Small duct primary sclerosing cholangitis (PSC) is less common than the classic "big duct" PSC. About 10% of patients with clinical PSC will have small duct disease, at least initially.
Features shared by small and big duct PSC:
(1) liver biopsy histology consistent with PSC
(2) cholestatic liver function tests
(3) exclusion of other liver disease (autoimmune, PBC, hepatitis, etc)
A differentiating feature is the appearance of intra- and extra-hepatic bile ducts on cholangiography:
(1) in large duct PSC these show a characteristic pattern of strictures
(2) in small duct PSC these are absent
Additional features of small duct PSC:
(1) Some patients may progress to large duct PSC over time.
(2) The risk of cholangiocarcinoma is low for a patient with persistent small duct PSC. However, cholangiocarcinoma can occur in a patient who later develops large duct PSC.
(3) It can recur after liver transplantation.
(4) The prognosis for persistent small duct PSC is better than that seen with big duct PSC with a slower progression.
(5) Patients with small duct PSC may show HLA-DRB1*13:01 or HLA-B*08, with the latter associated with inflammatory bowel disease.
