There are two types of gallstones that may be present in the gallbladder (cholelithiasis) or bile ducts (choledocholithiasis) :
- Cholesterol stones
- Pigment stones
Most gallstones are cholesterol stones but is usually mixed in composition as it contains some of the chemical characteristics of pigment stones as well. Biliary or gallbladder sludge is a form of gelatinous bile that can sometimes develop into a gallstone.
Cholesterol is a product of fat metabolism in the liver and is passed out with bile. Since it is insoluble, cholesterol binds with water-soluble bile salts and water-insoluble lecithin to exist as small micelles or vesicles (‘bubbles’) in the bile. Bile enters the gallbladder where it is stored and concentrated.
If the cholesterol concentration within the bile is too high, it may exceed the solubilizing capacity of the bile salts and precipitate. In addition, there is a diminished movement of bile, and mucus secretion within the gallbladder that allow the sediments to aggregate. For this reason, cholesterol stones almost always exist in the gallbladder.
Over time, the presence of bacteria and the action of the body’s immune cells breaks down bilirubin within the cholesterol stone and the chemical composition partially resembles a pigment stone. This is known as a mixed stone.
Features of Cholesterol Gallstones
- Apart from cholesterol, these gallstones also contain calcium salts – calcium phosphate, calcium bilirubinate, calcium carbonate. Mucin glycoprotein is also present and is the main compound that cements the stone.
- Cholesterol gallstones appear yellow in color if they are made of pure cholesterol, which is rare, but due to the presence of calcium salts and hydrolization of conjugated bilirubin, it can appear dark yellow to brownish-yellow.
- Cholesterol stones usually do not occur in isolation. Multiple stones may fill up the gallbladder and can be as large as several centimeters. Rarely, a single large cholesterol stone may be found.
- Cholesterol stones are almost always found in the gallbladder.
- Most cholesterol stones are radiopaque and are detectable upon x-ray.
- A large stone made up of purely cholesterol, which is rare, can be radiolucent and therefore not detected upon x-ray.
Pigment stones occur when there are high amounts of bilirubin, particularly unconjugated bilirubin, in the bile. While bile does contain small amounts of unconjugated bilirubin, most of these bilirubin is conjugated. Refer to Bile Production for more information.
Unconjugated bilirubin occurs as a result of hemolytic anemias, where large amounts of red blood cells are broken down, or when bacteria within the biliary tree is able to hydrolyze the conjugated bilirubin due to the action of bacterial enzymes.
Black and Brown Pigment Stones
Composition, Color and Consistency
- Pigment stones can be classified as brown or black pigment stones. The changes in color may be due to the amount of calcium salts, like calcium bilirubinate, and cholesterol which also affects the consistency. Mucin glycoprotein is also present and is the main compound that cements the stone.
- Brown stones usually have a higher amount of cholesterol compared to black stones.
- Black stones typically have a higher concentration of bilirubin compared to brown stones.
- Brown gallstones are softer and and more greasy in consistency.
- Black stones are firmer but crumble easily.
Size and Quantity
- Pigments stones are usually smaller than cholesterol stones and are rarely larger than 1.5 centimters.
- Brown stones are more likely to occur in isolation while several black stones may be found in the gallbladder.
- Black pigment stones are usually found in the gallbladder and is more likely to occur in cases of hemolytic anemias.
- Brown pigment stones are more frequently seen in the bile ducts and is commonly due to an infection.
- Most black pigment stones are radiopaque and can be detected upon x-ray.
- Brown pigment stones are often radiolucent and may not be detected upon x-ray.