Changes in bile composition

Pathogenesis of black pigment stones: Pigment gallstones invariably contain a mucin glycoprotein matrix (“scaffolding”)[26]. Black pigment stones develop in the sterile bile supersaturated in calcium bilirubinate. Supersaturation occurs in the presence of an increased concentration of unconjugated bilirubin or of an increased concentration of free ionized Ca2+ in the bile[27-30]. The unconjugated bilirubin fraction represents in physiological conditions less than 1% of the total amount of bilirubin in bile. It increases significantly in case of: (1) increased excretion of unconjugated bilirubin due to defective conjugation or hemolysis; (2) increased hydrolysis of conjugated bilirubin in bile due to enhanced beta-glucuronidase activity; (3) defective acidification of the bile due to mucin hypersecretion, resulting in increased ionization of unconjugated bilirubin and precipitation of Ca2+; (4) decreased solubilization of bilirubinate anions in the presence of reduced bile salt concentration; and (5) induced enterohepatic cycling of unconjugated bilirubin.

Increased hemolysis and/or hydrolysis of conjugated bilirubin in bile lead to a shift in the ratio of bilirubin conjugates in the bile of cirrhotic patients in favour of bilirubin monoconjugates, especially monoglucuronides[24]. Bilirubin monoglucuronide is more easily deconjugated in bile by the β-glucuronidase secreted by hepatic parenchymal[31] or biliary epithelial cells, or is deconjugated through non-enzymatic hydrolysis.

Most mechanisms involved in pigment lithogenesis are also present in liver cirrhosis. A higher prevalence of hypersplenism and hemolysis was found in cirrhotics with gallstones than in those without gallstones[10,25]. Hemolysis could be promoted in advanced liver disease by hypersplenism, Kupffer cell destruction and altered membrane lipid composition.

The very low bile salt/unconjugated bilirubin molar ratio found in cirrhotic patients as compared to controls is an independent physico-chemical factor predisposing to pigment gallstone formation[24]. The reduction of the global bile acid pool size in cirrhotic patients is due to the impaired bile acid synthesis in the liver. Solubilization of the unconjugated bilirubin in bile, which is dependent on its interaction with bile salts, is reduced in liver cirrhosis. Vlahcevic et al[32] found a decreased cholic acid, but relatively preserved chenodeoxycholic acid synthesis in cirrhotic patients. They explained the fact that cirrhotic patients form rather pigment than cholesterol stones by demonstrating a reduced secretion of phospholipids and especially of cholesterol in their bile[33].

The decreased biliary secretion of phosphatidylcholine and cholesterol diminishes the detergent effect on membranes of the bile salts, potentially leading to bile salt-induced injury of the gallbladder mucosa. This favors pigment lithogenesis not only by production of mucosal β-glucuronidase from the biliary epithelial cells, but also by mucin glycoprotein hypersecretion and possibly by reactive oxygen species (ROS) production[34].

An induced enterohepatic cycling of unconjugated bilirubin, favored by alcohol abuse and low-protein diets[35] might contribute to gallstone formation in liver cirrhosis.

Pathogenesis of cholesterol stones in liver cirrhosis: Cholesterol gallstones occur more rarely in cirrhotic patients. Coelho et al[21] in their study on 369 transplant recipients with liver cirrhosis and gallstones observed on direct examination of the explanted livers that 318 patients (86.2%) had pigment stones, and 51 patients (13.8%) had cholesterol stones. The type of gallstones was not evaluated in relation to the etiology of liver cirrhosis. Viral C infection was in 132 patients (33% of the transplanted patients) the cause of liver cirrhosis[21].

Cholesterol gallstones occur in liver cirrhosis mainly in patients with viral C and non-alcoholic fatty liver disease (NAFLD) cirrhosis, and are due to the cholesterol supersaturated bile. Chronic HCV infection seems to be a risk factor for GD in patients with liver cirrhosis: gallstones are more frequent in patients with viral C as compared with viral B or alcoholic cirrhosis[20,36]. An increased incidence of gallstones in subjects with chronic HCV infection was found not only in cirrhosis, but also in the stage of chronic viral C hepatitis[37].

Non-alcoholic fatty liver diseases is associated with an increased prevalence of gallstones[38-40] due to obesity and increased insulin resistance. The risk of GD increases with the severity of liver disease; the highest prevalence of gallstones was found in the more advanced stages of fibrosis (cirrhosis) in NAFLD patients[39].

Enhanced nucleation in bile

Both cholesterol and pigment stones form on a matrix of mixed mucin glycoproteins secreted by the epithelial cells lining the biliary tree. Mucin hypersecretion, favored by gallbladder wall inflammation, accounts for the enhanced nucleation in cirrhotic patients.

Advanced liver disease is associated with a reduced apolipoprotein (apo) A1 and apoAII secretion in alcoholic patients with liver disease[41,42] and also in cirrhosis of other etiology. This might contribute to the enhanced crystal nucleation in cirrhotics’ bile, as apo A-I and A-II act as antinucleating factors.

Family history of GD

GD is a complex disease, resulting from the interaction between environmental factors and numerous genetic influences. The familial aggregation of gallstones supporting the genetic influence on gallstone formation has been known since decades. A large family study, comprising 358 families with 1038 subjects having symptomatic gallstones, suggested that the genetic factors account for at least 30% of the etiology of symptomatic GD[58].

The first human susceptibility genes for cholesterol gallstone formation were recently identified. A genome-wide association study (GWAS) detected a highly significant association of GD with the DH19 polymorphism in the ABCG8 gene, the gene controlling the cholesterol transporter in the bile[59]. This variant of the ABCG8 gene was found to be associated with GD in a linkage and association study in siblings with gallstones[60], and it was later confirmed in many populations. An update inventory of human gallstone genes can be found in two recent reviews[61,62].

The Gilbert syndrome variant rs6742078 in the promoter of the UGT1A1 gene, the gene encoding uridine 5'-diphosphate (UDP)-glucuronyltransferase 1A1 (UGT1A1), that is responsible for bilirubin conjugation, was identified as a candidate gene for GD in a genome-wide association study of Sardinian subjects having increased serum bilirubin levels[63]. This variant promotes formation of pigment gallstones. It was confirmed as a candidate gene for pigment stones in German and Chilean patients[64], especially in men, and was shown to increase the risk not only for pigment gallstones, but for all types of gallstones. The association of the variant of UGT1A1 not only with the stone bilirubin content but also with the global risk for gallstones confirms the presence of common factors in the pathogenesis of cholesterol and pigment gallstones[64]. An increased gallstone risk was later found in Swedish twins carriers of this variant[65], supporting also the nucleation in the bilirubin supersaturated bile as an initial step in cholelithogenesis. Buch et al[64] calculated that the population-attributable fraction of the common ABCG8 and UGT1A1variants in men was 21.2%. If estimated for all European gallstone carriers, this fraction was between 15% and 20%[62].

It would be of interest to evaluate the presence of these variants in patients with liver cirrhosis and gallstones. A genetic susceptibility might represent an independent risk factor for the occurrence of gallstones in cirrhotic patients. It has been already demonstrated that a positive family history of GD increases the risk of developing symptoms in cirrhotic gallstone carriers[57].

Etiology of liver cirrhosis

All prevalence studies agree that the lithogenetic risk in patients with liver cirrhosis is related to the cirrhotic change of the liver, which develops as the end stage of chronic liver diseases of any etiology. However, for some liver diseases, the lithogenetic risk was demonstrated to be higher.

Chronic alcoholism: Friedman et al[66] did not find an association between chronic alcoholism and lithogenesis. Trotman and Soloway[67] observed that a history of alcoholism in cholecystectomized patients did not influence gallstone type, cholesterol or pigment. Some clinical studies found a protective effect of alcohol in moderate consumers (39 g/d), suggesting a reduced bile lithogenicity as responsible for this effect[68]. Other studies noted an association between pigment stones and chronic alcoholism without cirrhosis[69], which was explained by the direct effect of chronic alcohol consumption on the liver, bile and red blood cells (macrocytosis) leading to an increased proportion and decreased solubilization of unconjugated bilirubin in the bile. Alcohol consumption was shown in one study to reduce the risk of symptoms in noncirrhotic women with gallstones[70]. In summary, epidemiological studies have been contradictory regarding the protective/favoring effect of alcohol for gallstone formation in patients without liver cirrhosis.

But all studies agree that alcohol-related liver cirrhosis is associated with an increased prevalence of gallstones. And previous alcohol abuse was found to be an independent risk factor for gallstone formation in a prospective follow-up of cirrhotic patients of all etiologies[16]. Regarding the risk for gallstone symptoms, this is significantly lower in patients with alcoholic versus viral cirrhosis (OR = 0.23, P = 0.0116)[57].

Chronic HCV infection: HCV infection represents a major cause of liver cirrhosis in the developed countries, where its prevalence is rising[1]. Some prospective[71] and retrospective[19,72] studies evidenced a higher gallstone risk in patients with chronic HCV infection in the stage of liver cirrhosis. A study derived from a populational survey in the United States (NHANES III) found that anti-HCV positive men had a higher prevalence of gallstones than the HCV-negative, and that gallstone prevalence was higher in those with severe disease[35]. Stroffolini et al[20] noted a significantly higher prevalence of GD in viral C versus viral B or alcohol-related cirrhosis.

A study performed on 453 patients with chronic HCV-infection (cirrhotics excluded) demonstrated that HCV infection represented an independent risk factor for gallstone formation[37]. Prevalence of GD was higher in patients than in controls, gallstones occurred at a younger age and central obesity and fatty liver (steatosis) were the significant risk factors for their occurrence. A causal link between chronic HCV infection and GD was thus proved, at least for the subgroup of obese subjects with liver steatosis.

The increased insulin resistance, commonly present in obese subjects and in those with fatty liver disease, could represent the link between chronic HCV infection and cholesterol GD. Increased insulin resistance favors cholesterol lithogenesis via increased biliary saturation in cholesterol. Although gallstone composition has not been evaluated in these patients, one can presume that cholesterol stones are the prevalent type in patients with viral C liver disease.

NAFLD: NAFLD is characterized by the fat accumulation in the liver in the absence of alcohol abuse and includes a large spectrum of liver changes, from simple fatty liver to non-alcoholic steatohepatitis (NASH) and liver cirrhosis. Cholesterol GD and NAFLD share many common risk factors, such as insulin resistance, type 2 diabetes mellitus, central obesity, hypertriglyceridemia and metabolic syndrome. These common factors account for the higher prevalence of cholesterol GD in patients with NAFLD[38-40]. Given this frequent association, it was even suggested that routine liver biopsy for diagnosing and staging NAFLD might be justified during cholecystectomy[73].

A recent study by Fracanzani et al[39] showed a progressive increase of gallstone prevalence with the severity of fibrosis in NAFLD (P for trend = 0.0001): from a gallstone prevalence of 15% in fibrosis stages 0-2, to 29% in stage 3 and 56% in stage 4 (cirrhosis). Female gender, prediabetes/diabetes, central obesity, older age and metabolic syndrome were significantly more frequent in NAFLD patients with gallstones than in NAFLD patients without gallstones.

Obesity and type 2 diabetes mellitus

Abdominal (central) obesity, type 2 diabetes mellitus and hypertriglyceridemia are risk factors for cholesterol gallstones in the general population, and have also been found to be independent risk factors for GD in patients with liver cirrhosis[13,17,22]. Increased insulin resistance represents the link between these disorders, being also responsible for NAFLD development. The increased gallstone risk in cirrhotics with obesity and type 2 diabetes mellitus might thus mainly refer to patients with NAFLD-induced liver cirrhosis, but to date, no study has evaluated this aspect.

Prophylactic cholecystectomy

For the time being, there are no published randomized trials to evaluate the better approach for patients with silent gallstones: cholecystectomy or expectant management[78]. Prophylactic cholecystectomy is generally not recommended for gallstone carriers in the general population, except for some special situations. It should be also not recommended in cirrhotic patients given their higher surgical risk as compared to patients without liver disease. The management of asymptomatic gallstones found incidentally in these patients during abdominal surgery for another indication is controversial. Concomitant cholecystectomy might be a reasonable option for patients with well compensated cirrhosis undergoing elective abdominal surgery for other conditions[77]. However, in a small study on 34 patients with liver cirrhosis, all patients, even those in Child A or B class, who underwent additional cholecystectomy during the non-shunting operation for esophageal varices required blood transfusion[79].