Couinaud classification of hepatic segments

The Couinaud classification (pronounced kwee-NO) is used to describe functional liver anatomy. It is the preferred anatomy classification system as it divides the liver into eight independent functional units (termed segments) rather than relying on the traditional morphological description based on the external appearance of the liver. The segments are numbered in romanRoman numerals I to VIII.

The delineation of the segments is based on the fact that each segment has its own dual vascular inflow, biliary drainage and lymphatic drainage. In general each segment can be thought of as wedged shapedwedge-shaped with the apex directed towards the hepatic hilum (porta hepatis). At the apex a single segmental branch of the portal vein, hepatic artery and bile duct enter; whereas along the sides of each segment there is venous outflow through the hepatic veins so that a hepatic vein drains two adjacent segments. These veins run in 3 vertical planes that separate the segments:

• right hepatic vein located in the right intersegmental fissure, divides the right lobe into anterior and posterior parts
• middle hepatic vein lies in the main lobar fissure, divides the liver into right and left lobes (or right and left hemiliver): this vertical plane runs from the inferior vena cava to the gallbladder fossa and is known as Cantlie's line
• left hepatic vein located in the left intersegmental fissure, divides the left lobe into medial and lateral parts

A horizontal plane further divides the liver, known as the portal plane where the portal vein bifurcates and becomes horizontal, dividing the liver into superior and inferior units.

These 4 planes (3 vertical and 1 horizontal) divide the liver into the 8 segments.

Segments

• segment I is the caudate lobe situated posteriorly around the IVC and different to the other 7 segments. It may receive its supply from both the right and the left portal vein and is drained directly into the IVC by one or more small hepatic veins

The remainder of the segments (II to VIII) are numbered in a clockwise fashion starting superiorly in the left hemiliver:

• segments II and III are to the left of the left hepatic vein and falciform ligament with II superior and III inferior to the portal plane
• segment IV lies between the left and middle hepatic veins; it is subdivided into IVa (superior) and IVb (inferior) subsegments
  • easy tip: IVa is above and IVb is below the portal plane
  • segment IV includes the quadrate lobe
  • the falciform ligament is variable in location hence is not routinely used to identify segmental boundaries

Segment V to VIII make up the right hemiliver and are easier to describe:

• segment V is located below the portal plane between the middle and right hepatic veins
• segment VI is located below the portal plane to the right of the right hepatic vein
• segment VII is located above the portal plane to the right of the right hepatic vein
• segment VIII is located above the portal plane between the middle and right hepatic veins

Each hepatic vein therefore drains multiple adjacent segments, those that are bounded by the hepatic vein.

The division of the liver into self-contained units allows the surgical resection of individual segments and sections (two or more adjacent segments) without damaging those segments remaining. Hence for the liver to remain viable, resections occur along the hepatic veins and portal veins in the planes that define the boundaries of these segments.

History and etymology

It was first described by the French Surgeon Claude Couinaud in 1957. The notion of the Couinaud liver segments being based on the arrondissements (administrative districts) of Paris is a radiological urban myth ⁴, but sounds cool nonetheless.