Gallstone Disease - Pathology PDF

65 Gallstone Disease David Q.-H. Wang, Nezam H. Afdhal CHAPTER OUTLINE cause symptoms and complications.15 An estimated 700,000 cho-...

65 Gallstone Disease David Q.-H. Wang, Nezam H. Afdhal CHAPTER OUTLINE cause symptoms and complications.15 An estimated 700,000 cho- lecystectomies are performed for gallstone disease, and medical TYPES OF GALLSTONES 1016 expenses for the treatment of gallstones exceeds $6 billion annu- ally.2 In addition, unavoidable complications of gallstones result EPIDEMIOLOGY 1016  in 3000 deaths (0.12% of all deaths) per year.1 In the USA, per- Risk Factors  1017 sons with gallstone disease have increased cardiovascular disease, Protective Factors 1020 cancer, and overall mortality.16 COMPOSITION AND ABNORMALITIES OF BILE 1020 TYPES OF GALLSTONES  Physical Chemistry of Bile 1020 Hepatic Secretion of Biliary Lipids 1023 Based on chemical composition and macroscopic appearance, PATHOPHYSIOLOGY 1024  gallstones are divided into 3 types: cholesterol, pigment, and rare Hepatic Hypersecretion of Biliary Cholesterol 1025 stones.3, 4, 17 The majority (≈75%) of gallstones in the USA and Rapid Cholesterol Nucleation and Crystallization 1025 Europe are cholesterol stones,15 which consist mainly of cho- Imbalance of Pronucleating and Antinucleating Factors 1026 lesterol monohydrate crystals and precipitates of amorphous Gallbladder Dysfunction 1027 calcium bilirubinate, often with calcium carbonate or phosphate in one of the crystalline polymorphs. These stones are usually Intestinal Factors 1028 subclassified as either pure cholesterol or mixed stones that con- Growth of Gallstones 1028 tain at least 50% cholesterol by weight. The remaining gallstones GENETICS 1028  are pigment stones that contain mostly calcium bilirubinate and PIGMENT STONES 1032 are subclassified into 2 groups: black pigment stones (≈20%) and brown pigment stones (≈4.5%). Rare gallstones (≈0.5%)  Black Stones 1032 include calcium carbonate stones and fatty acid–calcium stones. Brown Stones 1033 Gallstones also are classified by their location as intrahepatic, gall- NATURAL HISTORY 1034  bladder, and bile duct (choledocholithiasis) stones. Intrahepatic stones Asymptomatic Stones 1034 are predominantly brown pigment stones. Gallbladder gallstones Stones in Patients With Diabetes Mellitus  1034 are mainly cholesterol stones, with a small group of black pig- Symptomatic Stones 1034 ment stones. Bile duct stones are composed mostly of mixed cho- Special Patient Populations 1034 lesterol stones. DIAGNOSIS 1034 EPIDEMIOLOGY  US 1035 EUS 1038 Investigations of gallstone prevalence are more common than Oral Cholecystography 1039 those of gallstone incidence because of the nature of the statisti- Cholescintigraphy 1039 cal analyses. Prevalence is often defined as the number of cases ERCP 1039 of gallstones at any one point or period of time divided by the CT and MRI 1040 population at risk of forming stones. Incidence is usually defined as the number of new cases of gallstones occurring in a time period CLINICAL DISORDERS 1040  divided by the population at risk of forming stones. Therefore, Biliary Pain and Chronic Cholecystitis  1040 the determination of incidence requires that investigation for Acute Cholecystitis  1041 gallstones be performed at a minimum of 2 different times—that Choledocholithiasis 1043 is, at the beginning and at the end of an interval of time. By con- Cholangitis  1044 trast, prevalence can be determined by sampling at only one point in time—for example, at US screening or autopsy. UNCOMMON COMPLICATIONS 1045  Although determining the true incidence of gallstones in a Emphysematous Cholecystitis 1045 given population is not easy, a large study of the incidence of Cholecystoenteric Fistula 1045 gallstones in the Danish population has been performed.18 The Mirizzi Syndrome 1046 5-year incidence of gallstones was 0.3%, 2.9%, 2.5%, and 3.3% Porcelain Gallbladder 1046 for Danish men, and 1.4%, 3.6%, 3.1%, and 3.7% for Danish women ages 30, 40, 50, and 60, respectively. Women have a higher incidence than men at ages 30 and 40 years, but the dif- ference declines with increasing age. These incidence rates may Cholesterol cholelithiasis is one of the most prevalent and costly reflect an interaction between genetic and environmental factors digestive diseases in Western countries. At least 20 million on gallstone formation in the specific populations studied because Americans (≈12% of adults) have gallstones.1-11 The prevalence they are in accordance with estimated prevalence rates reported of gallstones appears to be rising due to the epidemic of obesity, for Denmark and other populations.19 In a major Italian study, associated with insulin resistance and the metabolic syndrome. the incidence of gallstones was obtained at 10 years’ follow-up Each year, roughly 1 million new cases are discovered.12-14 in an originally gallstone-free cohort in the town of Sirmione.20 Although many gallstones are “silent,” about one third eventually This study revealed that new cases of gallstones developed at a 1016  CHAPTER 65 Gallstone Disease 1017 Women 65.6 65 Men 40 Prevalence (%) 30 20 10 0 l nd pan isia ina esh om dia ssia Iran ark razi cks eru ites Italy way tina any nics hile ans la h d In u ha i Ja Tun C lad Kin R e n m B Bla P h..W r n m a C ric No rge Ger isp e T g d D S S Fig. 65.1 Prevalence rates of cholesterol n Ba nite U. U. A S. H Am. e gallstones by gender in 18 countries U U tiv based on US surveys. Na rate of 0.5% per year. Although age, female gender, parity, obe- Risk Factors sity, and hypertriglyceridemia were associated with gallstones in the cross-sectional prevalence study of Sirmione, multivariate Age and Gender analysis of risk factors for the formation of gallstones in the lon- Epidemiologic and clinical studies have found that cholesterol gitudinal study identified only age and obesity as risk factors. gallstones occur infrequently in childhood and adolescence, Differences in the incidence of gallstone formation among and the prevalence of cholesterol gallstones increases linearly different populations are striking, suggesting that genetic fac- with age in both genders and approaches 50% at age 70 years tors play a crucial role in the pathogenesis of cholesterol gall- in women.22,23 Furthermore, older adults are at higher risk for stones. Pathogenic factors are likely to be multifactorial and to complications of gallstones, and mortality from surgery is often vary among populations. Most relevant studies have found that unacceptably high in patients older than 65 years. Cholesterol the prevalence of gallstones in women ranges from 5% to 20% saturation of bile is significantly higher in older Swedes and between the ages of 20 and 55, years and from 25% to 30% after Chilean women than in younger controls, and age correlates the age of 50 years. The prevalence in men is approximately half positively with an increased hepatic secretion rate of biliary that of women of the same age. cholesterol.24,25 In animals, aging has been shown to be associ- US screening or autopsy data are often used to estimate the ated with increased cholesterol gallstone formation as a result of prevalence of gallstone disease in different populations, as illus- increased biliary secretion and intestinal absorption of choles- trated in Fig. 65.1. Although US screening cannot be used to terol, decreased hepatic synthesis and secretion of bile salts, and distinguish cholesterol from pigment stones, 70% to 80% of reduced gallbladder contractility.26 detected gallbladder gallstones are assumed to be cholesterol Epidemiologic investigations have found, and clinical studies stones. have confirmed, that at all ages, women are twice as likely as men The prevalence of gallstones in American Pima Indians was to form cholesterol gallstones. The difference between women investigated by oral cholecystography (OCG).21 The well-studied and men begins during puberty and continues through the child- Pima Indians in southern Arizona exhibit a high prevalence of gall- bearing years because of the effects of female sex hormones15 and stones, which occur in 70% of the women after the age of 25 years. differences between the sexes in metabolism of cholesterol by the Subsequently, real-time US was used for screening in nationally liver in response to estrogen. Human and animal studies have representative samples of civilian Mexicans, Hispanic white Amer- shown that estrogen increases the risk of cholesterol gallstones by icans, non-Hispanic white Americans, and non-Hispanic black augmenting hepatic secretion of biliary cholesterol, thereby lead- Americans of both genders ages 20 to 74. The cross-sectional ing to an increase in cholesterol saturation of bile.27-30 prevalence rates of gallstones were found to be highest in certain tribes of Native Americans (e.g., Pima Indians), higher in Hispanic Americans than in whites, and lowest in black Americans.14 Diet Fig. 65.2 shows the world distribution of cholesterol gall- Epidemiologic investigations have shown that cholesterol chole- stones. American Pima Indians are an extremely high-risk lithiasis is prevalent in populations that consume a Western diet population. Other high-risk populations include Native Ameri- consisting of high amounts of total calories, cholesterol, saturated can groups in North and South America and Scandinavians, of fatty acids, refined carbohydrates, proteins, and salt, as well as a whom 50% develop gallstones by age 50. By contrast, African low amount of fiber. The prevalence of cholesterol gallstone dis- populations show the lowest risk of gallstones. The prevalence ease is significantly higher in North and South American as well as of gallstones in Asian populations is intermediate. Within a European populations than in Asian and African populations.3,31 given population, first-degree relatives of index cases of per- Several clinical studies have found an association between the sons with gallstones are 4.5 times as likely to form gallstones increased incidence of cholesterol gallstones in China and west- as matched controls, thereby underscoring the importance of ernization of the traditional Chinese diet.32 In Japan, cholesterol genetic predisposition. cholelithiasis was once rare, but since the 1970s, the adoption 1018 PART VIII Biliary Tract High Intermediate Low No data Fig. 65.2 Prevalence of cholesterol gall- stones around the world. of Western-type dietary habits has led to a markedly increased TPN, biliary sludge often forms in the gallbladder because of incidence.33 prolonged fasting. In addition, the sphincter of Oddi may fail to relax, leading to preferential flow of bile into the gallbladder. Approximately 45% of adults and 43% of children form gall- Pregnancy and Parity stones after 3 to 4 months of TPN.38,39 Because patients who Pregnancy is a risk factor for the development of biliary sludge receive TPN often have serious medical problems and are not and gallstones.34 During pregnancy, bile becomes more litho- good candidates for abdominal surgery, prophylactic treatment genic because of a significant increase in estrogen levels, which to prevent gallstones should be prescribed if no contraindication result in increased hepatic cholesterol secretion and supersatu- exists. CCK octapeptide administered twice daily via an IV line rated bile. In addition, gallbladder motility is impaired, with a to patients on long-term TPN has proved to be safe and cost resulting increase in gallbladder volume and bile stasis. These effective40 and should be used routinely in TPN-treated patients. alterations promote the formation of sludge and stones in the gallbladder.35 Increased progestogen concentrations also reduce gallbladder motility. Because plasma concentrations of sex hor- Biliary Sludge mones, especially estrogen, increase linearly with duration of Biliary sludge is a crucial intermediate stage in the pathogenesis gestation, the risk of gallstone formation is high in the third tri- of both cholesterol and pigment gallstones because it facilitates mester of pregnancy. Increasing parity is probably a risk factor crystallization and agglomeration of solid plate-like cholesterol for gallstones, especially in younger women. monohydrate crystals, as well as precipitation of calcium biliru- binate, and ultimately develops into macroscopic stones.41,42 In addition, biliary sludge can induce acute cholecystitis, cholangi- Rapid Weight Loss tis, and acute pancreatitis. Furthermore, biliary sludge is associ- Rapid weight loss is a well-known risk factor for the formation of ated with many conditions that predispose to gallstone formation, cholesterol gallstones.36 As many as 50% of obese patients who including pregnancy, rapid weight loss, spinal cord injury, long- undergo gastric bypass surgery form biliary sludge and eventu- term TPN, and treatment with octreotide.3 Although biliary ally gallstones within 6 months after surgery. Gallstones also sludge is reversible in most cases, it persists or disappears and develop in 25% of patients who undergo strict dietary restric- reappears in 12% to 20% of affected persons and eventually leads tion. Furthermore, about 40% of these patients display symp- to gallstones.43 UDCA treatment of patients with persistent bili- toms related to gallstones within the same 6-month period. The ary sludge decreases the frequency of clinical complications of mechanisms by which rapid weight loss causes gallstone forma- biliary sludge (see later). tion include enhanced hepatic secretion of biliary cholesterol during caloric restriction, increased production of mucin by the gallbladder, and impaired gallbladder motility. Gallstones may Drugs be prevented in this high-risk population by prophylactic admin- Estrogens istration of UDCA, which, in a dose of 600 mg/day, has been Most, but not all, relevant clinical studies have shown that use of reported to reduce the prevalence of gallstones from 28% to 3% oral contraceptive steroids and conjugated estrogens in premeno- in obese patients on a very-low-calorie diet (see later).37 pausal women doubles the prevalence of cholesterol gallstones.15,44 Moreover, in a large French study of 45,984 postmenopausal women, use of hormone replacement therapy was associated with TPN an increased risk of cholecystectomy (hazard ratio [HR], 1.10); TPN is associated with the development of cholelithiasis and the increase in risk was limited to women who received unop- acalculous cholecystitis. As early as 3 weeks after initiation of posed estrogen (HR, 1.38).45  CHAPTER 65 Gallstone Disease 1019 Administration of estrogen to postmenopausal women and patients experience biliary symptoms.53 The sludge usually disap- estrogen therapy to men with prostatic carcinoma have similar pears after ceftriaxone is discontinued. 65 lithogenic effects.44,46 Therefore, estrogen has been proposed to be an important risk factor for the formation of cholesterol gall- stones. In mice, the hepatic estrogen receptor α, but not β, plays Lipid Abnormalities a crucial role in cholesterol gallstone formation in response to Epidemiologic investigations have shown that plasma HDL estrogen.29 The hepatic estrogen receptor α, which is activated by cholesterol levels are inversely correlated with the prevalence estrogen, interferes with the negative feedback regulation of cho- of cholesterol gallstones.54 By contrast, hypertriglyceridemia is lesterol biosynthesis by stimulating the sterol-regulatory element positively associated with an increased prevalence of gallstones.55 binding protein-2 (SREBP-2) pathway, with the resulting acti- These seemingly independent variables are actually interrelated vation of the SREBP-2–responsive genes in the cholesterol bio- because high plasma TG levels tend to increase with increasing synthetic pathway.30 These alterations lead to increased hepatic body mass and are inversely correlated with plasma HDL levels. secretion of newly synthesized cholesterol and supersaturation Interestingly, high plasma total and LDL cholesterol levels are of bile, thereby predisposing to precipitation of solid choles- not likely to be risk factors for the formation of gallstones. terol monohydrate crystals and formation of gallstones. More- over, genetic analysis in mice reveals that the G protein–coupled receptor 30 (GPR30), a novel estrogen receptor, is a gallstone Systemic Diseases gene named Lith18. GPR30 exerts a synergistic lithogenic action Obesity and Insulin Resistance with estrogen receptor α to enhance estrogen-induced gallstone Obesity is a well-known risk factor for cholelithiasis, and the formation.47 In addition, estrogen induces a decrease in plasma prevalence of gallstone is rising with the worldwide obesity epi- LDL cholesterol levels and an increase in plasma HDL choles- demic and the increasing incidence of insulin resistance.56,57 A terol concentrations. The decrease in plasma LDL levels is a large prospective study of obese women demonstrated a strong result of increased expression of the hepatic LDL receptor, which linear association between BMI and the prevalence of choleli- increases the clearance of plasma LDL. The increased uptake of thiasis.58 In this study, the risk of gallstones was 7-fold higher LDL by the liver may also result in increased secretion of cho- in women with the highest BMI (>45 kg/m2) than in nonobese lesterol into bile. High levels of estrogen may induce gallbladder control women. Obesity is associated with increased hepatic hypomotility and consequently bile stasis. secretion of cholesterol into bile, possibly because of higher enzymatic activity of HMG-CoA reductase and increased cho- Lipid-Lowering Drugs lesterol synthesis in the liver. As a result, gallbladder bile is more Lipid-lowering drugs may influence the formation of gallstones lithogenic in obese than in nonobese persons, and a higher ratio because they regulate key pathways in cholesterol and bile salt of cholesterol to solubilizing lipids (bile acids and phospholipids) metabolism. Clofibrate is a lipid-lowering drug associated with is observed in the former group. These alterations predispose to gallstone formation. Clofibrate induces cholesterol supersatu- cholesterol crystallization and gallstone formation. Gallbladder ration in bile and diminishes bile salt concentrations by reduc- motility is often impaired in obese persons, thereby promoting ing the activity of cholesterol 7α-hydroxylase (the rate-limiting mucin secretion and accumulation, as well as cholesterol crystalli- enzyme in bile salt synthesis of classical pathway) (see Chapter zation. The effect of pronucleating and antinucleating factors on 64).48 The 3-hydroxy-3-methylglutaryl coenzyme A (HMG- cholesterol crystallization and gallstone formation in gallbladder CoA) reductase inhibitors (statins) reduce the biliary cholesterol bile warrants further investigation in obese and nonobese sub- saturation index (CSI), but their role in the prevention or therapy jects. of gallstone disease requires further investigation in humans.49 The potent cholesterol absorption inhibitor ezetimibe prevents Diabetes Mellitus formation of cholesterol gallstones and facilitates dissolution of Patients with diabetes mellitus have long been considered to be at gallstones in gallstone-susceptible C57L mice. Ezetimibe may increased risk of developing gallstones because hypertriglyceride- also act as a potent biliary cholesterol-desaturating agent in mia and obesity are associated with diabetes mellitus and because patients with gallstones.50,51 Cholestyramine and nicotinic acid gallbladder motility is often impaired in patients with diabe- have no association with gallstone formation. tes mellitus.59 Proving that diabetes mellitus is an independent risk factor for gallstones has been difficult, however. Mice with Octreotide hepatic insulin resistance induced by liver-specific disruption The somatostatin analog octreotide increases the prevalence of of the insulin receptor are markedly predisposed to formation gallstones when administered to patients as treatment for acro- of cholesterol gallstones.60 Hepatic insulin resistance promotes megaly, with approximately 28% of treated acromegalic patients hepatic secretion of biliary cholesterol by increasing expression forming gallstones. Acromegalic patients who are treated with of the hepatic cholesterol transporters Abcg5 and Abcg8 through octreotide display dysfunctional gallbladder motility, sluggish the forkhead transcription factor FoxO1 pathway. It also reduces intestinal transit, and increased colonic deoxycholic acid forma- expression of the bile salt synthetic enzymes, particularly oxys- tion and absorption,52 all of which facilitate formation of choles- terol 7α-hydroxylase, thereby resulting in a lithogenic bile salt terol gallstones. profile. Ceftriaxone Diseases of the Ileum The third-generation cephalosporin ceftriaxone has a long dura- Disease or resection of the terminal ileum has been found to be tion of action, with much of the drug excreted in the urine. a risk factor for gallstone formation. For example, intestinal bile Approximately 40% of the drug, however, is secreted in an unme- salt absorption is often impaired in patients with Crohn disease, tabolized form into bile, where its concentration reaches 100 to who are at increased risk of gallstones.61 The loss of specific bile 200 times that of the concentration in plasma and exceeds its salt transporters (e.g., ileal apical sodium-dependent bile acid saturation level in bile. Once its saturation level is exceeded, cef- transporter) in the terminal ileum may result in excessive bile salt triaxone complexes with calcium to form insoluble salts, thereby excretion in feces and a diminished bile salt pool size, presumably resulting in formation of biliary sludge. Up to 43% of children with a consequent increase in the risk of cholesterol gallstones. who receive high doses of ceftriaxone (60 to 100 mg/kg/day) have These changes may also lead to formation of pigment gall- been reported to form biliary sludge, and about 19% of these stones because increased bile salt delivery to the colon enhances 1020 PART VIII Biliary Tract solubilization of unconjugated bilirubin, thereby increasing bili- celiac disease nor clinical studies of the impact of celiac disease rubin concentrations in bile.62 on the pathogenesis of gallstones have been reported, whether celiac disease is an independent risk factor for gallstone disease Spinal Cord Injuries remains largely unknown. Spinal cord injuries are associated with a high prevalence of gall- stones, which have been reported in some 31% of such patients, who have an annual rate of biliary complications of 2.2%. Protective Factors Although the complication rate associated with gallstones in patients with spinal cord injuries is at least 2-fold higher than Statins the rate of gallstones in the general population, the relative risk Use of statins has been associated with a decreased risk of gallstone is still low enough that prophylactic cholecystectomy is prob- disease in 2 large case-control studies. The first study compared ably not justified. The mechanisms responsible for the associa- 27,035 patients with gallstone disease who required cholecys- tion between spinal cord injuries and gallstone formation remain tectomy with 106,531 matched controls and showed a benefit to unclear. Gallbladder relaxation is impaired in these patients, long-term statin use (>20 prescriptions filled and use of statins but gallbladder contraction in response to a meal is normal. for >1.5 years)75; statin use was associated with a decreased risk of Therefore, the increased risk of gallstones is unlikely to be due gallstone disease requiring cholecystectomy (adjusted odds ratio to biliary stasis alone. [OR], 0.64). Similar results were observed in a population study from Denmark of 32,494 patients with gallstone disease matched NAFLD with 324,925 controls.76 The odds ratios of having gallstone dis- Both gallstone disease and NAFLD are highly prevalent in the ease in current and prior users of statins (>20 prescriptions filled) general population and often co-exist in the same populations were 0.76 and 0.79, respectively, compared with controls. (see Chapter 87). These epidemiologic and clinical studies raise the possibility that both disorders could be casually related, similar risk factors influence the natural history of NAFLD and Ascorbic Acid gallstone disease, or NAFLD is indeed an independent risk fac- The observation that deficiency of ascorbic acid (vitamin C) is tor for cholesterol cholelithiasis. Although many clinical studies associated with the development of gallstones in guinea pigs have investigated the association between NAFLD and gall- prompted investigation of the relationship between ascorbic stone disease, the results have been variable.63-68 The relation- acid levels and gallstones in humans. Serum ascorbic acid lev- ship among insulin resistance (evaluated with the homeostatic els have been correlated with clinical or asymptomatic gallstones model assessment), liver fibrosis, NASH, and gallstone disease in 7042 women and 6088 men who were enrolled in the third has been studied in morbidly obese patients with NAFLD NHANES.77 Among women, but not men, each standard devia- before bariatric surgery.67 The prevalence of NASH is 18% in tion increase in serum ascorbic acid levels was associated with a a morbid obese population with gallbladder disease. The third 13% lower prevalence rate of clinical gallbladder disease. large U.S. National Health and Nutrition Examination Survey (NHANES) between 1988 and 1994 investigated 12,232 sub- jects by US and reported an association between gallstone dis- Coffee ease, with a prevalence of 7.4% for gallstones and 5.6% for In a 10-year follow-up of 46,000 male health professionals, sub- cholecystectomy, and NAFLD, with a prevalence of 20.0%.63 jects who consistently drank 2 to 3 cups of regular coffee per day The prevalence of NAFLD was significantly higher in the group were approximately 40% less likely to develop symptomatic gall- that underwent cholecystectomy (48.4%) and in the gallstone stones.78 Drinking 4 or more cups per day was even more ben- group (34.4%) than in the gallstone-free group (17.9%). These eficial (relative risk 0.55), but there was no benefit to drinking findings suggest that both conditions are tightly associated with decaffeinated coffee. A similar benefit to regular coffee was noted metabolic disturbances such as obesity, insulin resistance, dys- in a cohort study involving 81,000 women.79 lipidemia, and the metabolic syndrome. Celiac Disease Celiac disease is a chronic, small intestinal, autoimmune enter- COMPOSITION AND ABNORMALITIES OF BILE opathy caused by an intolerance to dietary gluten in genetically Physical Chemistry of Bile predisposed individuals (see Chapter 107). Clinical studies have found that, because of defective CCK release from the proximal Chemical Composition of Bile small intestine caused by enteropathy in patients with celiac dis- Cholesterol, phospholipids, and bile salts are the 3 major lipid ease before they start a gluten-free diet, gallbladder emptying in species in bile, and bile pigments are minor solutes. Cholesterol response to a fatty meal is impaired.67-72 Lack of CCK markedly accounts for up to 95% of the sterols in bile and gallstones; the enhances susceptibility to cholesterol gallstones via a mecha- remaining 5% of the sterols are cholesterol precursors and dietary nism involving dysmotility of both the gallbladder and the small sterols from plant and shellfish sources. intestine.73 Because a gluten-free diet can significantly improve Concentrations of cholesteryl esters are negligible in bile and celiac enteropathy, early diagnosis and therapy in celiac patients account for less than 0.02% of total sterols in gallstones. The is crucial for preventing the long-term impact of CCK defi- major phospholipids are lecithins (phosphatidylcholines), which ciency on biliary and intestinal function. When gluten is rein- account for more than 95% of total phospholipids; the remain- troduced in the diet, clinical and histologic relapse often occurs der consists of cephalins (phosphatidylethanolamines) and a trace in patients with celiac disease. Moreover, some patients do not amount of sphingomyelin. Phospholipids constitute 15% to 25% respond well to a gluten-free diet. Patients with celiac disease of total lipids in bile. Lecithins are insoluble amphiphilic mole- should routinely undergo US to determine whether gallblad- cules with a hydrophilic zwitterionic phosphocholine head group der motility function is preserved and whether biliary sludge (a and hydrophobic tails that include 2 long fatty acyl chains. Biliary precursor to gallstones) is present in the gallbladder. Impaired lecithins possess a saturated C-16 acyl chain in the sn-1 position intestinal CCK secretion is the link between celiac disease and and an unsaturated C-18 or C-20 acyl chain in the sn-2 position. cholesterol gallstone disease.74 Because neither epidemiologic The major molecular species of lecithins (with corresponding investigations of gallstone prevalence rates in patients with frequencies) in bile are 16:0 to 18:2 (40% to 60%), 16:0 to 18:1  CHAPTER 65 Gallstone Disease 1021 (5% to 25%), 18:0 to 18:2 (1% to 16%), and 16:0 to 20:4 (1% cholesterol. They can also solubilize and incorporate phospholip- to 10%). Lecithins are synthesized principally in the endoplas- ids to form mixed micelles that are capable of solubilizing at least 65 mic reticulum of the hepatocyte from diacylglycerols through the triple the amount of cholesterol compared with that solubilized cytidine diphosphate-choline pathway. The common bile salts by simple micelles. Mixed micelles (4 to 8 nm in diameter) are typically contain a steroid nucleus of 4 fused hydrocarbon rings large, thermodynamically stable aggregates composed of bile with polar hydroxyl functions and an aliphatic side chain conju- salts, phospholipids, and cholesterol. Their size depends on the gated in amide linkage with glycine or taurine. In bile, more than relative proportion of bile salts and phospholipids. The mixed 95% of bile salts are 5β,C-24 hydroxylated acidic steroids that are micelle is a lipid bilayer with the hydrophilic groups of the bile amide-linked to glycine or taurine in an approximate ratio of 3:1. salts and phospholipids aligned on the “outside” of the bilayer, Bile salts constitute approximately two thirds of the solute mass interfacing with the aqueous bile, and the hydrophobic groups of normal human bile by weight. The hydrophilic (polar) areas on the “inside.” Therefore, cholesterol molecules can be solubi- of bile salts are the hydroxyl groups and conjugated side chain of lized on the inside of the bilayer away from the aqueous areas on either glycine or taurine, and the hydrophobic (nonpolar) area is the outside. The amount of cholesterol that can be solubilized is the ringed steroid nucleus. Because they possess both hydrophilic dependent on the relative proportions of bile salts, and the maxi- and hydrophobic surfaces, bile salts are highly soluble, detergent- mal solubility of cholesterol occurs when the molar ratio of phos- like, amphiphilic molecules. Their high aqueous solubility is due pholipids to bile salts is between 0.2 and 0.3. Furthermore, the to their capacity to self-assemble into micelles when a critical solubility of cholesterol in mixed micelles is enhanced when the micellar concentration is exceeded. concentration of total lipids in bile is increased. The primary bile salts are hepatic catabolic products of cho- When model and native biles are examined by quasi-elastic lesterol and are composed of cholate (a trihydroxy bile salt) and light-scattering spectroscopy and electron microscopy, it is found chenodeoxycholate (a dihydroxy bile salt) (see Chapter 64). The that, besides micelles, vesicles solubilize cholesterol in bile. secondary bile salts are derived from the primary bile salt species Biliary vesicles are unilamellar spherical structures that contain by the action of intestinal bacteria in the ileum and colon and phospholipids, cholesterol, and little if any bile salts. Vesicles are include deoxycholate, ursodeoxycholate, and lithocholate. The substantially larger than either simple or mixed micelles (40 to 100 most important of the conversion reactions is 7α-dehydroxylation nm in diameter) but much smaller than liquid crystals (≈500 nm of primary bile salts to produce deoxycholate from cholate and in diameter) that are composed of multilamellar spherical struc- lithocholate from chenodoxycholate. Another important conver- tures. Because vesicles are present in large quantities in hepatic sion reaction is the 7α-dehydrogenation of chenodeoxycholate to bile, they could be secreted by hepatocytes. Unilamellar vesicles form 7α-oxo-lithocholate. This bile salt does not accumulate in are often detected in freshly collected samples of unsaturated bile but is metabolized by hepatic or bacterial reduction to form bile and are physically indistinguishable from those identified in the tertiary bile salt chenodeoxycholate (mainly in the liver) or its supersaturated bile. Dilute hepatic bile, in which solid cholesterol 7β-epimer ursodeoxycholate (primarily by bacteria in the colon). crystals and gallstones never form, is always supersaturated with Bile pigments are minor solutes and are formed as a meta- cholesterol because vesicles solubilize biliary cholesterol in excess bolic product of certain porphyrins. They account for roughly of what could be solubilized in mixed micelles. Cholesterol-rich 0.5% of total lipids in bile by weight. They are mainly bilirubin vesicles are remarkably stable in dilute bile, consistent with the conjugates with traces of porphyrins and unconjugated bilirubin. absence of cholesterol crystallization in hepatic bile. The unila- Bilirubin can be conjugated with a molecule of glucuronic acid, mellar vesicles can fuse and form large multilamellar vesicles (also which makes it soluble in water. In human bile, bilirubin mono- known as liposomes or liquid crystals). Solid cholesterol monohy- glucuronides and diglucuronides are the major bile pigments. drate crystals may nucleate from multilamellar vesicles in concen- Other bile pigments are monoconjugates and diconjugates of trated gallbladder bile. xylose, glucose, and glucuronic acid and various homoconjugates Vesicles are relatively static structures that are affected by sev- and heteroconjugates of them. eral factors, including biliary lipid concentrations and the relative Proteins and inorganic salts are also found in bile. Albumin ratios of cholesterol, phospholipids, and bile salts. The relative appears to be the most abundant protein in bile, followed by concentrations of these 3 important lipids in bile are influenced immunoglobulins G and M, apolipoproteins AI, AII, B, CI, and by their hepatic secretion rates, which vary with fasting and feed- CII, transferrin, and α2-macroglobin. Other proteins that have ing. For example, during the fasting period, hepatic output of been identified but not quantitated in bile include EGF, insulin, biliary bile salts is relatively low. As a result, the ratio of choles- haptoglobin, CCK, lysosomal hydrolase, and amylase. Inorganic terol to bile salts is increased, and more cholesterol is carried in salts detected in bile include sodium, phosphorus, potassium, cal- vesicles than in micelles. By contrast, with feeding, hepatic output cium, copper, zinc, iron, manganese, molybdenum, magnesium, of biliary bile salts is increased, and more cholesterol is solubi- and strontium. lized in micelles than in vesicles. In addition, when the concentra- tion of bile salts is relatively low, especially in dilute hepatic bile, vesicles are relatively stable, and only some vesicles are converted Physical States of Biliary Lipids to micelles. By contrast, with increasing bile salt concentrations Cholesterol is nearly insoluble in water, and the mechanism by in concentrated gallbladder bile, vesicles may be converted com- which cholesterol is solubilized in bile is complex because bile is pletely into mixed micelles. Because relatively more phospho- an aqueous solution. The 2 main types of macromolecular aggre- lipids than cholesterol can be transferred from vesicles to mixed gates in bile are micelles and vesicles, which greatly enhance the micelles, the residual vesicles are remodeled and may be enriched solubilization of cholesterol in bile. in cholesterol relative to phospholipids. If the remaining vesicles Bile salts are soluble in an aqueous solution because they are have a relatively low ratio ( HDL > chylomicron remnants) be solubilized by the available bile salts and phospholipids, solid plus de novo hepatic cholesterol synthesis. Output is related to plate-like cholesterol monohydrate crystals precipitate in bile. the amount of cholesterol disposed of within the liver by con- Furthermore, the proportional distance outside the micellar zone version to cholesteryl ester (to form new VLDL and for stor- directed along an axis joined to the cholesterol apex is often cal- age) minus the amount of cholesterol converted to primary bile culated as the CSI (or lithogenic index).86 Therefore, the degree salts. An appreciable fraction of cholesterol in bile may also be of saturation of bile with cholesterol can be quantitated. A CSI derived from the diet via apolipoprotein E–dependent delivery for a sample of bile can be estimated directly from the diagram or of chylomicron remnants to the liver. Under low or no dietary calculated by using a formula. The CSI is the ratio of the actual cholesterol conditions, bile contains newly synthesized choles- amount of cholesterol present in a bile sample to the maximal terol from the liver and preformed cholesterol that reaches the amount of cholesterol that can be dissolved in it. Bile that has liver in several different ways. Approximately 20% of the cho- a CSI of 1 is saturated; bile with a saturation index less than 1 lesterol in bile comes from de novo hepatic biosynthesis, and is unsaturated; and bile with a saturation index greater than 1 is 80% is from pools of preformed cholesterol within the liver. De supersaturated. The degree of saturation can also be expressed novo cholesterol synthesis in the liver uses acetate as a substrate as percent saturation by multiplying the saturation index by 100. and is mainly regulated by the rate-limited enzyme HMG-CoA For example, at the boundary of the micellar zone, bile is satu- reductase. This enzyme can be up- or down-regulated depending rated, and the CSI is 100%. Supersaturated bile has a CSI above on the overall cholesterol balance in the liver. An increase in the 100%, and unsaturated bile has a CSI below 100%. The CSI val- activity of this rate-limiting enzyme leads to excessive cholesterol ues are also useful for predicting the proportion of lipid particles secretion in bile. The major sources of preformed cholesterol are and the metastable and equilibrium physical states in bile. hepatic uptake of plasma lipoproteins (mainly HDL and LDL through their receptors on the basolateral membrane of hepa- tocytes). Consistent with their central role in reverse cholesterol Hepatic Secretion of Biliary Lipids transport, HDL particles are the main lipoprotein source of cho- lesterol that is targeted for biliary secretion. Under conditions Source of Lipids Secreted in Bile of a high cholesterol diet, dietary cholesterol reaches the liver The supply of hepatic cholesterol molecules that can be recruited through the intestinal lymphatic pathway as chylomicrons and for biliary secretion depends on the balance of input and out- then chylomicron remnants, after chylomicrons are hydrolyzed put of cholesterol and its metabolism in the liver (Fig. 65.4) (see by plasma lipoprotein lipase and hepatic lipase. The synthesis of BLOOD HEPATOCYTE BILE Acetate LDL LDLR ABCB4 HMGCR HDL Canalicular Biosynthesis SR-BI membrane CMR PL CMRR ABCG5/G8 ACAT CH ester CH CH Basolateral Esterification membrane NPC1L1 BS Vesicle CYP7A1 Catabolism CYP27A1 ABCB11 VLDL Nascent HDL ABCA1 BS Fig. 65.4 Uptake, biosynthesis, catabolism, and biliary secretion of cholesterol at the hepatocyte level. Hepatic uptake of cholesterol is mediated by the LDL receptor (LDLR), by scavenger receptor class B type I (SR-BI) for HDL, and by the chylomicron remnant receptor (CMRR) for chylomicron remnants (CMR). Biosynthesis of hepatic cholesterol (CH) from acetate is regulated by the rate-limiting enzyme 3-hydroxy-3-methylglutaryl- coenzyme A reductase (HMGCR). Part of the cholesterol is esterified by acyl-coenzyme A:cholesterol acyl- transferase (ACAT) for storage in the liver. Some of the cholesterol is used for the formation of VLDL, which is secreted into the blood. The ABC transporter ABCA1 may translocate, either directly or indirectly, cholesterol and phospholipids to the cell surface, where they appear to form lipid domains that interact with amphipathic α-helices in apolipoproteins. This interaction solubilizes these lipids and generates nascent HDL particles that dissociate from the cell. A proportion of cholesterol is used for synthesis of bile salts (BS) via the classical and alterative pathways, as regulated by 2 rate-limiting enzymes, cholesterol 7α-hydroxylase (CYP7A1) and sterol 27-hydroxylase (CYP27A1), respectively. Hepatic secretion of biliary cholesterol, bile salts, and phospholipids (PL) across the canalicular membrane is determined by 3 lipid transporters, ABCG5/G8, ABCB11, and ABCB4, respectively. The Niemann-Pick C1-like 1 (NPC1L1) protein may have a weak role in taking cholesterol back from hepatic bile to the hepatocyte. A vesicle is shown in the canaliculus. 1024 PART VIII Biliary Tract new cholesterol in the liver is reduced and comprises only about formation of cholesterol gallstones. The Niemann-Pick C1-like 5% of biliary cholesterol. Overall, the liver can systematically 1 (NPC1L1) protein is expressed in the canalicular membrane of regulate the total amount of cholesterol within it, and any excess hepatocytes as well as the apical membrane of enterocytes; how- cholesterol is handled efficiently. ever, its expression levels are significantly lower in the liver than Although biliary phospholipid is derived from the cell mem- in the small intestine in humans. These observations suggest that branes of hepatocytes, the composition of biliary phospholipid hepatic NPC1L1 may have a weak role in the regulation of bili- differs markedly from that of hepatocyte membranes. The mem- ary cholesterol secretion.96 In addition, scavenger receptor class branes of hepatocytes contain phosphatidylcholines (lecithins), B type I (SR-BI) is localized in sinusoidal and possibly canalicular phosphatidylethanolamines, phosphatidylinositols, phosphati- membranes of hepatocytes, and in transgenic and knockout mice dylserines, and sphingomyelins. The major source of phospha- fed a chow diet, biliary secretion of cholesterol varies in pro- tidylcholine molecules destined for secretion into bile is hepatic portion to hepatic expression of SR-BI and to the contribution synthesis. A fraction of biliary phosphatidylcholines may also of SR-BI to sinusoidal uptake of HDL cholesterol destined for originate in the phospholipid coat of HDL particles. From 10 to secretion into bile.97,98 Attenuation of the SR-BI, however, does 15 g of phospholipids are secreted into bile each day in humans. not influence gallstone formation in mice. These results suggest More than 95% of bile salt molecules, after secretion into that although HDL cholesterol is a principal source of biliary bile, return to the liver through the enterohepatic circulation by cholesterol in the basal state, uptake of cholesterol from chylo- absorption mostly from the distal ileum via an active transport micron remnants appears to be the major contributor to biliary system such as apical sodium-dependent bile acid transporter cholesterol hypersecretion during diet-induced cholelithogenesis and organic solute transporters α and β (see Chapter 64). Conse- in the mouse.97 quently, newly synthesized bile salts in the liver contribute only a Deletion of the Abcb4 gene completely inhibits hepatic secre- small fraction (G — + + ↓ Intestinal bile salt absorption sodium-dependent bile salt transporter) (rs9514089) SLCO1B1 (OATP1B1) Solute carrier organic anion transporter 12p12 p.P155Thr — — + ↓ Intestinal bile salt absorption family, member 1B1 (rs11045819) TM4SF4 Transmembrane 4 3q25.1 — TBD TBD superfamily member 4 Lipid Regulatory Enzymes CYP7A1 Cholesterol 7α-hydroxylase (Cytochrome 8q11-q12 Promoter + — + ↓ The rate-limiting enzyme for bile P450 7A1) SNP−204A>C salt biosynthesis in the classical pathway UGT1A1 Bilirubin UDP-glucuronyl transferase 2q37 Promotor — — + ↑ Hepatic bilirubin conjugation A(TA)7TAA SULT2A1 Sulfotransferase 19q13.33 rs2547231 — — + ? sulfate conjugation and detoxification of bile salts family 2A, member 1 GCKR Glucokinase 2p23.3 rs1260326 — — + ↑ Altered glucose homeostasis, ↑ cholesterol synthesis regulatory protein Intracellular Lipid Regulatory Transporter CETP Cholesteryl ester transfer protein 16q12-q21 RFLP — — + ↑ Hepatic cholesterol uptake from increased HDL catabolism Lipid Regulatory Transcription Factor NR1H4 (FXR) Nuclear receptor 1H4 (Farnesoid X 12q23.1 Promoter SNPs — — + ↓ Conversion of cholesterol into receptor) −1G>T and bile salts −20647T>G, ↑ Biliary cholesterol secretion IVS7−31 A>T Lipoprotein Receptors and Related Genes APOA1 Apolipoprotein A1 11q23-q24 −75G>A, RFLP — — + ↑ Biliary cholesterol secretion secondary to increased reverse cholesterol transport APOB Apolipoprotein B 2p24-p23 c.2488C>T, + — + ↑ Biliary cholesterol secretion c.4154G>A secondary to reduced hepatic VLDL synthesis ↑ Intestinal cholesterol absorption APOC1 Apolipoprotein C1 19q13.2 RFLP — — + ↑ APOC1 remnant-like particle cholesterol LRPAP1 LDL receptor–related protein-associated 4p16.3 Intron 5 insertion/ — — + ↑ Hepatic cholesterol uptake from protein 1 deletion chylomicron remnants via LRP (rs11267919) Hormone Receptors CCK1R (CCKAR) Cholecystokinin 1 receptor (CCK A receptor) 4p15.1-p15.2 RFLP + — + ↓ Gallbladder and small intestinal motility ESR2 (ERβ) Estrogen receptor 2 14q23.2 c.1092+3607(CA)n — — + ↑ Hepatic cholesterol biosynthesis AR Androgen receptor Xq12 c.172(CAG)n — — + ↓ Gallbladder motility ADRB3 β3-Adrenergic receptor 8p12 p.R64W (rs4944) — — + ↓ Gallbladder motility Black Pigment Stones ANK1 Ankyrin 1 8p11.1 Multiple — + — Spherocytosis → hemolysis CFTR (ABCC7) CF transmembrane regulator 7q31.2 Δq31 + — — ↑ Enterohepatic bilirubin circulation ↓ Bile pH ↑ Fecal bile salt excretion G6PD Glucose-6-phosphate dehydrogenase Xq28 Multiple + + + ↑ Hemolysis GPI Glucose-6-phosphate isomerase 19q13.1 p.Leu339Pro TBD TBD PKLR Pyruvate kinase 1q21 p.R510Q TBD TBD HBA1/2 Hemoglobin alpha chain complex 16p13.3 HbH — + + α-Thalassemia/ β-thalassemia intermediate/minor/sickle cell disease → hemolysis HBB Hemoglobin beta chain complex 11p15.5 p.E26K (HbE) TBD TBD p.E6V (HbS) UGT1A1 Bilirubin UDP-glucuronyl transferase 2q37 Promotor — — + ↑ Hepatic bilirubin conjugation A(TA)7TAA Biliary Tract Stones COMT Catechol-O-methyltransferase 22q11.21 Exon4−76C>G — — + ↑ Estrogen levels (rs4818) CXCR2 Chemokine (C-X-C motif) receptor 2 2q35 c.811C>T — — + TBD (rs2230054) c.1235T>C (rs1126579) IL8 Interleukin-8 4q13-q21 −351A>T (rs4073) — — + ↑ IL8 expression → inflammation NOS2 Nitric oxide synthase 2 17q11.2-q12 Exon16+14C>T — — + TBD (rs2297518) RNASEL Ribonuclease L 1q25 Exon1−96A>G — — + TBD (rs486907)    LRP, Low-density lipoprotein receptor-related protein; RFLP, restriction fragment length polymorphism; SNP, single nucleotide polymorphism; TBD, to be determined; UDP, uridine diphosphate. Adapted with permission from Krawczyk M, Wang DQ, Portincasa P, et al. Dissecting the genetic heterogeneity of gallbladder stone formation. Semin Liver Dis 2011;31:157–72.    CHAPTER 65 Gallstone Disease 1031 65 1032 PART VIII Biliary Tract some ethnic groups at high risk of gallstones. This study strongly humans.220 The UGT1A1 promoter variant increases the sus- suggests that inhibiting both hepatic synthesis and intestinal ceptibility to pigment stone formation in patients with sickle cell absorption of cholesterol to reduce biliary output of cholesterol disease or CF.221-224 A regression analysis has shown that serum may be a therapeutic strategy for genetically defined subgroups of bilirubin levels and the prevalence of gallstones are strongly persons at high risk for gallstones.208 associated with the number of UGT1A1 promoter [TA] repeats The factors that regulate intestinal membrane lipid trans- in patients with sickle cell disease, with each additional repeat porters, lipid regulatory enzymes, intracellular lipid transport- correlating with an increase in serum bilirubin levels of 21% ers, and lipid regulatory transcription factors may influence and in cholelithiasis risk of 87%.222 Moreover, UGT1A1 gene the amount of cholesterol of intestinal origin that contributes variants in linkage disequilibrium with the variant are associated to biliary secretion by the liver. Direct evidence for the role with the risk of developing cholesterol gallstones. These find- of intestinal factors in mouse gallstones comes from a study of ings imply that the supersaturation of bile with bilirubin may be ACAT2-knockout mice.209 Because of the deletion of the Acat2 a risk factor for the formation of both pigment and cholesterol gene, the lack of cholesteryl ester synthesis in the small intestine gallbladder stones. As discussed earlier, increased biliary bili- significantly reduces intestinal cholesterol absorption and leads rubin levels and enhanced precipitation of calcium bilirubinate to complete resistance to diet-induced cholesterol gallstones. in bile provide a critical nidus for cholesterol nucleation and Furthermore, the potent cholesterol absorption inhibitor crystallization. ezetimibe prevents gallstones by effectively reducing intesti- The frequency of gallstones in patients with CF is 10% to nal absorption and biliary secretion of cholesterol and protects 30% compared with less than 5% in age-matched control sub- gallbladder motility by desaturating bile in mice.50,210 More- jects, but biliary cholesterol saturation does not differ between over, ezetimibe significantly reduces biliary cholesterol satura- patients with and without gallstones. In fact, gallstones in patients tion and retards cholesterol crystallization in bile of patients with CF are generally black pigment stones (i.e., composed of with gallstones.50 Therefore, reduced intestinal absorption of calcium bilirubinate with an appreciable cholesterol admixture) cholesterol or hepatic uptake of chylomicron remnants may but rarely cause symptoms. In a mouse (ΔF508 mutant) model of induce a decrease in biliary cholesterol secretion and satura- CF, increased fecal bile salt loss induces more hydrophobic bile tion. In addition, reduced expression levels of the genes that salts in hepatic bile and augments enterohepatic cycling of bili- encode the ileal apical sodium-dependent bile acid transporter rubin.225 These alterations lead to hyperbilirubinbilia and signifi- (ASBT), the cytosolic ileal lipid binding protein (ILBP), and cantly higher levels of all bilirubin conjugates and unconjugated organic solute transporters α and β (OSTα and β) may contrib- bilirubin, followed by hydrolysis and precipitation of divalent ute to gallstone formation by decreased ileal bile acid reabsorp- metal salts of unconjugated bilirubin in bile. In addition, lower tion and an altered bile acid pool and composition in female gallbladder bile pH values and elevated levels of calcium biliru- and nonobese patients with gallstones compared with control binate ion products in bile increase the likelihood of supersatu- subjects (see Chapter 64).211,212 The single nucleotide polymor- rating bile with bilirubin and forming black pigment gallstones. phism rs9514089 in the ASBT gene (gene symbol SLC10A2) The pancreatic duodenal homeobox gene-1 (Pdx1) is required for has been identified as a susceptibility variant for cholelithiasis in proper development of the major duodenal papilla, peribiliary humans,213 although the effect of rs9514089 genotype on gall- glands, and mucin-producing cells in the bile duct and for main- stone risk was not replicated in Sorbs.214 Further analyses in tenance of the periampullary duodenal epithelial cells during the larger cohorts are required to evaluate the role of genetic vari- perinatal period (see Chapter 62). Loss of the major duodenal ants of SLC10A2 as a risk factor for gallstone formation. papilla allows duodenobiliary reflux and bile infection, resulting in formation of brown pigment stones in Pdx1-knockout mice, and treatment with antibiotics significantly reduces the frequency PIGMENT STONES of brown pigment stones.226 Although the pathogenesis of black and brown pigment gallstones is not as well understood as that of cholesterol gallstones, and each type of stone probably has a distinctive pathogenesis, both Black Stones types of pigment stones result from abnormalities in the metabo- Black pigment stones are formed in uninfected gallbladders, lism of bilirubin and are pigmented as a result of bilirubin pre- particularly in patients with chronic hemolytic anemia (e.g., cipitation.215-217 In general, the bile of patients with either type β-thalassemia, hereditary spherocytosis, sickle cell disease), inef- of pigment stones contains an excess of unconjugated bilirubin, fective erythropoiesis (e.g., pernicious anemia), ileal diseases analogous to the saturation of bile with cholesterol in patients (e.g., Crohn disease) with spillage of excess bile salts into the with cholesterol stones.218 Also, both types of pigment stones large intestine, extended ileal resections, and liver cirrhosis. are composed primarily of bile pigment and contain a matrix of These alterations promote formation of black pigment stones mucin glycoproteins. In black stones, however, the pigment is because higher colonic bile salt concentrations enhance the solu- predominantly an insoluble highly cross-linked polymer of cal- bilization of unconjugated bilirubin, thereby increasing bilirubin cium bilirubinate, whereas in brown stones, the main pigment is concentrations in bile.227 The resulting unconjugated bilirubin monomeric calcium bilirubinate. The 2 types of pigment stones is precipitated as calcium bilirubinate to form stones.228 This also differ in radiodensity, location within the biliary tract, and type of stone is composed of either pure calcium bilirubinate or geographic distribution. polymer-like complexes consisting of unconjugated bilirubin, Results of studies of susceptibility genes for pigment stones calcium bilirubinate, calcium, and copper. Mucin glycoproteins are summarized in Table 65.1. Several candidate genes enhance account for as much as 20% of the weight of black pigment the formation of pigment stones by increasing enterohepatic stones. A regular crystalline structure is not present in this type cycling of bilirubin. Persons with Gilbert syndrome have mild, of stone. chronic, unconjugated hyperbilirubinemia in the absence of For hepatic secretion, bilirubin is first mono- or digluc- liver disease or overt hemolysis because of reduced expression uronidated by UGT1A1 and subsequently secreted by ABC of bilirubin uridine diphosphate glucuronyl transferase 1 (gene transporter C2 (ABCC2), also called multidrug-resistance associ- symbol UGT1A1), which is due to an abnormality in the pro- ated protein 2 (MRP2) (see Chapters 64 and 77). Under normal moter region of the gene for this enzyme (see Chapter 21).219 physiologic conditions, unconjugated bilirubin is not secreted A genome-wide association study has identified a variant of the into bile. Although bilirubin glucuronides are hydrolyzed by UGT1A1 gene as a major risk factor for gallstone disease in endogenous β-glucuronidase, unconjugated bilirubin constitutes  CHAPTER 65 Gallstone Disease 1033 less than 1% of total bile pigment, primarily because the activ- especially in intrahepatic bile ducts. Formation of brown pigment ity of the enzyme is inhibited by β-glucaro-1,4-lactone in the stones requires the presence of structural or functional stasis of 65 biliary tract.229,230 The unifying predisposing factor in black bile associated with biliary infection, especially with Escherichia pigment stone formation is hepatic hypersecretion of bilirubin coli.231 These stones are quite prevalent in Asia, where Clonorchis conjugates (especially monoglucuronides) into bile. In the pres- sinensis and roundworm infestations are common, and parasitic ence of hemolysis, hepatic secretion of these bilirubin conjugates elements have been considered to be kernels of brown pigment increases 10-fold. Unconjugated monohydrogenated bilirubin stone formation (see Chapter 84).232 Bile stasis predisposes to is formed by the action of endogenous β-glucuronidase, which bacterial infection as well as accumulation of mucins and bacte- coprecipitates with calcium as a result of supersaturation. A 1% rial cytoskeletons in the bile ducts. Bile stasis may be induced hydrolysis rate may give rise to high concentrations of unconju- by bile duct stenosis and bacterial infection caused by infestation gated bilirubin that often greatly exceed the solubility of biliru- of parasites and their ova.233 As the incidence of biliary infec- bin in bile. A defect in acidification of bile may also be induced tions has decreased in Asian populations prone to development of by gallbladder inflammation or the reduced buffering capacity of brown pigment stones, the ratio of cholesterol stones to pigment sialic acid and sulfate moieties in the mucin gel. The reduction in stones has also changed in these populations. The percentage of buffering capacity facilitates supersaturation of calcium carbon- brown pigment stones in Japan has fallen from 60% to 24% since ate and calcium phosphate that would not occur at a more acidic the 1950s, and similar changes have been reported from other pH. Gallbladder motility defects are not observed in patients with Asian countries.234-236 black pigment stones. Enteric bacteria produce β-glucuronidase, phospholipase A1, and conjugated bile acid hydrolase. Activity of β-glucuronidase results in production of unconjugated bilirubin from bilirubin Brown Stones glucuronide; phospholipase A1 liberates palmitic and stearic acids Brown pigment stones are composed mainly of calcium salts from phospholipids; and bile acid hydrolases produce unconju- of unconjugated bilirubin, with varying amounts of choles- gated bile salts from glycine or taurine-conjugated bile salts. Par- terol, fatty acids, pigment fraction, and mucin glycoproteins, as tially ionized saturated fatty acids, unconjugated bilirubin, and well as small amounts of bile salts, phospholipids, and bacte- unconjugated bile salts may precipitate as calcium salts. Mucin rial residues. Brown pigment stones may be easily distinguished gel can trap these complex precipitates and facilitate their growth grossly from black pigment stones by their reddish brown to into macroscopic brown pigment stones. Fig. 65.6 shows the pos- dark brown color and lack of brightness. Their shape is irregular tulated mechanisms underlying the formation of brown pigment or molded and occasionally spherical. Most of the stones are stones. Under normal physiologic conditions, bilirubin in bile muddy in consistency, and some show facet formation. Brown exists mainly as bilirubin glucuronide, which is soluble in aque- pigment stones are either smooth or rough without any surface ous media. Bile also contains β-glucuronidase of tissue origin, luster and are soft, fragile, and light in comparison with other the activity of which is inhibited by β-glucaro-1,4-lactone, which gallstones. The cut surface is generally a stratified structure is also formed in the liver. If infection with E. coli occurs, the (lamellation) or is amorphous without the radiating crystalline concentration of bacterial β-glucuronidase increases significantly structure seen in cholesterol stones. Almost invariably, brown and exceeds the inhibitory power of β-glucaro-1,4-lactone. As a pigment stones have a lamellated cross-sectional surface with result, bilirubin glucuronide is hydrolyzed to produce unconju- calcium bilirubinate-rich layers alternating with calcium palmi- gated bilirubin and glucuronic acid; the former is water-insoluble tate-rich layers. and combines with calcium to form calcium bilirubin at its car- Brown pigment stones are formed not only in the gallblad- boxyl radical, thereby leading to the formation of brown pigment der but also commonly in other portions of the biliary tract, gallstones. Bilirubin Phospholipids Conjugated Fig. 65.6 Proposed mechanisms for the patho- glucuronides bile salts genesis of brown pigment stones. Under normal physiologic conditions, unconjugated bilirubin -Glucaro-1,4- is not secreted into bile. Although modest hy- lactone drolysis of bilirubin glucuronides by endogenous Bacterial Bile salt β-glucuronidase occurs, unconjugated bilirubin con- -glucuronidase (–) Phospholipase A1 hydrolase stitutes less than 1% of total bile pigment, mostly because the activity of β-glucuronidase is inhibited Endogenous Glucuronic acid -glucuronidase by β-glucaro-1,4-lactone in the biliary system. The presence of excess bacterial β-glucuronidase, however, overcomes the inhibitory (−) effect of β- Free bilirubin Free fatty acids Free unconjugated glucaro-1,4-lactone, which results in hydrolysis of bile salts bilirubin glucuronide into free bilirubin and gluc- uronic acid. Free bilirubinate combines with calcium to yield water-insoluble calcium bilirubinate. In addition, phospholipase A1 liberates free fatty acids such as palmitic and stearic acids from phospholip- Brown pigment ids, and bile salt hydrolases produce unconjugated Calcium bilirubinate stones bile salts from glycine or taurine-conjugated bile salts. Dead bacteria and/or parasites could act as nuclei that accelerate precipitation of calcium bilirubinate. The mucin gel in the gallbladder can Calcium Dead bacteria Mucin gel trap these complex precipitates and facilitate their and/or parasites growth into macroscopic stones. 1034 PART VIII Biliary Tract NATURAL HISTORY Symptomatic Stones The natural history of gallstones is typically described in 2 sepa- The cardinal symptom of gallstones is biliary pain (“colic”), rate groups of patients: those who have symptoms and those who which is described as pain in the RUQ often radiating to the back, are asymptomatic. Autopsy studies clearly show that the vast with or without nausea and vomiting. The pain is usually not true majority of patients with gallstones are asymptomatic and remain colic (see Chapter 11) and is almost never associated with fever. so. Ascertaining the true frequency of complications in persons The natural history of symptomatic gallstones has a more aggres- with asymptomatic stones (as well as those with symptomatic sive course than that of asymptomatic stones. The U.S. National stones) is critical to providing rational, cost-effective recommen- Cooperative Gallstone Study showed that in persons who had an dations regarding therapy (see later). Unfortunately, the informa- episode of uncomplicated biliary pain in the year before enter- tion available on the natural history of gallstones has been sparse ing the study, the rate of recurrent biliary pain was 38% per and somewhat varied.237-239 year.243 Other investigators have reported a rate of recurrent biliary pain as high as 50% per year in persons with symptom- atic gallstones.244 As noted earlier, biliary complications are also Asymptomatic Stones more likely to develop in persons with symptomatic gallstones. The study that changed our understanding of the course and appro- The risk of biliary complications is estimated to be 1% to 2% per priate therapy of gallstone disease was performed by Gracie and year and is believed to remain relatively constant over time.245 Ransohoff.237 They monitored 123 University of Michigan faculty Therefore, cholecystectomy should be offered to patients after members for 15 years after they had been found to have gallstones biliary symptoms develop. In patients with high operative risk, an on routine screening US. At 5, 10, and 15 years of follow-up, 10%, alternative approach is close observation, because 30% will have 15%, and 18% of the patients, respectively, had become symptom- no further episodes of biliary pain. atic, and none had experienced serious complications. The inves- tigators suggested that the rate at which biliary pain develops in persons with asymptomatic gallstones is about 2% per year for 5 Special Patient Populations years and then decreases over time. Biliary complications devel- The clinical manifestations of gallstones are shown schematically oped in only 3 patients in this study, and all complications were in Fig. 65.7 and summarized in more detail in Table 65.2.246-250 preceded by episodes of biliary pain. In fact, biliary pain, not a Biliary pancreatitis is discussed in Chapter 58. Although the stan- biliary complication, is the initial manifesting symptom in 90% of dard approach to asymptomatic gallstones is observation, some people with previously asymptomatic gallstones.237 Therefore, in patients with asymptomatic gallstones may be at increased risk patients with asymptomatic stones, the frequency of complications of complications and may require consideration of prophylactic is low, and prophylactic cholecystectomy is not necessary. cholecystectomy. Subsequent studies have reported slightly higher rates of bili- An increased risk of cholangiocarcinoma and gallbladder car- ary pain and complications in patients with initially asymptom- cinoma has been associated with certain disorders of the biliary atic gallstones,238 but only 1 was a long-term and prospective tract and in some ethnic groups (e.g., Native Americans) (see study.239 The Group for Epidemiology and Prevention of Chole- Chapter 69). Risk factors include choledochal cysts, Caroli dis- lithiasis (GREPCO) in Rome reported the courses of 151 subjects ease, pancreaticobiliary malunion (also referred to as anomalous with gallstones, 118 of whom were asymptomatic on entering the union of the pancreatic and biliary ducts, in which the pancre- study. In those who were initially asymptomatic, the frequency of atic duct drains into the bile duct), large gallbladder adenomas, biliary pain was 12% at 2 years, 17% at 4 years, and 26% at 10 and porcelain gallbladder (see Chapters 55, 62, and 67). Patients years, and the cumulative rate of biliary complications was 3% at at increased risk of biliary cancer may benefit from prophylac- 10 years.239 tic cholecystectomy. If abdominal surgery is planned for another In a 1987 study, incidental gallstones were discovered in 285 indication, an incidental cholecystectomy should be performed. (21%) of 1371 patients from Norway who had not had a cholecys- Pigment gallstones are common and often asymptomatic in tectomy.240 Twenty-four years later, a follow-up study included patients with sickle cell disease. Prophylactic cholecystectomy is 134 of the patients who had gallstones.241 Gallstones were pres- not recommended, but an incidental cholecystectomy should be ent on US in 25 of 89 patients (28% overall, 31% of women and considered if abdominal surgery is performed for other reasons. 25% of men), and there was no correlation between initial size Some authorities recommend combined prophylactic splenec- and number of gallstones and persistence of stones on follow-up. tomy and cholecystectomy in young asymptomatic patients with Nine of 134 patients (7%) had undergone cholecystectomy, as hereditary spherocytosis if gallstones are present. had 5 of 91 patients who had died prior to follow up (6%). Dur- Morbidly obese persons who undergo bariatric surgery are ing follow up, abdominal pain developed in 44%, and 29% had at high risk of complications of gallstones (see Chapters 7 and what were deemed to be functional abdominal complaints. This 8). These patients have a frequency of gallstones of greater than study illustrates again both the frequent resolution and relatively 30%. An incidental cholecystectomy is recommended at the time benign nature of asymptomatic gallstone disease. of surgery. Some investigators have proposed that patients with inciden- tal cholelithiasis who are awaiting heart transplantation undergo Stones in Patients With Diabetes Mellitus a prophylactic cholecystectomy irrespective of the presence or Diabetic patients have been considered at increased risk of gall- absence of biliary tract symptoms because they are at increased stone complications; however, the natural history of gallstones risk of post-transplant gallstone complications.251 A retrospective in diabetic patients follows the same pattern observed in non- study that addressed this issue in renal transplant recipients, how- diabetic persons. A prospective study of patients with insulin- ever, concluded that complications of gallstones could be man- resistant diabetes mellitus showed that after 5 years of follow-up, aged safely after symptoms emerged.252 symptoms had developed in 15% of the asymptomatic patients.242 Moreover, the complication and mortality rates were compa- rable to those in studies of nondiabetic patients with gallstones. DIAGNOSIS Therefore, prophylactic cholecystectomy is not recommended in Imaging studies play a central role in the diagnosis of gallstones patients with insulin-resistant diabetes mellitus and asymptom- and associated conditions. Table 65.3 shows the wide array of atic gallstones. imaging techniques available to evaluate the biliary tract.253-256  CHAPTER 65 Gallstone Disease 1035 Stone intermittently obstructing Stone impacted in the cystic the cystic duct, causing intermittent duct, causing acute cholecystitis (10%) 65 biliary pain (20%) 3 2 Stone in the cystic duct compressing or fistulizing into the common hepatic duct, causing Asymptomatic stone (75%) 4 Mirizzi syndrome (10 mg/dL suggests Serum alkaline phosphatase hepatic infection malignant obstruction level is usually elevated Mild serum amylase and lipase or coexisting Blood cultures are usually elevations are seen even in the hemolysis positive, especially during absence of pancreatitis A transient “spike” chills or a fever spike; If serum bilirubin is >4 mg/dL or in serum 2 organisms are grown amylase or lipase is markedly aminotransferase or in cultures from half of elevated, a BD stone should be amylase (or lipase) patients suspected levels suggests the passage of a stone Diagnostic US US ERCP ERCP studies (see Oral cholecystography Hepatobiliary scintigraphy EUS Percutaneous THC Table 65.3 Meltzer-Lyon test (see Abdominal CT MRC for details Chapter 67) Percutaneous THC on imaging studies) Natural history After the initial attack, 30% 50% of cases resolve The natural history is A high mortality rate if of patients have no further spontaneously in 7-10 days not well defined, but unrecognized, with death symptoms without surgery complications are from septicemia Symptoms develop in the Left untreated