Involvement of small intrahepatic bile ducts is a characteristic but underexplored feature of autoimmune liver diseases (AILD) in children. A distinctive aspect of pediatric AILD is the overlap between autoimmune hepatitis (AIH) and primary sclerosing cholangitis (PSC), which complicates differential diagnosis – especially in cases with isolated small-duct injury and absence of typical PSC features on MRCP.

The aim of the study was to assess the spectrum and frequency of biliary injury in children with autoimmune hepatitis, autoimmune sclerosing cholangitis, and primary sclerosing cholangitis using the modified Nakanuma scoring system and additional morphological criteria of cholestasis.

Materials and methods. Sixty-nine children with AILD were included: 26 with AIH, 29 with ASC, and 14 with PSC. Liver biopsies were evaluated using the Nakanuma classification. Additional features assessed included periductal fibrosis and signs of cholestasis (biliary interface, rosettes, ballooning / feathery hepatocyte degeneration, portal / periportal edema, and mucobilia).

Results. The median age at diagnosis was 11 years (IQR: 8–14). Advanced liver fibrosis (F3–F4) was present in 54 % of AIH, 79 % of ASC, and 43 % of PSC cases. Cirrhosis was observed in 24 % of ASC, 19 % of AIH, and 7 % of PSC patients. Inflammatory bowel disease was common in biliary phenotypes – detected in 72 % of ASC and 71 % of PSC patients, and in only 3.8 % of AIH cases. According to MRCP, large-duct involvement was seen in 71 % of PSC and 72 % of ASC patients, while isolated small-duct injury was found in 29 % and 28 %, respectively. ASC and PSC patients showed a broader range of biliary lesions compared to AIH. Statistically significant differences were observed in the frequency of ductopenia ≥ grade 2 (ASC – 52 %, PSC – 64 %, AIH – 19 %), advanced periductal fibrosis ≥ grade 3 (ASC – 62 %, PSC – 71 %, AIH – 8 %), and mucobilia (ASC – 66 %, PSC – 36 %, AIH – 4 %; p < 0.001). Cumulative scoring of biliary features reliably distinguished AIH (mean 3.4 ± 1.4) from ASC (6.4 ± 1.6) and PSC (6.7 ± 1.9, p < 0.001).

Conclusions. Extended morphological analysis using the Nakanuma classification and additional biliary injury criteria improves diagnostic accuracy for identifying the biliary phenotype of AILD in children, including small-duct variants of ASC and PSC. This approach supports more precise diagnosis and individualized treatment strategies.