Caroli disease is one of non-obstructive biliary diseases that cause biliary dilatation,Once you have excluded obstruction, you have to think about nonobstructive biliary diseases as:
1- Caroli disease
2- Choledochal cyst
3- Recurrent pyogenic cholangitis
4- Primary sclerosing cholangitis
Caroli disease is an autosomal recessive disease secondary to malformation of the ductal plate .
It is associated with polycystic kidney disease, medullary sponge kidney and medullary cystic disease.
So looking at the kidneys can sometimes help you make this diagnosis.
In images below Notice the intrahepatic duct dilatation and the normal caliber of the choledochal duct (extrahepatic bile duct).
**The hallmark of Caroli disease is intrahepatic duct dilatation.
The dilatation can be very large and saccular as seen in the image or it can be very linear.
The duct dilatation in Caroli disease is due to a congenital malformation of the ductal plate, which is the precursor of the intrahepatic bile ducts.we can see in this diagram the normal development of the ductal plate .
Embryologically each bile duct starts as a single layer of cells that surrounds a portal vein.after that this layer duplicates.
Portions of this double layer fuse and resorb leaving unfused portions that become the bile ducts.
So in the normal condition each portal vein is surrounded by interconnecting bile ducts .However if the patient has ductal plate malformation, the bile ducts are too numerous and they are ectatic (as seen below).
Whether or not we see this on imaging depends on which portion of the bile ducts is affected.
If the large ducts are involved, we see this as Caroli disease.
However if only the very small ducts are involved, the result is congenital hepatic fibrosis.
If all ducts are involved, then there is a combination of fibrosis and Caroli disease, which is also known as the Caroli syndrome.