GALLSTONE DISEASE, PATHOGENESIS AND MANAGEMENT ELDON A. SHAFFER DONALD M. SMALL
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TABLE
EPmEMIOLOGY
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NATURAL HISTORY CHEMICAL PIGMENT
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CLASSIFICATION GALLSTONE
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GALLSTONES
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CHOLESTEROL GALLSTONE FORMATION
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Bile Composition and Cholesterol Solubility
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D e t e r m i n a n t s of Biliary L i p i d C o m p o s i t i o n .
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Stages of Cholesterol Gallstone F o r m a t i o n and Their Diagnosis . . . . . . . .
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Possible Etiologic M e c h a n i s m s of Cholesterol Gallstone F o r m a t i o n . . . . . . .
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Associated P a t h o g e n e t i c F a c t o r s
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MANAGEMENT
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QUESTIONS OF CLINICAL USEFULNESS ANSWERED IN THIS iSSUE
1. J u s t how common are gallstones? Page 6 2. Are gallstones becoming more common? Page 7 3. W h a t kinds of populations are afflicted with gallstone disease? Are some populations resistant? Page 8 4. Do age, sex, social class and genetic factors determine the prevalence of ;Tallstones? Page 9 5. W h a t clinical conditions are associated with gallstones? Page 9 .
W h a t is the natural history of gallstone disease? Do all patients develop symptoms? Page 11
7. W h a t are the chemical types of gallstones? Page 12 8.
W h a t are the pigment stones and how do they form? Page 14
9.
W h a t are the major chemical constituents of bile? Page 17
10. How do bile salts solubilize cholesterol in bile? Page 25 11. W h a t are the limits of cholesterol solubility in bile? Page 27 12. How does one use triangular coordinates to plot bile composition? Page 27 13. How does concentration of lipids in bile affect the solubility of cholesterol? Page 29 14. How are bile salts formed and metabolized? Page 31 15. W h a t is the enterohepatic circulation of bile salts? Page 37 16. Hew is cholesterol synthesized, metabolized and secreted into bile? Page 41 17. How does choline influence the synthesis of biliary phospholipid? Page 44 18. W h a t is the relationship between bile salt secretion rate and the secretion of phospholipid and cholesterol? Page 44 19. W h a t are the stages of gallstone formation and how are they diagnosed? Page 45 3
20. Does cholesterol gallstone disease have a single etiology? Page 47 21. How does excessive bile salt loss cause gallstones? Page 49
22. What is "oversensitive bile acid feedback?" Page 49 23. Can excessive cholesterol secretion cause gallstones? Page 53 24. What is the metabolic defect afflicting the American Indians in the Southwest to give them such a high prevalence of gallstones? Page 53 25. Are hormones related to gallstone disease? Page 55 26. What is the relation between hyperlipidemia and gallstones? Page 56 27. Do some drugs that decrease serum cholesterol cause gallstones? Page 57 28. How do we manage gallstones today? Page 58 29. What is the status of medical treatment of gallstones? Page 58
4
is Assistant Professor of Medicine at McGill University a n d Assistant Physician at the M o n t r e a l General Hospital. After g r a d u a t i n g from Queen's University, Kingston, Ontario, he received training in internal medicine and gastroenterology at the M o n t r e a l General Hospital and subsequently became a Fellow in the Royal College of Physicians and Surgeons and a Dipl~,mate of the A m e r i c a n Board of I n t e r n a l Medicine. A w a r d e d a Medical Research Council of C a n a d a fellowship, he initiated his research in the m e c h a n i s m of gallstone :formation as a research associate at Boston University School of Medicine and has p u r s u e d this i n t e r e s t while at McGill University.
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is Professor of Medicine and Biochemistry a n d Chief of the Section of Biophysics at Boston University School of Medicine. Doctor Small received his M.D. degree from the University of California, Los Angeles Medical S c h o o l i n 1960 and obtained p o s t g r a d u a t e training in internal medicine at Boston University Medical Center. F r o m 1963 to 1965 he served as a N a t i o n a l Institutes of H e a l t h Fellow in Physical C h e m i s t r y at the Service de Biophysique I n s t i t u t e Pasteur, Paris, France. F o r his studies on cholesterol gallstone form a t i o n a n d the enterohepatic circulation of bile acids he was a w a r d e d the 1972 American Gastroenterological Association Distinguished Achievement Award, the N a t h a n H a t field Award, the U C L A Alumni Association Distinguished Achievement A w a r d in Science in 1974 and the E p p i n g e r Prize of the I n t e r n a t i o n a l Liver Society in 1976. Doctor S m a l l ' s present work involves the physical structure, physiology and metabolism of serum lipoproteins and the arterial wall. T h e long t e r m goal of this work is to discover the physical chemical basis of lipid deposition a n d removal in atherosclerosis.
G A L L S T O N E S have afflicted man since antiquity, being found by the Egyptians in their dead and used as charms, potions and paint. 1 Present knowledge began 200 years ago when cholesterol was first identified as the major component of gallstones. Indeed, "cholesterine" means "bile solid. ''2 Soon after, bile salts were discovered and thought to be important in dissolving cholesterol 6
leading to the nfid-19th century suggestion t h a t a deficiency of bile salts relative to cholesterol in bile was the central problem in stone formation, d, 4 Modern concepts have evolved from the physicochemical definition of cholesterol solubility in bile ~ and the events leading to gallstone formation. 6 Such advances have elicited a resurgence of interest in the mechanism of gallstone disease, and the therapeutic rationale t h a t logically followed 7 has evoked a new era in management, s EPiDEMIOLOGY
Epidemiology, "the study of disease occurrence in h u m a n populations," is crucial to understanding disease causation. .~ The available data on gallstones are fragmentary and the overworked aphorism "female, fair, fat, fertile, forty and flatulent" is not particularly associated with gallstone disease. TM 11 Estimates of occurrence and information about associated factors have come primarily from clinical and autopsy studies t h a t contain both selection and detection bias. Unfortunately, such a data base underestimates the true frequency and tends to obscure real associations by failing to identify asymptomatic disease. PREVALENCE. "Prevalence" describes the number of people who have gallstones at a certain point in time 9, 12 and is best determined by performing a random cholecystographic survey of a population at risk. The true prevalence of gallstone disease has been determined only in 600 Pima Indians living on a reservation in Arizona 13 (Fig. 1,A). Gallstones are very common among these people, so t h a t only a small sampling population was necessary. Seventy per cent of the women were found to have gallstones by the age of 30 whereas, in males, this high frequency was reached only in later life. The stones appeared soon after the age of 20 in the females b u t developed only gradually in males during middle age. There was no relation between the prevalence of radiologic cholelithiasis and such factors as parity, obesity, diabetes, serum cholesterol and atherosclerosis. 13 As for obesity and diabetes, subtleties may have been obscured by the high preponderance of these two conditions in this Indian population. "Clinical" prevalence, in contrast to " t r u e " prevalence (Fig. 1,A), suggests t h a t symptoms do not appear until several years after the development of the stones. Also, nearly half the women and twothirds of the men with gallstones never had clinical symptoms. The n u m b e r of persons who have experienced symptoms of biliary disease has been reported as 10 per 1000 population randomly surveyed over a 6-month period. This represents 10% of all digestive complaints recorded in this United States survey. 14 In those over 65 years of age, the figure triples. 6
PIMA INDIANS
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Fig. 1,--A, the "true" (solid line) and "clinical" (dashed line) prevalence of gallbladder disease in the Pima Indians. Vertical axes represent the percentage of the total group who h~ve gallstones; horizontal axes, the ages. Stones develop at an earlier age in females, but the prevalence in the older age groups approaches 80% in.both sexes. The lower curves for clinical prevalence confirm the general impression that symptomatic disease appears some time after radiographic demonstration of the presence of gallbladder disease. The difference, at any age, represents "silent" cases. (Drawn from data of Sampliner et a1.13) B, the estimated prevalence of gallbladder disease in the general U. S. population shown for male and female. Closed circles give definite prevalence from J. Chronic Dis. 19:273, 1966, ref. 11, and open circles the calculated "true" prevalence. X refers to autopsy fate of gallstones.
Prevalence of gallstones has also been radiologically examined in Canadian Micmac Indian and Caucasian women (C. N. Williams personal communications). Gallbladder disease prevalence was 185/1000 for these Indians, which corresponded closely to t h a t found in the Pima Indians of 215/1000. I3 Disease also tended to start early in life---in the 20s and 30s. The Caucasian women in Nova Scotia appeared l/o have a prevalence rate of 108/1000, which was higher than t h a t of 39/1000 reported in Framingham, Massachusetts. 11 True prevalence of cholelithiasis is otherwise unknown for the general population. If, however, at least half of all people with gallstones are symptomatic, the " t r u e " prevalence can be estim a t e d from the "clinical" prevalence on the basis of the Framingham study (Fig. 1,B). 11 This estimate suggests t h a t 12 million females and 4 million males in the United States will harbor gallstones. About 800,000 new cases of cholelithiasis develop each year, and approximately half of these ultimately undergo operation. Gallstone disease accounts for about 5000--8000 deaths per year, and the resultant morbidity costs approach 1 billion dollars. There is no d o u b t t h a t gallbladder disease is an i m p o r t a n t health problem both medically and financially. INCIDENCE.- " I n c i d e n c e " refers to the rate of development of new cases over a certain time period. 12 The incidence of overt biliary tract disease is rising in some autopsy studies ~5 b u t not in others. ~6 Operative reports have shown an increasing fre7
quency. '7 Large increases in biliary operations have occurred over the past 30 years on both sides of the Atlantic, 18 but the number of cholecystectomies still is 6 times higher in North America than in Western Europe. ~9 Such large differences in tile rates at which operation is performed may reflect either a heightened tendency to the disease, improved diagnostic expertise or better health-care delivery. Conversely, this could imply that therapeutic usefulness has been exceeded and eventually may lead to excess mortality. 2°. 21 Care must be taken not to misinterpret the number of deaths attributed to cholecystectomy. 22 In any event, biliary disease has become an increasing burden on surgical facilities. 2~ GEOGRAPHIC PREVALENCE.~The over-all frequency of gallstones in a given country or community commonly is derived from autopsy data that are selective and not truly representative of the population at large, ls,24 The reported frequency in postmortem material is an approximation that allows a reasonable geographic classification (Table 1). Prevalence has an exceedingly wide range: from the North American Indian population, in whom the frequency of gallstones reaches epidemic proportions, ~3 to the Masai tribe in sub-Saharal Africa, who apparently do not have gallstones. 25 Significant differences can exist even within the boundaries of one country, perhaps related to socioeconomic class and dietary factors. 2~ Immigration also may influence the development of cholelithiasis.27 Such studies, which may disclose pertinent information on the geographic distribution of gallstones, often fail to document the type of gallstones found. Cholesterol gallstones characteristically are the most common in modem Western civilization. DET~,RMINANTS OF PREVALENCE: The factors associated with gallstones are constantly being re-evaluated from both an epidemiologic and a pathophysiologic basis.
Age.~Gallstones are uncommon in childhood but may occur in association with chronic hemolytic states. 2s Frequency rises progressively with age in both sexes and all races. ~s, 2t. 2~,~9,30 AITABLE
1.--FREQUENCY COUNTRIES
VERY COM~VION
COMMON
OF GALLSTONES IN DIFFERENT (AUTOPSY STUDIES) INTERMEDIATE
B~IE
(> 30-50%) (10-30%) ( < 10%) (~0%) U. S. ( I n d i a n s ) 13. ~oo U. S. ( w h i t e s ) ll. 15 U. S. (blacks) Is. lo3 E. A f r i c a 2~. los S w e d e n 1°1 N o r w a y 29 J a p a n 1°4 E s k i m o s (Canada) lo7 C z e c h o s l o v a k i a 3° U . K . le. 35. 44 ThailandlO~ W e s t e r n E u r o p e '-'4 A u s t r a l i a 1 o~
8
though gallbladder surgery (and hence the supposed occurrence of clinical disease) is more common in middle age, ~7 the actual formation of the stone may have occurred several years earlier. 13
Sex. Female predominance in clinical gallstone disease has been well established. ~1 Its prevalence has been further related to parity in some reports 9, 22.33 but not in others. 13,34,35 This discrepancy may be related to the difference between " t r u e " prevalence and "clinical" prevalence; the latter, apparent as symptomatic gallstone disease, may be diagnosed more frequently (i.e., detection bias) with multiple pregnancies, 36 because such patients are seen more often by a physician. More convincing evidence has come from epidemiologic surveys concerning women receiving female sex hormonal therapy. These demonstrate a significant association between gallbladder disease in both young women on oral contraceptives 37 and older women taking estrogens. 38 Social class.--In the past century, sedentary occupations were associated with a predilection for gallstones, but presently this feature is not apparent in Western countries, although socioeconomic differences may occur elsewhere, such as in India "-'~and Japan. TM Genetic factors.--The general impression that gallstone disease has a tendency to run in certain families is supported by the few studies available. 39 Concordance Studies in identical twins and familial clustelings have only been suggestive because of the very common nature of the disease. Stronger evidence has come from a study of bile composition in siblings of young Caucasians with cholelithiasis. Even those relatives without radiologic evidence of gallstones possessed bile, which put them at risk for developing gallstones. 4° The racial differences noted previously (Table 1) incriminate genetic factors, but the incidence changes rapidly in migrants. 27 Therefore, a polygenic predisposition may exist in the general population, only to be e!icited under the influence of certain environmental events. Clinical associations. The factors associated with the occurrence of gallstones either have been inadequately examined by observational epidemiology or have resulted in conflicting interpretations. Certain clinical associations presently are considered to be real and not just artifactual, is. 41 a) Definite associations: 1. Pancreatitis. 24, 4t Gallstones, in their migration down the biliary tree, may be an important precipitating cause of acute and recurrent attacks of pancreatitis. 42 2. Carcinoma of the gallbladder, but the cause-effect relationship has not been established. 3°' 41 9
3. Pigment gallstones in both hemolytic states 24 and cirrhosis.48.44, s7 4. Chronic ileal disease or extensive ileal resection undoubtedly isrelated to gallstoneformation. 4~. 4G 5. Obesity. B o t h autopsy24.:a0. 3~,a5 and clinical 11 surveys have supported this strongly positive correlation. Dietary assessment has suggested t h a t gallstone patients, a t least in France, consume s i g n i f i c a n t l y m o r e calories than do control subjects. ~' In Pima Indians, a relat~ionship to body weight, was not demonstrated, perhaps because obeslt~y is very common in this population. 13 b) Probable associations: 1. Diet. Excessive dietary retake induces the diseases of overconsumption (obesity, diabetes and coronary artery disease) and probably is a major determinant of gallstone formation. Qualitative dietary differences may explain differences in incidence between geographic regions and within countries2' In J a p a n , where the majority of gallstones were composed of pigment material, a rising incidence of cholesterol gallstones has been attributed to an increasing fat content in the diet. 4s A conflicting report from Australia has suggested t h a t patients with cholelithiasis had a reduced fat intake2 9 Bias occurs in such retrospective dietary surveys because patients may reduce fat intake because of socalled fatty food intolerance after the disease has developed. The overriding importance of genetic and population differences is exhibited by the Masai tribe a n d Eskimos. Despite their large consumption of animal fat, these people rarely develop gallstones. In fact, a diet low in unsaturated fats, given to lower serum cholesterol and prevent atherosclerosis, appeared to increase the prevalence of cholelithiasis in a Los Angeles study. ~° Last, the highly refined carbohydrate diet in affluent Western civilization, where gallstones are so common, has led Burkitt ~1 to suggest t h a t fiber deficiency m a y be an important feature. 2. Atherosclerosis. There is no true association between coronary artery disease and cholelithiasis, despite some shared etiologic factors, such as age and obesity. ~2 Because most gallstones are largely composed of cholesterol, it was naturally assumed t h a t cholesterol gallstones reflected disordered lipid metabolism. Hypercholesterolemia does not appear related to gallstone disease, ~,~2 but the " n o r m a l " range of serum cholesterolin Western Caucasians is higher than in certain populations where gallstones are uncommon (Japanese) or rare (Masai). 25 Almost paradoxically, the North American Indian, who commonly has cholesterol gallstones, tends to possess a low serum cholesterol. ~3 3. Hyperlipidemia. Familial hyperlipoproteinemia i n general 5~ and hypertriglyceridemia (type IV) 54 in particular m a y be associated with a high incidence of gallbladder disease. Other contributing factors in this heterogenous entity include obesity, 10
diabetes meUitus and dietary sucrose all of which m a y be associated with gallstones. 4. Diabetes mellitus. Autopsy evidence has suggested an increased risk of gallstones in diabetic patients,15.2~, ~5--57 but a recent autopsy study failed to confirm such an association. ~° Clinical population studies, both in Caucasians I~ and in Pima Indians, 13 have been inconclusive. The FramJngham study conrained too few diabetics to justify statistical analysis whereas the very high prevalence of both diabetes and gallstones in the Pima Indians may have obscured any discernible correlation. Therefore, this relationship has not been established. 5. Peptic ulcer disease and gastric surgery. The consequences of peptic ulcer disease and operation for this condition on gallstone formation are not known. Most likely there is no association between peptic ulcer and gallstones except for detection bias i.e., peptic ulcer patients are more likely to receive an oral cholecystogram. The postulated association between gallstones and gastric surgery (.specifically truncal vagotomy) remains unproved. 4~, 5s 6. Other associations. Cholelithiasis has been related to a myriad of other disorders, including pheochromocytoma and idiopathic late-onset immunoglobulin.deficiency. .59 The meaning and validity of such findin-gs remain to be clarified. NATURAL HISTORY
The natural course of gallstone disease is poorly defined, but autopsy evidence has suggested that most patients have no significant symptoms. 60 Further information on the frequency of these clinically "silent gallstones" has come from retrospective studies using cholecystography. 61, 62 In Figure 1,A, the lower curves for clinically symptomatic patients may be contrasted to the higher incidence of gallstones seen radiographically. The difference represents asymptomatic or silent cases. The obvious difficulty resides in a clinical definition of asymptomatic cases t h a t are found fortuitously. 63 Why was the radiographic examination done in the first place? W h a t about gallstones "incidentally" discovered at the time of abdominal s u r g e r y ? Despite this, the available data suggest t h a t only one-third to one-half of patients will develop significant symptoms during a 10-20-year period. 13 The tendency to cause symptoms appears u n r e l a t e d t o size, number or type of gallstones present. 64 The silent gallstone, in fact, really is not a specific lesion, but merely represents the asymptomatic stage of cholelithiasis. In the older patient, the incidence of symptomatic cholelithiasis may rise to become a major problem. 15, 65 The danger engendered by nonoperative management of silent gallstones, particularly in 11
the elderly, is the development of acute cholecystitis, with its higher mortality a n d morbidity. Those patients who develop acute cholecystitis usually have premonitory Symptoms, 6~ and more severe complications tend to occur when the gallbladder is radiologically nonfunctioning. 62 Although the risk-benefit ratio of cholecystectomy is not fully known, eGthe elderly patient with definite symptoms of gallbladder disease or with a radiologically nonfunctioning gallbladder is the best candidate for surgery. The other potential danger in a gallbladder harboring gallstones is the development of carcinoma. The association between carcinoma of the gallbladder and gallstones has led to the suggestion of early elective operation in the geriatric population, eS, 6e There is n o evidence that gallstones, per se, predispose to gallbladder carcinoma or that elective cholecystectomy even reduces the mortality. 67 Actually, the 1% risk of attendant malignancy is not an indication for prophylactic cholecystectomy, which by itself carries at least a 1% mortality. G~ Well-designed studies to define the natural history of gallstone disease are urgently needed as a guide to therapy, especially in the older age group. Indeed, not only do gallstones appear, but occasionally they can disappear spontaneously, es CHEMICAL CLASSIFICATION OF GALLSTONES
Biliary calculi rarely are examined other than by gross description in the pathology laboratory. An early clue to their metabolic origin came from chemical analysis of gallstone composition. Cholesterol was found to be the major component of most human gallstones in the West 1 whereas bile pigment commonly comprised those in the Orient. 69 Crystallographic studies such as x-ray diffraction 7° and x-ray powder method vl have demonstrated that more than one physical form of cholesterol may exist in stones collected under different conditions. Cholesterol usually is present as single crystals whose axes are radially oriented with. respect to the nucleus. 71 The crystals of cholesterol in the stone, as it exists in the gallbladder, occur onlyas the cholesterol monohydrate (C. Loomisupersonal communication). Cholesterol gall.~tones also contain small quantitiesof other substances, such as calcium carbonate and calcium palmitate. Calcium palmitate can even be the sole crystalline component, the palmitic acid likely arising from hydrolysis of biliary lecithin.72 Calcium carbonate can be present in three different physical forms calcite, aragonite or vaterite. 71 Calcium and bicarbonate ions are normal constituents of gallbladder bile. Bicarbonate absorption across the wall of the gallbladder controls pH and hence calcium carbonate solubility. The association of calcium carbonate with dark pigmented areas has implied that 12
coprecipitation of calcium carbonate and pigment material may occur, perhaps as a result of a pH change in bile. 73 This situation does not exist with "limy bile," which is just precipitated calcium carbonate and does not necessarily proceed to stone formation. Conversely, pigment material can precipitate in the absence of calcium carbonate. Organic material, rarely, is the only ingredient in pigment gallstones (rather than pigmented calcium carbonate or calcium bilirubinate). An organic matrix commonly remains in the majority of pigment stones, once the crystalline material is dissolved out. TM This mucus-like substance appears as a constituent of most gallstones and m a y be involved in stone formation. Mucosubstances could act at various s t a g e s - - i n the form of a nidus to initiate stone formation, 75 as a mucous plug to entrap precipitared crystals within the gallbladder or as a scaffold for crystal aggregation. 76 Their physical and chemical properties in bile are not known and their importance has yet to be established. 77 The central portion of the stone represents events occurring during the initial stages of stone formation; the periphery probably reflects factors involved with stone growth. Differential microanalysis, therefore, has a t t e m p t e d to show regional differences in composition. Bile acids in the center of cholesterol gallstones have a composition with relatively less deoxycholic acid and taurine conjugates, suggesting t h a t stone formation was initiated during a time when the bile salt secretion around the enterohepatic circulation was disturbed. 7s Other materials found a t the center include pigment matter, protein and parasites. Stone centers often consist of a mass of small crystals and pigment apparently trapped in strands of fibers, supporting the thesis t h a t nucleation begins on biliary debris or mucosubstances. 7~ The peripheral layers of gallstones have also undergone analytic scrutiny, b u t with conflicting results. Some have found no compositional differences 79 whereas others demonstrated a successive change in gallstone layers (and presumably bile composition) t h a t probably occurred during growth, s° Mixed stones containing both cholesterol and pigment material tended to have differences in composition when layers were sequentially analyzed, sl Analysis of the layered structure, which presumably was built up, step by step, around the central core, has unfortunately not disclosed the basis for periods of stone growth. Pigment gallstones h a v e been investigated extensively in J a p a n because of their high hlcidence. Two types have:been recognized: calcium bilirubinate stones commonly found i n the Orient associated with infestations of Ascaris or Escherichia coli and pure pigment stones occurring without any infection b u t sometimes with hemolysis, s2 Morphologically, calcium bilirubinate stones are fragile, with a dark o r reddish brown color and 13
may form a m u d d y substance anywhere in the biliary tree. The pure pigment stones are typically black or dark green. These gross features can be readily used to accurately distinguish pigment from cholesterol gallstones 1.s3 whereas radiographically visible opacity is not necessarily pathognomonic, s4.s~ Calcium, the major mineral in all gallstones, determines the opacity of stones. When calcium is largely absent, as commonly occurs in pure cholesterol or occasionally with pure pigment stones, gallstones will not be radiologically visible on plain films. Analysis of pigment stones has suggested that unconjugated bilirubin, calcium and protein are the major constituents, sl, s2 although calcium bilirubinate material may not be the predominant type in the West. s'~ Unfortunately, analytic deficiencies have led to incomplete extractions so t h a t the principal component remains unidentified. PIGMENT GALLSTONE FORMATION
Pigment stones with only trace amounts of cholesterol comprise about 15% of all gallstones in the Occident but are common in Asiatics. The percentage of gallstones composed of cholesterol may be increasing in Japan, a reflection of dietary and environmental changes2 9 In certain disease states such as chronic hemolysis, the frequency of pigment gallstones is increased to almost epidemic proportions, s6 Cirrhosis is associated with over a twofold increase in gallstones, predominantly of the pigment variety. 44,s7 Retrograde cholangiography recently has disclosed a high prevalence of gallstones in primary biliary cirrhosis, ss Furthermore, if these small gallstones produce biliary obstruction, the clinical picture that ensues can create a complex management problem. The possible mechanisms by which these gallstones form include: secretion of an abnormal pigment t h a t readily precipitates in bile, excessive hepatic-secretion of a normal pigment t h a t surpasses its solubility in bile or extrahepatic conversion of a normal constituent into an insoluble precipitate. The last thesis was developed to explain the genesis of calcium bilirubinate gallstones in Asiatics, who commonly have associated parasitic infestations a n d / 9 r bacterial infections (Fig. 2). Bilirubin normally is esterified in the liver and secreted into bile as a watersoluble glucuronide, s9 Conjugated bilirubin then can be hydrolyzed into free (unconjugated) bilirubin and glucuronic acid by the enzyme fl-glucuronidase. The biliary tree normally is not only free of fl-glucuronidase b u t also contains an apparent inhibitor of fl-glucuronidase, glucaric acid (D-glucaro-l,4-1actone). Glucaric acid, curiously, is a metabolic p r o d u c t o f glucuronic acid. Coliform bacterial infections of the biliary tree, however, 14
f
. •' ~ BILIRUBIN DIGLUCURONIDE( D-glucar,c ~ C ac,d
J /~.glucuronidase=
--E. coh
GLUCURONIC/ ACID FREE BILIRUBIN ÷ Ca
~ALC'uM..BIL]RUB'INATE(insoluble)] Polymerization
l Ca BILIRUBINATE STONE
O
Fig. 2.: Mechanism of calcium bilirubinate stone formation. The presence of excess fl-glucuronidase overcomes the natural inhibiting factor, D-glucaric acid, hydrolyzing bilirubin to the free form. Free bilirubin combines with calcium. The resultant calcium bilirubinate either can polymerize to form a pigment stone or may act as a nidus for precipitation of cholesterol.
NIDUS for CHOLESTEROLSTONE
elaborate fl-glucuronidase, s2 which results in the formation of unconjugated bilirubin. The free bilirubin is capable of combining with the calcium in bile to form calcium bilirubinate, molecules of which then aggregate to create a biopolymer. T h a t is, chemical alteration of conjugated bilirubin within the bfliary tree results in an insoluble precipitate. This sequence theoretically could be aborted by the oral administration of D-glucaric acid, which markedly reduce;~ fl-glucuronidase activity in bile. 9° These hypotheses implicating the formation of free bilirubin within the biliary tree or its secretion from the liver into bile are, as yet, largely unproved. Unlike in the Asian populations, most gallbladders removed from the Occident contain sterile bile and thus would lack glucuronidase activity. Presently there is no evidence of significant unconjugated bilirubin in gallbladder bile harboring pigment gallstones, s3 and if any were present it likely would be absorbed by the gallbladder. 91 Analysis of such gallbladder bile has failed to demonstrate any distinguishing features in composition with respect to bilirubin or other biliary constituents. The black pigment in the stone itself has not been completely extracted or identified. 88 Characterization of bile pigments has been limited by their instability and complex nature, so that investigators have resorted to examining more stable azopigment derivatives. The diazo reaction splits t h e tetrapyrrole ring of bilirubin at the central methylene bridge, resulting in the formation of two isomeric "o
15
azopigments. Although bilirubin previously was thought to be secreted into bile as bilirubin diglucuronide, a plethora of bilirubin conjugates have been described recently, s9 Patients with liver disease may have altered bilirubin conjugates, 92 but the relationship of bilirubin conjugates to gallstone formation remains unexplored. Indeed, there probably is no association between pigment gallstones and abl.~orrnal conjugates of bilirubin (E. Shaffer and E. Gordon--personal communication). From a different point of view, patients with chronic cholelithiasis appear to have depressed UDP--glucur'onyl transferase activity, a microsomal enzyme that catalyzes glucuronide conjugation of bflirubin as a preliminary for bilirubin secretion. 93 Of the constitutional unconjugated hyperbilirubinemias associated with severe (Crigler-Najjar syndrome) or moderate (Gilbert's syndrome) reduction in glucuronyl transferase activity, only the original description by Gilbert indicated a predilection for stone forrnation. 94 In the Crigler-Najjar syndrome, type 2, the pigment material secreted into bile is largely bilirubin monoglucuronide, but some 10% is in the unconjugated form. 95 Unconjugated bilirubin does not appear to constitute such a major bile component in either hemolysis or Gilbert's syndrome. Not only is the importance of bile pigment composition in gallstone formation unknown but, more important, their solubility in model systems has not been elucidated. Although conjugation of bilirubin to form polar compounds may be the key to its secretion into bile, very little is known T A B L E 2. C H E M I C A L AND P H Y S I C A L PROPERTIES
Common Name
Abbreviation
Systematic Name
(5fl-cho]anoic acid) (3~-hydroxy-5flLA cholanoic acid) (3~, 7~-dihydroxy-5flCDCA Chenodeoxycholic choIanoic acid) acid (3~, 12a-dihydroxy-5fiDCA Deoxycholic acid cholanoic acid) (3~, 7a, 12a-trihydroxyCA Cholic acid 5fl-cholanoic acid) (3, 7, 12-triketocholaDHCA Dehydrocholic acid noic acid) *Compiled from references 109-113. Cholanic acid Lithocholic acid
16
Molecular Weight
Melting Point ° C
360.56 376.56
163-4 184-6
392.56
143
392.56
176-8
408.56
198
402.51
237
about the factors regulating bilirubin secretion in man. A linked bilirubin-bile salt t r a n s p o r t mechanism has been suggested in animal studies, 9Gand bilirubin m a y be secreted in bile in association with macromolecular aggregates of mixed micelles2 7 Bflirubin secretion rates have never been accurately studied in man under physiologic conditions, so t h a t variable results have been reported in the past. 98 Bilirubin secretion, as measured by a duodenal perfusion technic, 95 has established t h a t bilirubin secretion, as expected, really is significantly increased with hemolysis (E. A. Shaffer personal observation). In cirrhosis without significant cholestasis, bilirubin secretion also tends to be high, most likely related to increased bilirubin production either from erythroid sources due to chronic hemolysis or from non-erythroid heine released by chronic liver injury. 99 F u r t h e r investigation obviously is required to discern if abnormal bilirubin metabolism is responsible in pigment gallstone formation so t h a t the critical defect m a y be characterized.
CHOLESTEROL GALLSTONE FORMATION BILE COMPOSITION AND CHOLESTEROL SOLUBILITY
Normal bile is a liquid whereas bile from patients with cholesterol cholelithiasis also contains cholesterol in crystalline form t h a t has precipitated from solution. "~,10s This indicates t h a t the solubility of cholesterol in bile is limited. T h e solubility of choOF F R E E B I L E ACIDS AND SODIUM SALTS*
CMC Critical CMT Micellar Critical concentraMicelle tion (raM) Density Tempera- at 37° C ~;m/cm ~ ture ° C in 0.15 ot Na Salt Na Salt M NaC1, ii, aq. sol. in aq. sol. pH--8
pK'a in Water at CMC and above CMC
Ag# Number of pH of bile salt precipitamolecules tion of in micelle bile acid (36 ° C, from 1% 15 M NaCI sol. of pH--=8)20 mM Na Salt bile salt
100 ° C 63 ° C 1.28
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TABLE
EPmEMIOLOGY
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NATURAL HISTORY CHEMICAL PIGMENT
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CLASSIFICATION GALLSTONE
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6
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11
GALLSTONES
FORMATION
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12
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14
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17
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CHOLESTEROL GALLSTONE FORMATION
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Bile Composition and Cholesterol Solubility
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]7
D e t e r m i n a n t s of Biliary L i p i d C o m p o s i t i o n .
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31
Stages of Cholesterol Gallstone F o r m a t i o n and Their Diagnosis . . . . . . . .
45
Possible Etiologic M e c h a n i s m s of Cholesterol Gallstone F o r m a t i o n . . . . . . .
47
Associated P a t h o g e n e t i c F a c t o r s
54
MANAGEMENT
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QUESTIONS OF CLINICAL USEFULNESS ANSWERED IN THIS iSSUE
1. J u s t how common are gallstones? Page 6 2. Are gallstones becoming more common? Page 7 3. W h a t kinds of populations are afflicted with gallstone disease? Are some populations resistant? Page 8 4. Do age, sex, social class and genetic factors determine the prevalence of ;Tallstones? Page 9 5. W h a t clinical conditions are associated with gallstones? Page 9 .
W h a t is the natural history of gallstone disease? Do all patients develop symptoms? Page 11
7. W h a t are the chemical types of gallstones? Page 12 8.
W h a t are the pigment stones and how do they form? Page 14
9.
W h a t are the major chemical constituents of bile? Page 17
10. How do bile salts solubilize cholesterol in bile? Page 25 11. W h a t are the limits of cholesterol solubility in bile? Page 27 12. How does one use triangular coordinates to plot bile composition? Page 27 13. How does concentration of lipids in bile affect the solubility of cholesterol? Page 29 14. How are bile salts formed and metabolized? Page 31 15. W h a t is the enterohepatic circulation of bile salts? Page 37 16. Hew is cholesterol synthesized, metabolized and secreted into bile? Page 41 17. How does choline influence the synthesis of biliary phospholipid? Page 44 18. W h a t is the relationship between bile salt secretion rate and the secretion of phospholipid and cholesterol? Page 44 19. W h a t are the stages of gallstone formation and how are they diagnosed? Page 45 3
20. Does cholesterol gallstone disease have a single etiology? Page 47 21. How does excessive bile salt loss cause gallstones? Page 49
22. What is "oversensitive bile acid feedback?" Page 49 23. Can excessive cholesterol secretion cause gallstones? Page 53 24. What is the metabolic defect afflicting the American Indians in the Southwest to give them such a high prevalence of gallstones? Page 53 25. Are hormones related to gallstone disease? Page 55 26. What is the relation between hyperlipidemia and gallstones? Page 56 27. Do some drugs that decrease serum cholesterol cause gallstones? Page 57 28. How do we manage gallstones today? Page 58 29. What is the status of medical treatment of gallstones? Page 58
4
is Assistant Professor of Medicine at McGill University a n d Assistant Physician at the M o n t r e a l General Hospital. After g r a d u a t i n g from Queen's University, Kingston, Ontario, he received training in internal medicine and gastroenterology at the M o n t r e a l General Hospital and subsequently became a Fellow in the Royal College of Physicians and Surgeons and a Dipl~,mate of the A m e r i c a n Board of I n t e r n a l Medicine. A w a r d e d a Medical Research Council of C a n a d a fellowship, he initiated his research in the m e c h a n i s m of gallstone :formation as a research associate at Boston University School of Medicine and has p u r s u e d this i n t e r e s t while at McGill University.
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is Professor of Medicine and Biochemistry a n d Chief of the Section of Biophysics at Boston University School of Medicine. Doctor Small received his M.D. degree from the University of California, Los Angeles Medical S c h o o l i n 1960 and obtained p o s t g r a d u a t e training in internal medicine at Boston University Medical Center. F r o m 1963 to 1965 he served as a N a t i o n a l Institutes of H e a l t h Fellow in Physical C h e m i s t r y at the Service de Biophysique I n s t i t u t e Pasteur, Paris, France. F o r his studies on cholesterol gallstone form a t i o n a n d the enterohepatic circulation of bile acids he was a w a r d e d the 1972 American Gastroenterological Association Distinguished Achievement Award, the N a t h a n H a t field Award, the U C L A Alumni Association Distinguished Achievement A w a r d in Science in 1974 and the E p p i n g e r Prize of the I n t e r n a t i o n a l Liver Society in 1976. Doctor S m a l l ' s present work involves the physical structure, physiology and metabolism of serum lipoproteins and the arterial wall. T h e long t e r m goal of this work is to discover the physical chemical basis of lipid deposition a n d removal in atherosclerosis.
G A L L S T O N E S have afflicted man since antiquity, being found by the Egyptians in their dead and used as charms, potions and paint. 1 Present knowledge began 200 years ago when cholesterol was first identified as the major component of gallstones. Indeed, "cholesterine" means "bile solid. ''2 Soon after, bile salts were discovered and thought to be important in dissolving cholesterol 6
leading to the nfid-19th century suggestion t h a t a deficiency of bile salts relative to cholesterol in bile was the central problem in stone formation, d, 4 Modern concepts have evolved from the physicochemical definition of cholesterol solubility in bile ~ and the events leading to gallstone formation. 6 Such advances have elicited a resurgence of interest in the mechanism of gallstone disease, and the therapeutic rationale t h a t logically followed 7 has evoked a new era in management, s EPiDEMIOLOGY
Epidemiology, "the study of disease occurrence in h u m a n populations," is crucial to understanding disease causation. .~ The available data on gallstones are fragmentary and the overworked aphorism "female, fair, fat, fertile, forty and flatulent" is not particularly associated with gallstone disease. TM 11 Estimates of occurrence and information about associated factors have come primarily from clinical and autopsy studies t h a t contain both selection and detection bias. Unfortunately, such a data base underestimates the true frequency and tends to obscure real associations by failing to identify asymptomatic disease. PREVALENCE. "Prevalence" describes the number of people who have gallstones at a certain point in time 9, 12 and is best determined by performing a random cholecystographic survey of a population at risk. The true prevalence of gallstone disease has been determined only in 600 Pima Indians living on a reservation in Arizona 13 (Fig. 1,A). Gallstones are very common among these people, so t h a t only a small sampling population was necessary. Seventy per cent of the women were found to have gallstones by the age of 30 whereas, in males, this high frequency was reached only in later life. The stones appeared soon after the age of 20 in the females b u t developed only gradually in males during middle age. There was no relation between the prevalence of radiologic cholelithiasis and such factors as parity, obesity, diabetes, serum cholesterol and atherosclerosis. 13 As for obesity and diabetes, subtleties may have been obscured by the high preponderance of these two conditions in this Indian population. "Clinical" prevalence, in contrast to " t r u e " prevalence (Fig. 1,A), suggests t h a t symptoms do not appear until several years after the development of the stones. Also, nearly half the women and twothirds of the men with gallstones never had clinical symptoms. The n u m b e r of persons who have experienced symptoms of biliary disease has been reported as 10 per 1000 population randomly surveyed over a 6-month period. This represents 10% of all digestive complaints recorded in this United States survey. 14 In those over 65 years of age, the figure triples. 6
PIMA INDIANS
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20
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Fig. 1,--A, the "true" (solid line) and "clinical" (dashed line) prevalence of gallbladder disease in the Pima Indians. Vertical axes represent the percentage of the total group who h~ve gallstones; horizontal axes, the ages. Stones develop at an earlier age in females, but the prevalence in the older age groups approaches 80% in.both sexes. The lower curves for clinical prevalence confirm the general impression that symptomatic disease appears some time after radiographic demonstration of the presence of gallbladder disease. The difference, at any age, represents "silent" cases. (Drawn from data of Sampliner et a1.13) B, the estimated prevalence of gallbladder disease in the general U. S. population shown for male and female. Closed circles give definite prevalence from J. Chronic Dis. 19:273, 1966, ref. 11, and open circles the calculated "true" prevalence. X refers to autopsy fate of gallstones.
Prevalence of gallstones has also been radiologically examined in Canadian Micmac Indian and Caucasian women (C. N. Williams personal communications). Gallbladder disease prevalence was 185/1000 for these Indians, which corresponded closely to t h a t found in the Pima Indians of 215/1000. I3 Disease also tended to start early in life---in the 20s and 30s. The Caucasian women in Nova Scotia appeared l/o have a prevalence rate of 108/1000, which was higher than t h a t of 39/1000 reported in Framingham, Massachusetts. 11 True prevalence of cholelithiasis is otherwise unknown for the general population. If, however, at least half of all people with gallstones are symptomatic, the " t r u e " prevalence can be estim a t e d from the "clinical" prevalence on the basis of the Framingham study (Fig. 1,B). 11 This estimate suggests t h a t 12 million females and 4 million males in the United States will harbor gallstones. About 800,000 new cases of cholelithiasis develop each year, and approximately half of these ultimately undergo operation. Gallstone disease accounts for about 5000--8000 deaths per year, and the resultant morbidity costs approach 1 billion dollars. There is no d o u b t t h a t gallbladder disease is an i m p o r t a n t health problem both medically and financially. INCIDENCE.- " I n c i d e n c e " refers to the rate of development of new cases over a certain time period. 12 The incidence of overt biliary tract disease is rising in some autopsy studies ~5 b u t not in others. ~6 Operative reports have shown an increasing fre7
quency. '7 Large increases in biliary operations have occurred over the past 30 years on both sides of the Atlantic, 18 but the number of cholecystectomies still is 6 times higher in North America than in Western Europe. ~9 Such large differences in tile rates at which operation is performed may reflect either a heightened tendency to the disease, improved diagnostic expertise or better health-care delivery. Conversely, this could imply that therapeutic usefulness has been exceeded and eventually may lead to excess mortality. 2°. 21 Care must be taken not to misinterpret the number of deaths attributed to cholecystectomy. 22 In any event, biliary disease has become an increasing burden on surgical facilities. 2~ GEOGRAPHIC PREVALENCE.~The over-all frequency of gallstones in a given country or community commonly is derived from autopsy data that are selective and not truly representative of the population at large, ls,24 The reported frequency in postmortem material is an approximation that allows a reasonable geographic classification (Table 1). Prevalence has an exceedingly wide range: from the North American Indian population, in whom the frequency of gallstones reaches epidemic proportions, ~3 to the Masai tribe in sub-Saharal Africa, who apparently do not have gallstones. 25 Significant differences can exist even within the boundaries of one country, perhaps related to socioeconomic class and dietary factors. 2~ Immigration also may influence the development of cholelithiasis.27 Such studies, which may disclose pertinent information on the geographic distribution of gallstones, often fail to document the type of gallstones found. Cholesterol gallstones characteristically are the most common in modem Western civilization. DET~,RMINANTS OF PREVALENCE: The factors associated with gallstones are constantly being re-evaluated from both an epidemiologic and a pathophysiologic basis.
Age.~Gallstones are uncommon in childhood but may occur in association with chronic hemolytic states. 2s Frequency rises progressively with age in both sexes and all races. ~s, 2t. 2~,~9,30 AITABLE
1.--FREQUENCY COUNTRIES
VERY COM~VION
COMMON
OF GALLSTONES IN DIFFERENT (AUTOPSY STUDIES) INTERMEDIATE
B~IE
(> 30-50%) (10-30%) ( < 10%) (~0%) U. S. ( I n d i a n s ) 13. ~oo U. S. ( w h i t e s ) ll. 15 U. S. (blacks) Is. lo3 E. A f r i c a 2~. los S w e d e n 1°1 N o r w a y 29 J a p a n 1°4 E s k i m o s (Canada) lo7 C z e c h o s l o v a k i a 3° U . K . le. 35. 44 ThailandlO~ W e s t e r n E u r o p e '-'4 A u s t r a l i a 1 o~
8
though gallbladder surgery (and hence the supposed occurrence of clinical disease) is more common in middle age, ~7 the actual formation of the stone may have occurred several years earlier. 13
Sex. Female predominance in clinical gallstone disease has been well established. ~1 Its prevalence has been further related to parity in some reports 9, 22.33 but not in others. 13,34,35 This discrepancy may be related to the difference between " t r u e " prevalence and "clinical" prevalence; the latter, apparent as symptomatic gallstone disease, may be diagnosed more frequently (i.e., detection bias) with multiple pregnancies, 36 because such patients are seen more often by a physician. More convincing evidence has come from epidemiologic surveys concerning women receiving female sex hormonal therapy. These demonstrate a significant association between gallbladder disease in both young women on oral contraceptives 37 and older women taking estrogens. 38 Social class.--In the past century, sedentary occupations were associated with a predilection for gallstones, but presently this feature is not apparent in Western countries, although socioeconomic differences may occur elsewhere, such as in India "-'~and Japan. TM Genetic factors.--The general impression that gallstone disease has a tendency to run in certain families is supported by the few studies available. 39 Concordance Studies in identical twins and familial clustelings have only been suggestive because of the very common nature of the disease. Stronger evidence has come from a study of bile composition in siblings of young Caucasians with cholelithiasis. Even those relatives without radiologic evidence of gallstones possessed bile, which put them at risk for developing gallstones. 4° The racial differences noted previously (Table 1) incriminate genetic factors, but the incidence changes rapidly in migrants. 27 Therefore, a polygenic predisposition may exist in the general population, only to be e!icited under the influence of certain environmental events. Clinical associations. The factors associated with the occurrence of gallstones either have been inadequately examined by observational epidemiology or have resulted in conflicting interpretations. Certain clinical associations presently are considered to be real and not just artifactual, is. 41 a) Definite associations: 1. Pancreatitis. 24, 4t Gallstones, in their migration down the biliary tree, may be an important precipitating cause of acute and recurrent attacks of pancreatitis. 42 2. Carcinoma of the gallbladder, but the cause-effect relationship has not been established. 3°' 41 9
3. Pigment gallstones in both hemolytic states 24 and cirrhosis.48.44, s7 4. Chronic ileal disease or extensive ileal resection undoubtedly isrelated to gallstoneformation. 4~. 4G 5. Obesity. B o t h autopsy24.:a0. 3~,a5 and clinical 11 surveys have supported this strongly positive correlation. Dietary assessment has suggested t h a t gallstone patients, a t least in France, consume s i g n i f i c a n t l y m o r e calories than do control subjects. ~' In Pima Indians, a relat~ionship to body weight, was not demonstrated, perhaps because obeslt~y is very common in this population. 13 b) Probable associations: 1. Diet. Excessive dietary retake induces the diseases of overconsumption (obesity, diabetes and coronary artery disease) and probably is a major determinant of gallstone formation. Qualitative dietary differences may explain differences in incidence between geographic regions and within countries2' In J a p a n , where the majority of gallstones were composed of pigment material, a rising incidence of cholesterol gallstones has been attributed to an increasing fat content in the diet. 4s A conflicting report from Australia has suggested t h a t patients with cholelithiasis had a reduced fat intake2 9 Bias occurs in such retrospective dietary surveys because patients may reduce fat intake because of socalled fatty food intolerance after the disease has developed. The overriding importance of genetic and population differences is exhibited by the Masai tribe a n d Eskimos. Despite their large consumption of animal fat, these people rarely develop gallstones. In fact, a diet low in unsaturated fats, given to lower serum cholesterol and prevent atherosclerosis, appeared to increase the prevalence of cholelithiasis in a Los Angeles study. ~° Last, the highly refined carbohydrate diet in affluent Western civilization, where gallstones are so common, has led Burkitt ~1 to suggest t h a t fiber deficiency m a y be an important feature. 2. Atherosclerosis. There is no true association between coronary artery disease and cholelithiasis, despite some shared etiologic factors, such as age and obesity. ~2 Because most gallstones are largely composed of cholesterol, it was naturally assumed t h a t cholesterol gallstones reflected disordered lipid metabolism. Hypercholesterolemia does not appear related to gallstone disease, ~,~2 but the " n o r m a l " range of serum cholesterolin Western Caucasians is higher than in certain populations where gallstones are uncommon (Japanese) or rare (Masai). 25 Almost paradoxically, the North American Indian, who commonly has cholesterol gallstones, tends to possess a low serum cholesterol. ~3 3. Hyperlipidemia. Familial hyperlipoproteinemia i n general 5~ and hypertriglyceridemia (type IV) 54 in particular m a y be associated with a high incidence of gallbladder disease. Other contributing factors in this heterogenous entity include obesity, 10
diabetes meUitus and dietary sucrose all of which m a y be associated with gallstones. 4. Diabetes mellitus. Autopsy evidence has suggested an increased risk of gallstones in diabetic patients,15.2~, ~5--57 but a recent autopsy study failed to confirm such an association. ~° Clinical population studies, both in Caucasians I~ and in Pima Indians, 13 have been inconclusive. The FramJngham study conrained too few diabetics to justify statistical analysis whereas the very high prevalence of both diabetes and gallstones in the Pima Indians may have obscured any discernible correlation. Therefore, this relationship has not been established. 5. Peptic ulcer disease and gastric surgery. The consequences of peptic ulcer disease and operation for this condition on gallstone formation are not known. Most likely there is no association between peptic ulcer and gallstones except for detection bias i.e., peptic ulcer patients are more likely to receive an oral cholecystogram. The postulated association between gallstones and gastric surgery (.specifically truncal vagotomy) remains unproved. 4~, 5s 6. Other associations. Cholelithiasis has been related to a myriad of other disorders, including pheochromocytoma and idiopathic late-onset immunoglobulin.deficiency. .59 The meaning and validity of such findin-gs remain to be clarified. NATURAL HISTORY
The natural course of gallstone disease is poorly defined, but autopsy evidence has suggested that most patients have no significant symptoms. 60 Further information on the frequency of these clinically "silent gallstones" has come from retrospective studies using cholecystography. 61, 62 In Figure 1,A, the lower curves for clinically symptomatic patients may be contrasted to the higher incidence of gallstones seen radiographically. The difference represents asymptomatic or silent cases. The obvious difficulty resides in a clinical definition of asymptomatic cases t h a t are found fortuitously. 63 Why was the radiographic examination done in the first place? W h a t about gallstones "incidentally" discovered at the time of abdominal s u r g e r y ? Despite this, the available data suggest t h a t only one-third to one-half of patients will develop significant symptoms during a 10-20-year period. 13 The tendency to cause symptoms appears u n r e l a t e d t o size, number or type of gallstones present. 64 The silent gallstone, in fact, really is not a specific lesion, but merely represents the asymptomatic stage of cholelithiasis. In the older patient, the incidence of symptomatic cholelithiasis may rise to become a major problem. 15, 65 The danger engendered by nonoperative management of silent gallstones, particularly in 11
the elderly, is the development of acute cholecystitis, with its higher mortality a n d morbidity. Those patients who develop acute cholecystitis usually have premonitory Symptoms, 6~ and more severe complications tend to occur when the gallbladder is radiologically nonfunctioning. 62 Although the risk-benefit ratio of cholecystectomy is not fully known, eGthe elderly patient with definite symptoms of gallbladder disease or with a radiologically nonfunctioning gallbladder is the best candidate for surgery. The other potential danger in a gallbladder harboring gallstones is the development of carcinoma. The association between carcinoma of the gallbladder and gallstones has led to the suggestion of early elective operation in the geriatric population, eS, 6e There is n o evidence that gallstones, per se, predispose to gallbladder carcinoma or that elective cholecystectomy even reduces the mortality. 67 Actually, the 1% risk of attendant malignancy is not an indication for prophylactic cholecystectomy, which by itself carries at least a 1% mortality. G~ Well-designed studies to define the natural history of gallstone disease are urgently needed as a guide to therapy, especially in the older age group. Indeed, not only do gallstones appear, but occasionally they can disappear spontaneously, es CHEMICAL CLASSIFICATION OF GALLSTONES
Biliary calculi rarely are examined other than by gross description in the pathology laboratory. An early clue to their metabolic origin came from chemical analysis of gallstone composition. Cholesterol was found to be the major component of most human gallstones in the West 1 whereas bile pigment commonly comprised those in the Orient. 69 Crystallographic studies such as x-ray diffraction 7° and x-ray powder method vl have demonstrated that more than one physical form of cholesterol may exist in stones collected under different conditions. Cholesterol usually is present as single crystals whose axes are radially oriented with. respect to the nucleus. 71 The crystals of cholesterol in the stone, as it exists in the gallbladder, occur onlyas the cholesterol monohydrate (C. Loomisupersonal communication). Cholesterol gall.~tones also contain small quantitiesof other substances, such as calcium carbonate and calcium palmitate. Calcium palmitate can even be the sole crystalline component, the palmitic acid likely arising from hydrolysis of biliary lecithin.72 Calcium carbonate can be present in three different physical forms calcite, aragonite or vaterite. 71 Calcium and bicarbonate ions are normal constituents of gallbladder bile. Bicarbonate absorption across the wall of the gallbladder controls pH and hence calcium carbonate solubility. The association of calcium carbonate with dark pigmented areas has implied that 12
coprecipitation of calcium carbonate and pigment material may occur, perhaps as a result of a pH change in bile. 73 This situation does not exist with "limy bile," which is just precipitated calcium carbonate and does not necessarily proceed to stone formation. Conversely, pigment material can precipitate in the absence of calcium carbonate. Organic material, rarely, is the only ingredient in pigment gallstones (rather than pigmented calcium carbonate or calcium bilirubinate). An organic matrix commonly remains in the majority of pigment stones, once the crystalline material is dissolved out. TM This mucus-like substance appears as a constituent of most gallstones and m a y be involved in stone formation. Mucosubstances could act at various s t a g e s - - i n the form of a nidus to initiate stone formation, 75 as a mucous plug to entrap precipitared crystals within the gallbladder or as a scaffold for crystal aggregation. 76 Their physical and chemical properties in bile are not known and their importance has yet to be established. 77 The central portion of the stone represents events occurring during the initial stages of stone formation; the periphery probably reflects factors involved with stone growth. Differential microanalysis, therefore, has a t t e m p t e d to show regional differences in composition. Bile acids in the center of cholesterol gallstones have a composition with relatively less deoxycholic acid and taurine conjugates, suggesting t h a t stone formation was initiated during a time when the bile salt secretion around the enterohepatic circulation was disturbed. 7s Other materials found a t the center include pigment matter, protein and parasites. Stone centers often consist of a mass of small crystals and pigment apparently trapped in strands of fibers, supporting the thesis t h a t nucleation begins on biliary debris or mucosubstances. 7~ The peripheral layers of gallstones have also undergone analytic scrutiny, b u t with conflicting results. Some have found no compositional differences 79 whereas others demonstrated a successive change in gallstone layers (and presumably bile composition) t h a t probably occurred during growth, s° Mixed stones containing both cholesterol and pigment material tended to have differences in composition when layers were sequentially analyzed, sl Analysis of the layered structure, which presumably was built up, step by step, around the central core, has unfortunately not disclosed the basis for periods of stone growth. Pigment gallstones h a v e been investigated extensively in J a p a n because of their high hlcidence. Two types have:been recognized: calcium bilirubinate stones commonly found i n the Orient associated with infestations of Ascaris or Escherichia coli and pure pigment stones occurring without any infection b u t sometimes with hemolysis, s2 Morphologically, calcium bilirubinate stones are fragile, with a dark o r reddish brown color and 13
may form a m u d d y substance anywhere in the biliary tree. The pure pigment stones are typically black or dark green. These gross features can be readily used to accurately distinguish pigment from cholesterol gallstones 1.s3 whereas radiographically visible opacity is not necessarily pathognomonic, s4.s~ Calcium, the major mineral in all gallstones, determines the opacity of stones. When calcium is largely absent, as commonly occurs in pure cholesterol or occasionally with pure pigment stones, gallstones will not be radiologically visible on plain films. Analysis of pigment stones has suggested that unconjugated bilirubin, calcium and protein are the major constituents, sl, s2 although calcium bilirubinate material may not be the predominant type in the West. s'~ Unfortunately, analytic deficiencies have led to incomplete extractions so t h a t the principal component remains unidentified. PIGMENT GALLSTONE FORMATION
Pigment stones with only trace amounts of cholesterol comprise about 15% of all gallstones in the Occident but are common in Asiatics. The percentage of gallstones composed of cholesterol may be increasing in Japan, a reflection of dietary and environmental changes2 9 In certain disease states such as chronic hemolysis, the frequency of pigment gallstones is increased to almost epidemic proportions, s6 Cirrhosis is associated with over a twofold increase in gallstones, predominantly of the pigment variety. 44,s7 Retrograde cholangiography recently has disclosed a high prevalence of gallstones in primary biliary cirrhosis, ss Furthermore, if these small gallstones produce biliary obstruction, the clinical picture that ensues can create a complex management problem. The possible mechanisms by which these gallstones form include: secretion of an abnormal pigment t h a t readily precipitates in bile, excessive hepatic-secretion of a normal pigment t h a t surpasses its solubility in bile or extrahepatic conversion of a normal constituent into an insoluble precipitate. The last thesis was developed to explain the genesis of calcium bilirubinate gallstones in Asiatics, who commonly have associated parasitic infestations a n d / 9 r bacterial infections (Fig. 2). Bilirubin normally is esterified in the liver and secreted into bile as a watersoluble glucuronide, s9 Conjugated bilirubin then can be hydrolyzed into free (unconjugated) bilirubin and glucuronic acid by the enzyme fl-glucuronidase. The biliary tree normally is not only free of fl-glucuronidase b u t also contains an apparent inhibitor of fl-glucuronidase, glucaric acid (D-glucaro-l,4-1actone). Glucaric acid, curiously, is a metabolic p r o d u c t o f glucuronic acid. Coliform bacterial infections of the biliary tree, however, 14
f
. •' ~ BILIRUBIN DIGLUCURONIDE( D-glucar,c ~ C ac,d
J /~.glucuronidase=
--E. coh
GLUCURONIC/ ACID FREE BILIRUBIN ÷ Ca
~ALC'uM..BIL]RUB'INATE(insoluble)] Polymerization
l Ca BILIRUBINATE STONE
O
Fig. 2.: Mechanism of calcium bilirubinate stone formation. The presence of excess fl-glucuronidase overcomes the natural inhibiting factor, D-glucaric acid, hydrolyzing bilirubin to the free form. Free bilirubin combines with calcium. The resultant calcium bilirubinate either can polymerize to form a pigment stone or may act as a nidus for precipitation of cholesterol.
NIDUS for CHOLESTEROLSTONE
elaborate fl-glucuronidase, s2 which results in the formation of unconjugated bilirubin. The free bilirubin is capable of combining with the calcium in bile to form calcium bilirubinate, molecules of which then aggregate to create a biopolymer. T h a t is, chemical alteration of conjugated bilirubin within the bfliary tree results in an insoluble precipitate. This sequence theoretically could be aborted by the oral administration of D-glucaric acid, which markedly reduce;~ fl-glucuronidase activity in bile. 9° These hypotheses implicating the formation of free bilirubin within the biliary tree or its secretion from the liver into bile are, as yet, largely unproved. Unlike in the Asian populations, most gallbladders removed from the Occident contain sterile bile and thus would lack glucuronidase activity. Presently there is no evidence of significant unconjugated bilirubin in gallbladder bile harboring pigment gallstones, s3 and if any were present it likely would be absorbed by the gallbladder. 91 Analysis of such gallbladder bile has failed to demonstrate any distinguishing features in composition with respect to bilirubin or other biliary constituents. The black pigment in the stone itself has not been completely extracted or identified. 88 Characterization of bile pigments has been limited by their instability and complex nature, so that investigators have resorted to examining more stable azopigment derivatives. The diazo reaction splits t h e tetrapyrrole ring of bilirubin at the central methylene bridge, resulting in the formation of two isomeric "o
15
azopigments. Although bilirubin previously was thought to be secreted into bile as bilirubin diglucuronide, a plethora of bilirubin conjugates have been described recently, s9 Patients with liver disease may have altered bilirubin conjugates, 92 but the relationship of bilirubin conjugates to gallstone formation remains unexplored. Indeed, there probably is no association between pigment gallstones and abl.~orrnal conjugates of bilirubin (E. Shaffer and E. Gordon--personal communication). From a different point of view, patients with chronic cholelithiasis appear to have depressed UDP--glucur'onyl transferase activity, a microsomal enzyme that catalyzes glucuronide conjugation of bflirubin as a preliminary for bilirubin secretion. 93 Of the constitutional unconjugated hyperbilirubinemias associated with severe (Crigler-Najjar syndrome) or moderate (Gilbert's syndrome) reduction in glucuronyl transferase activity, only the original description by Gilbert indicated a predilection for stone forrnation. 94 In the Crigler-Najjar syndrome, type 2, the pigment material secreted into bile is largely bilirubin monoglucuronide, but some 10% is in the unconjugated form. 95 Unconjugated bilirubin does not appear to constitute such a major bile component in either hemolysis or Gilbert's syndrome. Not only is the importance of bile pigment composition in gallstone formation unknown but, more important, their solubility in model systems has not been elucidated. Although conjugation of bilirubin to form polar compounds may be the key to its secretion into bile, very little is known T A B L E 2. C H E M I C A L AND P H Y S I C A L PROPERTIES
Common Name
Abbreviation
Systematic Name
(5fl-cho]anoic acid) (3~-hydroxy-5flLA cholanoic acid) (3~, 7~-dihydroxy-5flCDCA Chenodeoxycholic choIanoic acid) acid (3~, 12a-dihydroxy-5fiDCA Deoxycholic acid cholanoic acid) (3~, 7a, 12a-trihydroxyCA Cholic acid 5fl-cholanoic acid) (3, 7, 12-triketocholaDHCA Dehydrocholic acid noic acid) *Compiled from references 109-113. Cholanic acid Lithocholic acid
16
Molecular Weight
Melting Point ° C
360.56 376.56
163-4 184-6
392.56
143
392.56
176-8
408.56
198
402.51
237
about the factors regulating bilirubin secretion in man. A linked bilirubin-bile salt t r a n s p o r t mechanism has been suggested in animal studies, 9Gand bilirubin m a y be secreted in bile in association with macromolecular aggregates of mixed micelles2 7 Bflirubin secretion rates have never been accurately studied in man under physiologic conditions, so t h a t variable results have been reported in the past. 98 Bilirubin secretion, as measured by a duodenal perfusion technic, 95 has established t h a t bilirubin secretion, as expected, really is significantly increased with hemolysis (E. A. Shaffer personal observation). In cirrhosis without significant cholestasis, bilirubin secretion also tends to be high, most likely related to increased bilirubin production either from erythroid sources due to chronic hemolysis or from non-erythroid heine released by chronic liver injury. 99 F u r t h e r investigation obviously is required to discern if abnormal bilirubin metabolism is responsible in pigment gallstone formation so t h a t the critical defect m a y be characterized.
CHOLESTEROL GALLSTONE FORMATION BILE COMPOSITION AND CHOLESTEROL SOLUBILITY
Normal bile is a liquid whereas bile from patients with cholesterol cholelithiasis also contains cholesterol in crystalline form t h a t has precipitated from solution. "~,10s This indicates t h a t the solubility of cholesterol in bile is limited. T h e solubility of choOF F R E E B I L E ACIDS AND SODIUM SALTS*
CMC Critical CMT Micellar Critical concentraMicelle tion (raM) Density Tempera- at 37° C ~;m/cm ~ ture ° C in 0.15 ot Na Salt Na Salt M NaC1, ii, aq. sol. in aq. sol. pH--8
pK'a in Water at CMC and above CMC
Ag# Number of pH of bile salt precipitamolecules tion of in micelle bile acid (36 ° C, from 1% 15 M NaCI sol. of pH--=8)20 mM Na Salt bile salt
100 ° C 63 ° C 1.28
