What is lipomatosis of the perirenal fat?

Lipomatosis of the Perirenal Fat: Definition, Diagnosis, and Management

Lipomatosis of the perirenal fat, also known as renal replacement lipomatosis, is a rare condition characterized by excessive proliferation of fatty tissue that replaces the renal parenchyma, typically following severe atrophy or destruction of kidney tissue.

Definition and Pathophysiology

• Renal replacement lipomatosis involves the proliferation of fibrofatty tissue that replaces normal renal parenchyma, usually unilaterally 1
• The condition represents an end-stage process where kidney tissue is progressively replaced by fatty infiltration in the renal sinus, hilum, and perirenal spaces 2
• In severe cases, the entire renal parenchyma may be replaced by fat, hence the term "total renal replacement lipomatosis" 1

Etiology

• Most cases (>75%) are associated with chronic kidney inflammation, typically due to chronic calculous disease 3
• Other causes include:
  • Long-standing chronic inflammation or urinary tract infection 4
  • Renal atrophy from various causes 4
  • Aging-related changes 4
  • Post-transplantation changes (rare) 4
• Some cases are idiopathic, with no identifiable cause 5

Clinical Presentation

• Often asymptomatic and discovered incidentally on imaging 5
• May present as a renal mass clinically, mimicking other conditions 5
• Can be associated with symptoms of the underlying cause (e.g., renal stones, infection)
• Rarely causes spontaneous bleeding, unlike angiomyolipomas 6

Diagnosis

Imaging Characteristics

• CT scan: Most accurate for diagnosis, showing:

  • Low-density fatty tissue replacing renal parenchyma 5
  • Nodular masses of adipose tissue within kidney tissue 5
  • Possible coexisting calculi or signs of chronic infection 2

• MRI: Provides multiparametric assessment that can help characterize the fatty tissue 6

  • Preferred in younger patients to avoid radiation exposure 6
  • Useful for differentiating from other fat-containing renal lesions

• Ultrasound: May suggest the diagnosis but is less specific 2

  • Typically shows hyperechoic areas corresponding to fatty tissue 6
  • Limited in patients with large body habitus 6

Differential Diagnosis

• Angiomyolipoma (AML): Contains fat but also vascular elements and smooth muscle 5

  • Unlike lipomatosis, AMLs have fragile vessels that can bleed spontaneously 6
  • AMLs typically appear as discrete masses rather than diffuse replacement 6

• Lipoma: Benign fatty tumor without the replacement pattern 5

• Xanthogranulomatous pyelonephritis: Inflammatory condition that can coexist with lipomatosis 2

• Renal cell carcinoma: Critical to exclude, especially with fat-poor lesions 6

Management

• Treatment is generally conservative if asymptomatic and diagnosis is certain 3

• Surgical intervention may be considered when:

  • Diagnosis is uncertain and malignancy cannot be excluded 5
  • Complications from the underlying cause (e.g., recurrent infections, pain) 1
  • Symptomatic presentation requiring intervention 3

• Nephrectomy may be performed in cases of non-functioning kidney with complete replacement 1

• Follow-up imaging is recommended to monitor progression, using the same imaging modality for consistency 6

Clinical Pearls and Pitfalls

• Important distinction: Unlike angiomyolipomas, renal replacement lipomatosis rarely causes spontaneous bleeding as it lacks the abnormal vasculature characteristic of AMLs 6

• Diagnostic challenge: May be mistaken for other fat-containing renal lesions, particularly angiomyolipomas or renal cell carcinomas with fatty components 5

• Progressive nature: The condition typically develops over years, with gradual replacement of kidney tissue by fat 3

• Biopsy considerations: Generally not required if imaging is characteristic, but may be necessary to exclude malignancy in uncertain cases 6

• Awareness is key: The condition is often missed or misdiagnosed due to its rarity and similarity to other conditions 5