Lipomatous Hypertrophy of the Interatrial Septum (LASH): A Multimodality Imaging Case Report and Review

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 Lipomatous Hypertrophy of the Interatrial Septum (LASH): A Multimodality Imaging Case Report and Review

심방 중격 지방종성 비대(LASH): 다중 모달리티 영상 증례 보고 및 검토

Keywords: lipomatous hypertrophy, interatrial septum, FDG PET/CT, cardiac fat, LASH, atrial arrhythmia, heart imaging, benign cardiac lesion, aging heart

Introduction

Lipomatous hypertrophy of the interatrial septum (LASH) is a rare but clinically significant benign cardiac condition. Although usually asymptomatic and discovered incidentally, LASH can mimic malignant lesions on FDG PET/CT, creating diagnostic challenges in oncologic patients undergoing restaging. This article presents a comprehensive discussion of LASH, grounded in a real case of an elderly woman undergoing restaging for non-small cell lung cancer. Through this case, we explore the imaging characteristics, pathophysiology, differential diagnosis, and clinical implications of LASH.

Case Overview

Patient History

An 80-year-old woman with a prior history of segmentectomy and left upper lobectomy for non-small cell lung cancer presented for cancer restaging. A whole-body FDG-PET/CT was performed to evaluate potential recurrence or metastasis.

Imaging Findings

FDG-PET/CT Axial Image (Figure 1):
Demonstrated focal increased FDG uptake in the interatrial septum, exceeding blood pool activity.

CT Image (Figure 2):
Revealed a well-defined, non-enhancing, low-attenuation mass within the interatrial septum, consistent with fat (−80 to −120 HU). The lesion had the characteristic “dumbbell” or “hourglass” configuration, sparing the fossa ovalis.

Imaging Interpretation

Figure 1. FDG-PET image showing increased uptake in the interatrial septum (SUVmax elevated relative to blood pool).

Figure 2. Axial CT image showing a hypodense mass in the interatrial septum with fat attenuation, consistent with LASH.

Diagnosis

Based on the PET/CT findings and classic CT morphology, the diagnosis of lipomatous hypertrophy of the interatrial septum (LASH) was made.

Pathophysiology of LASH

Lipomatous hypertrophy of the interatrial septum is a benign, non-encapsulated mass characterized by fat accumulation in the interatrial septum, often sparing the fossa ovalis. Histologically, the lesion contains both mature white fat and metabolically active brown fat, explaining the elevated FDG uptake on PET.

  • Misnomer Clarification: The term “lipomatous hypertrophy of the atrial septum” is technically imprecise because the mass often excludes the fossa ovalis, which is a true component of the atrial septum.

  • Etiology: Remains unknown, though associations with advanced age and obesity are well documented.

  • Prevalence: Estimated at 2% to 8% in autopsy and imaging studies, increasing with age and BMI.

Clinical Presentation

LASH is generally asymptomatic and incidentally found during imaging for unrelated issues. However, it may occasionally present with:

  • Atrial arrhythmias, particularly atrial fibrillation or flutter

  • Rarely, syncope or conduction abnormalities

  • Sudden cardiac death, in exceedingly rare circumstances

Differential Diagnosis

Given its metabolic activity and appearance, LASH must be differentiated from malignant or metastatic lesions:

ConditionFDG UptakeCT AttenuationMRI Characteristics
LASHModerate−80 to −120 HU  High T1 signal with fat suppression
Cardiac MetastasisHighSoft tissue  Enhancing mass, non-fatty
Atrial MyxomaVariableMixed  Often mobile with a heterogeneous signal
RhabdomyomaMildSoft tissue  Isointense on T1, hyper on T2
LipomaNoneFatty   Encapsulated, homogeneous fat signal

Multimodal Imaging Characteristics

  • FDG-PET/CT: Increased uptake due to brown fat content, leading to potential misdiagnosis as malignancy.

  • CT: Fat-density lesion in the interatrial septum, sparing the fossa ovalis.

  • MRI: T1 hyperintense lesion with signal dropout on fat-saturation sequences.

Clinical Management

  • No intervention is required for asymptomatic LASH.

  • Awareness of LASH is essential during transseptal puncture procedures (e.g., atrial fibrillation ablation, left atrial appendage occlusion), as a thickened septum increases procedural risk.

  • In symptomatic patients with atrial arrhythmia, treatment is directed at the arrhythmia, not the LASH.

Quiz

1. What is the half-life of the radiotracer used in FDG-PET?

(1) 13 hours
(2) 6 hours
(3) 110 minutes
(4) 75 seconds

2. What is the photon energy of the annihilation event used in PET imaging?
(1) 140 keV
(2) 159 keV
(3) 364 keV
(4) 511 keV

3. What is the prominent abnormality in this case (ignoring post-surgical lung changes)?

(1) Anterior mediastinum
(2) Interatrial septum
(3) Interventricular septum
(4) Superior vena cava

4. What is the tissue density of the lesion on CT?
(1) Simple fluid
(2) Air
(3) Bone
(4) Fat

5. What is the most likely diagnosis?
(1) Interatrial lymph node metastasis
(2) Cardiac metastasis
(3) Cardiac myxoma
(4) Cardiac rhabdomyoma
(5) Lipomatous hypertrophy of the interatrial septum

6. What is a notable clinical implication of LASH?
(1) Requires chemotherapy
(2) Contraindication for CT
(3) Increases risk in transseptal procedures
(4) Requires surgical removal

Answer & Explanation

1. Answer: (3) 110 minutes, Explanation: F-18, the isotope in FDG, has a half-life of approximately 110 minutes.

2. Answer: (4) 511 keV, Explanation: PET imaging detects 511 keV photons resulting from positron-electron annihilation.

3. Answer: (2) Interatrial septum. Explanation: The FDG uptake and fat deposition are localized to the interatrial septum.

4. Answer: (4) Fat, Explanation: The lesion shows HU values consistent with fat (−80 to −120).

5. Answer: (5) Lipomatous hypertrophy of the interatrial septum. Explanation: Characteristic morphology and fat density are diagnostic.

6. Answer: (3) Increases risk in transseptal procedures. Explanation: The thickened interatrial septum can increase procedural difficulty and risk.

Conclusion

LASH is a benign but radiologically significant entity that requires careful differentiation from malignant cardiac lesions, especially in oncologic imaging. Recognition of this condition avoids unnecessary interventions and alerts clinicians to procedural risks associated with a thickened septum.

References

[1] C. M. Fan, A. J. Fischman, B. H. Kwek, S. Abbara, and S. L. Aquino, “Lipomatous hypertrophy of the interatrial septum: Increased uptake on FDG PET,” AJR Am J Roentgenol, vol. 184, no. 1, pp. 339–342, 2005.

[2] L. D. McLeod, R. Donnino, E. E. Kim, R. Benenstein, R. S. Freedberg, and M. Saric, “Lipomatous atrial septal hypertrophy: A review of its anatomy, pathophysiology, multimodality imaging, and relevance to percutaneous interventions,” J Am Soc Echocardiogr, vol. 29, no. 8, pp. 717–723, 2016.

[3] A. Xanthopoulos, G. Giamouzis, N. Alexopoulos, T. Kitai, F. Triposkiadis, and J. Skoularigis, “Lipomatous hypertrophy of the interatrial septum: A case report and review of the literature,” CASE (Phila), vol. 1, no. 5, pp. 182–189, 2017.

[4] D. M. Murphy et al., “Lipomatous hypertrophy of the interatrial septum: CT and MR features,” Radiographics, vol. 18, pp. 1097–1104, 1998.

[5] B. J. McKay and T. M. Wall, “Lipomatous hypertrophy of the interatrial septum: A benign cardiac pseudotumor,” Chest, vol. 101, no. 4, pp. 1141–1143, 1992.

[6] K. A. Miller, “Cardiac imaging of benign tumors: CT and MRI findings,” Clin Radiol, vol. 59, no. 7, pp. 576–585, 2004.

[7] J. M. Higgins and S. B. Newhouse, “Multimodality imaging of the heart,” J Nucl Med Technol, vol. 39, no. 3, pp. 161–169, 2011.

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