Chemical Pathology - Semester 4 Summary PDF

Chemical Pathology Liver anatomy Hepatic artery (500 ml/min) o Oxygenated o 100 mm Hg Portal vein (1000 ml/min) o Non-oxygenated o 10 mm Hg Hepatic vein o Venous drainage o 5 mm Hg Liver architecture St...

Chemical Pathology Liver anatomy Hepatic artery (500 ml/min) o Oxygenated o 100 mm Hg Portal vein (1000 ml/min) o Non-oxygenated o 10 mm Hg Hepatic vein o Venous drainage o 5 mm Hg Liver architecture Structural unit is hepatic lobule o Hexagonal unit o Surrounded by connective tissue septae o Portal tracts lie perpendicularly in the septae, containing branches of: ▪ Hepatic arteriole ▪ Portal venule ▪ Bile ductule o In the centre of each hepatic lobule is a branch of the hepatic venule Functional unit is hepatic acinus o Liver tissue lying between two centrilobular venules, with the portal tract in the centre. o Blood flow: ▪ From the portal tract ▪ Through the sinusoids ▪ Into the centrilobular venule o Therefore 3 functional zones based on oxygen content: ▪ I – Periportal, most O2, oxidative metabolism ▪ II – Mediolobular ▪ III – Centrilobular, least O2, CYP450 drug metabolism Functions of the liver 191 Bilirubin metabolism Source o Breakdown of RBCs in macrophages in spleen o > Hb -> haem -> biliverdin -> bilirubin Metabolism and excretion: o Unconjugated (hydrophobic) bilirubin carried to liver bound to albumin o In the liver, bilirubin is conjugated with glucuronic acid by UDP-glucuronyl transferase (UDPGT) enzymes I and II to BMG and BDG o Conjugated (water-soluble) bilirubin transported into bile canaliculi o Bile containing bilirubin secreted into GIT o In colon, conjugated bilirubin converted to urobilinogen -> urobilin (yellow) and stercobilinogen -> stercobilin (brown) o Up to 20% of urobilinogen undergoes enterohepatic circulation: ▪ From GIT re-absorbed into portal circulation ▪ Taken back to liver and re-absorbed and re-excreted ▪ Urobilinogen is therefore normally present in blood, and is filtered into urine Bile acid production Bile acids formed from breakdown of cholesterol Liver: o Produces primary bile acids (cholic acid; chenodeoxycholic acid) o Conjugates them with glycine or taurine (increases solubility) o Transports them into bile GIT: o Bacteria produce 2° bile acids (deoxycholic acid and lithocholic acid) Enterohepatic circulation of bile acids - 75% reabsorbed Structure of bile acids: Amphipathic Functions of bile acids: o Solubilise cholesterol for excretion o Stimulate bile formation and flow o Solubilise dietary fat for absorption in micelles 192 Circulating bile acids Normal o Bile secreted into GIT with a meal o Bile acids re-absorbed from GIT and appear in the blood o Bile acids transferred back to liver with enterohepatic circulation and gradually removed from blood Abnormalities 1. Obstruction in the biliary tree with reflux into circulation 2. Liver damage with impaired re-absorption of bile acids from the portal circulation 3. Ileal disease when bile acids are not re-absorbed Protein production The liver synthesizes practically all circulating plasma proteins except Ig and coagulation factor VIII Total protein in plasma: 60 - 80 g/L o Mainly albumin (35 - 50 g/L), synthesized in the liver o Immunoglobulins (approx. 15 g/L), synthesized by plasma cells o A variety of other proteins at low concentrations, synthesized by the liver Functions of some plasma proteins: o Albumin: oncotic pressure, buffering, general binding, and transport protein o Immunoglobulins: immune response o CRP: activates complement, binds to damaged tissues and pathogens, role in innate immunity ▪ Indicator of how well the body is handling infection and inflammation ▪ Increase shows infection or ongoing inflammation o Coagulation factors and fibrinogen: coagulation and clot formation o Transferrin and Ferritin: iron transport and iron storage o Caeruloplasmin: copper and iron metabolism Detoxification Ammonia o Produced from amino acid metabolism o Toxic to the CNS o Converted to urea in the urea cycle, in the liver Xenobiotics (foreign compounds, e.g., drugs) o Liver deals with hydrophobic compounds by making them water-soluble for excretion in urine & bile o Phase I ▪ Modification chiefly by cytochrome P450 enzymes o Phase II ▪ Conjugation to add polar groups ▪ Glucuronide, glutathione, sulphate, and glycine are the chief conjugates o Phase III ▪ Excretion ▪ The compound is moved from the liver into the bile by transporters 193 Biochemical tests of liver function and liver damage (NB) Two kinds of tests: those that show damage and those that show the liver is performing its function Bilirubin: liver function and damage Measurement of bilirubin o Conjugated – “direct reacting” o Unconjugated – “indirect-reacting” ▪ Stuck to albumin so it is not available to react with the test dye, need to add another reagent to measure unconjugated o Total – Reaction includes an extra step so that all bilirubin reacts ▪ Measure conjugated and then total, must calculate unconjugated by subtracting Normal levels in plasma o Total bilirubin 1 indicates more severe acute disease (greater cell injury affecting mitochondria) o AST:ALT ratio > 1 also more common in cirrhosis o AST:ALT ratio > 2 suggests alcoholic liver disease Level of transaminases in various hepatocellular diseases o Levels < 15 x ULN - usually seen in chronic liver disease, or milder forms of liver disease o Levels > 15 x ULN - usually seen in more severe forms of acute liver disease ▪ Levels > 25 x ULN - acute viral hepatitis or toxin-induced injury ▪ Levels > 50 x ULN - acute ischaemic hepatitis Jaundice Yellow appearance of skin and sclerae due to increased bilirubin o Pre-hepatic (before the liver) ▪ Excess bilirubin production o Intra-hepatic (in the liver) ▪ Decreased uptake, metabolism or excretion of bilirubin by the liver o Post-hepatic (after the liver) ▪ Obstruction to bile flow Pre-hepatic jaundice Causes o Haemolytic disorders ▪ Abnormal haemoglobins (e.g. sickle cell anaemia) ▪ RBC membrane defects (e.g. hereditary spherocytosis) ▪ RBC enzyme defects (e.g. G6PD deficiency) ▪ Autoimmune haemolytic anaemias ▪ Mechanical RBC damage (e.g. prosthetic heart valves) ▪ Blood group incompatibility ▪ Malaria o Ineffective erythropoiesis ▪ Megaloblastic anaemias Results of laboratory tests: o Increased unconjugated bilirubin (rate of bilirubin production exceeds maximum rate of conjugation) o Normal conjugated bilirubin (no obstruction) o Urine bilirubin negative o Urine urobilinogen increased (increased flux through pathway) o LDH enzyme elevated o Other liver enzymes (ALT, AST, ALP, GGT) normal Perform haematological tests: o Low haemoglobin, haematocrit and red cell count o Increased reticulocyte count o Perform tests to identify cause of haemolysis (RBC morphology, abnormal haemoglobin, Coombs test) Post-hepatic jaundice Causes of cholestasis o In the lumen (gallstones, worms) o In the wall (bile duct carcinoma, stricture, atresia) o Outside the wall (carcinoma of head of pancreas, lymph node enlargement at porta hepatis) Biochemical features: o Jaundice, dark urine, pale stools o Increased conjugated bilirubin in serum and urine o Decreased urobilinogen / urobilin in stools 196  o Increased obstructive enzymes (ALP and GGT) If obstruction is partial (e.g., space-occupying mass in liver) then obstructive enzymes are increased but bilirubin is normal or only mildly increased Intra-hepatic jaundice Inherited defects in bilirubin metabolism (rare) o Defects in bilirubin uptake / conjugation ▪ Gilbert’s syndrome AR inherited disorder; Decreased expression UDPGT I Mildly increased unconjugated bilirubin. Worsened by viral infection and fasting ▪ Criggler-Najjar syndromes: Type I: AR inherited disorder, absent UDPGT I activity. Severely increased unconjugated bilirubin Type II: AD inherited disorder, reduced UDPGT I activity. Moderately increased unconjugated bilirubin o Defects in excretion of bilirubin into bile canaliculi: ▪ Dubin-Johnson and Rotor syndromes Benign inherited disorders. Mildly increased conjugated bilirubin Acquired causes of intra-hepatic jaundice o Acute viral hepatitis o Alcoholic liver disease o Non-alcoholic fatty liver disease o Cirrhosis Acute viral hepatitis Common infection worldwide, especially hepatitis B Risk factors HBV: living in or travel to areas of high prevalence, contact through household or institutional facilities, multiple sexual partners or STDs, MSM, use of injected drugs. Cause of the majority of chronic liver disease Clinically: o Pre-icteric phase ▪ Flu-like illness, nausea, vomiting, diarrhoea, abdominal pain ▪ 2/3 of cases never develop jaundice ("anicteric or subclinical hepatitis") o Icteric phase ▪ Tender hepatomegaly ▪ Jaundice ▪ During later obstructive phase: dark urine (bilirubin) ▪ If intrahepatic cholestasis severe: pruritus, pale stools and steatorrhoea o Recovery phase ▪ Prolonged lassitude, depression Clinical severity varies. Can be asymptomatic; acute hepatitis with resolution; recurrent severe acute hepatitis resulting in liver failure; chronic hepatitis; cirrhosis 197 Acute hepatitis lab test results Early phase: Hepatocellular injury pattern; in late phase additional obstructive pattern (due to cellular swelling). Often mixed picture. Bilirubin: increased unconjugated bilirubin throughout and increased conjugated bilirubin during obstructive phase. Urine: early increase in urine urobilinogen due to impaired re-uptake of urobilinogen by damaged hepatocytes. During obstructive phase, urobilinogen disappears from the urine, but urine positive for bilirubin. Enzymes: early marked rise in plasma transaminases reflecting hepatocyte damage. Can start to rise before onset of jaundice. The more severe the liver damage, the greater the likelihood of mitochondrial damage and the more AST is increased. During obstructive phase, increased ALP and GGT. Plasma proteins: Albumin only slightly decreased. Increase in acute phase proteins. Immunoglobulins: early increase in IgM, later IgG. Alcoholic liver disease Fatty liver, alcoholic hepatitis, alcoholic cirrhosis Biochemical features include: o Raised GGT due to enzyme induction as well as cholestasis (GGT used to monitor alcohol intake) o Mild disease: nothing specific o Severe disease: transaminases are elevated, especially AST (therefore the AST:ALT ratio > 1) Non-alcoholic fatty liver disease (NAFLD) Hepatic steatosis after excluding alcohol and other causes Types: o Non-alcoholic fatty liver (NAFL) - hepatic steatosis is present without inflammation o Non-alcoholic steatohepatitis (NASH) - hepatic steatosis and inflammation Cause not known. Main risk factors for developing NAFLD: o Central obesity o Type 2 diabetes mellitus o Dyslipidemia o Metabolic syndrome Pathogenesis: Not known. Insulin resistance and oxidation damage Biochemical features: Mild elevation of ALT, AST or GGT Diagnosis: Non-invasive imaging (primarily liver ultrasound) in at-risk individuals Treatment: Lifestyle changes including diet (low cal/fat), exercise and gradual weight loss Cirrhosis The final common pathway of many chronic liver diseases Histologically: o Fibrosis o Regenerating nodules o Alcoholic, cholestatic and congestive cirrhosis is micronodular o Post-viral hepatitis cirrhosis is macronodular Derangement of microvasculature leads to portal hypertension – shunting of portal blood to systemic circulation The most common causes of cirrhosis include: o Chronic viral hepatitis (hepatitis B, C) o Alcoholic liver disease o Haemochromatosis o Non-alcoholic fatty liver disease Other causes include: o Autoimmune hepatitis o Medications (methotrexate, isoniazid) o Chronic cholestatic diseases (primary and secondary biliary cirrhosis, primary sclerosing cholangitis, biliary atresia) o Metabolic (Wilson disease, alpha-1 antitrypsin deficiency) o Infection (brucellosis, syphilis, echinococcosis, schistosomiasis) o Vascular (right-sided HF, hereditary haemorrhagic telangiectasia, veno-occlusive disease) 198  Clinical presentation o Symptoms of compensated (early) cirrhosis: ▪ Non-specific weakness, fatigue, fever ▪ Loss of appetite ▪ Loss of weight and muscle mass ▪ Nausea o Symptoms of decompensated cirrhosis: ▪ Jaundice ▪ Pruritus ▪ Dark urine ▪ Bruising ▪ Upper GIT bleeding: haematemesis or melena ▪ Fluid build-up: oedema or abdominal ascites ▪ Abdominal pain ▪ Diarrhoea ▪ Female anovulation: amenorrhoea or irregular menses ▪ Male hypogonadism: impotence, loss of libido, testicular atrophy, infertility ▪ Hepatic encephalopathy: Early symptoms: irritability, personality or behavioral changes. Later symptoms: ataxia, confusion, delirium and coma. 199 Pathophysiology of features and complications of cirrhosis Jaundice: o Shunting of blood past the liver o Impaired conjugation ability due to hepatocellular dysfunction o Obstruction Mixed picture of hepatocellular dysfunction and obstruction: increased unconjugated and conjugated bilirubin Liver enzymes mildly to moderately increased without a characteristic pattern. Ascites (major): o Portal hypertension. o Low plasma albumin due to hepatocellular dysfunction. o Secondary hyperaldosteronism due to low "effective arterial volume" Varices (major): o Portal hypertension causing thin-walled varices at systemic-portal anastomotic sites. o Can bleed if traumatised Bleeding tendency: o Impaired synthesis of clotting factors due to hepatocellular dysfunction. o Vitamin K deficiency due to fat malabsorption. o Thrombocytopaenia due to hypersplenism ▪ Patient not lacking fluid, but lacking in the right compartment, so spleen makes extra fluid Oesophageal varices are the commonest site for bleeding. Anaemia: o Disturbances in vitamin B12 and folate metabolism. o Bleeding. o Hypersplenism. Endocrine changes o Failure to conjugate oestrogens Malnutrition, vitamin deficiencies and loss of muscle mass: o Alcoholism. o Loss of appetite. o Malabsorption. o Decreased protein synthesis esp. albumin o Possibly increased protein breakdown Has negative impact on survival Infection: o Immune dysfunction: lack of liver clearance; reduced white cell number and function; reduced synthesis of complement o Ascites can lead to spontaneous bacterial peritonitis o Hospitalization associated with increased risk of hospital-acquired infection Hepatic encephalopathy (major): o Increased ammonia: ▪ Reduced liver metabolism of ammonia ▪ Increased GIT bacteria metabolism of GIT protein ▪ Portal blood from GIT bypasses liver and ammonia is delivered to systemic circulation ▪ GIT bleeding adds additional protein load o Ammonia crosses BBB (pathophysiology not critical but interesting) o Ammonia is metabolised in astrocytes by glutamine synthestase to form glutamine. Glutamine accumulation causes osmotic shift with astrocyte swelling and dysfunction. o Ammonia binds to GABA receptors and causes central neurotransmission imbalance. o Other factors involved in hepatic encephalopathy: Hypoglycaemia; GIT microbiome alterations; Inflammatory cytokines Hepatorenal syndrome (major): o Progressive rise in serum creatinine and low sodium (U Na 1. Normal levels don’t exclude cirrhosis.  ALP moderately elevated, usually no more than 2-3 x ULN  Albumin reduced. Level correlates with severity of cirrhosis.  INR prolonged. Level correlates with severity of cirrhosis.  Hyponatraemia. Increased creatinine if hepatorenal syndrome has developed  Polyclonal hypergammaglobulinaemia  Thrombocytopaenia and anaemia Results of laboratory tests in liver failure:  Bilirubin elevated  AST and ALT elevated. May decrease with worsening of liver failure due to total loss of hepatocytes.  Albumin low  INR prolonged  Creatinine elevated  Ammonia elevated  Glucose low Extra notes from here on: Unconjugated bilirubin  Main form found in plasma (~17umol/L)  Hydrophobic. Carried bound to albumin.  Cannot be filtered by glomerulus so not found in urine.  Can be displaced from albumin by drugs.  Free (unbound) form is neurotoxic. If it crosses BBB causes kernicterus. (Risk in neonatal hyperbilirubinaemia).  Delivered to liver. Dissociates from albumin at hepatocyte membrane.  Increased amounts of unconjugated bilirubin are found in plasma in: o Increased bilirubin production beyond the liver’s capacity to conjugate. o = “Pre-hepatic” cause of jaundice 201