What is the diagnosis and cause of fainting episodes in a patient with a small, circumscribed round mass in the left atrium?

Diagnosis and Management of Left Atrial Mass with Syncope

A. Diagnosis: Left Atrial Myxoma

The diagnosis is left atrial myxoma, the most common primary cardiac tumor, accounting for approximately 50% of all benign cardiac neoplasms. 1, 2

• Myxomas occur most frequently in the left atrium, typically attached to the interatrial septum 1
• They are most common in patients between the 4th and 6th decade of life, with female predominance 3
• The tumor appears as a small, circumscribed round mass on imaging (CT and echocardiography) 4, 5
• Cardiac myxomas have an incidence of 0.02% in autopsy series 1, 2

B. Mechanism of Fainting Episodes

The syncope results from mechanical obstruction of blood flow across the mitral valve during diastole, causing transient reduction in cardiac output and cerebral hypoperfusion. 1, 6, 5

Primary Mechanism: Obstructive "Ball-Valve" Effect

• The mobile tumor acts as a ball-valve, intermittently obstructing the mitral valve orifice during diastole 4, 5
• This dynamic flow obstruction causes sudden drops in cardiac output, leading to cerebral hypoperfusion and loss of consciousness 5, 7
• Syncope is particularly triggered by positional changes or exertion when the tumor shifts position 4

Additional Contributing Factors

• Arrhythmias (including symptomatic bradycardia) can occur due to tumor proximity to cardiac conduction structures 5
• The tumor may cause mechanical irritation of the atrial wall, triggering reflex bradycardia 5
• Larger myxomas (>6 cm) are associated with worse prognosis and more severe obstructive symptoms 3, 4

C. Microscopic Findings of Cardiac Myxoma

Cellular Components

• Myxoma cells (lepidic cells): Polygonal to stellate-shaped cells with eosinophilic cytoplasm, arranged singly or in small clusters
• Stromal cells: Spindle-shaped cells embedded in abundant myxoid matrix
• Endothelial cells: Lining vascular channels within the tumor

Structural Features

• Myxoid matrix: Abundant loose, gelatinous extracellular matrix rich in mucopolysaccharides (appears basophilic on H&E staining)
• Vascular channels: Thin-walled blood vessels of varying caliber scattered throughout
• Hemorrhage and hemosiderin: Common findings, especially in larger tumors
• Calcification: May be present, particularly in older lesions
• Surface thrombus: Friable surface with adherent thrombus (source of embolic potential)

Key Distinguishing Features

• Absence of mitotic figures (benign tumor) 3
• Villous or papillary surface architecture in some cases (associated with higher embolic risk) 1
• Immunohistochemistry: Positive for vimentin, calretinin; negative for cytokeratin, S100

Management Recommendations

Surgical excision is the definitive treatment and should be performed urgently to prevent life-threatening complications including sudden death, stroke, and recurrent syncope. 1, 2

Surgical Indications (Class I)

• All symptomatic left atrial myxomas require surgical resection 1, 2
• Surgical excision reduces stroke risk compared to conservative management 1
• Complete tumor removal with adequate margin of atrial septum is essential to prevent recurrence 2

Pre-operative Considerations

• Echocardiography (transthoracic and transesophageal) is the diagnostic modality of choice 2
• Cardiac MRI or CT provides additional characterization for surgical planning 2
• Avoid excessive manipulation during imaging to prevent tumor embolization 4

Post-operative Management

• Histopathologic confirmation is mandatory 1, 3
• Long-term follow-up with periodic echocardiography to monitor for recurrence (occurs in 1-5% of sporadic cases) 2
• Resolution of syncope and arrhythmias typically occurs immediately after tumor removal 5

Critical Pitfalls to Avoid

• Never delay surgery once diagnosis is established—the risk of sudden death from complete mitral obstruction or massive embolization is substantial 4
• Do not attempt pericardiocentesis if pericardial effusion is present, as this is contraindicated in cardiac tumors 1
• Anticoagulation or antiplatelet therapy alone is inadequate and does not prevent embolic complications 1
• The overall embolism rate with cardiac myxomas is 25%, with embolic stroke being the presenting symptom in up to 50% of cases 1, 2