PSC is divided into forms according to which bile ducts are involved and what the cholangiogram — an image of the biliary tree — shows.
Large-duct PSC
Large-duct PSC is the most common and best known form. It is more frequent in men, with a male-to-female ratio of about 3:2, and it usually appears in the third and fourth decades of life. In more than 60–80% of cases it occurs alongside inflammatory bowel disease (IBD), the chronic inflammation of the gut that includes ulcerative colitis and Crohn disease. Its natural history ranges from a rapidly progressive course to an indolent one that changes little over years, so the diagnosis alone does not tell you how fast the disease will move.
Large-duct PSC is also strongly associated with certain human leukocyte antigen (HLA) types, the inherited proteins that present antigens to the immune system. The types repeatedly linked to the disease are HLA-A*01, HLA-B*08 and HLA-DRB1*3 — the last two being the same markers written as B8 and DR3 in older texts.
Small-duct PSC
Small-duct PSC is defined by histological findings — what a liver biopsy shows — that are compatible with large-duct PSC, but without the ERCP (endoscopic retrograde cholangiopancreatography) findings of the large-duct form. In other words, the tissue looks like PSC while the large ducts, which ERCP images, look normal. Small-duct PSC accounts for 5–20% of all cases. In the past many of these patients were labelled AMA-negative — negative for antimitochondrial antibody, the marker of primary biliary cholangitis — or as having autoimmune cholangiopathy. The distinction is not permanent: in 15% of patients with small-duct PSC, classical large-duct PSC becomes visible on extended follow-up.