Analgesic Toxicity: Aspirin and Paracetamol

Analgesic Toxicity

• Analgesics, specifically aspirin and paracetamol, are the most common pharmaceutical agents involved in overdoses.
• Aspirin poisoning, also known as salicylism, involves acetylsalicylic acid. Its uses include antiplatelet effects, mild to moderate pain relief, anti-inflammatory properties, and fever reduction.
• Acute aspirin poisoning can result from a single overdose, with a 2% mortality rate, potentially inducing respiratory alkalosis followed by metabolic acidosis.
• Chronic aspirin poisoning occurs from prolonged high-dose use, exhibiting a 25% mortality rate, often more severe in children.
• Aspirin toxicity mechanisms involve the inhibition of oxidative phosphorylation, resulting in metabolic acidosis. Increased renal bicarbonate excretion is another characteristic effect. This leads to symptoms like nausea, vomiting, diaphoresis, tinnitus, deafness, vertigo, agitation, delirium, and hallucinations. Initially, respiratory alkalosis is prominent.

Paracetamol Toxicity

• Acetaminophen, a common analgesic, is generally safe, but a significant overdose can lead to acute hepatic necrosis (fatal).
• Daily intake of more than 4 grams over two weeks can induce toxicity.
• Acetaminophen poisoning causes about half the acute liver failures in the U.S.
• The metabolism of acetaminophen involves primarily hepatic glucuronidation and sulfation pathways to form inactive metabolites, with a smaller percentage being excreted unchanged.
• In overdose situations, the cytochrome P450 pathway becomes significant, creating NAPQI, a highly reactive metabolite that damages liver cells unless counteracted with glutathione.
• Acetaminophen toxicity is a saturation example of deactivation pathways. Excessive dosage depletes hepatic glutathione, binding the reactive metabolite to macromolecules, leading to hepatic necrosis.
• Early administration of sulfhydryl compounds like N-acetylcysteine (NAC) prevents liver damage.
• Paracetamol poisoning is characterized by three phases:
• Phase 1: (few hours to 24 hours): Nausea, vomiting, diaphoresis (sweating).
• Phase 2: (1-3 days): Elevated liver enzymes, increased serum bilirubin, pain.
• Phase 3: (3-5 days): Hepatic necrosis and encephalopathy; serious liver failure can develop within 4 to 5 days if hepatic damage is severe, leading to death.

Factors Affecting Paracetamol Toxicity

• Enzyme inhibitors, like omeprazole, decrease the hepatotoxicity of acetaminophen.
• Enzyme inducers, such as carbamazepine, increase the hepatotoxicity of acetaminophen.
• Patients with low glutathione reserves, due to factors like chronic alcohol abuse, fasting, malnutrition, or HIV infection, are at higher risk for paracetamol toxicity.
• Competition for glucuronidation pathways (e.g., sulfa drugs and azathioprine) can increase the likelihood of toxicity.

Management

• General measures (within 2 hours): Gastric lavage and activated charcoal.
• Specific measures: Intravenous fluids and infusions involving sodium bicarbonate (NaHCO3) to correct acidosis and enhance the elimination of paracetamol through the urine are preferred treatments.
• Antidote: N-acetylcysteine (NAC) – most effective within the first 8 hours.

NAC (N-acetylcysteine)

• NAC acts as an antidote by providing sulfhydryl donors to NAPQI to detoxify it.
• It acts as a precursor to cysteine, supporting glutathione production.
• It enhances sulfation of remaining acetaminophen.
• It acts as a free radical scavenger, enhancing oxygen uptake.

Test your knowledge of analgesic toxicity, focusing on aspirin and paracetamol overdose. This quiz covers the mechanisms of toxicity, symptoms, and mortality rates associated with both acute and chronic poisoning. Understanding these concepts is crucial for health professionals dealing with overdose cases.