This article was last reviewed and updated on May 31, 2018
by Stephanie Reid, MD FRCPC
Steatosis is liver disease where droplets of fat are found inside the cells normally found in the liver.
The causes of fatty liver disease include alcohol use, central obesity, diabetes, certain medications, diet, and genetic diseases (such as Wilson’s disease).
The liver is an organ found in the right upper part of your abdominal cavity. It is responsible for removing toxins, processing medications, and producing substances such as bile that are essential for breaking down and using food.
The liver can be affected by diseases other than tumours. These are broadly referred to as ‘medical liver disease’. In these diseases, there is damage to specific cell types or areas of the liver. The liver contains multiple types of cells which make up its structure and contribute to function. The main type of cell in the liver is called a hepatocyte. The liver also has biliary cells which line the inside of long channels called bile ducts, and endothelial cells which line the inside of blood vessels. There are also a variety of background of cells and materials that support and hold all of these other parts of the liver together. In medical liver disease, any of these structures may be affected or damaged.
Steatosis is one type of disease in a category of medical liver disease called ‘fatty liver disease’. In fatty liver disease there are fat droplets inside the hepatocytes. The causes of fatty liver disease include alcohol use, central obesity, diabetes, certain medications, diet, and genetic diseases (such as Wilson’s disease).
Steatosis usually has no symptoms but patients with steatosis may experience pain the right upper portion of their abdomen. Steatosis is commonly first discovered during an ultrasound or CT scan of the abdomen. If either of these tests sees fat inside your liver, your doctor may order a biopsy to determine the type of fatty liver disease.
If you were diagnosed with steatosis, that means your pathologist saw fat droplets inside the hepatocytes. Unlike normal hepatocytes, the hepatocytes in steatosis contain clear fat droplets which can be seen when the tissue is viewed under a microscope.
Pathologists use a scale to describe the amount of fat in a liver with steatosis. The scale is based on the percentage of liver cells that contain fat droplets:
The scale used by most pathologists includes:
Mild - fat droplets are seen within less than 33% of the hepatocytes in the biopsy
Moderate - fat droplets are seen within 33 - 66 % of hepatocytes in the biopsy
Severe - fat droplets are seen within greater than 66 % of hepatocytes in the biopsy.
The liver is divided into ‘zones’ and at the center of each zone is a structure called a ‘portal tract’. Portal tracts are important because they contain blood vessels and channels that move other substances such as bile in and out of the liver.
When examining a liver biopsy, your pathologist must first determine if the sample contains the minimum amount of portal tracts required to make an accurate diagnosis. The adequacy of the biopsy may be reported simply as “yes” or “no”, or the number of portal tracts seen may be stated.
The condition of the liver biopsy when viewed under the microscope is usually described. If the liver biopsy is brittle and has broken apart this will be described, as it may be a clue to specific liver conditions.
Ballooning is a word pathologists use to describe damaged or dying hepatocytes. They are called ‘ballooning’ because the hepatocyte swells to several times its normal size and the body of the cell becomes clear.
Ballooning hepatocytes are required for the diagnosis of several medical liver diseases. The amount of hepatocyte ballooning present will be described as mild, moderate, or severe.
Ballooning hepatocytes cannot be present for the diagnosis of steatosis and they are usually reported as absent in pathology reports for this condition.
The liver is made up of three main compartments:
The lobule which contains the hepatocytes
The portal tract which contains a vein, an artery, and a bile duct
The central vein which brings blood into the liver
Inflammatory cells can enter any of these areas and prolonged inflammation can damage the liver. If inflammatory cells are seen in your tissue, your pathologist will describe their location and the types of inflammatory cells present. The amount of inflammation will also be described on a scale of mild, moderate, or severe.
Steatosis is usually associated with no or mild inflammation.
Mallory bodies form as a result of damage to hepatocytes. When viewed under the microscope, they look like dense pink material inside of the cells. Mallory bodies are seen in specific forms of medical liver disease and their presence or absence helps guide pathologists to a diagnosis.
Fibrosis is a type of scar tissue that forms in the liver after damage. Most pathology reports comment on the amount of fibrosis and will assign it a ‘stage’. The stage is dependent on multiple factors including extent of initial injury, the length of time injury was occurring, and which parts of the liver were damaged. Too much fibrosis disrupts the architecture of the liver and prevents it from functioning properly.
There are several different classifications systems used to stage fibrosis but all of them include the type and amount of fibrosis seen. Cirrhosis is the last stage of fibrosis and it is characterized by large fibrous bands that form nodules in the liver. These nodules prevent the liver from carrying out its normal functions and may lead to a medical condition called ‘liver failure’.
The liver produces a substance called bile which is used to remove toxins from the body and digest food. The bile produced in the liver drains through channels called bile ducts into the small bowel. Each portal tract contains one bile duct.
There is a category of medical liver disease which effect the bile ducts. For this reason, your pathologist will examine the bile ducts to see if the normal amount of bile ducts are present and if any of the bile ducts are damaged.
Cholestasis is a word pathologists use to describe bile trapped in the liver. Trapped bile is important because it can cause liver injury. If cholestasis is seen, your pathologist will describe its location within the liver and the amount of trapped bile will be described as mild, moderate, or severe.
Iron can build up within the liver as a result of abnormal breakdown of iron, increased iron in the body (such as after multiple blood transfusions), or when the liver is not functioning properly (as in liver cirrhosis). This excess iron can be seen within the hepatocytes or within immune cells called macrophages. If iron is present in your tissue, your pathologist will report its location and severity.