)
• Forward flow through a narrowed or irregular orifice into a dilated vessel or chamber
• Backward or regurgitant flow through an incompetent valve

Often, more than one of these factors operates simultaneously. 1

Common Causes by Murmur Type

Systolic Murmurs (Most Common)

Midsystolic (Crescendo-Decrescendo) Murmurs:

• Aortic stenosis is the most frequent pathological cause requiring valve replacement in adults, with incidence increasing with age 2
• Aortic sclerosis (focal thickening without significant obstruction, peak velocity <2.0 m/s) is extremely common in older adults, particularly those with hypertension 1
• Physiological flow murmurs from increased cardiac output states (pregnancy, thyrotoxicosis, anemia) 1
• Pulmonic stenosis or flow into dilated great vessels 1
• Hypertrophic cardiomyopathy with left ventricular outflow tract obstruction 1

Holosystolic (Pansystolic) Murmurs:

• Mitral regurgitation (30% of referred patients with systolic murmurs) 3
• Tricuspid regurgitation 1
• Ventricular septal defect 1

Late Systolic Murmurs:

• Mitral valve prolapse with late systolic regurgitation 1
• Apical tethering and malcoaptation of mitral leaflets due to anatomic and functional changes 1

Diastolic Murmurs (Virtually Always Pathological)

Early Diastolic Murmurs:

• Aortic regurgitation (decrescendo, high-pitched) 1
• Pulmonic regurgitation with pulmonary hypertension 1

Middiastolic Murmurs:

• Mitral stenosis 1
• Tricuspid stenosis 1
• Increased diastolic flow across normal valves (severe regurgitation, shunts) 1
• Austin-Flint murmur (severe chronic aortic regurgitation causing functional mitral stenosis) 1

Presystolic Murmurs:

• Mitral or tricuspid stenosis in sinus rhythm 1
• Atrial myxoma 1

Continuous Murmurs

• Venous hum (innocent, disappears with jugular compression or supine position) 4
• Mammary souffle (late pregnancy/early puerperium, disappears with breast compression) 4
• Patent ductus arteriosus and other high-to-low pressure shunts 1

Clinical Context and Prevalence

Most systolic heart murmurs in adults do not signify cardiac disease and are related to physiological increases in blood flow velocity. 1 However, in a study of 100 adults referred for systolic murmurs of unknown cause, only 21% had functional (innocent) murmurs, while 79% had organic heart disease. 3

The most common organic causes identified were: 3

• Aortic stenosis (29%)
• Mitral regurgitation (30%)
• Combined valvular lesions (35% had more than one abnormality)

Critical Diagnostic Pitfalls

Physical examination alone has significant limitations: 3

• Aortic regurgitation is missed in 79% of cases (sensitivity only 21%)
• Combined aortic and mitral valve disease is correctly identified in only 55% of cases
• Intraventricular pressure gradients are detected in only 18% of cases
• Aortic stenosis severity can be misjudged when left ventricular ejection fraction is severely diminished

Diastolic murmurs virtually always represent pathological conditions and require further cardiac evaluation. 1 The only exceptions are continuous innocent murmurs (venous hums and mammary souffles). 1

When Echocardiography is Mandatory

Echocardiography should be performed for: 1, 5

• Any diastolic murmur (except confirmed venous hum) 4
• Holosystolic murmurs 4
• Grade ≥3/6 systolic murmurs 2
• Any murmur with associated symptoms (syncope, angina, heart failure, dyspnea) 1
• Abnormal ECG findings (ventricular hypertrophy, prior infarction) 1
• Clinical findings suggestive of endocarditis 1
• When significant heart disease is suspected but cannot be definitively excluded by examination 3

Asymptomatic young adults with grade 1-2/6 midsystolic murmurs and no other cardiac physical findings may need no further workup after initial history and physical examination. 1