Liver, biliary system and pancreas

Bilirubin

Jaundice becomes visible when the serum bilirubin is more than 40 µmol/L and is the most common sign of liver disease. It has several causes. Bilirubin pigment is a breakdown product of haemoglobin ( Fig. 16.2 ). It is normally present in low levels in the blood as an unconjugated water-insoluble form bound to albumin. This is converted to water-soluble conjugated bilirubin by hepatocytes and excreted in bile. If bile outflow is interrupted, conjugated bilirubin is resorbed and excreted in urine, making it appear dark. Once in the intestine, bilirubin is converted by bacteria to faecal urobilinogen (stercobilinogen), and its metabolites account for the brown colour of faeces.

Prehepatic jaundice (unconjugated hyperbilirubinaemia)

Increased breakdown of red blood cells ( haemolysis ) (see Ch. 23 ) can result in production of bilirubin that exceeds the capacity of the liver to conjugate and excrete it; the bilirubin is therefore unconjugated.

Intrahepatic jaundice

Jaundice due to failure of hepatocytes to conjugate and excrete bilirubin is a symptom of:

• acute diffuse liver cell injury

• liver cell failure in end-stage chronic liver disease

• inborn errors of bilirubin conjugation and bile salt excretion, including cholestasis in pregnancy.

The excess bilirubin is predominantly conjugated, reabsorbed into the blood, and is excreted in the urine, causing darkening.

Posthepatic jaundice

This is due to obstruction of the bile ducts — see ‘ biliary system ’ below. As with intrahepatic causes, the excess bilirubin is conjugated and darkens the urine. Conversely, the patient's faeces are pale because bile pigment does not reach the intestinal contents.

Liver cell enzymes

In hepatocyte injury, damage to the membranes of cells and their organelles allows intracellular enzymes to leak into the blood, where the elevated concentrations can be measured. Examples include alanine transaminase (ALT), aspartate transaminase (AST) and gamma-glutamyltransferase. The enzyme alkaline phosphatase is normally present in cells lining bile ducts. A raised level in the blood is seen in biliary obstruction. Many of these enzymes are not specific to the liver; therefore the results of diagnostic serum assays need careful interpretation.

Albumin

Albumin is a major serum protein synthesised by the liver cells. It has a relatively long half-life, so low levels indicate long-term insufficient liver function. Serum albumin may be low for other reasons such as leakage from glomerulus into urine (nephrotic syndrome) or protein malabsorption.

Clotting factors

Liver cells synthesise most clotting factors; therefore a bleeding tendency results from hepatic insufficiency. This can be detected in the laboratory by measuring the prothrombin time. Because the clotting factors have a relatively short half-life, coagulopathy develops in severe acute liver disease as well as the end stage of chronic liver disease.

Pathogenesis

In acute hepatitis, there is hepatocyte injury with cell death, inflammation and regeneration without structural change to the liver. The normally uniform liver cell plates are transformed into ‘ lobular disarray’ characterised by gaps in the liver cell plates, swollen surviving hepatocytes, apoptotic bodies, small regenerated hepatocytes, inflammatory cells and active Kupffer cells ( Fig. 16.3 ). Histological features are similar in acute hepatitis whether it is due to hepatotrophic viruses, drugs or autoimmune hepatitis.

The extent of liver cell destruction seen in the biopsy is classified as ( Fig. 16.4 ):

• death of individual liver cells by apoptosis (spotty necrosis, hepatocyte drop-out)

• death of adjacent hepatocytes, for example, in the zone around the hepatic vein (zone 3) ( zonal necrosis )

• death of a swathe of hepatocytes between a portal tract and hepatic vein ( bridging necrosis ); the supporting reticulin framework of the liver collapses. Care is needed to avoid mistaking the resulting condensation of reticulin fibres for scar tissue of chronic liver disease

• liver cell death affecting all hepatocytes between adjacent portal tracts ( panacinar and multiacinar necrosis ); when extensive ( massive necrosis ), this can result in liver failure and death.

Outcome of acute hepatitis

• Resolution — the cause does not persist and the liver returns to normal (e.g. hepatitis A, E, most cases of hepatitis B, most drug-induced hepatitis)

• acute liver failure — with risk of death (rare, any cause)

• chronic hepatitis, may progress to cirrhosis — the cause persists (e.g. autoimmune hepatitis, chronic hepatitis B and C, some drugs); the initial acute hepatitis may have been mild and unrecognised.

Histology

The histopathological abnormalities include specific features of particular diseases and nonspecific effects of injury, including some inflammation, fibrous scarring and liver cell regeneration.

Examples of specific patterns of liver disease include:

• fatty liver disease — seen in alcoholic liver disease and liver disease associated with obesity and diabetes (known as nonalcoholic fatty liver disease, NAFLD)

• chronic hepatitis with inflammation and injury to hepatocytes, especially those surrounding portal tracts — known as interface hepatitis — the pattern seen in chronic viral and autoimmune hepatitis

• liver injury consequent on diseases of the bile ducts — includes intrahepatic ducts and effects of large (extrahepatic) bile duct obstruction

• inborn errors of metabolism where substances accumulate in liver cells — for example, iron in haemochromatosis, copper in Wilson disease, alpha-1 antitrypsin deficiency

• vascular abnormalities, for example, consequences of thrombosis of portal or hepatic veins.

Those with biochemical and serological features are listed in Table 16.3 .

Histology

The spectrum of alcoholic liver injury which can be seen in liver biopsies includes ( Fig. 16.5 ):

• fatty change ( steatosis ) fat globules in the cytoplasm of liver cells, a disturbance of energy metabolism which on its own is reversible when alcohol consumption stops

• alcoholic steatohepatitis — a combination of fatty change with cell swelling (ballooned hepatocytes), Mallory-Denk bodies and inflammation. In contrast to pure fatty change, steatohepatitis leads to liver cell death and fibrosis

• progressive architectural damage ranging from pericellular fibrosis to cirrhosis.

Pathogenesis

Alcohol produces liver injury by a variety of mechanisms.

• Alcohol metabolism disturbs other metabolic pathways, such as carbohydrate and fat metabolism, so fat accumulates in the liver cells.

• Acetaldehyde, the main product of alcohol metabolism, is toxic to liver cell proteins, resulting in injured hepatocytes and an inflammatory reaction.

• Alcohol stimulates collagen synthesis in the liver, leading to fibrosis and eventually cirrhosis.

Sustained alcoholic liver injury in genetically susceptible people who continue to drink results in irreversible end-stage cirrhosis. Because the fibrosis is pericellular and abundant, the cirrhosis is micronodular, although if abstinence is maintained after cirrhosis has developed, this may evolve into inactive macronodular cirrhosis as a result of regeneration of hepatocytes.

Recommended safe limits for alcohol continue to be debated. Currently men and women in the UK are advised not to regularly drink more than 14 units of alcohol per week. (One unit of alcohol is 10 mL by volume, equivalent to: half a pint, or c . 300 mL, of beer; or 25 mL of spirits. A small glass of wine contains 1.5 units.)

Hepatitis viruses

The hepatotrophic viruses specifically infect and injure hepatocytes. The initial infection may result in a clinically acute hepatitic illness, or go unnoticed. Hepatitis B and C (and hepatitis E in immunocompromised adults) can establish chronic infection. The hepatotrophic viruses ( Table 16.4 ) are:

• hepatitis A virus (HAV)

• hepatitis B virus (HBV)

• hepatitis C virus (HCV)

• hepatitis E virus (HEV)

• delta agent, a defective virus which only infects people with hepatitis B.

The liver may also become infected by many other viruses, but these are not regarded as ‘hepatotrophic viruses’ because the infection is not confined to the liver; clinically significant hepatitis is usually seen only in immunocompromised patients. Examples include:

• infectious mononucleosis due to Epstein–Barr virus

• herpes simplex virus 1

• cytomegalovirus.

Hepatitis A virus

The main characteristics of hepatitis A are:

• ‘faecal–oral’ spread

• relatively short incubation period

• sporadic or epidemic

• does not cause chronic hepatitis

• mild illness, full recovery usual.

In most countries, infection by the virus is common, usually in youth; the resulting illness is often very mild and jaundice absent or slight. Very rarely, it causes severe hepatitis; death is extremely rare.

HAV passes from one individual to another by ‘faecal–oral’ transmission — usually indirectly, such as by the contamination of food and drinking water with sewage. Specific diagnosis is made by detecting IgM-class antibody to HAV in the patient's serum; this indicates recent infection.

Hepatitis B virus

The main characteristics of hepatitis B are:

• spread from people with chronic hepatitis B by blood, blood-contaminated instruments, blood products and venereally

• relatively long incubation period

• liver damage by antiviral immune reaction

• chronic infection when immune system fails to eradicate the virus.

Infection by HBV is transmitted by blood and blood products from people with chronic hepatitis B. In developed countries, HBV is most often transmitted by contaminated needles, such as may be used for tattooing or by drug addicts. Other routes are sexual contact, and spread from mother to child. This is a common route of infection in parts of the world where hepatitis B is prevalent. Protective vaccination against hepatitis B is given to people at risk of infection, for example, health care workers or to populations in high-incidence countries.

Diagnosis is by virology, using serological tests for hepatitis B antigens and antibodies and for the DNA of circulating viral particles detected by PCR test. IgM antibodies against hepatitis B are produced in acute hepatitis, followed by IgG antibodies against hepatitis B core and surface antigen if the acute infection is cleared. Persistent hepatitis B surface antigen (HBsAg) in the patient's serum indicates that chronic hepatitis has developed, and if accompanied by the ‘e’ antigen (HBeAg), indicates active viral replication and high infectivity of the blood.

The liver cell damage in hepatitis B is caused by the body's immune reaction to viral antigens expressed by the infected liver cells, and the outcome of hepatitis B infection depends on the immune response.

In most cases, a strong immune response results in acute hepatitis with eradication of the virus. Patients with an impaired immune response insufficient to clear the virus develop chronic hepatitis with production of excess HBsAg by hepatocytes; this is visible as inclusions with a ‘ground-glass’ appearance in the hepatocyte cytoplasm. The suppressed virus may mutate and develop altered antigens, resulting in flares of inflammation. Over time, there is progressive scarring, cirrhosis and risk of HCC.

In perinatal infection, there is tolerance to the viral antigens, and hepatitis B can proliferate in hepatocytes to release high levels of viral particles into the blood without any liver injury. Eventually, a limited immune reaction develops, resulting in chronic hepatitis

Hepatitis C virus

The main characteristics of hepatitis C are:

• spread by blood, blood-contaminated instruments, blood products, and possibly venereally

• usually asymptomatic and unrecognised initial infection

• fluctuating liver biochemistry

• chronic hepatitis in around 70% cases.

Hepatitis C was identified in the early 1990s; prior to this, patients having blood transfusion and the administration of clotting factor concentrates were at risk of infection. In developed countries, donors are now screened for HCV, and most cases are acquired by needle sharing among users of intravenous drugs. In developing countries, the prevalence of infection may be high (>10% in some areas) and routes of infection unclear. Most infections are clinically silent and in around 70% the initial immune response fails to eliminate the virus, resulting in chronic hepatitis. Progression to cirrhosis in untreated cases is on average about 10% per decade but is very variable; factors associated with progression are older age at acquisition, male, alcohol consumption, and coinfection with other viruses (hepatitis B, HIV). Biopsies often show steatosis as well as features of chronic hepatitis.

Hepatitis E virus

Hepatitis E is a water-borne virus, distinct from HAV, which is endemic in some developing countries where it can cause severe hepatitis during pregnancy. It has recently been recognised in developed countries (genotype 3) where it can be acquired from poorly cooked meat. It can cause acute hepatitis with jaundice in adults, and can cause chronic hepatitis in immunocompromised hosts (liver transplant recipients).

Delta agent (hepatitis D virus)

Delta agent is an incomplete RNA virus that can only infect hepatocytes in the presence of HBV. Its main effect is to aggravate the consequences of HBV infection, either as more severe acute infection, or a flare of inflammation in chronic HBV infection.

Pathophysiology

Bilirubin is just one constituent of bile, a complex excretory product of which bile salts are a major constituent (50% dry weight). Bile salts function in micelle formation and fat absorption from the gut, from which they are reabsorbed into portal blood and then reexcreted several times a day in an enterohepatic circulation. Diseases that damage peripheral branches of the biliary tree result in bile salt accumulation in the surrounding periportal hepatocytes, together with accumulation of copper, also excreted in the bile. Such bile duct injury is associated with raised serum alkaline phosphatase, disturbed serum lipids and characteristic changes on liver biopsy. Bile salt accumulation also causes itching ( pruritus ), a common and troublesome symptom of chronic biliary disease.

Histology

Liver biopsy may show bile duct lesions specific to the disease — granulomatous inflammation in PBC and periductal fibrosis in PSC, and these progress to destruction of the intrahepatic ducts ( ductopenia ). In addition, as the disease progresses, there is proliferation of marginal ductules around portal tracts (‘ ductular reaction’ ), followed by portal tract fibrosis, bridging fibrosis and cirrhosis. The periportal hepatocytes show swelling and accumulation of copper bound to copper-associated protein, which can be identified with the Shikata stain and is a useful early sign of chronic biliary disease ( Fig. 16.7 ). Other rare causes of acquired ductopenia include graft-versus-host disease, drug reactions and Hodgkin lymphoma.

Unrelieved large bile duct obstruction (see ‘bile ducts’ below) may result in similar liver biopsy appearances, although associated with bilirubin visible in the canaliculae between hepatocytes only rarely with ductopenia.