Mild Hepatomegaly with Fatty Infiltration and Focal Fat Sparing: What It Means

This imaging finding describes a slightly enlarged liver with diffuse fat accumulation (steatosis) but with localized areas where fat is absent—“focal fat sparing.” It’s a pattern, not a diagnosis.

Understanding the Components

• Mild hepatomegaly: Liver span >15.5 cm (US) or craniocaudal dimension >16 cm (CT/MRI).
• Fatty infiltration (hepatic steatosis): Triglyceride accumulation in >5% hepatocytes. Diffuse = uniform; geographic = patchy.
• Focal fat sparing: Well-defined region(s) of normal attenuation/signal intensity within fatty liver. Often near porta hepatis, gallbladder fossa, or falciform ligament.

Why Does Fat Sparing Occur?

• Altered venous flow (portal/hepatic) in spared region.
• Arterial supply variation (e.g., accessory left gastric artery).
• Mass effect from adjacent structures (colon, gallbladder).
• Glycogen deposition (glycogen spares fat).
• Prior radiation/chemotherapy field edge.

Associated Conditions

• NAFLD/MAFLD: Most common. Metabolic syndrome, insulin resistance.
• Alcoholic liver disease: Steatosis + sparing possible.
• Drug-induced (corticosteroids, tamoxifen, methotrexate).
• Viral hepatitis, autoimmune, metabolic disorders (Wilson’s, α1-antitrypsin).
• Pseudo-lesion mimic: Sparing can look like a mass on non-contrast CT; MRI (in/out-of-phase) or contrast CT clarifies.

Evaluation & Management

1. Confirm with MRI (chemical-shift) or contrast CT.
2. Assess etiology: LFTs, metabolic panel, viral serology, alcohol history, medication review.
3. Fibrosis staging: FibroScan, ELF test, or biopsy if indicated.
4. Treat underlying cause: weight loss, diabetes control, alcohol cessation, medication adjustment.
5. Surveillance: Periodic imaging/labs per etiology.

Key point: Focal fat sparing is benign but must be distinguished from focal fatty infiltration (which can mimic tumor). Correlation with clinical context and multiphase imaging avoids unnecessary biopsy.