How should a lipomatous mass in the interatrial septum be evaluated?

Evaluation of Lipomatous Mass in the Interatrial Septum

A lipomatous mass in the interatrial septum should be evaluated primarily with cardiac CT or cardiac MRI to confirm the characteristic dumbbell-shaped fat deposition sparing the fossa ovalis, which definitively distinguishes benign lipomatous hypertrophy from other cardiac masses requiring intervention. 1, 2

Initial Diagnostic Approach

Transthoracic Echocardiography (TTE)

• TTE serves as the initial screening modality to identify increased interatrial septal thickness and characterize the mass as "hyperlipomatous" according to ACC/AHA/ASE standardized terminology. 1
• Echocardiography can detect the presence of the mass but has limited ability to definitively characterize tissue composition. 1

Definitive Imaging: Cardiac CT or Cardiac MRI

Cardiac CT is the preferred definitive diagnostic modality for lipomatous hypertrophy of the interatrial septum (LHIS). 2, 3

Characteristic CT Features (Diagnostic):

• Nonenhancing, smoothly marginated, homogeneous mass of fat attenuation (negative Hounsfield units consistent with fat). 2
• Pathognomonic dumbbell configuration: Fat extends from the coronary sinus to just above the aortic root with characteristic sparing of the fossa ovalis. 2, 3
• Typical dimensions: Craniocaudal extent 6-9 cm, along interatrial septum 3.6-6.2 cm, perpendicular to septum 1.5-4.8 cm. 2
• Diagnostic threshold: Interatrial septal thickness ≥20 mm (median 32 mm in confirmed cases). 3

Cardiac MRI Alternative:

• Cardiac MRI with dedicated protocols provides equivalent diagnostic accuracy when CT is contraindicated. 1
• MRI demonstrates characteristic fat signal intensity on T1-weighted sequences with signal suppression on fat-saturated sequences. 1

Differential Diagnosis to Exclude

The characteristic dumbbell shape with fossa ovalis sparing on CT/MRI confidently excludes other cardiac masses, including: 2

• Atrial myxoma (typically arises from fossa ovalis, not spared)
• Thrombus (does not demonstrate fat attenuation)
• Metastatic disease (enhances with contrast)
• Liposarcoma (extremely rare in this location, shows heterogeneous enhancement)

Clinical Correlation Required

Assess for Associated Conditions:

• Atrial arrhythmias occur in 61.9% of patients with LHIS - obtain 12-lead ECG and consider ambulatory monitoring if symptomatic. 3, 4
• Evaluate for superior vena cava obstruction if the mass protrudes into the right atrium or SVC, causing symptoms of venous congestion. 5
• Document obesity and age - LHIS typically occurs in elderly, obese patients (mean age 72 years). 3

Associated Imaging Findings:

• Increased epicardial fat is present in 75-83% of cases. 2, 3
• Pulmonary emphysema occurs in 64.3% of patients. 3
• Mediastinal lipomatosis is seen in 50% of cases. 2

Special Consideration: FDG-PET Findings

If incidental increased FDG uptake is noted in the interatrial septum on PET imaging, correlate with CT or MRI to assess for LHIS and avoid false interpretation of malignancy. 6

• LHIS can demonstrate increased FDG uptake despite being benign fatty infiltration. 1, 6
• This finding is associated with atrial arrhythmias and should not be mistaken for malignancy. 1

Management Algorithm

If Imaging Confirms LHIS:

• No intervention is required for asymptomatic patients - LHIS is a benign condition. 2, 3
• Treat atrial arrhythmias medically if present, using standard antiarrhythmic protocols. 3, 4
• Surgical resection is reserved only for:
  • Hemodynamically significant obstruction of right atrial inflow or SVC. 5
  • Refractory arrhythmias directly attributable to the mass after failed medical management. 5

If Imaging is Atypical:

• Biopsy is NOT routinely indicated when imaging demonstrates classic features. 2
• Consider alternative diagnosis and further evaluation if the mass enhances with contrast, lacks fossa ovalis sparing, or demonstrates heterogeneous attenuation. 2

Key Clinical Pitfalls to Avoid

• Do not mistake LHIS for atrial myxoma - myxomas arise FROM the fossa ovalis, while LHIS SPARES it. 2
• Do not perform unnecessary cardiac surgery - LHIS is benign and asymptomatic in most cases (incidence 2.2% on screening CT). 3
• Do not overlook associated arrhythmias - ECG abnormalities occur in >60% of patients and require appropriate management. 3
• Do not misinterpret FDG-PET uptake as malignancy - always correlate with anatomic imaging. 6