The liver

Chapter 5 The liver



Introduction


The liver is of vital importance in intermediary metabolism and in the detoxification and elimination of toxic substances (Fig. 5.1). Damage to the organ may not obviously affect its activity, as the liver has considerable functional reserve and, as a consequence, simple tests of liver function (e.g. plasma bilirubin and albumin concentrations) are insensitive indicators of liver disease. Tests reflecting liver cell damage (particularly the measurement of the activities of hepatic enzymes in plasma) are often superior in this respect. The categorization of such tests as ‘liver function tests’ is clearly a misnomer, but seems likely to endure. Various tests have been devised to provide a quantitative assessment of functional hepatic cell activity (see p. 90), but they are not suitable for use in routine clinical practice.



The results of the standard biochemical liver function tests rarely provide a precise diagnosis on their own, as they reflect the basic pathological processes common to many conditions. However, biochemical tests are cheap, non-invasive and widely available, and are of value in directing the use of other diagnostic tests, notably imaging and liver biopsy. They are also useful in detecting the presence of liver disease and in following its progress. Serological tests (e.g. for autoantibodies and evidence of viral infection) are also important in the investigation of liver disease.


The liver has a dual blood supply: approximately two-thirds from the portal vein, which drains much of the gut and through which most of the nutrients absorbed from the gut reach the liver; and the remainder from the hepatic artery, which supplies most of the liver’s oxygen. Blood leaves the liver through hepatic veins, which drain into the inferior vena cava.


The metabolic activity of the liver takes place within the parenchymal cells, which constitute 80% of the organ mass; the liver also contains Kupffer (reticuloendothelial) cells and stellate cells (the major cell type responsible for fibrosis). Parenchymal cells are contiguous with the venous sinusoids, which carry blood from the portal vein and hepatic artery, and with the biliary canaliculi, the smallest ramifications of the biliary system (Fig. 5.2). Substances destined for excretion in the bile are secreted from hepatocytes into the canaliculi, pass through the intrahepatic ducts and reach the duodenum via the common bile duct.



The most common disease processes affecting the liver are:



Patients with liver disease often present with characteristic symptoms and signs, particularly jaundice, the yellow-orange discoloration of the skin caused by a high plasma concentration of bilirubin, but the clinical features may be non-specific and, in some patients, liver disease is discovered incidentally. Because of the intimate relationship between the liver and biliary system, extrahepatic biliary disease may present with clinical features suggestive of liver disease or may have secondary effects on the liver. For instance, obstruction to the common bile duct may cause jaundice and, if prolonged, a form of cirrhosis.



Bilirubin metabolism


Bilirubin is derived mainly from the haem moiety of haemoglobin molecules and is liberated when senescent red cells are removed from the circulation by the reticuloendothelial system (Fig. 5.3); the iron in haem is reutilized but the tetrapyrrole ring is degraded to bilirubin. Other sources of bilirubin include myoglobin and the cytochromes.



Unconjugated bilirubin is not water soluble: it is transported in the bloodstream bound to albumin. In the liver, it is taken up by hepatocytes in a process involving specific carrier proteins. It is then transported to the smooth endoplasmic reticulum, where it undergoes conjugation, principally with glucuronic acid, to form mono- and diglucuronides; this process is catalysed by the enzyme bilirubin-uridyl diphosphate (UDP) glucuronosyl transferase. The resulting conjugated bilirubin is water soluble and is secreted into the biliary canaliculi, eventually reaching the small intestine via the ducts of the biliary system. Secretion into the biliary canaliculi is the rate-limiting step in bilirubin metabolism. In the gut, bilirubin is converted by bacterial action into urobilinogen, a colourless compound. Some urobilinogen is absorbed from the gut into the portal blood. Hepatic uptake of this is incomplete: a small quantity reaches the systemic circulation and is excreted in the urine. Most of the urobilinogen in the gut is oxidized in the colon to a brown pigment, stercobilin, which is excreted in the stool.


Some 300 mg of bilirubin is produced daily but the healthy liver can metabolize and excrete ten times this amount. The measurement of plasma bilirubin concentration is thus an insensitive test of liver function: it is frequently normal in early or mild liver disease


The bilirubin normally present in the plasma is mainly (approximately 95%) unconjugated; because it is protein bound, it is not filtered by the renal glomeruli and, in health, bilirubin is not detectable in the urine. Bilirubinuria reflects an increase in the plasma concentration of conjugated bilirubin, and is always pathological.


Although jaundice is a frequent feature of liver disease, it may not be obvious clinically unless the plasma bilirubin concentration is more than two and half times the upper limit of normal; that is, more than 50 µmol/L. Hyperbilirubinaemia can be caused by increased production of bilirubin, impaired metabolism, decreased excretion or a combination of these. The causes of jaundice are listed in Figure 5.4.




The biochemical assessment of liver function



Plasma bilirubin concentration


Hyperbilirubinaemia is not always present in patients with liver disease, nor is it exclusively associated with liver disease. For example, it is not usually present in patients with well-compensated cirrhosis but it is a common feature of advanced pancreatic carcinoma.




Conjugated hyperbilirubinaemia


This condition is due to leakage of bilirubin from either hepatocytes or the biliary system into the bloodstream when its normal route of excretion is blocked. The water-soluble conjugated bilirubin entering the systemic circulation is excreted by the kidneys, as a result of which the urine develops a deep orange-brown colour. In complete biliary obstruction, no bilirubin reaches the gut, no stercobilin is formed and the stools are pale in colour. The differential diagnosis of jaundice due to conjugated bilirubin is considered on p. 97.


Hyperbilirubinaemia can be due to an excess of either or both conjugated and unconjugated bilirubin. The separate measurement of these entities is useful in the diagnosis of neonatal jaundice, where there may be some doubt as to the relative contribution of defective conjugation and other causes; it is less often required in adults and the chemical methods are anyway not wholly reliable at detecting small increases in either fraction. If the plasma bilirubin concentration is <100 µmol/L and other tests of liver function are normal, it can be inferred that the raised levels are due to the unconjugated form of the pigment. The urine can be tested to confirm this because with unconjugated hyperbilirubinaemia there is no bilirubin in the urine. In adults, severe jaundice is almost always a result of conjugated hyperbilirubinaemia.


A third fraction of bilirubin, consisting of conjugated bilirubin bound covalently to albumin, is found in the plasma of patients with longstanding conjugated hyperbilirubinaemia. This substance has a half-life similar to that of albumin. Its persistence in the plasma during the resolution of liver disease or after the relief of obstruction explains the persistence of jaundice in the absence of bilirubinuria that can occur in these circumstances.



Plasma enzymes


Enzymes used in the assessment of the liver include aspartate and alanine aminotransferases (formerly called transaminases and still abbreviated AST and ALT, respectively), alkaline phosphatase (ALP) and γ-glutamyl transferase (GGT). In general, these enzymes are not specific indicators of liver dysfunction. The hepatic isoenzyme of ALP is an exception, and ALT is more specific to the liver than AST.


Increased aminotransferase activities reflect cell damage; plasma levels may be 20 times the upper limit of normal (ULN) in patients with hepatitis. In cholestasis (obstruction to the flow of bile), plasma ALP activity is increased. This is due mainly to increased enzyme synthesis (enzyme induction), stimulated by cholestasis. In severe obstructive jaundice, the plasma ALP activity may be up to 10 × ULN.


In practice, however, increases in the plasma activities of both the aminotransferases and ALP are often present in patients with liver disease, although one may predominate. In primarily cholestatic disease, there may be secondary hepatocellular damage and increased plasma aminotransferase activities, while cholestasis frequently occurs in primarily hepatocellular disease. Increased GGT activity is found in both cholestasis and hepatocellular damage: this enzyme is a very sensitive indicator of hepatobiliary disease but is non-specific. Thus, although certain patterns of plasma enzyme activities are frequently observed in various types of liver disease, they are not reliably diagnostic.


The enzymes AST and ALT provide essentially the same information and many laboratories measure only one, but AST is sometimes disproportionately elevated in alcohol-related liver disease.


Plasma enzyme activities are very useful in following the progress of liver disease once the diagnosis has been made. Falling aminotransferase activity suggests a decrease in hepatocellular damage and falling ALP activity suggests a resolution of cholestasis. However, in severe acute hepatic failure, a decrease in aminotransferase activity may misleadingly suggest an improvement when it is actually due to almost complete destruction of parenchymal cells.


It is important to appreciate that there are many extrahepatobiliary causes of increased plasma activities of the aminotransferases, GGT and ALP. These are discussed in Chapter 13.



Plasma proteins


Albumin is synthesized in the liver and its concentration in the plasma is in part a reflection of the functional capacity of the organ. Plasma albumin concentration tends to decrease in chronic liver disease, but is usually normal in the early stages of acute hepatitis owing to its long half-life (approximately 20 days). There are many other causes of hypoalbuminaemia, as discussed on p. 226, but a normal plasma albumin concentration in a patient with chronic liver disease implies adequate synthetic function; a fall implies a significant deterioration.


The prothrombin time, usually expressed as a ratio (the international normalized ratio, INR) to a control value, is a test of plasma clotting activity and reflects the activity of vitamin-K-dependent clotting factors synthesized by the liver, of which factor VII has the shortest half-life (4–6 h). An increase in the prothrombin time is often an early feature of acute liver disease, but a prolonged prothrombin time may also reflect vitamin K deficiency (in which case, a single parenteral dose of vitamin K should normalize the prothrombin time within 18 h).


A polyclonal increase in immunoglobulins is a frequent finding in patients with chronic liver disease (particularly of autoimmune origin) and may cause an increase in plasma total protein concentration even when albumin concentration is decreased. Plasma immunoglobulin A (IgA) is often increased in alcoholic liver disease, IgG in autoimmune hepatitis and IgM in primary biliary cirrhosis, but these changes are non-specific. More useful diagnostic information may be obtained from measuring individual autoantibodies: anti-mitochondrial antibody is increased in almost all patients with primary biliary cirrhosis, and anti-smooth muscle and anti-nuclear antibodies in many patients with autoimmune hepatitis (Fig. 5.6). Viral infection, an important cause of both acute and chronic liver disease, can be detected by measurement of viral antigens and antibodies to them.



Diagnostically useful changes in the concentrations of other plasma proteins are shown in Figure 5.7.




Other tests of liver function


Given the imperfections of the simple tests of liver function that have been discussed above, it is not surprising that many tests have been devised with a view to providing greater diagnostic sensitivity and specificity. Various dynamic tests, which give an indication of functional hepatic cell mass, are available, but are infrequently used. They may be considered as analogous to the use of clearance measurements for renal function. Marker substances are used that are excreted or metabolized by the liver, and either the rate of their removal from the blood or the rate of formation of a metabolite is measured. However, these processes depend on hepatic blood flow as well as hepatic metabolism, more so for substances that are efficiently extracted from the blood at first pass. Substances used include aminopyrine, antipyrine, indocyanine green, galactose and lidocaine (lignocaine). These tests are more sensitive than conventional tests but are more time-consuming; their use is likely to remain limited to special situations (e.g. the monitoring of novel treatments, assessment of prognosis, etc.). The simplest of these quantitative tests of liver function (requiring only a single blood sample) is measurement of the formation of monoethylglycinexylidide (MEGX) after administration of a bolus of lidocaine. Unfortunately, the reference range is wide, and serial, rather than isolated, measurements are likely to prove more useful. Thus the standard panel of biochemical tests in hepatobiliary disease continues to be, as it has been for many years, albumin and total bilirubin concentrations and the activities of one or other aminotransferase, alkaline phosphatase and γ-glutamyl transferase, together with the prothrombin time.


Plasma bile acid concentrations are increased in liver disease but, while this is a highly specific finding, bile acid measurements are in general no more sensitive than conventional tests. They do, however, have a special role in liver disease developing during pregnancy (see p. 101). The use of biochemical tests to detect hepatic fibrosis is discussed on p. 93.



Non-biochemical investigation of hepatobiliary disease


Many other types of investigation can provide valuable information in patients suspected of having hepatobiliary disease. Imaging techniques provide primarily anatomical information. Transcutaneous ultrasound examination is low cost and safe, and is widely used as a first-line imaging investigation. It can reveal gallstones, dilatation of the biliary system, tumours and the characteristic hyperreflectivity of hepatic fatty infiltration. Endoscopic ultrasound is particularly good for visualizing the pancreas and portal vein. Oral cholecystography (to visualize the gallbladder) is no longer used in the UK. Cholangiography is used to examine the biliary system using an X-ray contrast medium given either endoscopically (endoscopic retrograde cholangiopancreatography, ERCP) or percutaneously into the liver (percutaneous transhepatic cholangiography, PTC); intravenous cholangiography has now been largely superseded. Arteriography can reveal the typical pathological circulation in hepatic tumours. Various techniques of computerized tomography (CT) and magnetic resonance imaging (MRI) can demonstrate structural abnormalities and space-occupying lesions in the hepatobiliary system. Magnetic resonance cholangiography is tending to replace contrast cholangiography where it is available. Isotopic scanning (nuclear medicine) techniques are of limited use but are used in the evaluation of tumours and to assess the patency of the cystic duct. The ‘gold standard’ of diagnosis, particularly in chronic liver disease and cancer, is histology, usually of tissue obtained by percutaneous biopsy.



Liver disease



Acute hepatitis


Acute hepatitis is usually caused by viral infection (particularly with hepatitis viruses A, B, C, D and E but also Epstein–Barr virus and cytomegalovirus) or toxins (e.g. alcohol, carbon tetrachloride, various fungal toxins and a host of drugs, of which the most frequently implicated is probably paracetamol (acetaminophen)). There is considerable variation in the severity and time course of the disease, but the pattern of changes in the standard liver function tests reflects the common underlying pathological process and is similar whatever the cause.


Patients may present with jaundice but there is often a pre-icteric stage with relatively non-specific symptoms such as anorexia and malaise. Infection with hepatitis A usually occurs in children and is often asymptomatic (it is less likely to be so in adults); hepatitis E is endemic in many areas of the world but is infrequently acquired in the UK.


Early in the course of acute hepatitis, bilirubin and urobilinogen are usually readily detectable in the urine by a simple dip-stick technique. For as long as the plasma bilirubin is raised, bilirubin continues to be excreted in the urine. Urobilinogen may disappear from the urine at the height of the jaundice, when, because of cholestasis, no bilirubin reaches the gut, but it reappears as the hepatitis resolves and biliary excretion returns to normal. These changes (Fig. 5.8) are of no practical value in the management of hepatitis, but the detection of bilirubin in the urine is a simple and valuable diagnostic pointer to hepatitis in the pre-icteric stage of the illness.



Many cases of viral hepatitis resolve completely. In severe cases, hepatic failure may develop (fulminant hepatitis), but most patients who survive the acute illness eventually recover completely, aminotransferase activities falling to normal in 10–12 weeks. In some cases of infection with hepatitis B and C viruses, aminotransferase activities remain elevated; antigenaemia persists and chronic liver disease ensues. Infection with hepatitis A or E never leads to chronic disease, although some patients experience prolonged cholestasis with hepatitis E. Infection with hepatitis E carries a particular risk (to the mother and the fetus) during pregnancy.




Chronic hepatitis


Chronic hepatitis is defined as hepatic inflammation persisting for more than six months. There are many causes. Autoimmune hepatitis, chronic infection with hepatitis B or C, and alcohol are particularly important.


Autoimmune hepatitis (formerly called chronic active hepatitis) has a median age of onset of 45 years, although it can occur at any age; it occurs three times more frequently in women than in men. It can also present acutely. The aetiology is unknown. There is a strong association with other autoimmune diseases. There is no single pathognomic test that applies to all patients: a viral aetiology must always be excluded. Autoantibodies (anti-nuclear and anti-smooth muscle) are frequently present in the serum in high titre. Anti-liver–kidney microsomal antibodies are characteristic of a type of autoimmune hepatitis that more frequently presents in childhood and is more often acute in onset with a more aggressive course. However, none of these antibodies may be detectable at first presentation in up to 10% of patients, and autoantibodies (particularly anti-smooth muscle) are present in 10–15% of patients with viral hepatitis.


Plasma aminotransferase activities are usually elevated in chronic hepatitis, but other liver function tests are often normal unless cirrhosis develops. Although the natural history of autoimmune hepatitis is of progression to cirrhosis, this is often preventable if immunosuppressive treatment (usually with azathioprine and/or corticosteroids) is started early in the course of the condition.



Acute liver failure


This term encompasses a range of clinical syndromes of severe liver dysfunction and encephalopathy (neuropsychiatric dysfunction) developing within six months of the first clinical evidence of disease. It is also referred to as fulminant hepatic failure.


Acute liver failure can be hyperacute (encephalopathy developing within seven days of the onset of jaundice), acute (7–28 days) or sub-acute (jaundice preceding encephalopathy by 5–12 weeks). It is a rare condition: toxins and drugs (e.g. paracetamol) and viral hepatitis are the most frequent causes. The underlying hepatic lesion is usually potentially reversible, as the liver has a considerable capacity for regeneration, but the metabolic disturbance is profound and the prognosis poor: acute hepatic failure is often accompanied by renal failure.


Metabolic features of acute hepatic failure include severe hyponatraemia, hypocalcaemia and hypoglycaemia. Hydrogen ion homoeostasis is frequently disturbed. Lactic acidosis may develop as a result of the failure of hepatic gluconeogenesis from lactate, but may be masked by a respiratory alkalosis caused by toxic stimulation of the respiratory centre. Generalized depression of the brainstem may lead to respiratory arrest. In some cases (although not usually with paracetamol poisoning), a metabolic alkalosis predominates: this is in part related to excessive urinary potassium loss, due to intracellular potassium depletion and secondary aldosteronism, and in part to the accumulation of basic substances, such as ammonia, in the blood.


Despite the fact that renal failure may also be present, the plasma urea concentration is often relatively low, reflecting decreased hepatic synthesis. The plasma creatinine concentration is theoretically a more reliable guide both to renal function and to whether the patient should be haemodialysed, but some methods of measuring creatinine are subject to interference by bilirubin and produce invalid results in patients with jaundice. The prothrombin time is greatly prolonged as a result of impaired hepatic synthesis of clotting factors, and bleeding is an almost universal clinical problem.


Hepatic encephalopathy is the term used to describe the reversible neuropsychiatric syndrome that can occur in both acute and chronic liver failure: it is discussed on p. 94. Patients with acute liver failure are at particular risk of developing raised intracranial pressure, for which the first-line treatment is intravenous mannitol. This acts by increasing plasma hyperosmolality and necessitates its monitoring.


Apr 3, 2019 | Posted by in BIOCHEMISTRY | Comments Off on The liver
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