What causes a dilated left atrium?

Causes of Dilated Left Atrium

Left atrial dilation results from chronic pressure or volume overload, with the most common causes being mitral valve disease (regurgitation or stenosis), left ventricular diastolic dysfunction, systolic heart failure, atrial fibrillation, and hypertension. 1, 2

Primary Mechanisms of Left Atrial Enlargement

The left atrium dilates through two fundamental pathophysiologic pathways that often coexist 1:

Volume Overload Conditions

• Mitral regurgitation causes direct volume overload as blood flows backward into the left atrium during systole, with the atrium receiving both normal pulmonary venous return and the regurgitant volume from the left ventricle 1, 3
• Primary mitral regurgitation from myxomatous degeneration, mitral valve prolapse, or flail leaflet creates chronic volume overload leading to progressive left atrial enlargement 1, 4, 3
• Secondary (functional) mitral regurgitation from left ventricular dysfunction or atrial dilation itself perpetuates a vicious cycle of worsening regurgitation and further atrial enlargement 1
• Aortic regurgitation causes left ventricular dilation which secondarily leads to left atrial enlargement 2

Pressure Overload Conditions

• Mitral stenosis creates direct left atrial pressure overload as the stenotic valve impedes forward flow from atrium to ventricle 2, 5
• Left ventricular diastolic dysfunction elevates left ventricular end-diastolic pressure, which transmits backward to increase left atrial pressure chronically 1, 2, 6
• Systolic heart failure with reduced ejection fraction commonly causes left atrial enlargement through elevated filling pressures 1, 2, 6
• Hypertension leads to left ventricular hypertrophy and diastolic dysfunction, chronically elevating left atrial pressure 2

Atrial Fibrillation as Both Cause and Consequence

Atrial fibrillation has a bidirectional relationship with left atrial enlargement, where AF causes progressive dilation through loss of coordinated atrial contraction and elevated pressures, while the dilated atrium creates substrate for AF perpetuation. 1, 2, 7, 8

• Patients with paroxysmal atrial fibrillation demonstrate left atrial dilation even during sinus rhythm, with mean left atrial dimension of 35.5 mm compared to 23.3 mm in normal subjects 7
• Chronic atrial fibrillation (>6 months duration) is associated with bi-atrial dilatation, reduced left ventricular shortening fraction, and functional atrioventricular valve regurgitation 8
• Loss of the "Venturi effect" from atrial contraction in AF impairs mitral leaflet coaptation, contributing to secondary mitral regurgitation and further volume overload 1
• Progressive atrial dilation causes myocyte disarray and fibrotic changes that compromise atrial contractile function and perpetuate arrhythmias 2

Specific Clinical Contexts

Heart Failure with Preserved Ejection Fraction

• Diastolic dysfunction is frequently the underlying cause of left atrial enlargement in patients with preserved ejection fraction, particularly those with hypertension 2
• The E/e' ratio (reflecting left ventricular filling pressures) is an independent determinant of left atrial volume in dilated cardiomyopathy patients 6

Post-LVAD Complications

• Left ventricular failure related to LVAD overloading manifests with increasing left ventricular size and increased left atrial volume 1
• LVAD-related continuous aortic insufficiency causes increased left ventricular size and secondary left atrial enlargement 1

Giant Left Atrium

• Giant left atrium (diameter >65 mm) most commonly occurs with severe mitral valve regurgitation, with or without atrial fibrillation 5
• The mechanism of isolated giant left atrium without valvular disease remains unknown but possibly relates to inherent weakening of atrial wall tissue 5

Critical Pitfalls to Avoid

Do not overlook diastolic dysfunction as the underlying cause, particularly in patients with hypertension or preserved ejection fraction, as this is frequently missed. 2

• Single anteroposterior diameter measurements are inadequate; volumetric assessment using biplane disk summation technique at end-ventricular systole is required for accurate evaluation 2
• Exclude the left atrial appendage and pulmonary veins from endocardial tracing to avoid overestimation of left atrial volume 2
• In patients with mitral regurgitation, distinguish primary (intrinsic valve pathology) from secondary (functional) causes, as this fundamentally changes management 1
• Recognize that left atrial enlargement in the setting of normal left ventricular size and function strongly suggests severe primary mitral regurgitation 1