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Crigler-Najjar syndrome type 2 (CN-2) is a rare disorder that causes elevated levels of bilirubin in the blood (hyperbilirubinemia). Bilirubin normally is made by the body when old red blood cells are broken down. However, people with CN-2 develop hyperbilirubinemia even when red blood cells are not excessively broken down, because they have too little of a liver enzyme needed for conversion and excretion of bilirubin.[14888][14889][14890]
The main symptom of CN-2 is persistent jaundice, which is yellowing of the skin, mucous membranes and whites of the eyes. Jaundice may become noticeable in infancy (particularly when an infant is sick or has not eaten for an extended time), but some people with CN-2 are not diagnosed until adulthood. Rarely, a person with CN-2 may develop bilirubin encephalopathy (also called kernicterus), especially during illness, prolonged fasting, or while under anesthesia.[14888][14889][14890]
CN-2 is caused by mutations in the UGT1A1 gene and inheritance is autosomal recessive. CN-2 responds to treatment with phenobarbital; however during an episode of severe hyperbilirubinemia, phototherapy may be needed. Not all people with CN-2 require treatment, but routine monitoring is still recommended.[14888][14889][14891]
Of note, mutations in the UGT1A1 gene can alternatively cause other disorders, such as Crigler-Najjar syndrome type 1 (CN-1) and Gilbert syndrome. CN-1 is characterized by near or complete absence of enzyme activity (versus partial absence in type 2) and severe, life-threatening symptoms. Phenobarbitol treatment is ineffective for people with CN-1, which is treated differently.[14888][14889][14890] Gilbert syndrome is considered a mild liver disorder that often does not cause symptoms or causes mild jaundice.[14889][14892] Sometimes it can be hard to distinguish between Gilbert syndrome and CN-2 because of considerable overlap in measured bilirubin levels. Genetic testing to identify the specific mutation present is sometimes needed for the correct diagnosis.[14892]
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