A 58-year-old man with obstructive jaundice undergoes imaging. Ultrasound shows dilated intrahepatic bile ducts with a normal common bile duct diameter. Which feature best distinguishes intrahepatic cholestasis from extrahepatic obstruction?

## Distinguishing Intrahepatic vs Extrahepatic Cholestasis ### Imaging as the Gold Standard Discriminator **Key Point:** The morphology of the biliary tree on ultrasound or CT is the single best discriminating feature between intrahepatic and extrahepatic obstruction. **High-Yield:** In intrahepatic cholestasis, the intrahepatic bile ducts dilate while the extrahepatic ducts (CBD, hepatic ducts) remain normal or even narrow. In extrahepatic obstruction, the entire biliary tree proximal to the block becomes dilated, including the CBD. ### Comparison Table | Feature | Intrahepatic Cholestasis | Extrahepatic Obstruction | |---------|--------------------------|---------------------------| | **Intrahepatic duct calibre** | Dilated (>2 mm) | Dilated (>6 mm) | | **CBD calibre** | Normal (<4 mm) | Dilated (>6 mm) | | **Hepatic ducts** | Dilated | Dilated | | **Imaging finding** | Dilated intrahepatic with normal extrahepatic | Dilated entire tree | | **Cause** | Hepatocellular injury, pregnancy, sepsis, drugs | Stone, tumour, stricture | **Clinical Pearl:** The "dilated intrahepatic ducts with normal CBD" pattern is pathognomonic for intrahepatic cholestasis and is the most reliable discriminator on imaging — more reliable than any single biochemical or clinical parameter. ### Why Imaging Trumps Biochemistry While both intrahepatic and extrahepatic obstruction can present with: - Elevated ALP and bilirubin - Pruritus and dark urine - Acholic stools (if complete obstruction) These features are **non-specific**. Only the **biliary tree morphology** definitively separates the two. **Mnemonic:** **IHCD** = **I**ntrahepatic ducts dilated, **H**epatic ducts dilated, **C**BD normal, **D**iagnosis = intrahepatic cholestasis.