Couinaud classification of hepatic segments
The Couinaud classification (pronounced kwee-NO) is currently the most widely used system 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 Roman 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 visualized as wedge-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 boundaries of each segment there is venous outflow through the hepatic veins so that a hepatic vein drains two adjacent segments (and each segment has multiple draining hepatic veins). These veins run in 3 vertical planes radiating from the intrahepatic IVC that separate 4 sections of the liver (a section is two segments on top of each other):
- right hepatic vein located in the right intersegmental fissure, divides the right lobe into right lateral (posterior) and right medial (anterior) sections.
- 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. To the right is the right medial section and to the left is the left medial section.
- left hepatic vein located in the left intersegmental fissure, divides the left lobe into left medial and left lateral sections.
A horizontal plane further divides the liver, known as the portal plane where the portal vein bifurcates and becomes horizontal, dividing each section (or sector) of the liver into superior and inferior segments:
- left lateral section: segment II above and segment III below the portal plane
- left medial section: segment IVa above and segment IVb below the portal plane
- right anterior (or medial) section: segment VIII above and segment V below the portal plane
- right posterior (or lateral) section: segment VII above and segment VI below the portal plane
segment I is the caudate lobe
- bounded posterolaterally by the fossa for the inferior vena cava, anteriorly by the ligamentum venosum, and inferiorly by the porta hepatis
- its inferior portion is subdivided into a lateral caudate process and a medial papillary process 6
- may receive its supply from both the right and the left portal vein
- is drained directly into the IVC by one or more small hepatic veins, explaining why it might undergo hypertrophy in certain pathologies
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.
A 'handy' mnemonic exists to remember the segments.
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
This anatomic division 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.