Segmental Liver Anatomy

Introduction

Historically, the liver has been divided into right and left lobes based on morphological appearance.¹ Now, the widley used Couinaud Classification System divides the liver into functional segments.¹ Having a good understanding of this classification system is critical for consistent and clear communication between radiologists and surgeons.² It is also useful for localising findings within the liver. For example, by the end of this article you will hopefully be able to identify which segment the tumour seen in Figure 1 is located in.³ Knowledge of the Couinaud System also helps in understanding the early pathological changes seen in segment 1 during early cirrhosis.

Couinaud Classification System

In 1957, a French surgeon, Claude Couinaud, proposed a system for dividing the liver into functional segments. This Couinaud classification is still used today and is based on the orientation of the hepatic veins and portal veins.²

There are three orders of division which result in 9 different segments, with segment 4 being separated into segment 4a and segment 4b. This results in the segments being labelled from 1-8.

The first order of division results in two hemi-livers (which can be referred to as the right and left liver). The second order of division results in four sections (lateral, medial, anterior, and posterior). Lastly, the third order of division divides the four sections in two, resulting in segments 2-8.⁴ The caudate lobe is classified as segment 1 and is not a part of either the right or left hemi-liver.²

Each individual segment can act independently as each has its own blood supply, venous drainage, and biliary drainage.¹ Therefore, this system is also helpful for determining liver resections since one segment can be removed without affecting the function of other segments.⁵ Figures 2-5 show the liver from different views with the segments labelled.^6-9

Segmental Division

The middle hepatic vein divides the liver into left hemi-liver and right hemi-liver (Figure 6).^{10, 11}

Next, the right hepatic vein divides the right hemi-liver into anterior and posterior sections, while the left hepatic vein divides the left hemi-liver into lateral and medial sections.^1,5,10

The portal veins are orientated horizontally as shown in Figure 7.¹² This horizontal orientation creates what is called the "portal plane" and this divides the sections of the liver (lateral, medial, anterior and posterior) into superior and inferior segments. Remember segment 4 is separated into 4a and 4b).^1,5,10

The resulting segments are numbered clockwise, starting from the superior lateral segment, segment 2.

Detailed Segmental Anatomy

Segment 1

Segment 2

Segment 3

Segment 4

Segment 5

Segment 6

Segment 7

Segment 8

Terminology

The terminology used to define the livers anatomy has been through a confusing and inconsistent history. The International Hepato-Pancreato-Biliary Association (IHPBA) Terminology Committee addressed this confusion by proposing nomenclature in 2000 referred to as the Brisbane 2000 Terminology of Hepatic Anatomy and Liver Resections.^2,4 Since then, the proposed terminology has become widespread and is commonly used. It is important to know as it allows clear communication between surgeons, radiologists, and other physicians.

The terminology is based on the Couinaud Classification (discussed above) as each segment can act independently and can be resected without damaging surrounding segments. The terminology proposed by the IHBPA Terminology Committee is shown digramatically in Figure 8 and is discussed below.¹⁵

Right Hemi-Hepatectomy

Left Hemi-Hepatectomy

Right Anterior Sectionectomy

Right Posterior Sectionectomy

Left Medial Sectionectomy

Left Lateral Sectionectomy

Segmentectomy

Bisegmentectomy

Right Trisectionectomy

Left Trisectionectomy

As you may have noticed, none of the above definitions include resection of segment 1. The convention for relaying the inclusion of the caudate lobe in hepatic resections such as a hemi-hepatectomy or a trisectionectomy is:

Since the Brisbane 2000 Terminology, the "New World" terminology has been proposed as an update to the nomenclature and is starting to be seen in the literature. You can read more about the "New World" terminology in this article.

Clinical Application

Understanding the segmental anatomy helps localise liver lesions and it also helps understand the physiology behind some pathological findings.

As mentioned above, the caudate lobe (segment 1) drains directly into the inferior vena cava. On the other hand, the other segments drain into hepatic veins before subsequently draining into the inferior vena cava. Because of this, when there is increased venous pressure in segments due to portal hypertension such as in cirrhosis, the segments typically atrophy. The caudate lobe is often spared and hypertrophies as a compensatory mechanism since there is not the same venous pressure as it bypasses the hepatic veins, draining directly into the inferior vena cava.⁵ This hypertrophy is a specific finding in early cirrhosis, and a caudate lobe to right hemi-liver over 0.65 is especially specific for cirrhosis.⁵

Common Variants

Named after German Surgeon Bernhard Riedel, Riedel's lobe is an anatomical variation of the liver. There is inferior extension of the right hemi-liver and is often mistaken for hepatomegaly (Figure 9).^1,14 This variant is seen in ~5-10% of females and is rarely seen in males.^1,14