What causes a lipoma?

Causes of Lipomas

Lipomas are benign, slow-growing fatty tumors that develop from mature fat cells, with no single definitive cause identified, though they likely result from a combination of genetic predisposition and possibly traumatic factors. 1

Characteristics and Prevalence

Lipomas are among the most common benign mesenchymal tumors, typically presenting as:

• Soft, doughy, mobile masses
• Usually painless and asymptomatic
• Slow-growing
• Most commonly found in subcutaneous tissues of the trunk, neck, shoulders, and back 2
• Usually appearing between 40-60 years of age 2
• Typically small (under 2 cm diameter), though some can grow larger 3

Potential Causes and Mechanisms

Genetic Factors

• Lipomas may have genetic components, as evidenced by:
  • Familial tendencies in some cases
  • Association with specific syndromes such as hereditary multiple lipomatosis, adiposis dolorosa (Dercum's disease), Gardner's syndrome, and Madelung's disease 2
  • Cytogenetic studies show rearrangements of chromosome 12 in solitary lipomas, which differ from multiple lipomas 4

Trauma-Related Mechanisms

There are two main theories for post-traumatic lipoma formation:

1. Mechanical Effect (Pseudolipoma):

   • Direct trauma may cause herniation of deeper fat through fascial planes
   • Fat prolapse through damaged fascia resulting from direct impact 5
   • This mechanism explains what are sometimes called "pseudolipomas" 6

2. Inflammatory/Cytokine-Mediated:

   • Trauma causing fat necrosis may trigger local inflammation
   • Cytokine release following soft tissue trauma and hematoma formation
   • These factors may promote preadipocyte differentiation and proliferation 6, 5
   • Clinical observation: extensive and slowly resolving hematomas often precede lipoma formation in trauma cases 5

Distinguishing Features from Malignant Tumors

It's important to differentiate lipomas from atypical lipomatous tumors (ALT) or well-differentiated liposarcomas:

• MRI can help differentiate in about 69% of cases 1
• Key MRI features suggesting ALT include nodularity, septations, and stranding 1
• The definitive diagnostic test is molecular demonstration of MDM-2 gene amplification by fluorescence in-situ hybridization 1
• True lipomas do not typically transform into malignant tumors 4

Clinical Implications

• Most lipomas are best left alone unless they are:

  • Rapidly growing
  • Painful
  • Causing functional limitations
  • Cosmetically concerning 2

• Treatment options range from:

  • Observation
  • Steroid injections
  • Surgical excision
  • Liposuction in select cases 2, 5

Lipomas are generally benign with excellent prognosis, but careful evaluation is needed to rule out malignant variants, particularly for deep, large, or rapidly growing lesions.